Building the C19 in Aluminum-Questions before buying plans.

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WolframM
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by WolframM »

The reality is that aluminum is very springy-as long as you stay in the elastic deformation stress range. I don't think keeping it fair and even will be an issue. It will take a crease, but so will plywood if you bend it beyond it's elastic limit!

I have been reading and re-reading through Gerr's book and working through the math, and ended up stuck in Chapter 19 with his words on aluminum boats under 20 feet; with a Scantling Number less than 0.5 he recommends quite a difference in material thickness compared to scantling numbers higher than 0.5, namely instead of 5mm bottom plate, he asks for 3mm bottom plate!

Now, 3mm bottom plates may be what he calls out as "the norm" but certainly that's a bit thinner than I would want, just for abrasion purposes more than anything. When I make radiator and pressure tanks for racing cars I still use 2.3mm thickness sheet, and even then it needs pressed in ribs for the modest 15psi pressures involved there.

I think I will run the numbers, buy the C19 plans, and model the entire boat, hull, sole, fittings and fixtures, in both composite and aluminum, and find the compromise between hull weight and sheet thickness, that I am comfortable with. By that, I mean I will model the whole boat exactly as per plans for the composite version, and adjust the composite density to match the published weight. I will then rework the plans according to Gerr's Scantling Number system, placing frames and stringers per his methods, and use only the hull geometry and general arrangement of the C19, as if the boat had a Sn = 0.5 or higher, aka thick plate. THEN, I'll do the same thing again, using his instructions for Sn's less than 0.5 and boats under 20ft OAL, and see how these things affect the weight, depth of water drawn, and center of mass. From there I can make better decisions on how to move forward.

TomW1
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by TomW1 »

Sounds like a good plan to me. Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

OneWayTraffic
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by OneWayTraffic »

Those small boat scantlings are riveted construction only I think. It's impossible to buy a new riveted boat in NZ larger than a dinghy. It might be ok for a lake boat, but I'd feel a lot happier with 4mm or 5mm welded.

I extracted the Gerr scantlings into a spreadsheet a while back along with a lot of other boat related random things. Feel free to have a look.

https://docs.google.com/spreadsheets/d/ ... ue&sd=true

Matt Gent
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by Matt Gent »

Building a boat in aluminum is on my goals list, looks so clean and industrial when finished. I've got a lot of skills to build before I am ready to take that on though.

There are a couple skiff builds in aluminum over at the microskiff forum. Designs for foam and glass converted over.

I bought and read "Boatbuilding with Aluminum" which has a bunch of great info. Boatbuilding with Aluminum: A Complete Guide for the Amateur and Small Shop https://a.co/d/3J3so7p

WolframM
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by WolframM »

Yeah, I'm still searching and reading up on aluminum construction, I've got a few books coming inter-library loan that I hope will help clarify some stuff.

School's ramping up and will be in session again august 1st, I'm planning to make some fishing spoons and some lead casting molds this year as student projects. We'll see how they get on with that.

Only 71 more days til I get back to the coast and go fishing again.

Matt Gent
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by Matt Gent »

TomTom wrote: Thu Jul 14, 2022 3:42 pm I agree 100% with a comment made by Matt Gent that CNC adds very little time savings on an overall build time. I had my C19 panels all measured up, and cut out in 2 days! It took more time to tape it for paint
For the record on an aluminum build I’d 100% do cnc panels and structure. You can build in tabs and slots to make it self-fixturing. There is no fairing compound (fore bare finish) nor fat epoxy fillets to make up gaps. Welding requires much smaller gaps than epoxy s&g.

I think a well-planned aluminum build could go super fast in large part by skipping much of the finishing.

The book I linked talks a lot to the processing.

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