Folding arm materials - Any reason not to use Aluminum?

Believe it or not, I'm trying to simplify this, not complicate it. I just threw up that picture as a sort of general idea of what I'd probably build. If I go that route, I'll just take a sheet of probably 3/8" aluminum plate, cut out 8 identical arms on a pattern torch (I can get that done for free - just the time to draw it and the cost of the plate). Each beam would consist of two parallel plates in a configuration much like the swing arm. I would then run two 2" or so sized tubes that would run all the way through both beams. One near the vaka, one near the ama. They would both act as support and double as mounting bars for the tramp.

I believe if they are spaced something like 4" apart and the the entire assembly is welded up, it will be very strong as compared to two fiberglass beams. I think it should also be lighter. And it would be cheap and easy for me to build. Once I have the plate cut, I think just a few hours and I would have both sides ready to go. If I go fiberglass, then I'll need to build the mold, then cut something like 50 patterns of glass and then build each arm 1 at a time. I think it would be much more time consuming.

The other alternative is simple round or square tubing. That's probably the cheapest and easiest option. It's also more than likely the lightest as well.

 

Quote "The other alternative is simple round or square tubing. That's probably the cheapest and easiest option. It's also more than likely the lightest as well". Quote

Having built a Buccaneer 24, two Buccaneer 28s and three Buccaneer 33s all with tubular 60/61T6 alloy crossbeams, I can only agree with that quote.

 

What size tubing was used on the 24?

 

The Turbo 6 catamaran 18 foot.

On the turbo 6 catamaran the cross beams are 100 mm sqaure tubng
(about 3 mm thick) with U channel linking the beams.
The beams are glassed into sleaves within the hulls.

It folds horizontaly.

 

Some additional information to the turbo 6 is that has limited accomadation fitted within the hulls, the normal of a berth each end. As well as having the provision for a pop top, between the berths.

 

The Turbo 6 does use a water stay system right? Although I guess the lower strut on on a vertical folding beam acts in tension much like the lower stay would.

 

4 1/2" with 1/8" wall. Design calls for 4 5/8" but Lock OK'ed mine and I had no problems with it. The same tube was used for both crossbeams and mast. Mast had SS sail slide track rivetted on. No cross trees needed.

 

That seems like a decent safety factor for a boat of that size with a good margin for shock

 

The turbo 6 is a catamaran that used a dolphin striker under the mast, with a outboard motor pod runiing along the center line. a u channel is used to proved a wider pivot point for the cross beams.

I thought the cross beams would be a reasonable guide as it is a similar size to the scarab 18.

The Sylph trimaran by Malcom Tennant is a 26 foot timaran.
Samez left a post #10 on its cross beam specifactions for the Sylph.

I would think it would be more complex to build up the cross beams,
how ever combining the two Ideas may work. Using a single lower strut.

 

Keep in mind jetboy that the bucc24 had waterstays so the tube on the 24 was a compression strut. If you cantilever your beams you may have to go to a thicker wall on the beam, square section is probably more appropriate due to less tendency to twist.

 

My original thought was 3" round .250 wall. I think the 18 should have considerably less forces than a 24' tri. It will be much lighter - hoping to end up around 1,000lbs building with honeycomb and epoxy.

 

Crossbeams on the Piver "Nugget" were Douglas Fir 4 X 4s, tapered on the outboard undersides to 2". No waterstays. Strong. Cheap. Simple. A bit heavier than alloy---but satisfactory. No trouble over four years of use.

 

From what I understand Malcom Tennat used oval or square tube was so it would not rotate, and sleave insread of bolts to eliminate point loading.

Would be possible to combine both with water stays, to hold the tube in grp tubes.

It may then be possible bond grp tubes into the main hull.

 

How would I go about calculating something like 3" .125 wall wrapped in say two layers of 6oz carbon fiber?

Would that be a better beam or just a waste of time? I'm just throwing out ideas here. I'm trying to simplify not add complexity.

 

What Malcom tennant sugested was to wrap the beam in polyporolene plastic, then to use 305gr/m2 chopped matt to form the initial sleeve.
That formed a partial frame in the hull that was built up.

That would have been the mid eighties