Net Skiff Build

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Net Skiff Build:sheer line curve

#26

Post by kmorin » Wed Feb 22, 2012 2:40 pm

netman, boat building is a series of decisions so how about thinking about one that is not crucial for the time being?

The sheer. This can be a curve or a line in profile but in 3D or as-built will always be some form of curve on the skiff you've proposed to build.

This curve is not critical to the bottom decisions that are immediately ahead but since it will affect the skiff's build and you'll have to draw this line in the not distant future let's begin introducing this curve as a way to further discuss the subject of curves?

Image
here is a look at the 'example' skiff that I've been using to illustrate the lines and curves posts so far. The sheer is a line in profile/side view and lots of boats use this sheer.

Image
same skiff shape with a hollow or 'traditional' sheer that is a long slow sweet curve bow to stern/transom top.

Image
and the powder horn or Western sheer as some refer to this set of two curves at the sheer line. I've used this shape extensively since the 'flattening' forward allows the tank top surface to be less inclined to the waterline and therefore less work to lay over when working the waterline below the sheer in the forward 1/3 of the hull.

The line not shown is the opposite of the second example above where some speed boats' sheer is a long bulging curve what becomes most attractive when the boat is running. Then that particular sheerline become flat in the forward 1/3 and the curve when pitched up by the bow a few degrees then has a rising line instead of 'hump back' shape. ( I didn't illustrate this shape as its not that functional for net skiffs and is usually used for the higher speed boat running slightly pitched up by the bow.)

Each of the sheerline types can be drawn on the net skiff but the choice mainly aesthetic; therefore a personal taste issue is one that needs its own consideration. I know fishermen who will not own a boat with each of these lines! So this choice is ahead and should have some consideration, and it works to give some good practice drafting curves to look at these lines in different proportions (the curves at least) and draw some variations in order to see which is most appealing.

[ Pop Quiz ! : can you spot the 'hog' or unfair point in each of these three profile views? I used a PC with SketchUP to draw these- not a hull modeling marine software suite. That has left a 'legacy flaw' from the method/type/order of tool use I employed to create these sketches, but this does illustrate a curve's flaw pretty clearly.]

Cheers,
Kevin Morin
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Net Skiff Build: reference shapes

#27

Post by kmorin » Wed Feb 22, 2012 3:18 pm

netman here are some links that will help with out discussion of the sheerline.
http://www.custom-aluminum-boats.com/boat-builders.html
this site has a few hulls of different sheers, one is probably a line in profile but shows as nearly flat in the pictures, then one is a stepped sheer that I didn't mention above. That is because the line is not all the helpful for net work, but the picture on this page does show that line atop a skiff.

http://zagfab.com/my_folders/Small_Boat ... soleSm.jpg
here is a hollow or traditional type of sheer on a skiff about the size you're proposing.

http://i00.i.aliimg.com/photo/v0/107273 ... _boats.jpg
pretty much a flat line on this sheer and the angle of the photography helps confirm that fact.

viewtopic.php?t=29
and of course, on the mast head here at AAB.com, a fine example of the powder horn sheerline when drawn by someone (not sure of designer?) that has a fine sense of proportion and line!

http://chuitna.org/wp-content/uploads/2 ... hing-2.jpg
hollow sheer net fishing in AK , not one of ours.

Image
and an outboard profile of a hollow sheer version of one our skiffs that is about 30 years old here.

cheers,
Kevin Morin
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Re: Net Skiff Build

#28

Post by tracy » Thu Feb 23, 2012 8:48 pm

this is the one we have been working on .

Image

Image

Image
E&D manufacturing ltd. http://www.whiskeycreekmarine.com/
.................... Image

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Re: Net Skiff Build

#29

Post by netman » Thu Feb 23, 2012 11:54 pm

Kevin, I have been studying your words for a couple days. I have not figured out what is meant by sheer. I looked up the definition and I believe its the shape of the boat using the bottom and top lines=sheer. Is that correct? The links you provided with the different boats shapes I see easily. The boat in the link of zagfab.com is nice looking as is the boat in the Chuitna.org and your thirty year old boat. I guess I like the hollow sheer type skiffs.
I am off this weekend and plan to make the bottom jig. With your experience is the 4" camber o.k for the boat I am wanting to build? Should I go with a lesser camber?
The mechainical engineerI work with asked if you could get this info sent by Auto CAD or DXF file. I told him we were building this skiff old school. Pencil and paper.
I will be working on the boat shapes too. I've got some one by one alum angle to use for the camber. I could also purchase a flexiable batten if you recommend it. I'm going to run down some lined paper for the drawing.

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Net Skiff Build:naming lines

#30

Post by kmorin » Fri Feb 24, 2012 12:54 am

Image

this is a planked row boat with two chines, one keel and a sheer line.

Image
this row boat is sort of a 'dory' but has a rounded stern but still there's a sheer line and a chine but the keel isn't visible

Image
here's a little 14'er I did with some line labels.

More to follow.
Cheers
Kevin Morin
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Net Skiff Build: edited images

#31

Post by kmorin » Fri Feb 24, 2012 1:10 am

netman, sorry I wasn't very clear but the sheer is the line at the top of the hull, and each of the photo links I posted had a line that was one type straight/flat, hollow/traditional, or reverse/powder horn/Western and I was pointing out the look of the boat from that line.

Image
I've labeled the lines a bit more here

Image
this is the traditional or hollow sheer line that is most associated with work boats but not everyone likes to look at this sheerline.

Image
and the double curved or powder horn again.

Cheers,
Kevin Morin
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Net Skiff Build: more lines images

#32

Post by kmorin » Fri Feb 24, 2012 1:47 am

Randy, hopefully I'm making more sense? here are some additional images to further our discussion.

Image

Image
I drew a big white line along this huge hull's sheer to show that all these hulls have a sheer.

Image
here is a little cellulose skiff that is as cute as the little girl standing next to it.

Image
and one of my power boat designs with some labels to help make these terms apply to different hulls.

let me know if I'm making sense.

What files does your friend want? I have not modeled these skiffs' hulls and the illustrations I'm using are mainly from other discussions over the years we're just using them here to try to make sense of the project. I'd send some ACAD files if I knew what they were to contain?

Cheers,
Kevin Morin
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Re: Net Skiff Build

#33

Post by netman » Fri Feb 24, 2012 10:43 am

Kevin, thanks for making the info more clear. I was hung up on the sheer. chine and keel lines as a whole. I knew what each was however when it comes time to putting the lines on paper in boat form I'm stumped. Figuring I will have a 30" side on my skiff and using 1" to 1' scale I can put the sheer line in.
How is the chine drawn in?
I did a little mock up in my shop today on figuring the camber. I'm thinking 2" camber will be better suited for a net fishing boat.
You mentioned the inside chine being steep had my mind working. The mock up confirmed what a wet slimy sheet of alum with four inches of camber could be. I'm thinking the camber is not that important in getting the skiff to ride better in rougher water.
Today would be a good day for this skiff on the river. Right now we have 20-30 mph west winds with gust up to and over 40 mph.

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Re: Net Skiff Build

#34

Post by netman » Fri Feb 24, 2012 10:52 am

I had my co worker to mark this site and thread down as he plans to look at this project on his time off. I will see him Monday night. In the mean time I am getting a flexible batten ordered.

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Re: Net Skiff Build

#35

Post by netman » Fri Feb 24, 2012 11:08 am

Kevin I went to order a flexible batten and searched for it through google. Upon searching it brought me to several boat building web sites. Alot of guys suggested using wood strips. If you or someone else could suggest a web site to order a batten I will be happy to get it on the way. However I just had a guy rip me a couple hundred seventy inch clear no knot white oak strips for building catfish slat traps. I believe I could pick out a dandy for drawing the sheer and chine lines.

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Net Skiff Build: battens for drawing

#36

Post by kmorin » Fri Feb 24, 2012 12:34 pm

Netman, I agree that wood battens can be fine, and will work but to make one from the pot strips you'll have to rip a narrow, uniform strip from the oak then plane/sand/fair the batten. The reason people use plastic is they're uniform and smooth but wood is great, and does a fine job.

The main question about any batten is the rigidity of the batten compared to the scale you're drawing the lines. If you were drawing a 24' boat on a 8" x 11" piece of paper the curves would be pretty tight (small); so the batten would have to be very thin to make the curves and still flex then return to its original form.

The tightest curve in this skiff is the forefoot in profile ( curve between the bow stem and keel) and the sheer in plan view both will require some good flexibility in the 24" long drawing in a 1"= 1'-0" scale.

http://www.macnaughtongroup.com/yds_dra ... r_form.htm these folks call them 'splines' and they come in different lengths, but I'd call to determine what they're made of and the cross sections?

http://www.boatsofwood.com/lofting%20du ... _ducks.htm spline/batten wts of lead

http://www.ainplastics.com/ainp/Product ... areRod.htm you can contact one of the main plastics suppliers and ask for acrylic rod in square cross section and they'll sell them in lengths.... just a thought?

http://www.draftingsuppliesdew.com/mall ... upID=10071
I'd buy these if I were buying! I have a set but I didn't call them "adjustable curves" they're fine for the work you'll be doing in the scale you'll be working. I think the 18" batten will do the forefoot in profile and the 36" will probably do the sheer line in plan view.

http://www.artstuff.net/Acu-Arc-Adjusta ... p_276.html other prices the same set of battens.

'ride' is term that can bring a long discussion to lots of different boat designers, builders and owners. The term is so widely used that its hard to pin it down and talk about the same actions of the hull. For example the sport oriented boater may use the term for how their hull 'feels' in the legs at speed in a given seaway? But the commercial fisherman may be discussing how a hull handles in a seaway at different speeds and still delivers solid steering to put the boat onto the buoys or gear?

Some will describe 'ride' as the amount of smoothness of the impact of the boat in head seas at speed while others will use the term to describe the amount of pitch and roll at a different speed. Hard to describe the word so everyone is using the same references.

The complication to the 'smoothness' definition of 'ride' is the boat's displacement. There is no way a 22' welded skiff that weighs 1,000 lb. even with a 25 degree deadrise bottom (!) can ride as smoothly as a 4,500 lb 22' made of that 'plastic goop' even if the aluminum is sharper entry. The reason is mass or displacement. Part of 'ride' is a factor of overall wt or the all up displacement of the boat in question.

In a chop or short steep sides seas that may have a wind-blown top, but are not full on breaking rollers- like surf, the average small boat will usually slow down regardless of the amount of V below the chine as the shape of the waves usually wets from the topsides all the way to the sheerline.

If the waves (swells) are 20' high but they're 100' apart as in the ocean, even a flat bottom boat can travel at high speeds and not have a 'bad ride'. As the period, or distance between waves gets shorter/closer together, the wave faces get steeper until they become 45 degrees or a short wall of water. In this condition only a larger hull would be able to travel very fast since that boat's chine would be deeper/taller than the wave height.

If a small boat travels very fast in a short chop, steep sided and close together waves then a different result happens. When the entire topsides of a boat are wetted, that is when the wave/swell/face of the seas reaches up the entire distance from the waterline/chine to the sheer; the drag on that hull, by 'wetted area', becomes a bigger factor on forward thrust than impact under the bottom.

The more the V the faster one could go in these conditions but then when the entire topsides is wetted by the shape of the oncoming seas, the boat will slow so fast you can be throw forward even if you're holding the helm. In order to compensate for this motion in a light wt boat (<28-30') in commercial net boats we usually put more spoon in the bow area.

Spoon is the description of the fullness of the forward 1/3 of the hull in plan view. A boat with lots of spoon will have the sheerline outside the chine line in Plan View. A boat with lots of spoon will throw water away from the hull but as the water travels up the sides when the bow is buried into the wave face the boat will slow drastically and pitch upward all keeping water from boarding and keeping the skiff 'dry'.

A dry skiff is one that will through water away from the hull and a wet skiff is one that allows water to run up the sides and either spill into the skiff or spray the helm. Of course in the right wind and heading the helm can be covered in spray in any boat, (pictures of battleships with spray over the conning tower?) but lots of flam (outward lean of the topsides from the chine in metal boats- sometimes called flare incorrectly) in the forward 1/3 of the boat will help a skiff run into a short head sea and remain in good control and as dry as the wind direction allows.

Again the concept of ride is confused by displacement. The empty net skiff at speed will bounce on a very small wind shop but that same hull with a couple hundred pounds of catch will 'settle down' in that same chop and this keeps happening. That skiff loaded will rise over the short 3-4' seas almost without impact when the skiffs wt starts to be 3,000 to 4,000 lb. instead of that skiff at 900 -1,200 lb. empty.

Cheers,
Kevin Morin
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Re: Net Skiff Build

#37

Post by netman » Fri Feb 24, 2012 2:57 pm

Kevin I went to the drafting supplies and ordered the 18" and the 48" and their on the way.

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Re: Net Skiff Build

#38

Post by netman » Fri Feb 24, 2012 3:06 pm

So the 2" camber will be o.k. ?
I also spoke to a friend today who I get rebar to make my anchors . While bs'ing I told him of my skiff in progress build. He said he had a really good hook up on sheet aluminum.
I had him to call in for a quote on
20'x72" 5086 3/16 bottom
20'x60" 5086 3/16 sides
[2] 4x8 5086 3/16 front
[2] 4x8 5052 1/8 bracing



just guessing on what will be needed as I know I will need angle,flat stock, tubing and ??. I told my buddy that I would need more but asked for a price on above. I can get a really good prices on alum sheet in La. I am trying to save me a trip to LA

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Re: Net Skiff Build

#39

Post by welderbob » Fri Feb 24, 2012 5:31 pm

your probably going to need a 72" sheet for the sides. By the you decide on a sheer line and add a little camber and wrap it around the bow your going to lose a few inches. Something to think about as you lay out your drawing.

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Re: Net Skiff Build

#40

Post by welderbob » Fri Feb 24, 2012 5:45 pm

The price of aluminum gets cheaper the more you buy. I find it easier to compare quotes if there all by the pound. Don't for get about the shipping. Driving to LA to pick -up a pallet of AL can get expensive when gas is heading for $5 /gallon.

Mandel Metals, Inc.
11400 W. Addison Ave
Franklin Park, IL 60131

Toll-Free: (800) 962-9851
Phone: (847) 455-6606
Fax: (847) 455-3330

E-mail: sales@mandelmetals.com
Website: www.mandelmetals.com

They also have a warehouse in Ohio. They should quote you a price delivered. I don't know if they can cut a file. I think that the exercise here is to loft the boat on full sheets and cut with a circular saw.

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Re: Net Skiff Build

#41

Post by netman » Fri Feb 24, 2012 6:51 pm

Thanks welderbob! I'm not sure on how much alum or what size I was going to need so I just threw out some sizes to see what prices are at. welder I figured the excess will be used up elsewhere on the skiff.

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Net Skiff Build: partial BOM

#42

Post by kmorin » Fri Feb 24, 2012 10:22 pm

netman, yes a 2" bottom camber aft will be fine, the chines will rise to the bow stem as they curve in and that rise gives the bottom shape, if the working area forward is too steep you can extend the foredeck aft to cover the bottom further.

I'd plan on using the 20x6 for the bottom plate and then using 4x20's for the sides as they will allow most of the curvature of the side panel when its laid flat. If you draw an extremely leaned out/flammed/flared bow then you may need a wider sheet (5' ?? !!) to get the two sides' shapes. All the offcuts will brake to ribs, bottom or side ribs blanks which can be fit to the varying camber over the limber holes/liners.

I'd make all effort to see if you can get coil sheet in 25' long pieces? We get 25 and 30 coil sheet re-roll here in Alaska; so it seems like it would be available there? It just saves the topsides seam work, which is not a big deal but is easier to buy in one piece (if possible) than butt splice the topside panels.

It will take a few other sheets 4x20 or 5x20 for the rest of the framing, and it will take some for the air tanks/box beam sides too. Welderbob estimates that two 30" tall sides will come from one 6' sheet leaving about 3' per side and that shows his good eye. But, I think it will be closer to the truth that the sides will come from one 4' wide sheet- each. Thickness is a matter of welding skill and preference- not necessarily strength.

I liked the 1/8" sides I put on most skiffs but the majority of my (welding) employees couldn't put that side to rib weld on with enough speed and control to give a clean, single 3' long, down-hand bead. So, we did many skiffs with 0.160" sides and some longer skiffs with taller 3'+ sides of 3/16" too. I'd say the 0.160" was plenty but 3/16" will sure do the job it's just more to buy and push around, but the thicker sides will not cause any problems in the build. (they are both a little more work to wrap onto the hull at tack-up)

I tend to use 3" of camber in the stern of a 6' bottom but that 1" of camber (between 2" and 3") in the bottom will not make a radical change in how she handles. I think you can see why I asked you to go to the effort to mock up this (re)curved deck?

Black Beard, I'm the only one who's built most of my designs, and the skiff model above is one I'm building now, with some modifications to that exact model image. I'll have a post here on the entire build when she's finished.

Image
Image
shop is sort of small to get back enough to show the boat except for some odd angles

Cheers,
Kevin Morin
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Re: Net Skiff Build

#43

Post by netman » Fri Mar 02, 2012 11:59 am

Kevin I received the battens today. Should have some kind of skiff drawn up in a day or so. Randy

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Re: Net Skiff Build

#44

Post by kmorin » Tue Mar 06, 2012 12:02 am

Randy small emergency here. I have lost part of my drive and the photo-editing software was in that sector. I can't get the illustrations mounted until I get some recovery done so I can edit down the huge files into 'postable' sizes.

Will be back soonest- I'm working on this ASAP. Please bear with me; it looks like I will need another drive and to re-buy old Photoshop as well as a couple other applications?

serious pain in the stern.

Cheers
Kevin Morin
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Re: Net Skiff Build

#45

Post by kmorin » Thu Mar 08, 2012 12:04 am

netman,

Put some raked keel and transom lines down. I'd suggest you rake the transom aft 20deg's and the bow forward about 3' in 3' of rise. Then put in a keel. First draw the keel in profile and then 2" above that (in scale) draw a straight flat horizontal chine in the after 6' of the hull. These lines can all begin as straight lines and be subtly curved as the design progresses or during the build.

Image

Once you've drawn one set of lines using the first draft rake of the bow and stern you can make another set with less (or more?) rake and see what shapes result.

The chine rises to the bow stem in the profile view by about a foot to 16 inches. In your case I'd use the larger number but that is my choice, in the profile view you'll draw this line rising for the forward 2/3's the boat. The rise is slow and even so this curve's 'tangent point' as shown earlier is the end of the 'straight' chine at 6' forward the transom.

The keel is straight to within about 5-6 of the bow then it curves up and ends into the bottom of the bow stem which will need a curve from bottom to top of about 1".

Image

Next locate the sheer's lowest point (in profile) about 2/3 aft the bow, as your 30" high sides. Then let the stern rise to about 34- and the bow nearly the same maybe a little taller, say 36" above the keel? This will give a curve of the sheer. We've already discussed some lines so you can work these out in profile.

Essentially locate the keel in plan, then make the low point as a line about 3' long then the stern tip and bow tip and make a sweet curve bow to low point the up to the transom. I'd suggest the bow and stern be higher than the 30" low point and then I'd draw and redraw the bow stem rake, the sheer and transom (20deg's rake aft) a few different times?

Next, do the same lines in plan view below the profile on the same grid. Make sure the chine intersects the stem at the same distance aft the bow stem top or 'forward end' of the boat. I don't make full length reverse chines in these skiffs. So the reverse chine is a kind of 'half moon' along the outside of the chine in the forward 1/3 of the boat.

To draw the chine in plan, first make the centerline for the Plan View below and parallel to the Profile keel, then add the chine as a line parallel to the keel at (5-11-1/2")/2 or 35-3/4" from the keel per side. There is a slight narrowing of the bottom due to the camber but not much. For now, just draw this straight, but it will change a bit later on. (we can make this change without impact later)

Image

Then at 2/3 forward the transom of the 20' bottom sheet (about 13- to 14' from the stern in Plan) draw the curve toward the center and intersect the stem in plan view. Now, draw a wider curve OUTSIDE that inner curve and this is the shape of the reverse chine plate in the forward 1/3 of the bottom. (read that til it makes sense!)

The intersection point at the stem is the same. The beginning point (tangent to the inner chine) is the same place along the chine in PLAN view but the curve is a wider turn outside the inner chine. [This shape gives the nearly flat bottom skiff the change in the waterplane displacement that lifts the bow so well in a head sea.]

So your drawing of the Plan View will have one chine line for the after 12-15 feet but then in the forward 1/3 of the bottom it will have two chine lines! That is the inner and outer chine and that is critical to making the 'spoon' or bow 1/3 of the hull so full that it will lift 4 tonnes of fish over a swell.

Then draw the sheer in plan view. This would be the beam at the transom, then get about 6" wider till amidships then curve to the tip of the bow stem. So this curve begins at the bow stem forward, and curve outward at the forward third and then begins a very long slow (16' long) curve to the transom. This is not parallel to the chine. This curve is the top of the side and it curves inward toward the chine all the way to the transom. So at the stern these skiffs only have a few inches of side lean outward and the sides are nearly plumb at the stern.

to draw these lines as described make straight lines in each view that are the "limits" of your curves then make the curves touch (tangent) these "limit lines" and as the curves are fair(ed) the skiff will being to take shape.

There are an infinite number of shapes within this verbal description and therefore working these 'lines' more than one time is critical to drawing a clean and useful shape that will look fair and work well.

Image

I'm sorry to be without images in this post (of all posts) but if you look at what's posted before and read this a few times I think the sequence will start to come through? [edited to add images]

For anyone else following along, the process here is completely limited to this type of skiff hull form and is not [NOT] the way to draw (most) hull forms, but we're working on one particular form with a very nearly 'box-like' after 2/3 hull section. So, here we're able to take shortcuts and work with methods that would not work well with different marine shapes.

What is described is a method of drawing a very limited hull form in a 'short cut' methodology that will end up with a clean fair and workable net skiff.

Cheers,
Kevin Morin


The chine
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Re: Net Skiff Build

#46

Post by kmorin » Fri Mar 09, 2012 10:32 pm

Randy, AAB.com- I doubt you've missed me... but, I'm back with the digitally connected!
I ended up with a hard drive sector problem, a DSL modem problem and work related preoccupation! That was enough to slow down my already limited communications skills and abilities like going aground on the Naknek bar on a falling tide in following sea with a South/Southwest blow; at full throttle. (read- no headway and lots of water on the lazarette deck! with the air blue with sailor like terminology- you know "Let's keel haul Bill Gates!" things like that.... :thumbsup: )

Now that I'm back to using photo editing AND online access I'll post on the preliminary lines that were feebly described above hopefully we'll get back to an 'even keel' here in the net skiff design&build?

Cheers,
Kevin Morin
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Re: Net Skiff Build

#47

Post by netman » Tue Mar 13, 2012 7:15 pm

Kevin good to hear your back in action. Tonight is my last night of work for six weeks as I will be full blown wide open fishing until May. Got my graph paper,battens and working on the drawing. I have been looking at the nose or front of several skiffs trying to decide on which way to go. I have also been on the prowl for alum locally. Ready for more skiff talk myself. Randy

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Net Skiff Build: lines- around and around

#48

Post by kmorin » Sat Mar 17, 2012 12:07 pm

netman, here are two images that should begin to look pretty familiar by now?

Image

This one shows the plan and profile views of and some 'working lines'. In the profile view there is a line at the ht of the bow, the ht of the low point in the sheer and one at the top of the transom. All lines are parallel to the waterline and the keel in this particular shape and just a reminder.... THE WATERLINE is just a reference at this time it is not involved with the actual waterline that will be determined much later- if at all?

This is a significant difference to 'normal' or 'regular design methods, so if you're reading others' books on this subject (very good idea for anyone who wants to fully understand what we're doing here) then make note of this difference, please.

Next the other lines are more or less labeled so they can be described in the narrative and discussed a bit more.
the chine and keel in profile will be lines that are parallel in that view in the after half of more of the bottom. These are shown with two different curves forward.

the keel line is shown straight for a longer run forward from the transom than the chine then the keel line curves up into the bow stem line in the tighter curve called the forefoot. Above it the chine rises to the stem in a longer, slower curve and this is one area that will require you to redraw the hull four or five times -at least.

for the sake of this initial sketch of the hull both the sheer and chine are shown as straight lines in the plan view but we will deal with the subtle changes in those two curves farther along in the design process for now they are just parallel lines.

Image

this image shows two chine lines forward with the same keel, in profile view. The reason is that the V in the bow sections will be controlled/formed/established/created by the relationship between these two lines (curves) in profile. The farther they are spread, chine farther and farther above the keel, the deeper the V and sharper or more narrow the bottom and therefore the less pounding and better entry into the waves/wakes/chop/seas.

On the other hand (and in a boat there is ALWAYS 'another hand'), the higher the chine is above the keel in in profile view the less the boat will carry forward before the bow is down and water is coming over in a swell. The sharper bottom with a greater vertical separation between the two curves/lines will run faster and 'carry less' and will also require a longer bow deck further aft to 'walk on' the bottom surfaces.

the deeper bow,with higher chine over the keel in profile will also usually make less area in the bow to work nets and gear. The shallower lines, where the chine is closer to the keel in profile view will result in a hull that is more 'workable' but will slam at a slower speed in less sea/waves/wake height but will also lift a larger load over those same seas.

SO.... I hope that I am making clear these lines are related to more than one aspect of the hull's performance? Net fishing requires you to make this set of compromises, by going around and around this design cycle aspect and drawing a few different bottoms so you can 'balance' entry with load carrying- so you can balance narrow faster speeds without pounding with the work area and height off the water you'd stand to work the bow.

In the second drawing there are more than one forward chine curve. THESE ARE EXAMPLES ONLY; the reason they are here is to look at the relationship formed by the chine to keel separation in plan view - this is directly related to the profile view.
You will learn by drawing a few versions that the same V can be drawn by having one vertical separation of the chine and keel in profile and another way to get that V is to change the amount of distance (offset) of the chine and the keel line in plan view.

That statement, above is why you have to do this more than one time to get a balance that you can accept. People who draw boats all the time have acquired a 'feel' for these two lines when viewed from side (profile) and top (plan). If you're not used to looking at these lines and deciding the balance then you will have to draw them more than one time.

cheers,
Kevin Morin
kmorin

kmorin
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Net Skiff Build: Bottom Shape Forward

#49

Post by kmorin » Sat Mar 17, 2012 12:20 pm

netman, we're discussing the forward bottom shape and I want to review a bit of what's been shown, here and in other topics and posts.
Image
these two skiff hull forms are being used to illustrate this aspect of your current set of decisions about the skiff form you want for your river net fishing use.

One skiff is 'more full' or has 'rounder' lines at the chine in plan view. The other skiff is 'more narrow' or has a 'sharper entry' and they will both perform slightly different in the same sea state with the same loads. This discussion relates to your decision about which lines to draw for your net skiff.

The left hull is 'more full' at at the chine and the sheer therefore it will enter a wave and change the waterline/waterplane's volume more quickly than the more narrow hull to the right. Therefore the left hand hull will 'hit harder' than the right hand hull in the same loading and same sea, BUT.... the tradeoff is that the left hand hull will lift a load over any sea more easily with less waterline immersion than the right hand shape.

Image
here is another way of looking at this concept; the 'stations' or sections of the two skiffs are show in these glass panes passed through each hull. The skiff on the left shows wider leaning sides, a wider chine distance (offset) from the keel and an overall bigger 'cross section' (station, body plan, section) for any given point along the hull aft the bow compared to the skiff on the right.

This illustration is intended to review that the plan view of the chine to keel and sheer to keel curves will result in a hull that is 'more full' but that increased volume means the same hull cannot go as fast without impact in a head sea.

Going around this cycle a few times requires you to create a series of skiff lines (sets of lines) that have different curves to make the comparison between them. It would help for you to take some rough measurements from other skiffs and rough out a sketch of those lines to see if you can begin to relate the lines to the performance you already know so you're able to decide how fine the bow of your skiff should be drawn?

Cheers,
Kevin Morin
Last edited by kmorin on Sat Mar 17, 2012 12:21 pm, edited 1 time in total.
Reason: spelling
kmorin

kmorin
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Net Skiff Build: Bottom Shape Forward

#50

Post by kmorin » Sat Mar 17, 2012 12:38 pm

netman, in order to 'see' how 'sharp' or narrow the bottom is you'll need to "do a construct" of the body plan or section view of the lines at the stations you've used in your grid.

Image

I hope this illustration is worth lots of words I won't type? It shows the method used to create the 'cross sections' of the skiff at each station by using your dividers- NOT A RULER- to measure the distances of the offset (plan view) and elevation (profile view) at each station.

I suggest you add a 'baseline' to the profile view parallel to the keel and waterline, but below the keel, as a reference line for the elevations.

This step may take a few views to make sense but we're just 'taking off' the information created by the two views to see what the shape is at each station. This will allow you to see the V of the keel to chine shape and the lean of the sides away from the chine(s).

Once you have the intersecting lines on a separate sheet showing the vertical keel line, (the keel plane is on edge in the body plan so its a single vertical line) and the base line ( a horizontal line at 90deg to the keel in body plan) then the keel point will be above the baseline and the inner chine above that, but at an offset to the keel.

A line can be drawn from each point (starting at the keel) to the (inner) chine then outward to the outer chine (in the forward 1/3 to 1/2 of the hull) then up the topsides to the sheer. This would be reduced by one point (outer chine) if you use a plan similar to the one shown here? In the after half of the hull there would only be one keel point, then a single chine then upward and outward to the sheer.

The straight line from the keel to the chine will be replaced with a curve/camber line of the bottom when the drawings are a little farther along.

A stack of stations will appear that resemble this - middle of the lines plan here are the two body plans with stations shown.
Image
now this set of lines is not from a V bottom or even a camber bottom skiff-this is a dory. But the set of lines (from the lines article posted before) shows how this third set of lines relate to the profile and plan views. This set of lines shows the waterlines in profile view as part of the 'grid' and then as a set of curves around the topsides as they are intersected or 'cut' above the bottom.
This also shows the buttock lines or planes intersecting the hull parallel to the keel but offset away from the keel plane. this are just grid lines in the plan view however they become curves through the topside in profile view and they revert to grid lines, again, in body plan, as do the waterlines.

These lines may be reviewed at the article on lines already mentioned, where I tried to show the 'cut planes' as a series of visible glass panes intersecting the hull.
Image

Image

this body plan, stack of stations, is more like what your skiff will look like at this stage. Here the chine flats go all the way aft so they appear in each station, but the overall information provided by making this view is still shown; you will be able to see what depth of V you are drawing, and given the offset of the chine from the keel you will be able to see what angle of V is being drawn as well.


Cheers,
Kevin Morin
Last edited by kmorin on Sun Mar 18, 2012 12:31 pm, edited 3 times in total.
Reason: add link; 2nd edit-add text; 3rd edit-added image
kmorin

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