Cross Beam thoughts for small Tri

[revintage]

Google is your friend:
nacra 18 square - Google Search https://www.google.se/search?q=nacra+18+square&sca_esv=559959589&sxsrf=AB5stBi9xyfsm6nO2YH0Comy_TI50iSRtw%3A1692977770432&ei=asroZOD7Gb6Qxc8Pg-C5gAk&ved=0ahUKEwjg2PnUkfiAAxU-SPEDHQNwDpAQ4dUDCA8&uact=5&oq=nacra+18+square&gs_lp=Egxnd3Mtd2l6LXNlcnAiD25hY3JhIDE4IHNxdWFyZTILEAAYgAQYsAMYywEyCxAAGIAEGLADGMsBMgsQABiABBiwAxjLAUjYBlAAWABwAXgAkAEAmAF2oAF2qgEDMC4xuAEDyAEA4gMEGAEgQYgGAZAGAw&sclient=gws-wiz-serp

 

[Air Tahiti]

That’s the sail. I assumed wrong. It is set up for a boom. It’s the same with the little 18^2 at the top. The photo in the ad wasn’t great.

 

[revintage]

Go back to the beams, waterstays and ama dihedral. The other stuff is not that hard to solve.

 

[Air Tahiti]

I think I have settled on standard 4 inch tubing 1/8 inch wall thickness. Will tighten it up with w stays. Can’t wait to get cranking on this one….

 

[revintage]

How long tubes can you get? I would go for 15-16”. Note the forward beam needs to be placed ca 10-15cm above deck level, right above the center hull beam pocket. The rear beam has to be adjusted to get correct nose down. Is your Tornado hull with bolts through the beams or with beam straps like the Nacra?

 

[Air Tahiti]

Tube length is theoretically unlimited when there is a high end fab shop around the corner. But the sticks of tubing typically come in 20ft or 24 ft. From what I can see poking around a good rule of thumb for trimaran beam is about 2/3 of length. 14/20=0.7. Keep in mind my target is sandwich eating type sailing, not Everglades Challenge. You see I have 5 kids and just getting to the water is a huge victory all on its own.

I understand your statement about ama positioning and plan to do as you mention. I have seen that discussion play out several times in these types of threads.

 

[revintage]

Plenty of knowledge to get here, amongst them how to find the correct beam ratio.
Trimaran Design Issues https://smalltridesign.com/Trimaran-Hull-Issues.html

Good luck with your build!

 

[Air Tahiti]

Thanks for the link I will check some of that out for sure. I recognize as I turn 50 and am an engineer, I better not get over tempted by analysis. I promised myself - measure once and cut, move forward. I plan to combine some common sense with what I have read over the past years, get some feedback here, and plow forward!

 

[revintage]

Calculated your ideal beam width and arrived at 15.4 ft, using Tornado waterline length of 5.94m. Ratio is 0.79.

 

[Air Tahiti]

That’s great. 15.4 it shall be. Thanks!

 

[Jonathan Fowkes]

Love this thread. Any updates or photos. On progress,

You look to have visited all the same sites I have to research. You are a few months ahead of me.

I'm currently doing the same only fixing crossbeams made from fibreglass.

I have H18 amas on a 20ft centre hull
Sockets needed rebuilding in amas. Crossbeams need some reinforcement as they seem to flexy. They are fibreglassed so a little difficult to calculate the cross sectional measurements required. Currently they are 100x100mm chop strand. Will be reinforcing with epoxy resin and eglass matting. Cross section is upside down U shape. Thinking I need to make it at least a box section to build the bottom compression (downward force) and tension (for upward forces). But that then impedes on the folding mechanism arm that folds inside the U when extended.

I'll be same use as air Tahiti as far as use. Just family fun (no racing in hectic conditions).

 

[Air Tahiti]

Thanks for the reply Jonathan. You will end up ahead of me. I have a home addition happening such that I do all the work. It’s 1000ft2. I plan to be done maybe in 2 months. Plan is to begin the tri work just after so I can get it wet this coming summer.

I have decided, subject to more changes in thought, that I will cut up the center section a bit to accommodate more width for feet etc. for me if it’s not comfortable then it’s a no-go. Performance is a distant second. Although I do believe performance will be solid if not very good. Please feel free to use this thread for your photos. I would be very interested in seeing your progress. I am in SC, what part of the country are you in?

 

[Air Tahiti]

Jonathan - I am nearing the end of my home addition (basically a small house built solo) and am once again able to think about my three hulls staring at me every time I pull up into the driveway. I plan to get them to my barn soon. I have the cross beams and all the gear. Can’t wait to get cracking on this. Mid to late summer I would guess.

 

[Air Tahiti]

Forgot to mention the reason for the reply. If there any new pictures please post!