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I have been building my Tiki 21 for too many years. I started with plenty of time and no cash... now I have the opposite. If I need to make the beams I may never finish the boat! So ....

Aluminum Cross Breams ... What standard cross section is best, and what dimension?

Looking forward to any advice, ideas, opinions and experience!

Tom

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Hi Tom,

The beams on "Little Cat" are welded aluminum otherwise to plan. I am not an engineer, but this seems to work very well - they are very stiff with no flex. I think it would be expensive to have done though.

Roger

Yes that is why I am interested in modifying (as little as possible) a standard cross section. I also doubt the extra work for following the plans is needed.

One question... are the beaks heavier or lighter than the wooden version?

Tom

Tom,

I have not handled a wooden beam, but the alu ones are light - I pick them up and carry them around with one hand. One drawback is that they will not float in a catastrophic failure.

Roger

For the 21 tiki wich is ligth weigh , my wondering is if you don't have time to built beams ,  why not standard big alu tube or square section ? There is a lot small catamaran with alu beam  ?

Laurent ... yup, we are on the same tack... The questions are

  1. what dimensions? So if it is square would 100 x 100 mm with a wall thickness of 5mm be strong enough?
  2. the same for all three beams, or does the mast carrying beam need to be larger?
  3. The only modification I need to make to the beam is to be able to drill holes so I can bolt all the other fittings onto it (Cleats, Mast base, deck etc) ... how does this effect strength? Probably negligible?
  4. Does any one recommend a specific aluminum alloy?
  5. Anodized, painted or untreated?

Looking forward to your ideas...



Roger said:

Tom,

I have not handled a wooden beam, but the alu ones are light - I pick them up and carry them around with one hand. One drawback is that they will not float in a catastrophic failure.

Roger

Thanks Roger ... I hope to avoid catastrophic failure... In that case I hope to step up into my life raft :-)



Roger said:

Tom,

I have not handled a wooden beam, but the alu ones are light - I pick them up and carry them around with one hand. One drawback is that they will not float in a catastrophic failure.

Roger

Roger ...would you be so kind as to measure, or estimate the aluminum wall thickness? And ... your cross beams are 'H' beams?

The Tiki 21 beam is an H beam. I would say from 4mm-6mm from memory - I will measure next time at the boat. There are more loads on the center and aft beams. The center beam has a dolphin striker - all as in the plans.

6061 is the usual marine grade in the U.S., it may be called something else in Europe.

PS if departing from the plans I would do a consult with a multihull designer.  The alu beams on similar sized cats are substantial  - take a look at a Hobie 21 for example, and this is a narrow tippy beachcat, not for the ocean. Usually a mast section of some sort is used, and again they are substantial. Perhaps take a look at some of Richard Woods designs under 25'. The problem with a mast section would be how to mount it - it may well be more drama than just building the standard beams. Also, alu section is hellishly expensive.

Thanks Roger ... I'll check out Richard Woods, a favorite designer of mine. Re mast section... I was hoping to bolt the fitting on to the alu... re cost ... its work time and money vs. another few hours in dust masks, plastic gloves and epoxy... tough call! 

the square beams 100 x 100 mm x 5 mm should make the business for the main beams . If you see some weakness or curve a dolphin striker is easy to add under the mast  . And one tube 80 mm  fwd , if you don't want to put the mask , the gloves ...  There is also the H beams industrial if you dont like alu... but you will have to glass them . I have read that the alu dont need to be anodized , it covers itself with its protective coat of oxydation  ? Anyway you can paint it with a good bi part poly .  All that are just ideas and some time i think to much and shall be better to finish my own refurbishing so slow
Tom Venning said:

Thanks Roger ... I'll check out Richard Woods, a favorite designer of mine. Re mast section... I was hoping to bolt the fitting on to the alu... re cost ... its work time and money vs. another few hours in dust masks, plastic gloves and epoxy... tough call! 

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