Calculating beam scantlings for catamarans

The other thing to consider is torsion, one hull going up, the other down in a quartering wave.

It is worth noting that high speed powered craft codes like DnV look at a "hollow landing", which assumes the two opposite corners are supported by buoyancy while the rest of the boat is not, with some amount of acceleration. (Tranverse and longitudinal loads are handled in much the same way.)

 

Bringing back an oldie cause I...

need to figure out what sort of beams to use on the double Tamanu cat I'm building. I broke the main beam from a H18 in the luff groove in just such quartering seas described above.

Gary Dierking provided me some scantlings for various beam scenarios--I'm going to keep it to a trailerable 8'6". I can use wooden box beams like woods and other designers use or go with aluminum tubing. The breakage was a crack in the luff groove--not catastrpphic failure, but enough for me to get some pressure off the beam by sailing under jib for the last day.


In the next iteration the beam will be reduced by about a foot, adding a bow beam as well, hard deck aft, and a tramp up front. Rig will be the Hobie 18 again. I don't mind making the structure monocoque, lashing on timber beams or going with aluminum tubing--whatever makes the most sense.

Basically, I'm looking to do a H21SC with a hard deck aft.

Ideas? Are the scantlings of the Seawind 24 over the top for the much lighter Tamanu/H18 combo?


Dan

 

Did you have any luck with this? I would love to see some numbers if you did.

If anyone is interested my 2x beams for my new tri are basically
boat weight 1200 kg inc crew
length 8.5m width 6.5m righting moment about 3200kg/m
300mm deep x 200mm x 6m long.
8mm H80 foam box
450gm db glass inside and 600db glass outside
2000 grams carbon unis top and bottom
main hull supports 1.5m of the beams

I had two independent composite engineers check it and

apparently the moment of the beam will is around 11500kg/m
3x safety factor.

or the other calculation appears to be moment 40000nm with a SF of 6.5 on the carbon and about 30 for the walls of the beam, either way it is easily strong enough.

 

Trimaran beams....

This is an ask for help, please. I know, the thread is catamaran beams, but the earlier discussion seems to be one of the best informed around and has already helped me a great deal.

So here is my problem. What is the worst case scenario on which to design the beams for a trimaran?

My design is for a 38' light cruiser, design displacement 6100 lbs, overload displacement 7265 lbs. The usual crossbeam layout, that is two beams per side. I want to use round ali 6061 tubes for building space, transport and very occasional de-mount reasons. Using Beam Boy to do the number crunching I get fairly good results using 12" tube, 1/4" wall (304.8mm x 6.35mm). I'm aiming to have max bending stress less than the fatique stress of 96.5 MPa, and acceptable deflection. The weights, approx 500lb for 4 tubes seem okay.

Is it over the top to take as worst case / max possible load the case of one beam takes it all? That is, one beam loaded by total vessel displacement? And in this case is the fatique stress criterion relevant?

Or is it more rational to accept the case of one beam loaded by half the total vessel displacement?

Getting the right answer has implications for increasing or reducing beam.

Any input or help and advice gratefully received.