Net Skiff Build

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Net Skiff Build

#1

Post by kmorin » Sat Feb 18, 2012 12:37 pm

Welder, AAB.com, netman, let's begin the river net fishing skiff here then welder can move the entire thread to a location suited for one-off builds?

I'd like to preface this thread with some notes which may need to be kept in mind as we move along in the build. I will be explaining this skiff beginning with its hull shape, or form, in narrative (words) and sketches (illustrations) with the goal of helping netman create a net fishing, welded aluminum boat for the rivers of the Midwest USA. Netman's cost will be his time to post the build progress here for the AAB.com readers and visitors to follow.

I have used this method of construction and kept to the reasoning guidelines I'll show, to build lots of boats of this type. I am sure others can do it as well once they're shown how. What I can't do is predict all the variations that may arise if these methods are not followed or what problems a hull may experience if lots of innovations result in a waste of metal. So if you follow the methods shown, I'm confident you'll have a good skiff, and on the other side of that coin, if you decide to make changes as you go along, then I'm not sure where you'll end up with your boat project.

Further, I am not selling designs. This design is essentially public knowledge due to the number of past associates/students/co-workers who have built this set net skiff in the Cook Inlet area of South Central Alaska, and therefore I do not support the design by providing problem solving or telephone support! I offer this article to help one fisherman create his own skiff and others are welcome to use the information for their own use, but I am not offering to support other builds.

To begin, I suggest that anyone with interest in this build take a few minutes to read the 'Helpful Info' here on AAB.com regarding the Lines of a hull form. This article uses a dory hull to describe the various hull intersection lines and planes, but I think it will make the initial discussion of netman's skiff hull easier to 'see'.

cheers,
Kevin Morin
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Re: Net Skiff Build

#2

Post by tracy » Sat Feb 18, 2012 12:51 pm

What would you be looking for, for size. I'm working with a cad program, and would be posting them.
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Re: Net Skiff Build

#3

Post by kmorin » Sat Feb 18, 2012 1:54 pm

tracy, netman's skiff will be based on the 6' bottom sheet and is essentially a flat bottom design. The bottom is cambered and has the longs outside as 'rails' or small keels and impact plates and the transverse structurals are all inside made of bent/press braked sheet (1/8") and continuous welded to the bottom and sides to form box beam like structures on the hull panels.

We'll begin this post by reviewing some of the very basics that need to be clear so the bottom shape makes sense and Randy clearly see's what can go wrong, and what the result of the most common errors in this method of building would be.

If you've had the chance to do a quick review of the lines of a hull, then you'll recall the keel plane or keel, is a vertical plane at the center of the long axis of the boat. The Buttock Lines, or butt lines are planes taken through the hull parallel to the keel and offset toward the chine(s)?

In welded boats using a single piece bottom, there is a risk of a butt line rising toward the stern. If the butt lines are not straight in the after third or quarter of the planing hull, that hull will not balance fore and aft (bow to stern) when planing.

Image

Here, the butt lines rise aft, so that is the best explanation of why dories won't plane when they are powered.

Image

Here is another hull type. A displacment hull (tug in this case), and the bottom shape is designed to go slowly through the water, not fast OVER the water, so the buttock lines rise as you follow them (not drawn in this sketch) aft.

Image

Here is a typical low V (small deadrise) hull bottom, with a continuous chine plate ('reverse chine' as called by some), and a single keel plane shown to make this reference more clear.

Image
This sketch shows the buttock lines as glass panes, and their intersection to the hull forms lines that begin in the bow with a slow smooth curve downward, and end in straight lines in the after 1/3 to 1/4 of the hull.

If these lines are not kept relatively straight, they will allow the bow up or force it down. If the lines form a bulge or bump in the stern, like the tug hull above, then the bow will usually rise and fall (porpoising) as the waterline shifts aft and the hull can't balance evenly between the push forward and the weight of the bow pushing down. So, a boat with bulged butt lines usually lifts the bow too high, then lets it down, and depending on speed and hp to wt ratio, this continues. So keeping these lines straight is important.

Next, if the lines were to form a hollow or curved upward and then curve back down, not shown in the sketch, this cupped bottom will usually work to force the bow downward as the entire aft end of the skiff is acting like a huge 'trim tab' and the harder and faster the hull is pushed, the more downward force this 'cup' or hollow in the butt lines will force the bow down.

So, the shape of a flat bottom skiff is just as important as a V bottom skiff in regard to these surface intersection lines being straight. OK, for the purist boat form readers, very high performance boats made with extreme tolerances of accuracy may have very small butt line contours to achieve specific performance on that hull, however, for our relatively slow speed and mainly load carrying skiff, we'll stick to the adage that these lines are best kept "straight".

cheers,

Kevin Morin
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Net Skiff Build: more lines

#4

Post by kmorin » Sat Feb 18, 2012 2:13 pm

OK, more lines so we're all on the same page of the hymnal as I start to explain the choices Randy has in front of him to 'design' his version of this skiff building method.

Image
A plane intersecting the hull in the horizontal is a waterline or waterplane. Here's one that passes through the generic shallow V with chine bottom shape for example.

Image
Here are more planes stacked up forming the waterlines which can be seen in the plan view or looking down (without perspective) on the drawing of the hull shape.

We'll need to discuss these shapes a little, so netman can make some choices before we start cutting sheets so the sketches are intended to reduce the words and still get the idea across when we talk about hull shape.

So each of the 'hull intersections' are a series of lines on the hull just like a topographic land map. It does take some time to learn to 'read' these lines quickly, but everyone can 'see' them if time is taken to give a bit of study to the hull's lines and intersections.

Image

This sketch introduces the stations, or cuts through the hull at right angles to the keel plane. This small skiff is being pushed through a glass sheet to show how the stations would be drawn/established/recorded/observed. This view is taken bow on, or stern on, so the station planes or sections are flat, and not in perspective. My view here, is to help make more of a picture of the station plane and the entire skiff hull.

Again, like buttock lines and water lines or water planes these lines are viewed in a stack that shows the hull in even spaced sections from bow to stern. [Yes, for those who see lines regularly, many times the the Body Section Plan is shown with the bow half on the right, and stern half of the hull on the left of the vertical keel plane.]
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Net Skiff Build; comparing hull forms

#5

Post by kmorin » Sat Feb 18, 2012 2:36 pm

Now, to compare shapes of boats, here is how you can use these lines to we've reviewed above, to see how one hull's shape is different from another.

Image
From the bottom, here are a pair of skiff forms that are not identical but are very similar. This view is almost the plan view, but the none of the hull intersections are included - yet. Notice that we can see the shape of the line of intersection between the bottom and the topsides? (chine), and the shape of the top of the sides (sheerline), both of which are curves. However, one set of curves is "more full," or has more rounded hollow in the forward half of the left hand skiff? The right hand skiff is sharper in the bow and topsides.

This comparison will have some impact on how each hull hauls a load and performs with increased displacement which is a kind of special case for small boats primarily for commercial fishing.

Image

This sketch has the two skiffs bow to bow. If you were to compare the sections/stations/Body Plan views of these shapes, you'd find the left hand skiff had more volume the higher you go above the keel. What will happen to a pair of skiffs, one with a sharper bow and more plumb sides, and one with wider full sides and bow when the carry an equal load?

Image
The wider the shape, and the more full the curves, the more water that's displaced (pushed aside) as that 'rounder' bow is buried into a wave face, or just down into the water. That condition would also be true if we look at the waterlines and butts, not just the stations/sections.

What is important is the relationship these sets of hull intersections show us as a means to compare these hull forms. If the boat's purpose is to haul a fixed load, crew, gas and gear, then take on a small amount of fish, then the sharper, narrower boat will offer the best performance. (**** best will be discussed below****). On the other hand, if a boat will begin with one displacement, then take on two or three times that displacement, as in commercial fishing, then the more full shape begins to offer the 'reserve,' or added buoyancy needed for the heavier loads not regularly experienced by recreational boats.

What we need is a shape with a good compromise between a sharp enough lower (bottom) shape to go along at planing speeds without being entirely 'pool table' flat, and the 'filled out' or 'spooned' bow sections so that same hull will lift a huge load over a wave/wake/swell when the boat is slowed by a large added weight.

If this is not making sense, (?) I'd like to hear about it so I can make some edits or clear up questions as we go.

cheers,
Kevin Morin
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Re: Net Skiff Build

#6

Post by tracy » Sat Feb 18, 2012 6:55 pm

what is the load? what is the water? 6' wide, bow rollers, side rollers, speed.
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Re: Net Skiff Build

#7

Post by kmorin » Sat Feb 18, 2012 7:55 pm

tracy, netman fishes the Illinois, and maybe another river. (?) He mentions this in his previous thread(s). I'd say the sheer would be as wide as he can trailer, (8'-6"?) the bottom at least 6'. He fishes a fresh water net called a hoop net (or something), ring net? anyway, it's a net around a series of rings that allow catfish to swim in but not out.

I don't know if they use rollers, powered or otherwise, but I did see a net bar, or picking bar on one of his posts in the past.

He's using a 200 now, I think?, so he's moving in the 50's I'd guess running out, dry. I don't know what a full load is for netman's river fishing. I guess it would depend on species and gear and if the fish are running or not for any given day.

Randy, here are the main choices for the 'runners,' or longs on the hull bottom. http://www.alaskancopper.com/pdf/al/marine.pdf. The 'hull stiffener' is toughest when welded to the hull, but adds the greatest depth for grounding. Below, on this page, is the half pipe(s) that are the least structural contribution but in 2-1/2" or 3", still add a good deal of stiffness to the bottom when welded on as longs and 'keels'.

http://www.alaskancopper.com/pdf/al/6061_angle.pdf, and of course you can put angle on the bottom as the longs and they do help steering as they provide a series of mini-keels to the cambered bottom.

Turning at speed with the half pipes will have more side slip than either of the other shapes with the hull stiffeners tracking like the hull is on rails. So you'd probably want to go around this bush a few times? Deeper is stronger, and more lateral area for tracking, but that shape also grounds earlier/shallower, and so the more shallow shapes may have adequate stiffness while giving those extra few inches of thin water near a shore or low angled loading ramp?

This decision will make some difference in the build, the set up, and surely your BOM, so I mention it early in the project to allow you to spend time on this item's shape.

Cheers,
Kevin Morin
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Re: Net Skiff Build

#8

Post by kmorin » Sat Feb 18, 2012 8:09 pm

Well we're completing an initial discussion of the bottom of a (cambered bottom) net skiff and there are couple more images I've found that will help close this part of the explanation so we can begin talking about the building ideas.

Image
Here is a little rowing skiff with 'rocker,' or fore and aft curvature of the bottom. Hopefully, everyone sees by now these buttock lines rise aft? So, if we put an outboard engine on this skiff, and pushed, the bottom would go down and the bow would go up. The harder we push, the more this will happen.

Image

Here she is from the stern below the waterline, to make this shape more obvious. If someone were to weld a metal hull out of sequence, they could create this shape even if the framing and lines of the boat called for the butts to be straight and parallel in the after half of the bottom.

Image
We also talked about the shape of a net skiff as more full or rounder than the fixed load skiff of the same size. The reason was that, as the waterlines get deeper on the hull, that is, the load line is greater, the skiff has more volume the deeper it gets in the water. Here a couple of shapes are superimposed on one another to show the idea of rounder more full shaped curved (when viewed from the top or plan view) gives a hull shape that is wider, leans out more, (flam) and therefore has a wider, rounder waterline, in plan view for each depth of the hull.

Image
A topic we haven't really covered too much, is the cross section/station/body plan shape of the bottom of a net skiff. The V bottom skiff will ground earlier in deeper water than the cambered (almost flat) bottom skiff. The reason for this is shown in the diagram here, a series of V bottom shapes inside the rounded or cambered shapes with the same beam and depth.

Of course, the V will have an easier motion in a chop at higher speeds, but, if the boat doesn't have at least 10-12 degrees of V, almost all welded aluminum boats will pound in some condition of speed, waves, and load. So, as long as we're discussing a skiff that planes on the outward trip and may (hopefully) return with a load it can't plane, the bottom shape as a curve is most desirable for landing heavy loads on a beach, bar or ramp.

Cambered bottoms allow a commercial skiff to sit broach to the breakers and 'walk' up the beach by rolling as it lifts to the rising surf's run. Then it will 'stay' on the beach and keep moving its load upward higher on the beach. Completely flat bottom skiffs of the same size, do not provide this behavior in the same conditions.

The overall difference in displacement, is only the areas shaded between the curve and the V lines, but that does add up to plenty of flotation over the bottom of a 20' skiff.

The cambered bottom generally allows easier turning since the shape is more of a bowl than a prism, but tight turns may not be all that helpful in a river? The shape of the net skiff in this project, will have most hull features that were developed for net fishing the Cook Inlet in South Central Alaska, where setting in a high speed running tide requires agility and quick turning on a wave face in order for the bowman to be able to grab the buoy and either start or end the set.

Cheers,
Kevin Morin
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Re: Net Skiff Build

#9

Post by netman » Sat Feb 18, 2012 9:52 pm

I fish the Ohio and Wabash Rivers here in Southern Indiana. I live about ten mins from Kentucky and forty mins from Illinois. Kevin and Tracy, I run up and down the river with little needs for a agile boat. I will run about a mile or two and will have a set of gill nets in eddys along the river bank. Some eddys are big, and I will fish four to six one hundred foot long gill nets that are twenty to twenty four feet deep spaced about twenty feet apart. One end of the gill net is tied to the bank, the other end has a sixty to seventy foot anchor rope and anchor. I tie the net to the bank, back out from the bank, and attach floats and net weights to each end of the net and one float/net weight to the middle. Once the floats and weights are attached and I'm backing out away from the bank, I pull the net straight or at an angle, depending on the current, and drop the anchor. The anchor is designed to right itself and dig in. Since the net is in an eddy, the net bows up river creating a pocket. If the eddy is big enough, I will put out as many gill nets as needed to cover the eddy. Most eddys only need one or two gill nets. However, due to competition, you have to put in a couple extra nets to guard the hole. If not, someone else will drop a net or two in your eddy, and compete for the fish that are migrating up river as the water warms. Its better to deal with your own nets than another fisherman's net.
I fish hard and I fish aggressive.
Depending on the wind and current, I run down the gill net from inside out. This means starting at the end closest to the river bank. I will grab the floating jug and pull it into the boat. I have the motor running to adjust the boats attitude. Once the net is alongside the boat, I pull up the bottom of the net, and pull down the net by the top and bottom line, since there is approximately thirty feet of netting bagged up between the top and bottom line. As I pull up to the fish in the net I flip him in the boat and use a net pick to work the fish out.
Since I fish for paddlefish Nov 1 through April 30, I am after females with eggs/roe. I take these eggy fish to my processing facility and process the roe into caviar.
I also fish for shovel nose sturgeon by using hoop nets. Hoop nets are also used for catfish and buffalo fish. Hoop nets are three to three and one half feet in diameter and most are 14' to 16' in length. You can pack a ton of catfish in hoop nets. Hoop nets are mainly fished in the 'blind'. That means no jug or float to mark the net. A hoop net has a anchor tied to the upstream end. I use around seventy five feet of rope from the anchor to the hoop net. When I find a desirable location for a hoop net, I will toss the self righting anchor out, and wait for it to dig in. I then let the current or motor power down river and feed the hoop net out over the side. The hoop net has two throats which face down river. This allows fish swimming up river to enter the bigger throat and up into the tighter throat where they remain until I remove them. I fish these hoop nets in shallow water and deep water depending on water temp and time of year. To locate these hoop nets I throw a net rope drag, or hook, and drag it across the river bottom to catch the anchor rope. Once I pull the anchor rope up to the boat side I put the rope over a vertical pipe and back the boat up, or let the current push me down river. This allows the net to be pulled up the the boat. Once the net is alongside the boat, I roll it up into the boat or power llift it up depending on current and fish load.
During winter/spring gill net season, I may have anywhere from couple of fish to a couple hundred fish. So weight varies each day. Some days fifty pounds, and others, twelve to fifteen hundred pounds.
I live fish for catfish spring, summer and fall to stock private and pay lakes where people sit around a lake in a lawn chair and reel in monster catfish. When doing live hauling, I have three one hundred fifty gallon tanks hooked up to a Sweetwater blower aerating the water to keep the catfish alive. I will have up to a couple thousand pounds of live catfish and water in the boat. However I will say that I have a boat set up for live hauling, and don't plan to use the skiff for this type of work.
The skiff I would like to build will be for winter/spring gill net season when the water is rough. Or, what I believe to be rough. Our prevalent wind direction during the gill net season is west to southwest. Those wind directions turn the Ohio River to an angry river. Three foot swells/waves are a daily event. I run twenty to forty miles a day on the river. So when there is rough water I have to go slow and just ride the waves out. I am looking to build a boat that is made for rough water and soften up the waves, allowing me to travel at a faster speed.
I have a Yamaha F90 that will be used to power this boat, It is a 20 inch transom model. I plan to install an Atlas Jack plate on this skiff.
I am reading the above post and studying them. I hope the above info will help in getting my commfish boat designed.
Thanks a bunch
Randy

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Re: Net Skiff Build

#10

Post by netman » Sat Feb 18, 2012 9:59 pm

Kevin, I will put angle on the bottom as hull stiffeners. Since I rarely beach the boat, I plan to use at least two hull stiffeners from angle. I currently have two on my plate boat, and she steers fine and does not slide. My partner has a boat similar to mine with no stiffeners or mini keels, and his boat power slides all over the river.
In regards to offloading fish from the boat, I have a triple axle trailer and pull the loaded boat up on it and head home. Once there, we offload the egg fish in separate tubs for processing, then tub the other fish separately for different processing.

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Re: Net Skiff Build

#11

Post by tracy » Sat Feb 18, 2012 11:56 pm

how long , what size of load 2000 lb ?
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Re: Net Skiff Build

#12

Post by netman » Sun Feb 19, 2012 1:38 am

Tracy I'm looking at 20-24'. I'm not locked in on length however a 20' with 2' extended motor bracket would fill the bill. I'm hoping you guys will be able to tell me what is the best length so I don't box myself in. Around twenty feet or so.

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Net Skiff Build; PC or hand drawing?

#13

Post by kmorin » Sun Feb 19, 2012 12:24 pm

This post discusses CNC cutting versus hand layout and cutting for the one off boat. Computer Numerical Control systems have to be told what to cut in a language very similar to Computer Aided Drafting. They are fast, very accurate and can be cost effective if you’re paying wages. But the instructions or ‘cut files’ take time to create and require computer software that performs several tasks.

First the software has to create a hull model and ‘fair’ the hull lines. Next the software has to provide the shapes of the hull panels and any framing, and last some software has to put all the pieces onto plates (scaled to real sheet and plate) and convert these outlines to cut paths that a given CNC controller understands.

This means there is considerable time involved to produce cut files and they are not inexpensive as a result of the time and computing power required to produce them.

CAD drawings of boats are faster, or take less time to create, than the older traditional methods of drawing with pencil or pen no paper or Mylar especially where the shapes of hull panels are concerned. These outlines are called ‘developed surfaces’ and are worth the cost of the marine design software if you build regularly.

All the parts of a boat need to be designed and drawn to ‘fair’ curves; that is the lines need to be curves without kinks, bend points, ‘hogs’ or flat spots where they don’t belong.

Doing these drawings on paper is one skill and takes some ‘design time’. Recreating these ‘lines’ again full sized is called "lofting" which is really no more than just drawing the boat plans/lines full size in preparation to building. Lofting is time consuming both because its usually done full size, so you’re crawling around on your knees on a 24’ skiff plan, and because this is where any lines plans errors or scaling errors are corrected.

I do not have CNC cut files for my design of a net skiff. The skiffs were designed and mainly built before PC (except MacSurf by the author of MaxSurf; Andrew Mason who is clearly a genius programmer) software of today was available. I do have a method of creating the outline panels which is like lofting but is done in small scale that takes a day or so of work. I am going to illustrate the method of “finding out” the sheet outlines of the bottom and the sides, many parts and transom of this skiff without expense of PC marine design software, OR the time to draw the boat full sized by lofting.

This is a one-off build not a production run of boats, and netman or others may not have any marine design software or the time to learn to use it? I want the method of building to come through here on AAB.com not to show a PC based procedure for this design-and-build method of creating a welded boat.

I personally use marine design software (Rhino & Delftship Pro together with Sketchup) to do most of my work on the PC after initial sketches, but it has taken more than the hours required to build one skiff (or three) to learn to use software to do the steps shown. And further.... I use Sketchup to make most illustrations I provide to create images that will be used to help guide the whole process, so I'm very much involved in PC CAD, D, D (Computer Aided Drafting, Drawing and Design).

I even wrote a 10 exercise tutorial for ‘very beginning DefltShipPro’ that I email free to those who ask, as a help in getting people going in D’ship. I am not making any inference against software, as I’ve stated that I use it to do my own designs; I’m simply going to show the new builder, or one-off builder how they can work off paper and plywood or sheet aluminum to get the information needed to form this method of building.

So, with that disclaimer, please don’t misunderstand in my next posts and infer that I am not fully supportive of computer based designed boats! In fact I’m very in favor; but in my experience there is a large time commitment to learn to effectively use the marine design software that will produce the information needed and here I’d like to provide a method that can be done by anyone willing to take the time to follow these steps.

Is there a time effort to draw and model the skiff? Yes, but I believe it is shorter by far than the time to buy and learn the software that will give the same information. Will the software be more accurate? Not necessarily, there are many errors possible with software. Will the software provide CNC cut files? Only some design software provides outlines which then have to be converted to cut files (which is not hard, but) again requires the new builder to begin study of CAD and CNC not boat building.

With all that said, I’ll begin next with a review of hand drafting/drawing and layout of sheet metal boats.

Cheers,
Kevin Morin
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Re: Net Skiff Build

#14

Post by tracy » Sun Feb 19, 2012 12:54 pm

http://www.svendsenmarine.com/23_setnet_skiff#Next is the what we are trying to create.
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Re: Net Skiff Build

#15

Post by tracy » Sun Feb 19, 2012 1:25 pm

E&D manufacturing ltd. http://www.whiskeycreekmarine.com/
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Net Skiff Build-image of skiff

#16

Post by kmorin » Sun Feb 19, 2012 1:42 pm

tracy this is the skiff netman, wants to build, and this thread is intended to give him the info to layout it out, cut it and weld it together.

Image

We're at the drawing stage now, and then will move to the modeling stage and then full scale layout.

Cheers,
Kevin Morin
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Re: Net Skiff Build

#17

Post by netman » Sun Feb 19, 2012 8:23 pm

Kevin, in regards to the bottom and cross section/body plan, I would like to go with a happy medium + for running rough water. Since I already have a flat bottom net boat I'm thinking the four/five inch would most likely be middle of the road. Am I thinking correctly?
Tracy, those hoop nets are what I run too. Except my hoop nets are seven hoop and fourteen to sixteen feet long. After seeing the video of those channel catfish can you imagine what a net twice as long with fifteen forty pound flathead catfish would be like :thumbsup: \
This boat we are in the process of designing and building will be for running gill nets on the Ohio and Wabash Rivers.
This is what I'm after

Image


My wife posing with a decent flathead
Image

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Net Skiff Build: drafting 101 review

#18

Post by kmorin » Mon Feb 20, 2012 5:01 pm

Randy, here's a quick review of some drafting points to set up that will be needed as we move along in this skiff build.

first you can draw on freezer paper as easily as drafting paper but it won't erase as well. Next we're going to take a quick look at some drafting steps to set up the base and grid lines and then we'll talk for a bit about 'curves' so they're more manageable.

If anyone is interested in the countless missing details in this very simplified version of drawing this skiff then I'd suggest reading some books. Sam Devlin is a designer builder in the Puget Sound who has a couple of good books, Wooden Boat magazine has lots of books on the subject, and John Gardener has showed very similar information in his books on wood boats.

I'm not providing all the info just enough to make sure Randy is on the right course.

We'll need to draw the boat. To do that you'll need a 'grid' to draw that you'll need some drafting tools but not necessarily the ones shown in my illustrations. A framing square will work, so will scribes instead of pencils and you can draw on paper, painted plywood, masonite or anything that will take a line and keep it.

First you need a size and a grid. I'd suggest 2' x 3' or 24" x 36" paper/ply/sheet goods but that is my preference not the only choice.
Image

I don't show the drawing table or drafting surface but it sure helps to tape the paper to a sheet of something smooth enough to draw on with at least two square edges that is 90 deg to one another.

The base lines are where you'd layout the grid or station lines.
Image
If you use a ruler or tape to make layout points its generally accepted that you'd make more mistakes than if you use a dividers so I show the dividers (called a compass by some but usually only when it has a pencil/pen on one leg) so you can choose your own poison.

This could be done with graph paper and a tablet of larger size graph (ruled in lines both directions) paper is probably available at a Kinko's or a drafting supply house?

Station lines are at 90 to the baselines, and they're usually 11 lines but any odd number will do. They're counted from "0" (zero) so the first station, say at the tip of the bow, OR at the forward edge of the Load Waterline in more formal work, is Station 0. (not 1). then with and even number more stations there will be 10 divisions evenly along the boat. It won't matter if you use the waterline or the Length OverAll (LOA) to divide the stations for this skiff as it has minimal shape change and is simple to draw.

Next is to figure your scale. If the boat is 24' LOA say a 20 bottom with some bow and stern overhang ? then a scale of 1" = 1'0" of 'boat' will take up 24" of drawing. The next most common scale is 1-1/2" = 1'0" and that would make a drawing 36" long so there won't be much paper left at the ends.

One thing to note is that the bigger the drawing and model from the drawing, the more accurate the modeling process will be. But, the other hand is the more area of work, time to draw and so there's some sort of compromise needed.

It will work fine to draw this set of lines 1" = 1'-0" and then scale up to a larger size for the sheet model, coming next.

So Randy the decisions here are the drawing scale, assembling the tools and drawing surface(s) and some thought to the spacing of stations.

One last drafting tool you'll need is a 'batten' but this can be from lots of commonly available items. You will need a batten or more than one batten to draw curves. Battens are used to draw curves line T squares are used to draw straight lines.

A drafting batten is a tool used to draw curves and they are used in this method of building extensively only in different sizes. Almost "anything" that is uniform in cross section, not kinked or bent, that is flexible and as long as the curve to be drawn; can be used as a drafting batten.

When we build the boat full sized the battens will be the aluminum extrusions used to build into the boat, but sometimes we have to buy specific angles just for layout purposes.

One place to get drafting battens is on line at a drafting drawing supply, others can be made on the band saw by slicing some clear wood to a 1/4" x 1/4" ( or less) cross section then sanding with a block - a nice wooden clear cedar batten is a fine drawing tool. There are many plastic strips at the Home Box store that can be made into serviceable battens.

Image
here is an image of a series of drafting wts holding a batten along a curve, but neither the wts or the battens have to be specially purpose built to work. I have the drafting ducks' or 'whales' and a couple of sets of plastic battens for this type of work but it is not the only way to draw these curves -needed for this skiff.

Of course this can be done in the PC where the programs for marine design have 'electronic' battens or Bezier curves with all sorts of digital routines to effect the curves, but here we're going with the old fashioned hand drawn skiff using some form of batten to create curves.

Cheers,
Kevin Morin
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Re: Net Skiff Build

#19

Post by netman » Tue Feb 21, 2012 7:19 am

Kevin , there are two guys I work with that have college degrees in architecture and drafting . I am going to show them this thread and see what they are capable of doing. I will get the paper lined out and ready to draw. I also need to study your post some more. Randy

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Re: Net Skiff Build

#20

Post by netman » Tue Feb 21, 2012 7:34 am

Kevin you mentioned that the bigger the scale the better. With these guys I work with what would be a good scale for me to suggest to them? I took drafting in high school and have a idea of whats going on here but by no means am I a draftsman.

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Re: Net Skiff Build

#21

Post by kmorin » Tue Feb 21, 2012 11:54 am

netman, you don't need to be a good draftsman, only just draw the lines a few times, errors in most of the curves will all be faired out full size, and the reminders here are mainly just to get everyone ready to model the skiff's shape.

2'x3' paper is called D size (not going into ANSI vs Arch sizing) and will work fine. I'd say drawing in 1" =1'-0" scale is fine then scale up the model to say 2-1/2" = 1'-0" or so, where the final model becomes about 5' long would give great detail to your final build.

Image
As long as you're going to check with some drafters then they may prefer to work with a back ground grid and transparent sheet ontop? This leaves the grid on one page that can have more than one page on top so if you do a couple of versions of the skiff/boat you can avoid making new grids from scratch.

here is a piece of film or tracing paper above the grid so the two main view are lined up with the same bow point, the same stern point but the lower base line (keel in plan view) is the center of the plan view (half breadths as they're called) and the upper base line (side view of the keel plane) is just below the keel viewed from the side (profile view).

Image

What we're going to draw is the net skiff in two views, the plan and the profile or from the top and one side. Its important that you realize the example drawings are not the net skiff. This skiff is just a generic planing hull shape common in welded aluminum and is used to show the ideas not as the design you'll be drawing.

But, the same basic information will be needed and the same curves will be needed the distances and curves will be for the net skiff instead of for the generic V bottom skiff I've drawn for illustration purposes.

Image

Lets get going, first you've already drawn the two parallel base lines and made 'stations' full width of the page, as shown and they're spaced so there are say 10 divisions along the LOA. (bow= 0; stern= last station line) Next you'll need to begin by placing the the keel line (in profile view) and IT WON'T matter where/how deep below a water line you place this line. I'd suggest that the keel be drawn 1/2'- 6"- half a foot below the waterline to begin, this may get adjusted but won't matter now.

Next the chine line is a line next to to the keel line. The boat has a straight keel line and the chine is straight when viewed from the side/profile view so the are parallel lines in the profile view.

Now comes the first decision that may take a minute or two. How deep is the curvature of the hull? This is the depth of the camber in the after parts of the boat. You've suggested the depth be 4" of curve in 72" of plate, chine to chine. That is depth of 4" in 36" chine to keel.

at this point you may need to cut a curve out of two pieces of 2x6 and lay some plywood in that curve and see if you want that depth? If boat is open, no decking except the foredeck, some fisherman consider this curvature too much slop to work on? Others find it works fine and are not bothered by that curvature underfoot.

I can't say, but I can say that creating a curve to make a bottom 'mock up' is simple to do and a good exercise to begin making this hull decision. The curvature of the bottom is not critical to the performance, or strength, the curve will influence the performance but not totally change it in the smaller depths we're discussing. If you were discussing or exploring 8" of curvature or something then the working surface is not reasonable and the performance does become much different but the difference between 2", 3" , & 4" is not that drastic.

What is important is the CHINES WILL RISE FORWARD. As they rise the curvature will increase and the bottom's walking areas near the sides will become more steep and harder to stand or work over the sides. At some point there will need to be a foredeck to allow you to work those forward areas of the hull.

Therefore; the deeper the camber/cup/curve aft the deeper longer the foredeck will need to be in order to work the forward parts of the boat. 4" is fine, but I'd suggest you consider mocking up that curve to make sure you're familiar with the shape of the bottom?

Next I'll stop the overall lines part and review drawing a camber or "expanding a circle".

Cheers,
Kevin Morin
kmorin

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Re: Net Skiff Build

#22

Post by kmorin » Wed Feb 22, 2012 12:03 am

netman, the bottom building jig for the cambered bottom skiff will require a curve cut in a 2x8 or 2x10 maybe. This is a simple layout so please don't get apprehensive about the layout or batten until you try it and see how simple this is.

You'll need to make a decision on your bottom camber, and that decision is how will it 'walk' or what does it feel like underfoot? To do this you can make a bottom forming 'jig' to begin and then make a separate one if the first is two deep?

What you'll build looks something like this.
Image

There are several different items in the image to discuss. First is the background fixture/jig is what you'd use to hold this skiff's bottom plate while you form it and add internals. Next is the shape of a V bottom skiff using a similar method but with two bottom halves instead of a single plate. In the foreground is an old board used to and it has small squares/strips of aluminum dry wall screwed to the face so the bottom can be "tacked to the board".

The curve you'll need to cut in the 2x material is begins with the depth of cut you want, in our case you'd make it 4".

Image
I drew these during a discussion a few years ago about deck camber but they will work just fine if you'll adjust image text with notes from my description here. First note that the circle's radius is 6" and its right side up, and yours will be used upside down, and will be 4" but this illustration will work to show how.

The first step is to draw a baseline, make a center point and this can all be done on a small piece of clean aluminum with scribe tip dividers and tungsten for a scribe to make the accuracy high. Next all the numbers/measurements you will get can be transferred to the 2x cross pieces once you have this part of the layout done.

To divide the 1/4 of the arc of the circle into equal arcs, walk the dividers up and down that 1/4 and make small adjustments upward or downward in the arc length of the dividers until you can walk exactly evenly beginning at 12:00 and then evenly tip to tip four times until you get to 9:00. Once the dividers are set to exactly divide the arc into even segments then strike them along the 1/4 of the original 4" radius circle.

Image
you'll end up with something like the arc here, then draw vertical lines to base line. These are the heights of the curve above the base line at the 1/4 locations along the 1/2 breadth of the 72" bottom. (might read that a few times, it's important)

Image
In this illustration I was making a 6" curve for a 16'6" wide hull's deck beams but you'll use the 4" depth and the distance to one side (keel to chine) will be 36" for 1/2 the 6' bottom. Notice that this illustration shows an image that is stated in words as the last line in my previous paragraph?

Image
now a curve is drawn along the points with the center point being 4" LOWER than the outside point. In other words you'd use the edge of the board as the 'baseline' so the cut for your bottom former is a curve 'into' the 2x8.

So... in this image the board would be upright and the bottom edge would be sitting along the line under the blue rectangles that are shown to make the image more understandable.

Next you'd need to batten the curve through the points laid down.
Image
here is how I do it. The batten shown is an angle extrusion but for your shallow and short curve I'd use a 1/4" x 1" flat bar since it will flex and not bend in that curve, and it will bend easier than an angle.

You can use visegrips, clamps or anything you have to batten the curve through the points you've set down. Notice here that I've expanded the pionts to BOTH sides of the curve even though you only need to do the layout on one side of the original 4" half circle to get the information. You spacing will be 9" from (chine to chine) along the full 72" wide 2x8's for each of the upright lines you established with the above 'divider walking' along the 1/4 arc of the 4" radius half circle.

Image

It likely you'll need more clamps as I was just making the image to show the idea, and this is not going to be enough to make a clean curve in most cases.

Cheers,
Kevin Morin
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Re: Net Skiff Build

#23

Post by netman » Wed Feb 22, 2012 6:35 am

Kevin, how many of these boards with curves cut into them will I need?

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Re: Net Skiff Build

#24

Post by kmorin » Wed Feb 22, 2012 11:30 am

Randy, you'll need at least two for the test to with plywood/sheathing. These two will be enough to see if you can walk comfortably on the curvature. If that experiment works then you'll need a 3rd one for the after bottom of the boat.

We want the aft 1/3-1/4 of the bottom as close a possible to a cylinder so the butt lines are all parallel and that's done by #1 making the former/jig/fixture a cylindrical curved surface and #2 by adding the longitudinal hull stiffeners/angles to the 'jig' so they're ready to be tacked on while the bottom sheet is warped into the cylinder.

But for test 'walk test' you could just make a pair of these and get by fine. I'd use 1/4" plywood or 5mm floor underlayment doubled to get the surface if I were building the test surface, I think both are available from the home box store or building supply. If you have a bunch of 1x3-1x4 on hand you could plank the curve but I'm not sure of the spring from using boards instead of plywood? I'd put the two formers about 3' apart and use the 4' wide material cut 6' long to form the hull bottom for the walk test/confirmation.

one thing to note on this layout is that just putting the batten at 4" in the middle of a curve will not (necessarily) give a curve that is cylindrical; it will probably be parabolic. Which will work, but does make the hull shape different than what is planned.

When I was building full time I had a series of 2", 3" , 4", 5" and 6" camber layouts scribed on aluminum. Each block was only about 8" x8" and hung on a nail to be used when that amount of curve was needed in a layout. (1970's-80's) This meant all we had to do for any given expansion was to select the camber's rise (radius) then divide the half breadth (keel to chine; or sheer to keel plane for decks) by four and set up the verticals to begin the process of battening that curve.

Next some points about battening curves.

Cheers
Kevin Morin
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Net Skiff Build: drawing curves w/ a batten

#25

Post by kmorin » Wed Feb 22, 2012 1:54 pm

netman, we're at the point where we need to discuss curves for a few different reasons. First you'll need to make the bottom test platform or "camber confirmation cell" where you'll need to draw the curve of the bottom camber to cut it on the band saw or with a jig saw in wood. ( This curve is set up by a series of points which are the drawing guides for the batten's lay on the material. )

Next, you'll need to use this same skill set and similar methods to draw the long curves of the hull. The chine in plan and profile is a baseline curve where the curve begins with a line and transitions into the curve; as shown below. The sheer can be a baseline curve and there are several other lines which will require this kind of drawing/modeling/layout in the final boat material.

These two types of curves and methods will be used over and over again in this building method so I think its a good time to review curves in concept and practice.

The curve line of the camber of the bottom, some decks and cabin tops all have symmetry where the two sides of the keel plane are mirrored curves so these curves have a middle and two ends. Other curves, just as important begin with a straight line or begin with another curve. The chine viewed in plan view and some sheer lines viewed in profile are curves that begin with a straight line and curve off that 'baseline'.

IN the case of the 'baseline' curve, the point where the curve begins (assuming we're beginning at the line and moving into a curve) is the tangent point, and that point it should be 'invisible' to the eye. Your eye is the key to this entire design/build/layout method of building.

So we need to train your eye if you're not already aware of these visual 'facts'.

Image
here are the elements in the discussion, the baseline is drawn alone to the left so we can see its a line. Next, above the first line, there's another baseline but the batten covers it up to the tangent point or the point where the curve and the line meet/overlap/coincide.

This point is where the curve 'begins' in the terms I'm using.
Image
to show that, I've cleaned up the sketch so there is only a baseline, tangent point and the curve. Then, as is good practice, I've put the same sheet of paper almost 'end on' that is; the view is not 'at the bottom' of the sheet of paper/plywood, it is instead at the end where the curve and line are seen in a different perspective.

This last point is very important, curves on your boat will look different from different points of view and the 'flaws' of a curve will show too. All curves should be checked for smooth transitions, so there are no kinks, hogs, flat spots, hard spots and other visual points that 'look' out of the ideal curvature.

Image

Here, the lower line and curve are reasonably 'fair' (not hogged, clean tangent point, and both look fair to one another) but the next set up- the curve drops below the baseline at the tangent point so the curve is 'hogged' at the 'kink' or 'hard spot' at the tangent point. The next line up shows the curve hogged at the tangent point because the curve does not smoothly flow into the baseline. The upper most curve begins OK but then has a hard spot or flat spot about half way along the curve's length.

All these are flaws and they will most often be eliminated by using the 'right' batten for that curve. But, the 'right' batten is not always at hand so your eye is the final decision about the curves' "cleanliness" or also said as 'the curve is fair'.

Since most people that don't build boats rarely pay much attention to these concepts, it does take a few minutes study and then some hours of practice to draw a fair curve.

The right batten has a cross section that will flex smoothly to that curve without so much pull and force the batten is permanently 'bent'. If a 2" x 2" x 1/4" angle is bent to the curve of the chine in plan view - it will likely be bent or deformed permanently in the forward 1/3 of the bow as that area has a much tighter curve than the after 2/3 of the chine in plan view. So to batten the bottom plate the 'right' batten for the after chine (in plan view) may be a 2" x 2" x 1/4" angle but the forward 1/3 of the chine may be drawn better with a 1-1/4" x 1-1/4" x 1/8" angle which is more flexible due to the smaller cross section and therefore it will assume a tighter curve without permanently bending or deforming.

Generally if a batten is permanently deformed by a curve it will not form that curve very clean and may distort the line so finding the 'right' batten is important otherwise the curves will not be 'fair'. Drafting battens come is different cross sections and lengths, and different cross sections usually make for different 'stiffness' of the battens just like various sizes if aluminum extrusions will flex without permanently bending due to the difference in shape, thickness and overall size.

Cheers,
Kevin Morin
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