Triple tube sliding beam

I am building beams for my small 19ftx15ft beachtri using Nacra 5.8&5.5 catamaran hulls as can be seen i n another thread here. Got the idea to make the front beam sliding to 8.5ft width for road transport. I have 90x3mm and 100x3mm tubing and will order the center 110x3mm tube, if this idea has a chance of work IRL.

The sliding plastic tubing I have gives a play of 0,2mm between the tubes. The double waterstays will be synthetic and the bolts connecting the tubes are 12mm.

What worries me is the 100x3mm tube that neither is connected to the ama nor the center hull. One could suspect torsional problems.

The rear beam will be simpler as I am using a 2.5m Harken sheet track. The 90x3mm tube is removed and the ama clamped with the existing Nacra beam straps and bolts to the 100x3mm center beam.

 

I would have two areas of concern which are a sort of opposite to each other. The first is the rigidity of the tube, with a short length of overlap when extended, and this issue is multiplied by the double telescoping. The second is the potential difficulty of the snugly fitting tubes getting jammed, particularly if sand, salt, dirt or dust get in. I have had this problem with my Dart 18 beams and my Solway Dory mast. Would having two beams front and two beams rear, which slide next to each other, and give a longer overlap in their housings (like the Astus 20.5 and others) work for you? You might find this keyway mechanism by Solway Dory interesting too. The 2 narrow brackets round the tube but spaced well apart are much less likely to jam from dirt.

 

Hi Tiny T,
I agree with you about the Astus designs. It would be interesting to see how they have solved the friction problem, though. Unfortunately I can not rebuild the hulls as all three have the beam pockets at equal distance to each other lengthwise, see first image. If you look at the Astus 16.5 they do not have the side by side sliding.

Checked the Osprey video and was impressed by the ingenuity, especially the key lock beam arrangement.

Maybe I didn´t make my self clear about my idea. The rear beam is not sliding, as the beam straps are loosened from the 90x3mm beam tube before the front beam is pushed together, so the problem with parallell sliding like on the Dart should hopefully not exist.
To further avoid jamming, maybe an O-ring could be added to the inner ends of the 90x3 and 100x3 front beam tubes to center them, second image.

I also suspect some diagonal stabilization might be needed.

 

I’ve been sailing and hanging on forums since the beginning of the internet and I can’t think of a successful sliding mechanism for marine use, short of tiller extensions which have lots of CRC applied regularly.
Between salt sand and oxidation it’s just not practical.

 

Hi rr,
Thanks for pointing this out, but the question was not about friction in the sliding mechanism. It was about if the triple design of the front beam would cause stability problems. No need to worry about neither salt water nor sandy beaches where I live.

 

Delrin O rings or slides[?] and being able to split /loosen the centre /largest tube would be good for cleaning or if it jammed and shouldn't lose too much strength with good [hose?] clamps or bound with dyneema in spots , Lanolin grease is pretty effective, clean and harmless also. An interesting project revintage.

 

Lanolin grease attract dirt silicone is more betterer.
I live in Perth where sand is a curse.

 

REDESIGN of the idea above!
Unfortunately 110x3mm tubing is not available from Sweden or Germany in lengths shorter than 6m. As prices for aluminium has been doubled in less than a year, probably due to Ukraina and greedflation, the following solution will be more economical, as well as simplified, without tubes sliding inside each other, when the boat is "folded".
As the beam pockets in the amas are for 100mm tubes, the beam straps can be loosened, the hulls be slided over the central 100x3mm tubes and locked for transport. You only remove the 12mm connecting bolts, slacken the synthetic waterstays and push the hulls inwards.
When going down to 90x2mm outer tubes for the rear beam, the waterstays need to be double like on the front beam.
This arrangement will be 3kg lighter than my first.

 

This build is continuing in my old thread:
Small trimaran with Tornado centerhull https://www.boatdesign.net/threads/small-trimaran-with-tornado-centerhull.66449/page-3#post-934343

 

We have used these very successful for frictional mechanisms fro sliding parts:
CRX Roller https://www.harken.com/en/shop/crx-roller/

 

What’s a mainsheet traveler got to do with sliding beams ?

 

If you focus on its general use, you'll miss the point.



The roller allows the beam/tube to slide as required.

In design, it is a methodical step by step process. You can't answer all the questions at once, nor with a on liner.

Thus, to address the sliding issue, here is one solution.
Whether it satisfies the other aspects of the design's MO is another matter...but that is a different questions to...can't think of a sliding mechanism.

We have used these before, very successfully and nothing to do with a main-sheet either.
Our design MO - a frictionless means of sliding 2 structures to create an extension.

 

Always wanted to use one on a in wall glass door of sizes that really fox the normal floor mounted rollers