Advice needed for creating an aluminum frame for a powercat

[Dayneger]

I'm working on a design for an efficient 29' powercat with long, slender hulls, a 14' beam and a boxy superstructure. The concept is laid out in 3 modular blocks, composed of 2 simple hulls, a frame, and the living box on top of the frame. At the moment I'm scratching my head about lots of aspects to do with the frame. I know very little about working with aluminum, particularly in a marine environment.

Here's the link where I describe the design in more detail, including intended use: Feedback desired for unusual powercat.

My idea at the moment is to weld the 4 lengthwise beams and most of the cross beams out of 4 x 6 x 0.25" aluminum extrusion, with 2 x 2" and 3 x 3" used for the verticals, hand rails and davit.

I would be very grateful if you could give me some tips on any of the following questions:

1. Is this an appropriate dimension extrusion to use as the basis for the frame, and could the torsional stiffness be acceptable for this application?

2. Is there a particular extrusion alloy that works best for marine applications, and a good place to buy the materials?

3. Do all of the tubes need to be closed off (ends welded shut), or is it ok for some of the ends to be open?

4. How do you bolt things to the aluminum frame without causing corrosion points? I'm concerned about stainless bolts in contact with the aluminum frame.

5. Would I need to protect the frame with something (paint or whatever), or is the raw aluminum ok?

6. Any idea how much it would cost to have a frame like this made for me?

7. Any other things I should consider when making this design?

Thanks so much in advance!

I read this with interst
To start with you dont need beams at all in the sense that you mean, apart from your house beams, your cockpit sole beams and your foredeck beams
In A BOAT HIS SIZE YOU CAN BUILD HUGE STIFFNESS INTO THE , whoops caps sorry , into the structure by way of the hulls and breast plate, and sole and foredeck you can even press the bottom in a vee in one piece from the chine down (google bladerunner boats NZ) Through this bottom spacing floors and intercostals, with your tanks and inboards, if you have inboards From chine up you might use flat bar cut frames at say 1m centres with a couple of flat bar stringers
through the sole will be supported on say 75x38x4 mm box and the house beams 50x20x3 box as with foredeck
Yes this is a bit of a fast and brief bit of advice but its tea time!! here in the morning) the bottom sides 4mm plate, sole 4mm, roof 4mm good luck

[Dayneger]

Hi,

Thanks for the response, although I admit I had a bit of a hard time following all of it!

Did I understand you correctly that you're suggesting making the hulls and cabin out of aluminum also? In any case, it would be great if you had time for a longer explanation at some point!

To make it easier for others I've attached the 3-view pic of my design from the other post.

Cheers!