Aluminum to Plywood

[kharee]

How feasible is it to convert designs intended for aluminum to plywood? Hartley converts many of their ferro-cement plans to strip plank. I see some nice plans for aluminum and just wondered about the possibility.

[TerryLL]

Don't know about aluminum to plywood, but plywood to aluminum works well for certain designs. I worked in an aluminum fab shop for a while and we cranked out 19-23 foot skiffs based on Tracy Obriens 19'deadrise design. The boat was originally designed for stitch-and-glue in plywood.

http://www.tracyobrien.com/moreinfo.asp?id=30

Alum is essentially the same building technique as S and G, different "glue" of course. I would think that some alum designs, those without chine flats, could be built in plywood. Scantlings would be way different due to the mechanical properties of ply vs alum, so you'd have to know what you were doing.

[Andrew]

What design are you interested in? There's probably a plywood equivalent.

[Ray Frechette Jr]

Plywood is better than aluminum is some respects, and aluminum better in other.

You really need to either have a N?A's level of knowledge of the properties of the two products, hire a NA to engineer the scantlings for you, or get lucky.

[kharee]

I was just looking at some pilothouse motor sailors by Ted Brewer. I like pilothouse motor sailors and few small ones are designed for wood or ply. I was just curious as I want a boat but I want to build it myself.

Should I take the plunge it will probably be a small Hartley design.

[Cuyahoga Chuck]

I have seen numerous queries about doing this and it seemed to founder on the convertibility of plywood which is quite stiff and fairly light with a suitable alloy and gage of aluminum.
If you look at the common commercially produced aluminum recreational hulls they are almost all rivited and most have draw-formed panels and stiffeners to overcome the inherent flex of flat aluminum panels. To keep the weight within bounds the metal gage is limited so these alternative devices are necessary.
Once the designs get to a certain size the required gage gets thick enough oil-canning is not a problem and welding without big time warpage is possible.
I dug up a web page where a competant welder welded up an aluminum hull for himself in about a week but he was short on commentary as to why he could do it and most commercial outfits avoided it.
I know for a fact that Jacques Mertens has refused to sanction out of hand aluminum conversions to his S&G designs because a lot of necessary scantling changes and alloy specs would be required to build a correctly engineered hull.

[TerryLL]

Just to give you some points of reference:

The 19-23 foot aluminum skiffs we built to the Obrien Deadrise 19 design had 3/16 or 1/4 bottom plate, 5/32 or 3/16 sides, and the pilothouses were either .100 or .125 sheet. All mig welded. Aluminum requires extensive internal structure to prevent flexing that leads eventually to brittleness and cracking. 1/4 bottom plate is way heavy for a 19' boat, but these skiffs got brutal use.