Best Shape Aluminum Stock for Hull Frame?

[RockReeve]

Hi, I have designed and am working towards building a 21' aluminum motor catamaran. For the framework of the hull i am wondering wat the best construction material would be in terms of price, strenght and ease. Should i use square stock, agnle iron, t-bar or other.Thanks.

[Dutch Peter]

I think you should go for the T-bar, it has a higher Section modulus as the square stock(flat bar) and will deform less with bending as the angle bar.

[Arrowmarine]

Hmmm, I would have to agree with Dutch. The T-bar is very strong and can be easily formed to desired bends and shapes. And as stated it is more structually sound than the square bar, not to mention lighter. Stay away from angle, it can bend too easily. At the very least, I would say use a structual channel. But then again, catamarans are not one of my specialties.

My two Cents:-) Joey

[SolomonGrundy]

I disagree with the pervious posts. Yes, absolutely, T-bar and to a lesser extent angle are strongest. But unless yopu have more money than sense, anything but flat bar is overkill on such a small craft.

[Arrowmarine]

Hmmm.........

1-1/2" X 1-1/2" X 3/16" T- Bar weighs about 7.8 lbs per 12 foot stick
2" X 3/8" Flat bar weighs about 10.5 lbs. per 12 foot stick
Even 1/4" X 1-1/2 flat bar is 7 lbs.
I buy my aluminum by the pound. I see no appreciable difference in price and weight. Also the strength provided by the t-bar is FAR superior to the flat bar.
So, here's my thoughts: If the T-Bar weighs the same, costs the same and is stronger, which one do you choose? Your call.

Peace, Joey

[Dutch Peter]

Joey,

Your absolutely correct!
Might I add to your calculations that in order to obtain the same strenght/section modulus the flat bar will prove to be much heavier and the same goes for the angle bar, but with far less difference.

[Arrowmarine]

Thanks Dutch:-)

[Dutch Peter]

Just some other thoughts about this discussion:

Considering the size of the vessel a rectangular shaped frame might be enough, and easier to work with as the T-bar will become very small.
As flat bar will prove very difficult to bend, cut frames from plate is a good option, but expensive and hard to do yourself.

[MikeJohns]

At 21 feet flat bar frames should be fine, you may want T or angle for the deck beams if you can get the sizes to match the required section modulus so as to increase head height.
As Peter said above T bar will be the most proffessional solution but I think flat bar will be easier. It all depends on the scanting number relevant to the design ie how strong your frames have to be.