ply tp alu

i,ve been looking into the bolger advanced sharpie -29 as a candidate

what would the weight diff between ply and alu be

and cost diff because i wouldnt need so mush epoxy and glass

and i assume building would be much faster

any gustimates would be appreciated

 

There wouldn't be any reasonable "guestimates" as you haven't a reasonable scantling list to work from. Ideally the weights would be nearly the same, other wise the boat will not trim properly or sail on the displacement intended.

You're correct in that light is good, but you have to be designed for the weight. In other words, taking a 5,000 pound design and building it at 3,000 pounds will not do it's performance any good. It's just not as simple as it seems, both in scantlings conversion and performance. I do think you're onto something though, as this is a good design for this type of conversion. It wouldn't cost much to preform the conversion, but it should be done professionally.

 

i was thinking of keeping the weight the same
but that could be adjusted with the ballast

ive worked on aircraft structures and the are amazingly light

it seems to me that the as 29 could be made heavier out of alu than needed and still stay within weight

also it seems a good candidate for riviting

 

I'd weld the boat and I also wouldn't attempt to make a bullet proof hull with thick plates. I'd spec it for expected service and let the chips fall where they may, making up the difference with ballast/displacement ratio and rigging choices.

 

If you are thinking in terms of using aluminum that is thinner than the ply that Bolger specified, then the problems begin. These boats have a bunch of flat panels, Bolger liked to use really thick bottoms. In doing so he could eliminate much of the interior framing. Let us say that you might use 0.125 plate for the skin....... where Bolger will have used one inch thick ply. There is a collossal difference in stiffness between the two materials. Roughly, the stiffness of a plate will vary with the third power of its' thickness. That means that the ply will be hundreds of times stiffer than the thinner alloy plate. Aluminum has a better elastic modulus than ply but not nearly enough better to make up the difference. So scantlings are the main problem here. You can use the aluminum but you will need to add a whole mess of stiffeners, which will increase cost, labor and aggrevation. All that honeycomb of stiffeners also contribute to problematic bilge sanitation. If you build a different sort of boat with a lot of curvature in the panels, then the sheet metal becomes more practical.