Windknife trifoiler build

Discussion in 'Multihulls' started by revintage, Dec 7, 2018.

  1. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Just recieved this generous load of foil noses from Gareth at Windknife UK.

    Also located 3”x0.058” tubing for the rear beam, will save 2kg wrt to the 70x2mm I earlier thougt I was forced to use.



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  2. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Until recently I was into using an old T-mast, but decided to use it as flag pole in my garden instead:D.

    Bought one and was given three broken Goodall F18 masts and will instead splice two pieces from them, to make the 8,2m mast. If this works out as it should, I will also build a 9,5m mast for my widened Nacra 5.8 Frankencat with Inter 20 main. Got tremendous help from Canadian Tornado-guru Geoff Dobbs with the blind rivet diamond pattern and the splicing/sleeving technique.

    Made short experimental sleeves, the ordinary will be 60cm long when splicing the mast. The sleeve is made in two pieces due to the internal webs. Lots of grinding also needed on the forward sleeve.

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  3. revintage
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    revintage Senior Member

    To get grip of the beam configuration I laid the parts out on the lawn. Decided to go from 6 to 5.5m main beam. About the water stays I will go for 1.5m from center, the same width as the 3m rear beam.

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    Last edited: Aug 19, 2019
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  4. revintage
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    revintage Senior Member

    Step by step..........

    As the aluminium tubes are notched to the diameter of the tube they meet, they interlock perfectly in kombination with the PVC fittings. This solution also makes it easy to dismantle the boat for transportation. Have not found out how to fix the nets to the gunwales, but thinking of epoxied Dyneema soft eyes every 10cm along it. The green nets will probably be substituted with a heavier quality in black or with the same polyethylene fabric as used in beachcat trampolines. With the scales at 46.9kg including the right side PVC fittings, tube and net, the goal of sub 50kg will still be reached with the forestay bulkhead and carbon rudder gantry in place.

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    Last edited: Aug 26, 2019
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  5. waynemarlow
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    waynemarlow Senior Member

    Are you really sure you need it to be so wide. We found the wider you go, the more drag from that Ama / foil, the more the boat had to be steered, the more the boat skewed itself along the track, the more drag, it was a vicious circle of drag. I found about 3m to 3.5m total width was about the easiest on all aspects of sailing and ground maneuvers which has to be taken into consideration. I also found a smaller sail often was faster than a larger one as a smaller sail puts far less stress on the foil and lets it work much more effectively. Many of the small cats that are performing well on foils such as the S9 are only 2.5m wide.

    On the nets, you cannot just use Dyneema as you plan, it eventually wears away and will cut though ( been there, done that and it does fail pretty quickly ), epoxying the fibres will actually make it worse. I would suggest you use what is effectively a soft shackle but put a SS thimble in the eye and the Chinese knot as the preventer from pulling through the hull. Simply take the thimble out, pass the eye through the hole in the hull, put the thimble back in and pull it all tight and snug. You will need to fill the foam around the hole, drill a 8 mm hole through only the outer layer of glass, use a small 90 degree bend on a piece of 2mm wire on a drill ( use several different depths so that only a few mm is taken off each time ), angle the wire through the outer skin into the foam, get the bend perpendicular, give the drill a quick whizz and the foam gets munched the depth of the 90 degree bend, fill with thickened epoxy, put a piece of gaffa tape over the now filled hole to stop it sagging, wait until set and then drill your 4mm or what ever size you need through both inner and outer layers and voila you have a fully filled and hardened area around the hole.
     
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  6. revintage
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    revintage Senior Member

    The center to center distance of the hydrofoils, with the configuration chosen, will be in the ballpark of 4 to 4.5m at 20 knots. This is needed to balance the trimaran, when sailing it with both foils in the water.

    About Dyneema eyes, I have already tried this, when fitting the corners of the nets to the wings I use on my widened Nacra. They where made with the eye coming out of a single hole with a knot and a SS washer on the inside. No epoxy used as the wings where aluminium. Of course a thimble can be added, not a bad idea, even if I haven´t experienced the wear you mention. I have made a few experiments with 3mm Dyneema and West G/Flex and I trust this to work in combination with the knot in the image, maybe with your suggested thimble added.

    The gunwales are actually solid in the area where deck meets hull, so no worries about removing foam there. Still I need to examine the gunwale the 2 meters between the beams. The buildup looked a little peculiar, when I prepared for the forestay bulkhead we discussed in my thread at SA, where you participated with some good advices about bridle height.

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  7. Doug Halsey
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    Doug Halsey Senior Member

    I don't think I ever had this problem with Broomstick, but I rarely sailed with the ama deeply immersed. (I often sailed without the amas).

    Can you post a photo showing your setup?
     
  8. waynemarlow
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    waynemarlow Senior Member

    Doug, I had only gone as far as 3m with flattish windsurf type Amas and it was showing signs of the Ama drag being sufficient to over come the nice rudder set up I had, I think foil drag in the low speed sector before getting up onto foils would be probably worse ? The boat handled nicely so left it at that as I have an abhorrence to over wide boats mainly for ground handling problems. The intention was to fit foils at that stage, ie get the boat sailing nicely and then fit foils, you can always change sail shapes etc for the foils but if it won't sail getting up to speed, you will never get on the foils.

    I stopped any further development as I tipped it in a couple of times in strong winds and unlike the cats I have, I couldn't right it due to a couple of problems which I hadn't really thought of, for some reason when on its side you cannot get enough righting moment with the ama below you pushing you up slightly, it was all very twitchy and never felt like it would right. The Weta has the ability to flood the ama and thus the whole boat sits down on the water on its central hull and rolls across that axis. Its not been abandoned, I just need to rethink the righting on the water problem.
     

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  9. revintage
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    revintage Senior Member

    Actually there is no reason to attach the nets to the center hull except in the corners. The nets can as well be attached to each other over the deck, with lines in a zigzag pattern, as long as they do not interfere with the fittings on the deck.

    Doug, about the amas I discussed with Wayne about a windsurfing board cut in half and have gone that route but will make them pivoting around the main beam. My idea is they plane at higher speed and their volume do the work at low speed. Question is how much volume is needed, any ballpark figures? You seem to have tried everything from zero and up. Zero is not for me, though ;-) .

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    Last edited: Aug 31, 2019

  10. Doug Halsey
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    Doug Halsey Senior Member

    Broomstick doesn't have excessive lee helm for several reasons:
    1-At low speeds, or whenever the heel angle is small, the drag from the windward foil contributes a weather-helm that partially cancels the lee-helm from the leeward foil.
    2-The foil drag is relatively small, especially at low speeds.
    3-The foils are mounted farther forward than typical multihull daggerboards, and get their incidence by angling them even farther forward. So the side-forces on the foils tend to produce weather helm.
    4-It doesn't have such a huge jib as your foiler.

    If it has much weather helm when foiling, it's not very obvious. The helm is very light and is easily balanced by small changes in the relative trim of the main & jib.

    I agree that the extra-wide beam causes logistical problems. I don't use narrow launch ramps; only nice beaches. If something is wrong with the rig, I can't capsize while ashore to fix it like the narrower catamarans. It's a pain to assemble and disassemble.

    Sailing without the amas, it feels much more lively. Part of that is the smaller weight (the 7' amas weigh 10 lbs. each, and the 11' amas weigh 20 lbs. each). Also less windage. And I also believe the hydro drag is usually less (maybe I need to add, if sailed "properly"). Balancing the boat when it's not moving can be more challenging, resulting in more low-speed capsizes, but that's not a problem above speeds of just a couple of knots.

    Recovering from a capsize without the amas is also tremendously easier. If the boat turtles, standing at the end of the cross-beam sinks it easily and gets the mast horizontal again. (assuming the mast is sealed so it doesn't fill with water, and the crossbeams are uncapped so they do fill with water).

    I realize that going without amas may not be an option on other foilers. Amas can provide better support for the foils, and they're handy for controlling them. But amas are only useful at low speeds, so they can typically be relatively small. In my opinion, their total volume should be small enough that, when capsized, the boat floats with the main hull in the water, instead of several feet up in the air. Broomstick's small amas attach to the crossbeams with simple loops of bungee cord, so they can be easily removed in case of turtling, providing an easier alternative to the flooding of the amas that Wayne mentions. (This also may not be an option on larger boats).
     
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