Framing Metal Boats

Strong Backs, Keels, Frames, and Longitudinals

Framing Metal Boats

Postby KevinMorin » Mon Jan 05, 2009 4:32 am

One of the most lively topics of spirited discussion among metal boat builders is about framing. Some folks use a version combining transverse and longitudinals, others use exclusively transverse frames. Still others don't frame in the traditional sense, they build from their hull's panels and add structural elements at a later time in the build sequence.

Names like "Origami", and "StrongAll", are used to describe less traditional methods of metal boat building, and some builders use their own unique variations that don't even have a popular terminology to explain it.

Different metal boat Designers/Naval Architects have plans for each of the methods described and some folks feel strongly about one or another method. All the methods are in use, all of them have "floated a boat" so we can't really say one method is the "end all", if the others aren't included somewhere in the discussion.

Different geographic areas have differing preferences in this area of discussion, for example, the 'all-transverse-frames' with no longs method is more popular in Europe's metal boat yards than in Australia or New Zealand.

In the Pacific NW of the US, and farther North into Canada, there is a growing group of 'origami' advocates who don't use initial hull framing at all. They take the entire outer hull 'skin' from a sheet model and form the whole hull by pulling the edges of a pre-shaped cutout outline in steel, or aluminum, into a hull form.

Small sheet metal boat builders, that is boats smaller than 25', can often buy metal sheet and plate so large that entire hull sections can be cut without one joint, except for chines. This allows a method of building, very common among aluminum power boat builders, of a 'skin first' approach to construction. While very common among small power boat builders this method doesn't fall into any of the previous categories and isn't named.

Some metal boat hulls are rounded, some only rounded at the primary chine, and others have more or less flat panels meeting in edges at longitudinal chines. Different hull forms lend themselves to the various framing methods as demonstrated by the may different commercial builders' employment of these differing hull framing practices.

Whatever way you will frame your boat, or if you have a question about how to best frame a given boat (?) this is the place to ask about your boat, or explain your point of view.
Cheers,
Kevin Morin
"Nothing is half so much worth doing as simply messing about building metal boats." apologies to Kenneth Grahame
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Re: Framing Metal Boats

Postby alangluyas » Tue Jan 13, 2009 2:51 pm

Kevin

Thank you for making the effort to get this forum started.

I don't have any burning issues but I will espouse a bit of my philosophy on metal boat building to keep the forum moving.

I am just completing the design for my biggest metal boat to date, which is a 50 foot displacement diesel cruiser. I have been a sailboat sailor for most of my life and I have built two metal sailboats, a 30 footer and a forty footer, both in steel. My wife has always coastal sailed with me but she is developing reservations about crossing oceans and rather than trade her in on a more agreeable model, I am leaning toward a "troller yacht". I have built two alloy boats in the last 10 years, a 30 foor semi displacement diesel cruiser and a 22 foot centre console OB runabout.

My first steel boat, built in NZ in the mid 80's, was a Bruce Askew design with a long fin keel, cutter rig and bowsprit . I built her the conventional way, from the keel up, although I cheated a bit and built the hull on a dummy keel so I could build her in my garage at home (mostly in the garage) and fit the keel after the hull was built. Nevertheless, she was built frames first and all hull plates were templated in hardboard or ply and then fitted to the frames. The boat was very successful and we sailed her across the Cook Strait many times in some very big seas and winds. I do recall that when "horning" the hull to check for symetry, one side was 10mm longer than the other. No one ever noticed, apart from me.

When we decided to move to Western Australia, we decided to sell her and I got Bruce Askew to design me a bigger version of the boat. I wanted 38 feet but got a boat 39' 6" and to this day I am not sure why. I think we may have decided to eliminate the bowsprit and rake the stem a bit more to facilitate anchor handling but whatever, I wound up with a 40 foot yacht.

During the construction planning stage I became aware that Van de Stadt were doing some interesting things with their new steel/alloy 34 footer, in terms of "the frameless fairing technique". I played with this concept for a while and even wrote some spreadsheet based software (this was in 1989) to develop the plate shapes but I met a Western Australian designer, Dave Jackman, who was already working on this idea and I engaged Dave to reverse engineer Bruce Askew's plans and give me the developed skin data in x,y co-ordinates.

I re-engineered the hull construction to maximise the advantages of this new technique and I built the boat. The accuracy of the method was startling. I don't think I ever had to grind a plate edge to get a perfect weld fit-up. The hull was perfectly symetrical and the speed of construction impresssive. Working almost entirely single handed, part time, I assembled and finished the bare hull in a few months. I can't be precise, because I had to take a break and get my neck vertebrae fused half way through the project as a result of an accident suffered offshore in my marine surveying career.

Sadly, although I finished the boat and made several major changes to the hull and rig, I never launched it. I probably made a mistake in converting the boat from a marconi cutter to a junk schooner (for which Bruce Askew has never forgiven me, I think). The job of setting up the new rig became too hard, my business became too successful and I ran out of time and the dream died. I ended up selling the boat and buying a FRP Cabo Rico 38 in the US and shipping it home, ( I still have this boat) but that is another story.

I built a 30 foot alloy semi displacement boat to keep on my home jetty ( I can't get the Cabo Rico up the river) and run around in locally. During my Westlawn study I became interested in narrow hull forms and hull efficiency and wanted to try some theories on narrow semi displacement hull forms. This one I designed from scratch. I did build a 1/5th scale model but the design was largely developed with ProSurf and Autocad. I developed a plasma cutting plan and had all the panels NC cut. Construction was again very fast and the hull was a succcess from a construction point of view. She is bit more tender than I had intended and she now carries abit of ballast to keep the VCG low when the fuel tanks are empty. I did it again with the 22 foot centre console which also worked very well.

I have read the "frameless" vs fully framed discussions/arguement avidly. I think that almost everyone has valid points that are well worth making and I think tthe problem with a lot of the controversy is that some peopel don't listen fully to what the other side is saying. Like they say, "opinions are like assholes - every one has one but no one wants to look at anyone else's".

Despite having built the last three boats "frameless", that is only how I built them and they did not stay frameless very long. The reason I developed the methods I did in the first place was that I was aiming to allow the plates to take up their "natural" curvature and not have to deform around hard spots in the framing. Once the hull plates were tacked up I added full transverse framing in the bottom plates ( and up to the first longitudinal side plate stringer, about 300 mm above the chine). The hull sides had flanged deep frames avery 2 mtrs and angle stringers. The decks also had full framing. The hull was relatively light, as I used 4mm plate throughout, apart from the keel.

My two newer boats have less framing but are still very well framed. I used Australian and US codes to decide the scantlings and used a lot more more mathematics in calculating stresses and section moduli for stringers and frames.

I have become a fan of "frameless" construction methods but they are not without shortcomings. The fact that you allow plates to form their "natural" shape ( I will write more on this later) has a reciprocal issue, in that you cannot force much "unnatural" curvature whilst plating the hull. This was most noticeable in my two steel boats. Both boats had chined hulls with a pronounced knuckle and rubbing strake about 60% of the way up the side plates. They are both very pretty boats. On the first conventionally built hull, I was able to force the plates aft to form to the curved edges of the transom by bending the plating around the stringers, so that the finished hull looked to almost round bilged sided. I could not achieve this with the second hull, as plate shapes were controlled by the edges of the plates and the plates near the transon were quite flat. I am not saying that forcing curvature could not be done, just that I could not do it.

This problem recurs in different ways with "skin first" construction but the issues can be managed and I will use the same technique for the 50 footer.

I bought a copy of Brent Swains "Origami" book and really enjoyed it. I did not agree with everything that Brent says and I could not live with the standard of finish that some of his boats wind up with, but that is my problem, not the boat owner/builders. The book is still a contribution to boat building knowledge and Brent is to be commended for it. Brents boats seem to work and despite the bullshit that flows both ways, they do not seem to fail catastrophically or regularly. If there are some failures, (and I don't know whether there are or there are not) Brent is not the only designer (by any means) who has had skegs bend or fall off. Despite all the science and engineering involved in boat design and naval architecture, there is huge pool of empirical knowledge that all designers draw from and which they use to temper and evaluate strictly mathematical analysis. A lot of this pool of knowledge came from people who were designing and building things that either broke or did not.

Any way, that is my introductory contrribution. I shall enjoy discussing or arguing (politely) any of the points put forward.

Cheers

Alan Gluyas
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Fremantle Westrn Australia
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Re: Framing Metal Boats

Postby KevinMorin » Tue Jan 13, 2009 5:54 pm

Alan,
thank you for the post, it was extremely informative and since you were discussing the Miracle Metal and not just that 'other' metal I enjoyed it more than I would have otherwise!

I've used the skin first method, myself, in boats up to 40' but where this method really lends itself is the size boat that allows the hull to be four or five pieces. If the boat is less than 25' we can arrange for 26' sheets without much premium cost allowing the entire boat bottom to be taken from one sheet, and the topsides from two others. Add a chine strip and transom and the hull is tacked up. I build a few hundred open skiffs for commercial net fishing using this method and found it fast and 'close enough' accurate to produce a reliable work boat.

What have you found about predetermining the amount of 'natural' convex bulge in the bottom's forward 1/3, chine to keel there is usually a naturally occurring outward curved line instead of a line. I've never had trouble with this area building off of sheet/plate models (builder's model) but I have had some difficulty with pre-cutting my PC generated designs' transverse frames/bulkheads that fit in this area. Problem being that if I pull the hull back to close a gap with a frame the hull is unfair, or sometimes if the frame touches half way from chine to keel I'd have to force it down and that has bulged the bottom panel in some cases. This is the one area of PC generated designs in small boats that still seems a bit elusive to me. I've tried to establish a thickness to distance to curvature to being conic projections but haven't found any real consistent results?
Cheers,
Kevin Morin
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Re: Framing Metal Boats

Postby alangluyas » Wed Jan 14, 2009 1:06 am

Hi Kevin (and other addicts).

Ha! The issue of conic curvature is the bit I was referring to when I said I would get back to "natural" curvature. The short answer is no. That is, no, I don't have the answer to how the perfect geometrical development will actually come out in a finished boat. I wrote a post in "boat design forums" at

<http://www.boatdesign.net/forums/design-software/predicting-plywood-bending-behavior-stitch-glue-design-24551.html>

that was aimed at stitch and tape plywood design, but the same principles apply. In my experience, with somewhere in the order of a dozen boats built in ply on frame and stitch and tape construction, plywood is closer to being a "perfect" sheet material than alloy, as it has a much higher Youngs modulus than alloy or steel ( effectively it stretches less). Among other things, this means that ply is less likely than alloy to conform to compound curvature.

By the way, please forgive me if I tend to over explain things that are probably obvious to you, as I am aware that others who are just embarking on the addictive path to metal boat building may be completely bamboozled by some of these concepts.

I always use a conic development process to predict how conically developable a hull shape may be but I have come to expect that the forward third of the hull will have more curve that I have predicted. The form of the curve is generally parabolic with the most curve toward the keel. Is this your experience?

I start with a set of lines that I think will be developable (the unused part of the brain gets very good at this sort of thing - I understand that the mathematics that the brain performs when you walk forward and reach up to catch a thrown ball is as complex as computing a ballistic trajectory for a moon mission) then I use the process described in the boatdesignforums post to predict where the keel profile line needs to be. If I can’t get the profile I want by moving the cone apexes I might have to alter the chine line in either profile or plan view but not very often.

My theory, and I have not got an engineering degree, is that when you make a valid conic projection, it is only one possible solution to the geometrical problem. It is not necessarily the only solution or the best one. If you use your conically developed hull shape to build a hull “plate on frame”, you can force the plates to conform to your solution, and because alloy especially is a more ductile material than mild steel ( the E value for mild steel is normally around three times that given for marine aluminium alloys), alloy will conform to a surprising degree of forced compound curvature.

As an aside, Dennis Ganley was a NZ designer of steel boats who seemed to really understand what could be done with steel plates. His designs were late on frame but he used very strong male jigs to restrain plates as they were forced to accept compound shape that got locked in as the hull went together. Sadly, he died as a relatively young man a few years back and we cannot consult him on this.

Anyway, for this reason I normally allow between 10 and 40 mm trim allowance on the bottom edge of bulkheads and frames forward of amidships. I also make an allowance for induced curvature in the side plating, as I am more fussy about pulling in hollows where they are obvious. The amount of curvature induced is obviously a function of the change of angle in the sections, the “twist” if you like, in the hull. If there is no change of angle (this would equate to a constant deadrise in bottom plates) then the hull is cylindrical rather than conical in form, and there should be no transverse curvature in the hull plates, as viewed in section.

As a matter of principle, if I am working with an apparently flat panel, I normally pre-stress the plates by artificially forcing some curvature into the panel by creating curved frames or bulkheads. I use a 1:40 rise/chord ratio for the span involved and this seems to be easily achievable. I find that if I don’t do this, the small devious gods of alloy boat plating create edge shrinkage and I get sections of plate between frames that are uncommitted or slack and prone to “oil canning”. Prestressing minimise this. I have had cracks form in fuel tanks as a result of having areas of slack plating and I am more careful now.

I am a reluctant convert to alloy as a boat building material. I love working with it. It is clean, light, easy to grind, amazingly fast to weld, has a high scrap value, it does not really need painting above the waterline and it allows you to build things that you can’t seal inside of. This last feature makes designs for steel and alloy potentially quite different. Alloy boats have a higher resale value and have the potential to be stronger or perform better due to the higher weight/strength characteristics of alloy.

As of a few months ago (global financial disasters aside) it was actually cheaper to build in alloy than steel here in Australia. 6mm steel plate was $AUD2950/tonne, vs $AUD6500/tonne for grade 5083 plate in 4mm and 5mm. As alloy weighs 2660 kg/m3 against steels 7850 kg/m3 and as alloy thickness is typically 30 to 50% more than equivalent steel thickness, the actual material costs start just about level, even before you start waving a sandblasting hose or epoxy paint around.

On the down side, alloy just does not like holding paint, it has become environmentally or politically incorrect to use effective antifouling paints, welding is less forgiving of incompetence, alloy loses a fair bit of strength in the heat affected zones around welds, it is not normally practical to build outdoors or even under an open roof as shielding gases get blown away, alloy is very susceptible to several forms of electrical corrosion and alloy boats can be noisy if you are not very careful about isolating mechanical noises.

The clincher for me is that it is easy in Western Australia to order plates up to 10 mtrs x 2.2 mtrs from the supplier, who will plasma or router cut to an NC cutting plan. The router cuts plates are marvellous, apart from the razor sharp edges. I tacked up my 22 foot centre console runabout in a few hours, including breaking out the pieces from the plates. The pieces are pre-marked with an ink marker with frames, waterlines or anything else I want marked. I just email the supplier, confirm the availability of the plate sizes I want, email through a cutting plan in dxf format and post a cheque.

A lazy mans boat building at its best.

Cheers

Alan Gluyas
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Re: Framing Metal Boats

Postby BrianR » Wed Jan 14, 2009 3:04 pm

RE: Prestressing Panels

Very interesting thread!
Alan,
You said
As a matter of principle, if I am working with an apparently flat panel, I normally pre-stress the plates by artificially forcing some curvature into the panel by creating curved frames or bulkheads. I use a 1:40 rise/chord ratio for the span involved and this seems to be easily achievable. I find that if I don’t do this, the small devious gods of alloy boat plating create edge shrinkage and I get sections of plate between frames that are uncommitted or slack and prone to “oil canning”. Prestressing minimise this. I have had cracks form in fuel tanks as a result of having areas of slack plating and I am more careful now.


Could you elaborate on this a bit more? How are you forcing the curvature into the panels ( which type of panel?hull plating,bulkhead, tank wall?)? Is this similar to cross braking gauge thickness sheet metal to stiffen it? English wheel? Plate rolls? Are you referring to frameless construction when you prestress the hull plating?

Also, it is my understanding that by using 50xx series alloy extrusions rather than 60xx extrusions, in conjunction with 5086 or 5083 plating, that there is not HAZ at the welds because the 50xx is not heat treated. The welds and the base metal exhibit similar ductility and tensile strength. This, to me, makes the extra cost of the 50xx extrusions worth the price. But then again, perhaps it doesn't make any real world difference ( I discussed this on another post in a "different" forum)

Do you have pictures of your boats-we'd love to see them!

Cheers
Brian

Thanks
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Re: Framing Metal Boats

Postby alangluyas » Sun Jan 18, 2009 12:22 pm

Hi Brian

Sorry about the delay, but as a result of my unfortunate working habit, I am now just winding up a job at an unbearably hot place called Dampier in the Australian North West, where we are enjoying temperatures of 40 plus degrees way into the evening.

I will use the term "skin first" construction to to try and minimise the amount of flak that gets generated when anyone use term "frameless" which I do not actually subscribe to in large vessels. As far as I am concerned the jury is still out on "strongall" construction. I am not saying it does not work or that it does not produce fine and seaworthy boats - I am sure it does both, but I am not convinced that it is necessarily a very efficient way of building a boat of more than about 10 mtrs. This is only my opinion and I do not attack anyone else’s ideas on this. I do like to use thicker plating than is normally called for in alloy, as I have had some cracking from plate flexing and I think that thicker plating minimises this.

To answer you question, I normally design metal displacement boats entirely transversely framed in the hull bottom plates. With semi-displacment or planing hulls I use transverse frames and intercostal stringers fitted after all the frames have been tacked in. Fitting the stringers intercostally after the frames eliminates a lot of problems with "skin first" construction. For example, it is very difficult to spring in full length stringers without pre-rolling them and if you do prefit them you need to site them in exactly the correct location for your frame cutouts. Once the full length stringers are tacked in the boat it is then very hard to template the bulkheads (remember I normally allow some “trim” forward to allow for errors in development).


So, in the sorts of vessels I am designing, ie, single or multi chine alloy and steel hulls in the 20 to 50 foot range, I have found it practical to generate the “induced” curvature by spring an extra degree of curvature in the frame sections.

Bear in mind that after I have attempted to predict the conic development by the method described elsewhere and after establishing apparently conically developable chine and hull centerlines, I then draw a section plan of every frame station just to the right of the elevation drawing and use intercepts from the conic projection lines and buttock lines to determine just what the “natural” conic shape will be at each station. If this is too hard to comprehend and sounds like gibberish, please let me know and I will attempt to construct some explanatory autocad drawings and some clearer notes.

Where there is significant curve in the plates, I am happy to use what is predicted, but as you get further aft in most hulls, the deadrise becomes more constant (at least it does in mine, because I tend to favour constant deadrise and parallel buttocks aft for efficiency) and the plate development become quite flat or cylindrical rather than conic. This is where I find that plates become slack or uncommitted ( I have not found a proper word for this concept, you may have noticed) and need a bit of stressing. I have usually just drawn a spline curve with two control points (as well as the end points). I don’t use arcs because an arc will carry a constant curvature right to the edge of the plate, which I have found can be unnecessarily difficult to achieve and encourages bad language during construction.

As for the 5000 vs the 6000 alloys, in an ideal world I would prefer not to use the 6000 alloys in an immersed situation as there are potential galvanic issues between the 5000’s and the 6000’s. I am quite happy to use them above the waterline and inside the hull as stringers and frames. In fact, the world is not ideal and I have used 80 x 10mm extrusions in alloy 6061 T6 for chine bars on 5mm plated bottoms in 5083 H321. I have had no trouble so far but I still don’t encourage the practice.

My understanding, and it may be imperfect, is that the 5000 series alloys are (generally) work hardened while the 6000’s are (generally) heat treated.

My welding instructor and welding notes give a percentage strength loss of 15% for the HAZ on 5000 series plate. I have no reason to doubt this but no real support outside the reasons stated. The 85% strength rule does seem to widely accepted in alloy boat building circles.

I do know from personal experience that when we were bending 5083 “marine” grade plate, we were able to bend to much smaller radii without cracking after annealing the plate with a heating ring. I am embarrassed to say that I do not know how hot we used to heat it as there is a local trick in marking alloy plate with a spirit marker of a type known in Western Australia as a “Texta” and the properties of this ink marking are such that it fades to almost nothing at a very suitable heat for bending plate. A poor mans thermal indicating crayon.

I have not seen extrusions available in the 5000 series but bear in mind previous comments about us being remote here. Having said that, alloy boatbuilding is very advanced in Western Australia in sizes from 300 feet plus megayachts to 15 foot plate runabouts. I can order alloy plates in thicknesses from 3mm to 25mm in plate sizes to 2.200mtr x 10.000mtrs off the shelf.

Any clearer?

I have attached 5 pictures of two boats - the site won't allow more. I will add some more next post.

Cheers

Alan
Attachments
0001.JPG
Early construction of 40 Askew designed yacht - this was a learning experience in "plate first" contruction.
0001.JPG (158.48 KiB) Viewed 8390 times
0003.jpg
Internal framing of 40 ft ASkew design - not frameless!
0003.jpg (71.25 KiB) Viewed 8381 times
IM000100.JPG
40 foot Askew boat finished (well as finished as she got)
IM000100.JPG (35.73 KiB) Viewed 8379 times
chiron28 cropped.JPG
Early Rhino rendering of 28 foor alloy semi displacement cruiser.
chiron28 cropped.JPG (32.95 KiB) Viewed 8372 times
Chiron 28 - alloy.JPG
28 foot semi displacement design at near completion of alloy work.
Chiron 28 - alloy.JPG (102.23 KiB) Viewed 8373 times
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Re: Framing Metal Boats

Postby alangluyas » Sun Jan 18, 2009 12:45 pm

I could not resist it. Here are some more. I can't find the construction photos of the 6.5 mtr boat - I must have them stored on a computer in the office, so here are a couple of drawings. This boat is almost ready for turning over to weld the keel on.

Cheers

Alan
Attachments
skylark - sea trial 3a.JPG
"Skylark" - the prototype during sea trials. She does not have a boot strip or antifouling at this stage. She managed 17 knots with a 70hp Nissan diesel and cruised at 14 knots comfortably for a fuel burn of approx. 10 litres an hour.
skylark - sea trial 3a.JPG (92.23 KiB) Viewed 8353 times
Kagan in blue.JPG
Kagan - a 30 foot Askew design that I built the "proper" way, plate on frame, right way up. She has steel hull & decks and a timber (plywood) house.
Kagan in blue.JPG (84.41 KiB) Viewed 8348 times
5 mtr rowboat.JPG
A 5 metre lapstrake plywood rowboat to keep on the jetty at home. Not metal but but very similar techniques apply.
5 mtr rowboat.JPG (123.1 KiB) Viewed 8339 times
Vixen 5600 Deck Plan - OB Profile.JPG
Deck Plan and OB profile of 6.5 mtr alloy runabout.
Vixen 5600 Deck Plan - OB Profile.JPG (38.01 KiB) Viewed 8338 times
Vixen 5600 NC Cutting Plan.JPG
NC cutting plan for the Vixen 6.5 centre/side console boat.
Vixen 5600 NC Cutting Plan.JPG (74.05 KiB) Viewed 8334 times
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Re: Framing Metal Boats

Postby alangluyas » Sun Jan 18, 2009 1:07 pm

Brian

Sorry to be boring, but if you look on the picture of the cutting plan for the 6.5 centre console, you should be able to see the ghost lines that represent the straghtline section shapes and the altered lines to allow for extra curvature and trim. There is an inner and outer transom on this design. The outer transom is quite low and carries the OB motor, while the inner transom is a watertight bulkhead. Both these have the extra curvature and these two frames effectively set the bottom plating shape aft in this boat. Frames forward of this have a trim allowance and are fitted to curvature of the hull bottom.

Cheers

Alan Gluyas
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Re: Framing Metal Boats

Postby idlerboat » Sun Jan 18, 2009 1:30 pm

Hi Alan..... from Melbourne with a much more moderate temperature,

Thanks for joining in. I have both learned from, and enjoyed your posts. Look forwards to more

cheers
Martin
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Re: Framing Metal Boats

Postby BrianR » Mon Jan 19, 2009 3:42 am

Alan- Thanks for the information. It's a bit hard for me to understand all of it, but that's my fault, not yours! The pictures of the 40' being built outdoors showing the transverse floors installed helps. I'm still unclear how you are inducing the curvature on the " prestressed" panels .
In your experience about how much additional material should be left on the fore and aft panels for trim allowance?.In other words, generally how precise is the translation between a CAD drawing that predicts the size of the panels , and the real world fit up, assuming accurate cutting? I've also heard that one should strive to have the temperature differential between cutting the plates and fitting and welding the panels to as little as possible to avoid thermal changes ( Especially with alloy).

Skylark is one good looking boat! Do you have a web site we could add to our designers link section?

Thanks
Brian
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