19 ft cuddy

[catdieselpow]

I am starting a 19 ft cuddy plate boat. I want to build it with a subfloor with bulkheads and full aluminum floor no ribs. I dont know how the hull bottom and sides are made. Is this the way it is normaly done?

[Chaps]

Welcome to the forum!

Not sure I understand the question (I'm not a builder) but I really like a boat with a keelson per what you are doing there. If you want to carry that design further have your hull sides and transom meet the bottom plates slightly inboard . . . strong T joints everywhere. I'm biased 'cause my LaConner is built that way.

[catdieselpow]

Thank you, Yeah this is not mine. The keel is the part I need to find how it works and I wanted to know if this was standard. I found out its not, problem is getting a extruded keel and chines.

[tazmann]

Thank you, Yeah this is not mine. The keel is the part I need to find how it works and I wanted to know if this was standard. I found out its not, problem is getting a extruded keel and chines.

The Keel bar like that would be cut from plate, I would think you would need a strong back to firmly hold the keel bar and a basket mold otherwise that keel bar will start twisting all over the place when you start tacking and pulling the bottom plates to it
Tom

[kmorin]

Tom, CAT, Chaps, the VKB (vertical keel bar) is only one way of building a skiff, not the only way. They can be free floating, like the one shown, with the bottom halves tacked up to it by holding the stern 2/3 straight while tacking the after centerline then tacking equally spaced and evenly up the forefoot until the chine tips touch the stem (VKB).

Tom, I agree a set of V's of wood or metal the hold the bottom halves at the deadrise makes the tack up so easy and accurate that it's the best practice, if the boat is not framed with precut frames and longs. Then I'd build upside down anyway because the interior framed boat works like a building jig.

But the centerline piece (VKB) is not the only way to frame up, the entire keel tack up process can be done just 'sheet-edge-to-sheet-edge' and the result is the shape is the 'same'. After a sheet-only bottom is tacked up you can add many different framing elements to the shape and result in plenty of strength in the final hull.

Tom is right as far as I can see; the VKB, as shown, looks like it was cut from 1/2" or something close, but CAT- there are firms that have extrusions they form, roll, bend, pull into shapes for this purpose. I think it's pretty important to note that hardly any boat building company will sell their (proprietary) extrusions to others. That firm has gone to the costs to design the extrusion, to have fairly large volume runs of that shape made, and they're not very inclined to let other builders, commercial scale or not, have access to that investment. There is no economic gain to selling their extrusions unless they sold them at the price per pound of platinum - part of their boat's competitive edge in the market place is the innovation of extrusions.

Technically, if you cut through the keel that is formed by a large, thick VKB which is sided with plate it forms a "Y" sort of shape, with the base rising through the V. Maybe we could call that an I inside a V? But the VKB is often very thick because of the legacy of building keels from dead plants- not always a necessary final scantling in welded boats. Actually part of the reason is the to have a VKB is that it will hold frames without changing shape until the hull plate is on!

A metal boats' keel area could be built sheet-to-sheet, then a flat bar or sheared piece laid over the back to form a delta or triangular 'box' shape that had a very small weight compared to the thicker VKB but it could be as strong or stronger, than the VKB arrangement.

So, while a VKB method of building works fine, and is pretty common, it's not the only way to build a plate skiff hull, and it's not likely, unless it was provided in a kit boat sold as a ready to tack up package, that an extruded keel would be available for a one-off build.

CAT, unless you have a plans package already? the keel issue will probably be resolved when you buy your plan/design or a kit boat already cut? If you're drawing your own skiff hull, then when you develop the structure you can follow the traditional wood framing type of structure or, like many currently built welded skiffs, you could opt for a 'skin first' build where the framing is put inside after the initial tack-up. Both methods work but the techniques to assemble and weld out are a little different.

cheers,
Kevin Morin

[Chaps]

Kevin, on the extrusions many builders use I can see that they aid in fast assembly and high cosmetics but based on my experience prepping and painting many alloy boat bottoms I frequently see evidence of pitted extrusions and crevice corrosion down in the unwelded seam between plating and extrusions on boats left in the salt chuck year-round. Me thinks the alloy of a typical hull extrusion is not much more noble than a zinc anode. Secondly, it would seem that most back gouging labor is eliminated on designs that have the plating meeting as a T instead of a butt. Is that right and if so what is that worth in the overall build effort?

[kmorin]

Chaps, I don't advocate the use of slotted extrusions where one side is left un-welded because the expense of a decent weld is too high for the boat's market targets.... I can't excuse anyone that builds in a crevice/poultice cell in the design.

yes, I agree that the extruded chine or keel shapes often leave the boat second class for durability or quality and I don't want to be read as advocating them. I was replying to a post that dealt with the VKB element. I use VKB's flush to the inside of the plate and weld them fully and totally all 'three sides' of the joint. Inside port, inside starboard and outside fully and I typically TIG weld them carrying a 'double barreled keyhole' that I've been told by many welders is 'impossible' to do. I still do that weld, regularly, and fully fill the keel seam flush using that method at the root, but I'm not trying to use my preferences as the basis for how others choose to build.

I don't use extrusions inside or integral to any hull seams, I use plate edge-to-edge and weld inside and out, then in some cases I put extrusions over the chines, and sometimes over the keel, outside. In most cases the covering is to provide an initial wear surface, but that's just my preference, not everyone agrees with my building techniques or methods.

Either at the keel, the chine or the transom, different builders prefer to have a T fillet, or a fillet where one sheet meets the other- but back from the edge of the two sheets. I use the edge-to-edge fit in all hull joints because of my experience repairing boats. I like the outside fillet of two sheets, regardless if I weld inside first and back chip the outside or weld outside first and back chip inside; I prefer the weld to fuse the absolute edge of the two panels.

Backchipping is so exact and fast and takes so little effort, I can't see it as a consideration that would influence me to reshape my hull seams.

If you don't bevel the T's vertical element, sides along a T type chine, or lower edge of a transom- fully and evenly- the fillet and back weld leave a pocket of cold or incomplete fusion and that corrodes when the interstitial space, that small volume at the lower edge of the sheet that was the T in a T joint, vacuums salt water. I've seen rock impact under this type of a transom joint where the entire seam split due to the joint prep and welding technique. In my method of using an inside edge fit and fully welding through, then back welding fully, I've not seen that same failure point in the four decades my boats have operated.

So each builder chooses their joints' type and geometry and 'pays' for the implications of their building technique choices.

As to the effect of any given joint arrangement on the build? There are way too many variables to make a decent rule of thumb. Designers who would sell to one off builders of welded aluminum are in a tough spot. There's no good way to gauge the level of skills of the plans buyer. Therefore, the designer has to draw complexity for what he guesses the skills will be and provide joint designs to suit an assumed ability of the one off builder.

In the overall build effort welding is an important but small part of the project, if the skill level of the welder is so low they can't correctly handle an edge to edge fit and need a T joint instead; that skill level is too low for me. But a first time, one-off builder probably won't have the experience or welding skills to handle those welds? So that builder has to find some way to choose a seam they or their crew can handle and still build 'their' boat.

I was just trying to discuss the spectrum of methods and joints that would 'get 'er done' and in the water.

cheers,
Kevin Morin

[Chaps]

Thanks Kevin, as always an enlightening post

[catdieselpow]

Thanks I appreciate your writings, I want to go sheet and build sheet skin first and then frame and I also am making my own plans. I could probably figure out a way to set up the bottom and sides but I would like to know how it is done. Probably with some kind of jig. The part Im stuck on is how to set up the sheets bottom and sides to weld up.

[kmorin]

CAT, I did a post on a recent one-off skiff here: http://aluminumalloyboats.com/viewtopic.php?f=49&t=4143 of course, it's not the only way to build but does show the tack-up and build sequence I used.



Another method used by many one time builds is to create a wooden former for the after 1/2 to 1/3 of the bottom where the chines and keel are most often parallel- this type of jig is not as simply built if the chines and keel are not parallel to each other.

Here, the jig is for a shallow cambered bottom but the same idea works fine for V's its a matter of adapting this idea to the shape. By using small tabs of aluminum screwed to the wood you can 'tack to wood' so to speak. If your hull has external extrusions, often easier than putting them inside and very beneficial for the initial tack up, you can notch them into the wood formers and lay the sheets on them to begin work.

I've seen one set of plans that had an entire wood frame for the metal boat, keel up, where the four major hull panel sheets of that skiff were draped onto the wooden former and tacked up; the sole purpose of that former was to tack all the hull panel edges evenly and smoothly. I do that using other techniques along each seam- point being there are many methods to get a clean, fair tack up.



Here is a little 14' tacked up on the Davis Jig, where all the seams are tacked inside edge to inside edge and there are no
framing pieces until the hull is tacked. This presumes you know the hull panel outline shapes, but on internal framed designs those frames are often used to develop the pattern of the hull panels' outlines.



Example of the corner geometry I design to get the most weld penetration, keyhole being the indicator of the puddle's full envelopment of the sheet edges. This doesn't show a back weld as it was part of a TIG article on tanks but its the handiest image to show the inside edge to inside edge fit. A keel, or chine joint would not be a right angles as in this image. This is the theory of the fit and weld.



This is the practice of the fit and weld. Weld here is TIG, of course, not MIG which is more commonly used on the hull seams but still illustrates the type of joint fit I try for in all of the major hull's long seams, this doesn't address butt welds, however, I use this same weld at the transom to bottom seam as well.

Holding the hull parts in the best position to tack, can be done with jigs, or fixtures or it can be done with frames both temporary or permanent. Having a set of hands to help is about the best, even if you build alone- rigging to hang sides is a rodeo without help, even if they're just there for the day it usually takes to hang sides. Bottom tack-up is something you can do alone but it sure helps to have an extra set of hands.

If you'd like more input on your project(?) post your lines for your design and let us see what you're thinking in terms of shape, style and speed. What software are you using to design?

cheers,
Kevin Morin

[catdieselpow]

I am using NX 7 for the lines. I will put up my design when I am done with it.

[kmorin]

CAT, NX 7 is pretty high end to use for a skiff, (!) so not being familiar with that application (suite of applications?) are there marine design routines? It sure is handy to be able to find CG, CB, Lateral plane, waterplane area and all the design coefficients of a hull while you're editing the surfaces.

I will assume it will unwrap/develop the surfaces? Will the NX7 also show Gaussian irregularities so you can see if the surfaces of your lines are going to lay flat?

looking forward to your posts.

Cheers,
Kevin Morin

[catdieselpow]

Kevin- CG, CB- Center gravity , center of balance is what i think you mean, yes

Lateral plane, waterplane area and all the design coefficients of a hull while you're editing the surfaces
I dont know.

I will assume it will unwrap/develop the surfaces?
Yes it will with the convert to sheet metal tool.

Dude I couldn't find what a Gaussian irregularitiy is any were but it sounds like something from calculus 3.

It does have a ship design program built into it but i think it is for huge ships.

[kmorin]

CAT, when the chines, keel and sheer are resolved into 'plates' if they can be 'developed' then the surfaces will 'unwrap' into flat outlines that can be made into a boat hull shape. But since there are an infinite number of curves in all three axis, therefore an infinite number of shapes that could be defined by these three line (sets) in 3D space.

Of any couple million lines sets, only one or two will become a fair set of outlines for flat panels and one common test is the Gaussian curvature of the surfaces, and the regular curvature of those surfaces- but not the only test. So, most marine software has surface tests for 'developabilty' and the surface is tested for curvature irregularities using a color pattern. (Gaussian irregularities shown by a moire pattern or coloration zones) If the model is curved too much, here or there, due to surface mesh or nodes' locations then the color pattern can help the designer to clean the surfaces to make sure they're 'regular' or developable using standard flats, cylinders and cones and 'stress limited' transitions between those geometric elements.

(CG) Center of Gravity implies you've already got a hull shape, all the scantlings of all structurals and the full equipment list from a table of moments. Some software allows you to enter that value (longitudinal location) so you can see it in relation to the CB (Center of Buoyancy), and many designers consider it important to pay attention to these (relative) locations when you do the wt tables and moments of trim for a hull design.

if NX 7 has a marine app already there, it will probably have all the tools to do a skiff, and more. I suspect the 'unwrap' to sheet metal feature, you've mentioned, will have some test of developable surface of some form to tell you if the lines need adjusting to produce a clean set of outlines for hull panels.

Cheers,
Kevin Morin