convert wood plans to aluminum construction

[lynnburcher]

Hi all,

I'm looking to build my first boat, and am considering converting plans designed for plywood to build an aluminum boat. Seems like it would be pretty straightforward, by comparing the strength ratings of the two materials. And, I guess draft and CG would be different. Is it difficult to convert plans from ply to Al?

The main reason I'm considering the conversion is that someday I would like to build a catamaran that I can use as a Texas gulf dive boat. It seems to me that Aluminum would be the material of choice for this project, and I'm looking at the Glen-L plywood Party Boat as a "welding 101."

Also, I'm considering the "Boatbuilding in Aluminum" book by Pollard. Any recommendations?



Thanks.

[Arrowmarine]

ahhh, another aspiring aluminum welder/boatbuilder. I remember that day long ago when I said " Hey, maybe building aluminum boats would be cool. Yeah. Anyway,.......

Ha Ha ,I'm just playing.

Key to remember on a material conversion is "If the materials weigh the same, and are the same size, you should get very similar results, (draft c.g etc.) In other words, cut your parts from aluminum that weigh as close as possible to what the parts made of wood would weigh. I have the glen L book here somewhere, iI will look into your Party Boat and give you more details later today

[sorenfdk]

You would probably get similar results regarding draft, c.g. etc., but what about strength and stiffness? These are really much more important...

The proper way to convert from wood to aluminium is to calculate the scantlings all over again.

If the draft, c.g. etc. don't come out as for the wood design, you can always add some internal ballast.

Check out Dave Gerrs "The Elements of Boat Strength".

[Arrowmarine]

An excellent point, sorenfdk, and one I was going to cover untill my nine month old son decided my time would be much better served playing with him than typing :-)
I agree that strength and stiffness are very important, but they wont mean anything if the boat doesnt float. Recalculating the scantlings is the proper way, but it's doubtful that lynnburcher is ready for that, this being his first boat. Which brings me to my next question for lynnburcher: Are you planning on purchasing a pattern kit and duplicating EVERY part from aluminum? After looking at the bill of materials on the glen l party boat, I can see this would be a tremendous amount of overkill. Remember that most aluminum hulls similar to the one you are considering consist of a bottom plate, sides, transom, 1 bulkhead and a few of longitudinal stringers and thats it! At least in the Pacific Northwest they do. This is called monocoque constuction which means that the shell makes up most of the structual strength of the vessel. Indy cars are noted for this type of constuction. Strong and light. After the hull is formed, you can reinforce where needed depending on your application.
You wont need all of the exta parts a wooden boat would require. Confused yet?
Bottom line? A boat, built from aluminum, with the dimensions you are considering(20' x 8' with a 15 degree bottom somwhere around 72" wide will perform just fine. That is a very popular size. I have seen between 10 and 15 thousand of these boats roll off the line, including somewhere around 350 built by yours truly. I would recommend .190 5086 material for the bottom, And .125 5052 for everything else. As sorenfdk noted a little internal ballast will fix any small problems with c.g. but I have never had to use any in the designs I have worked with, both my own and from other manufacturers.
Check out www.alumaweldboats.com "super vee" to compare dimensions. You will also note that the weight, about 1500 lbs is comparable to the glen l design you are looking at.
I would also check out sites from fish rite, jetcraft, boicejet, willieboats, among others, to get an idea of contempory aluminum hull construction. Also, read everything you can get your hands on about boat design, materials, etc.
Good luck, Feel free to email me with questions if you would like (I live for shop talk :-))
Joey

[lynnburcher]

Thanks, guys, for all the great info!