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cracked_ribs

Building in aluminum?

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This is kind of an idle question, but locally to me aluminum is by far the most desirable boat material.

 

The thought occurs that the Marissa, being plywood panels rather than cold moulded, could in theory be welded from aluminum.

 

I did see where someone had built I think a CK17 from aluminum and it was quite heavy, but I wonder if a Marissa could be done from 1/8 with longitudinal stiffeners in place of the existing framing and end up fairly similar in weight? It might be the the stiffeners would need tighter spacing but then again, maybe not.

 

Anyway just curious to see if any builders or prospective builders had taken a look at the idea in any depth, or if anyone at B&B had ever looked into aluminum as a medium. 

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Hi Cracked Ribs.

 

There is absolutely no reason why you could not build an aluminum Marissa. It will be heavier, the CS17 built in Alaska finished out at 700# which is at least 50% heavier than a wood version. Aluminum size for size is about 6 times heavier than okume ply. As you suggest 1/8" is a good starting point, it will be about twice as heavy as 9mm okume ply. That is aluminum's finished weight for the shell whereas the ply has to be glassed, coated with epoxy and painted plus the fact that the framing can be less. Which will probably come out about 50% heavier than the bare Marissa hull but you are talking about a very light boat. An all welded aluminum boat is about as tough and as maintenance free as it gets. You are right, the Pacific North West is aluminum boat country, it seems that work boats are not built from anything else except for maybe tugs where weight is probably positive.

 

CS17 hull #1 was built in aluminum in New Mexico, I got to sail her in Florida and she performed quite well. About a decade or so ago I was hired to take a 50 footer from California to Puerto Vallata in Mexico. One of the incentives for taking the job was that hull #1 was residing a short drive north of PV. I had a great visit with the builder who is one of my favorite people and hull #1 was living on the beach along with the local Panga's. The owner was still enjoying her, saying that she was the best fishing machine he had ever owned, he would trim her for a beam reach to sail herself and was totally stealthy as he trolled to and fro for game fish. He laughed that he usually outfished the charter boats burning hundreds of gallons of fuel a day.  She had not spent a day of her life under shelter and I will bet that she is still going strong.

 

The second CS17 built in aluminum was in Alaska,. Here is one of his posts on this forum. You can search for the rest of his build. He said that 6065 was marine grade aluminum which is incorrect. The 5000 series is the marine grade. 5083 is a popular but not the only choice.

styge

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The weight of the boat is around 700lbs. So you won't be dragging it up any sandy beaches, but it doesn't sound like that will be a problem. As for building one, it may be easier to get a guy who's been welding, (and in particular, welding boats) for a while. I thought about doing the welding myself, but I figured my finished project should probably not be my practice piece, that gets expensive. It was all I could do to keeps up with my guy. I was cutting and grinding and he was welding, welding, welding!

But it turned out great. If you decide to proceed with it, online metal in seattle is great for souring the aluminum, and they were happy to deliver it to Alaska Marine Lines to get barged up here.

I went with 6065, I think, its a good marine grand and is a little more pliable (which isn't saying much) than other better grade. I have originally thought I would just do the hull and fit out everything inside in wood, but once the project was underway and we had a flow to the build process, it just seemed (at the time) to make sense carrying on in alumnium. The panels were coming together at a rapid rate, and I'm not the patient type, plus every additional panel meant less maintenance. Right now the only wood to be found on the boat is the tiller!

Anyway let me know if you decide to proceed as there are a few things I would have done differently were I to do another one.

Regards

Styge Smith

Juneau, AK

 

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Thanks very much for the reply - I am sort of kicking the idea around although I have not done any welding in years and have never welded aluminum at all.  Still I am not too bad with a torch and I am pretty confident I could get my head around a MIG setup.  My best friend is actually a welder, although he lives on the mainland and probably couldn't be relied on for actual welding work, I am pretty sure he could help me get rolling.

 

I have a set of Marissa plans here and my intent was to build a normal version but when I think about an aluminum one I just think that would be a fantastic platform for fishing here - just hauling crab traps alone, the ruggedness of aluminum is kind of tempting.  And since guys often end up putting 40-60 horse motors I think an extra couple of hundred pounds of hull weight might not be too much of a problem.  With the lighter framing, as you say, the increase might not be too great.

 

I hate to pester with questions that are a little outside the original design but how do you feel about the spacing of the stringers in terms of supporting the aluminum hull?  Glassed ply is pretty rigid - do you have thoughts about whether intermediate longtitudinal stiffeners might be necessary, between the stringers say, or between the inboard stringer and keel?  My understanding is that the framing of aluminum boats is ordinarily almost entirely longtitudinal, to avoid the "starved horse" look as the plates get pummeled in, but then the Marissa already has four stringers and a keel, plus the chine steps which I think would be quite rigid...just ballparking it in my head the egg crating of the hull looks like it would support the slightly floppier aluminum pretty well, certainly more so than if it had been designed with a single stringer per side and no keelson (if I am using that term correctly).

 

Instinctively I feel that if the transverse framing was rebated so that rather than contacting the hull, it only connected the longtitudinal supports, and that if the side stringer was turned into something like a 2"x .100 stiffener, the whole structure would be very close already.  But maybe even the transverse frames are fine as is, I'm not sure.

 

There are some aluminum boat manufacturers around me...maybe I will request a tour and see if I can gather any ideas about how they are framing up their lighter boats.

 

I might also pick up Stephen Pollard's book on aluminum boat building.

 

Well, thank you for humouring my idea...if you have any further thoughts on it I assure you I'll be paying rapt attention!

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