Cross Beam thoughts for small Tri

[Air Tahiti]

New to the forum. Like a few others, I am building a recreational use trimaran. I have a Tornado 20 center hull and two Nacra 5.5 for amas. Mast will be Tornado standard setup w 30ft mast. I ran across a pile of 4 inch aluminum tubing. It appears to have wall thickness of 2mm. Planning for beam of 14ft and will leave these pieces whole without any hinging mechanism. Would love to get some feedback regarding if this would be practical to use for cross beams. Thoughts?

 

[oldmulti]

Air Tahiti. As long as the tri is light and water stays are used 4 inch 2 mm walls will work. Look at Revintage work as he has designed and is building a very similar trimaran. The jpegs below give an idea of his excellent work. You will have to go to: Small trimaran with Tornado/Nacra 5.8 center hull https://www.boatdesign.net/threads/small-trimaran-with-tornado-nacra-5-8-center-hull.66449/
to follow his improvements to the beam construction, dimensions etc.
The first jpeg below is the original "wide cat" before he added a Tornado central hull.

 

[Air Tahiti]

Thanks for the reply. I have a large international machine shop just around the corner from my house. They can basically do anything and pricing is fair since they buy in huge bulk. If any size is available would the optimum size be 4inch tubing X 1/8 inch wall thickness? They can also powder coat them. Any and all comments appreciated.

 

[revintage]

2mm will probably work but the waterstays will be critical. I would go for 1/8"or 3mm thickness.

 

[fallguy]

Powdercoating is not recommended for aluminum in salt environment, and not really f/w either. The aluminum will do well on its own oxidation. Otherwise you can anodize as well as alodine the inside of the tubes if saltwater will sit in them.

 

[cando2]

I agree with fallguy on the powdercoating. When I was a metal fabricator we used it more for cosmetics and to lengthen the life of compromised materials used in low stress applications made in a high production environment. Perhaps of equal importance to diameter and wall thickness is the proper grade and hardness of aluminum. Find out this info and let this forum know; they know if it will work. Using a cat as center hull may lead to very shallow seastay angles; perhaps explore eliminating them and going with larger diameter and thicker walled tubes. Fallguy, never heard of alodine. What is it and how is it applied and how permanent are the results?

 

[Air Tahiti]

Great info. Thanks. I just assumed the aluminum would be best protected by powder coat. And look good too. Otherwise I would paint them I guess. Keep in mind the intended use is very recreational and not pushing to the limit. As much as I like to think this would get used all the time, realistically it’s probably more like half a dozen times, each being a few days on average. Certainly most will be lake, until comfort and confidence goes up. Which probably will be quick. I live in SC and like to think I could take it maybe once per year to an annual beach trip and leave it on beach for a week. The aluminum that have heard of is 6061. Any size and any wall thickness is available via the shop near me. They are an international fabricator and I know the owner so could get anything made. But thinking normal straight pipe 4 inch and build center structure such that amas are are right elevation and with a slight angle to be made with some spacers. Planning for 14 ft beam. I probably could crunch numbers with the best of them but I don’t have the time and have no desire to turn this into a complex problem. If the cross beams seem weak, I would just go get some 1/2 inch larger in diameter and roll on. Hopefully this info helps the discussion. Thanks again for the input!

 

[Air Tahiti]

I forgot thickness. Thinking 1/8 inch walls. But can be thicker if need be. Many thicknesses are available. The shop can get anything I want. They have a water jet that can cut 12 inch steel plate that is the size of a small house. Just to give perspective.

 

[fallguy]

Alodine can be poured into the insides of the tubes and swished around a/o applied with a stick/sock. It is just a coating that can help if salt will land in the tube. It also works well if you intent to strap or bond two aluminum things and cannot use a rubber or neoprene boundary.

Other ways to help are to barrier the aluminum from other metals with rubber or other means to reduce galvanic corrosion.

I was going to alodine my tubes, sorry main beams, but got the boat put together and did not turn back for anodizing or alodine.

I did anodized one of my beams. It was a pretty low cost exercise. The beam pictured was polished prior to anodizing. Not sure if you can make it out, but the beam is sitting in neoprene cushions to keep it away from the stainless seats/fixtures.

 

[guzzis3]

Bigger diameter tube will make it stiffer while wall thickness will make it stronger. Most cat/tri beams are 3mm 1/8" from beach cats up to 30' cruisers. 3" diameter would be good 4" is probably too big. You could probably get away with 2" but it'd be a bit floppy. No stays required. If you use a marine grade you won't need coatings assuming it's attaching to glass hulls. There are plenty of 30 yo beach cats still with their original beams. For your application coatings are probably more trouble than they are worth.

If you do decide to go with 2" beams if you have the option go thicker wall. You won't need it sailing but the boat will probably get knocked around given your comments and say 2/16 wall will make it a lot more rugged for not a lot more weight.

 

[revintage]

The easy way out is going for 4" as the beam pockets in the amas are 4". I have the same amas but actually use 90mm in the beam pockets with PVC adapters I turned in the lathe. The reason for using 90mm is that I have a telescopic design going from 106mm in the center via 100mm to 90mm in the ends.

Rather large volume amas on this one. I don´t have my work computer here, so I used a simple online calculator to calculate a 4"x0.125" alu tube. With 14´ beams L=72", load on main beam same as ama deplacement F=1100lbs. We then arrive at ca 5" deflection and 380MPa stress, ie well over 6063-T6/215MPa or 6061-T6/255MPa.

So waterstays are mandantory.

This is good thread about crossbeams:
Trimaran crossbeam calculations https://www.boatdesign.net/threads/trimaran-crossbeam-calculations.38345/

Post taken from the thread above, where "daiquiri" suggests that 150mm thinwall(=2mm?) beams w/o waterstays might be to weak for a W17 trimaran, about the same size as Air Tahiti´s project:
Trimaran crossbeam calculations https://www.boatdesign.net/threads/trimaran-crossbeam-calculations.38345/page-6#post-468492

 

[Air Tahiti]

Great input thanks. Yes the Nacra AMAs are all set up so will prevent work if I match things up to how they are originally built. The Tornado hull will get gutted and therefore I can do whatever.

changing gears, I ran across a sail that I plan to go get for super cheap. It’s 29ft (my mast is 30ft). But apparently is set up for use without a boom. There are battens. Looks to be in really great shape. It’s a giveaway price being sold as a shade over a pool, so thinking it’s worth it. I realize if I go this route there will need to be some custom rigging changes. I am one that typically enjoys doing things against the grain. I am trying to figure out where this sail may have come from. Anyone have an idea?

 

[Air Tahiti]

I don’t have a photo the ad is down. But it has a small clear window and 18 on it. So probably an 18 ft beach cat of some variety. Also the foot of the sail isn’t straight. The luff dimension would be less than typical and the clew a bit higher in terms of distance vertically from hull. Or so that’s what I could see in the photos.

 

[revintage]

I guess it is from a 18sq. Buy it if its cheap, be sure to get the battens.

 

[Air Tahiti]

Thanks. It has the battens. What’s an 18sq? I would like to pull up a photo to inspect the rigging.