Weight of wood vs. aluminum

I have plans for a 16' wooden stitch and glue boat that says the final weight of the finished boat would be 1000 pounds. I would like to build the boat using 3/8" aluminum. Is there a rule of thumb equation I could use to estimate the final hull weight using aluminum?

Thanks for your help.

 

I can't imagine a stitch and glue boat weighing 1000#.
If you are going to have a heavy keel you need to subtract that weight, the weight of any motors, and the rig (at least) before you compare weights.

What design are you going to build? The designer may be able to tell you the surface area, but it might not include interior structure.

If you can figure out the surface area of the boat, just figure out the area in sq. inches, multiply by 3/8, and multiply by 0.1#/cubic inchs.

3/8 is very heavy for a 16' boat I believe. Someone else will be along who knows.

 

Yeah, extremely.

 

My 1957 16' Star-craft Marlin weighed a little over 250#. It was about .080" thick in places. A little thicker in that Keel Piece.
The only problems were leaks at the Corroded Rivets.
For low-maintenance and longevity, I'd go with Aluminum.

 

My 40 year old stich and glue dinghy is still in good shape with no corroded rivets and it is quite lightly built. Good quality ply wood protected from water intrusion is a great building material and takes much less skill than aluminum. Amateur builders can get very good results you just have to live with some chines.

 

Unless there is a very good reason for using aluminum......don't.

As a matter of fact there are many good reasons for using aluminum: If You will leave the boat exposed to weather, rainfall, oak leaves, bacterial attacks, unwashed fish guts, and so on. In that case aluminum is the right way to go. If you intend to give your boat reasonable care, then wood is the better choice because it is lighter, less expensive, easier to build with, is not as noisy, has better thermal insulation qualities, and a few more pluses.

If your plans call for three eights ply, do not presume that the same thickness aluminum is appropriate. Aluminum is roughly five times heavier than wood. A wood boat that might weigh two hundred pounds would weigh somewhere around a thousand pounds in aluminum. That means that you'd need to use about 0.075 aluminum sheet to get an equivalent weight for the finished boat. But not quite. With a thin 0,075" skin you will need to use much more framing to maintain reasonable skin stiffness.

 

There is no doubt in my mind that small aluminium boat building in Australia is way ahead of anywhere else in the world. That was once almost exclusively with boats made of relatively light guage material ( roughly 1/16" or thereabouts) with various styles of pressings or swaging to give a stiff lightweight structure. These were more or less the province of larger concerns with the plant required. More latterly there has been a proliferation of small builders, almost a cottage industry, using plate. The cut-off point is around the 5-metre mark, few boats under that size are not pressed/swaged lighter guage material. A typical boat in the 6 metre range would be plate of 3-6mm with the heavier guage on the bottom only, as a rule. Anything thicker than that is a rarity, certainly 3/8" (10-11mm ?) is well outside the range, and makes no sense at all in a small boat. To preserve the shape without excessive "dishing" in a plate boat does require either a lot of framing, or thick-ish plate, or a suitable mix of both, and in a sub 5 metre boat that means quite a bit of weight, which is the reason few are built in that size range.

 

mr efficiency is spot on with with his post .I have to go and check over a 16 ft plate aluminium boat this week. it is a heavy duty hull , commercial spec with self draining cockpit. the bottom is 5mm and the sides 3mm. pressed hulls that size come right down to 3mm bottoms. so you could build the queen mary with 3/8.

 

I can't imagine a 1,000 pound 16' stitch and glue design. You must be looking at the displacement figure, not the hull weight. I'm building a 15' glued lap hull currently and I exspect the hull to weigh about 150 pounds.

As to a conversion, yes this can be done, but there's no simple formula that you can employ, mostly because the materials are quite dissimilar in physical properties. These types of conversions are best left to professionals, familiar with the materials and engineering issues involved.

 

Rule of thumb...
It will be five times as expensive and 1.2 times "better".
Stick with wood and with the plan.

 

.375 or 9.5mm would be fine on the keel, but not on the hull.

 

Many West Coast (read BC) builders are ahead of their time in aluminum work. Google "Origami" boat building for more answers. I've worked with all materiels incl aluminum for many years studying different methods. Swain's proposal of bending a sheet of alum or steel has its merits, vs conventional framing. It's not for everyone, but the tech is there.

 

I live in the Pacific Northwest and in my area welded aluminum boats are everywhere.
I have personally built 2 welded 16ft boats and can tell you from experience that 1/8" 5052 is a pretty standard gauge. My second boat did have a 3/16" bottom and was slightly heavier but they are in the 350-400 lb range (less power). They require almost no maintenance and are incredibly tough. That being said, they lack the charm of a wooden boat.

 

3/8" aluminum plate is more suited to something like a 170-ft crewboat than a 16-foot boat.

 

Maybe on a lightweight ship, but mostly the ones I built we used 5/8-7/8 on the keel and 3/8 on the hull sides. Decks are generally 1/4 as are the superstructures - pilot house.