Main beam waterstays on small foiling tri

Hi,
On all tri´s I have seen, the waterstays are anchored in the hull sides with reinforcements inside the hull. My idea is to arrange it like on a beachcat with a dolphinstriker coupled to the mast to unload the hull.

The tri is very small with an A-cat hull as aka and a cut off F18 mast section as main beam. Smyth might have done it this way, as you can see slots in the hull sides on Sizzor.

Has anyone else tried this solution?

Lars

 

Typically there is different tension on port and starboard sides of waterstays. In your setup this would cause major problems for a dolphin striker.
The reason for different loads is that typically shroudloads are distributed between fore and aft beams on windwardside, but on leeside there is much more load on front (main) beams. Thus the leeside waterstay on frontbeam has more tension than winward, while the windwardside waterstay on aft beam has more tension than on leeside.
When the daggerboard case is used as a dolphinstriker, it is also firmly attached to the hull, eliminating the problem.

 

Interesting, I almost(must let it sink in first!) get your point, but the shroud loads are very small at the rear beam as they are only there to prevent the mast from falling forward. See attached image of the foiler. The amas are only there to aid before foiling.

The other solution is to fix the waterstays at the hull sides and connect the points inside the hull with for example a turnbuckle. Below is what my friend cholsson did using the other of the two hulls.

But lets say we imaginary eliminate the center hull completely and sail only on the leeward foil? Kind of like on a foiling beachcat, except for the centerplaced rudder. But maybe the two beams of a beachcat being connected, makes the difference?

 

What problems do you foresee? Bending the dolphin-striker? Chafing the stays themselves? Or ???

 

Hi Fastsailing,
You probably assumed the waterstays on each side where separate and locked to the dolphin striker rod. But this one is intended to be like on a typical beachcat with a single line(band) from beam end to beam end, passing the striker rod with some prebend of the beam. I.e. it is always the same tension in leeward and windward waterstays.

 

Having now seen another drawing of the boat in question from a different angle none of that is valid anymore. But for a typical trimaran that arrangement would first cause dolphin striker to rotate around it's upper attachment and then bending. For a non-typical trimaran with only one beam there is no such problem.
But now the only beam might carry some torque that must be designed for. Same as for all beach cats I have seen. Not sure how much and if a composite mast section is strong enough in torque. Aluminium one probably would be depending on section size.

 

This is the section, 160x72 x1,5mm, 1,87kg/m and fits perfectly in the beam pocket. If it seems to weak I will use round 100x2mm. Rearbeam will be round 70x2mm. Will do some load tests with DS attached before fitting it to the hull.

What worries me most is torsion in the unstayed main beam outside the DS where the foils will be attached. Actually I am getting more reluctant to use the wing profile, see image below. The 100x2mm at just below 100USD will be both lighter and stiffer, but not for free as the F18 mast tube. Another option but at 200USD is 102x1,7mm. As this is a Frankenfoil, costs should be kept low.

EDIT: Did some calculations of the torsion constant for the three sections above and the 100x2 is clear the winner, far beyond the 4”x0.065” together with the F18 mast section that where on par with the latter. The mast tube was calculated like an oval, without taking the luff track or the reinforcements in the front and sides in account. If wall thickness had been 2mm of the two losers, the 100x2 would have finished last.

It will also be far easier to make foil and aka fittings for a round tube.

Note the beam will be tensioned also in the horisontal plane(light blue in the drawing). I have a 100m roll of 4mm Liros D-Pro Static:

LIROS GmbH | Details https://www.liros.com/en/products/productfinder/details/detail/liros-d-pro-static.html

It will be used for both standing rigging and vertical/horisontal beam tension, where it will be doubled at the dolpinstriker. A course guess is that only half the roll will be left when done, at 7gr/m still only 350 grams!



The daggerboard is for this summers tests without foils.

 

Hi, If I understand the issue properly I think the dolphin striker will need to allow the Dyneema to move back and forth with either a chaff preventing sleeve or a mounted small block[ so it can't jump off]. So it may as well go all the way to the bottom with a re enforcement pad to help brace.. maybe mast section instead of striker..? The wire water stays on a tremolino are mounted on the main hull in the centre of the two beams, so a pair for each side [4 total] for and aft. Anticipating flex affects the tension on the forestay which affects the jib shape, while load on the main sheet and traveller counteracts this. Setting the waterstay tension requires doing it up drum tight then backing it off to an acceptable load,. Stainless rod is used on an old Hobie 16 and 18 and it allows for a little movement as it passes through the dolphin striker,yours can't hit a dolphin so it's a vertical tension post ..sampson? .. maybe...ha, interesting project, hope this helps cheers

 

Hi,
Great idea to prolong the strut to the bottom to prevent it from moving!

I am actually an old Tornado guy, now with two Nacra´s. Built new beams for them last winter using 100x2mm alu tubing. Built one dolphin strut Tornado style and the other Nacra style. Used 40x3mm Sandvik stainless as striker bands with just a M6 bolt under the strut to prevent the band from slipping off. I use 15mm pre bend to prevent the beam from bending downwards under the force from the mast.

Will use less pre bend here since both RM and SA are halved. My original idea was a flared stainless tube welded to the end of a ss rod to lead the Dyneema through. If using a solid alu rod (15-20mm) I will just make a flared and polished hole through the strut.

EDIT: Did an alternative sketch wrt "trip the light fandango"´s suggestions. Bought the A-cat cup and cone a while ago. About the strut a 20x2mm ss tube is sligthly heavier than 20mm solid aluminium. Will go for aluminium. The guide can be made of the same 25x2mm GRP-tubing as used for the sides, no need for an extremely tight fit. The vertical tube will be 30x2mm.

 

I'm pleased the idea was useful, I'll be getting my own build going in the next couple of weeks and will be watching yours with interest.

 

Thanks, made me think a little further .

 

You might consider attaching the waterstays a little lower.

The loads can be reduced quite a bit that way, at the expense of some increased drag at times when sailing without the foils.

Whenever the foils are providing any lift at all, much lower waterstays would still be above the waterline.

Compromises, compromises...

 

Hi Doug, was just about to ask you about waterstay height. The sketch isn´t to scale, so they can be attached according to your recommendations.

 

It's really a judgement call on your part, but for comparison: Broomstick's waterstays exit the main hull about 3" above the static waterline, only about 9 3/4" below the bottom of the crossbeam.

In the 1st season of sailing I didn't have foils, but sailed it a couple of dozen times in displacement mode. At about 12-13 knots boatspeed, it became very noticeable that the bow wave was hitting the stays (or the tangs) & I always felt it was a big factor in limiting the top speed to something just under 15 knots. I tried various fairings on the wires, but the real solution was just to get the foils ready. Once I did that, I never noticed the bow wave hitting the waterstays any more.

Broomstick doesn't have a particularly fine entry however, so your experiences with the waterstays in displacement mode may not be as bad as mine.

 

How much is it from deck to bottom? My hull is 46cm but have a beam pocket 3cm below deck, with 9 3/4"(~25cm) it would only be ca 18cm to the bottom. Maybe I should move the beam a few cm above deck?