Small Tri's under 20', any mention of foils is banned..

Discussion in 'Multihulls' started by waynemarlow, Jan 13, 2015.

  1. waynemarlow
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    waynemarlow Senior Member

    That range of 10 - 26 degrees is the problem together with the flip flopping of the boat at rest. A number of the NZ 8.5 went the route of lots of dihedral only to find the boats so uncomfortable at rest that they now have all gone back to much less. Equally the American 24 footer has had problems with too much dihedral and owners were complaining of too high Amas and mast angle whilst sailing.

    To that extent we are allowing owners to have some control of the angle they want with a simple methods of adjustment, once we have a boat on the water we will know more. This is something that computers and models can't predict and it will come down to personal useage and comfort as the Pulse 600 found.
     
  2. waynemarlow
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    waynemarlow Senior Member

    Interesting that you think the F18 hulls are heavy, they are heavy in purist terms, but probably only 10kgs over weight. I have built 17.5 ft hulls out of a mould with full vacum bagging and minimilist cloth weights but without the volume needed, at around the 18kgs. If we increase the area to get the extra volume needed, then I would predict most hand layed Amas will be in that 25 to 30 kgs.

    Sometimes the package of extra weight creates toughness and durability, which in my view is a desirable, together with cheap availability, can out weigh the 2 packs of 10 beer bottles of weight we are talking about.
     
  3. Doug Lord
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    Doug Lord Flight Ready

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    You seem to misunderstand what I'm talking about. Clearance of the ama at static doesn't necessarily reflect what the heel angle of the boat is when the main hull is .1mm above the water. Heel angle seems like it is a design choice/characteristic of momentous importance to the moderate to heavy air performance of the boat. It's primarily a characteristic of the how much beam the boat has coupled with how much rocker the main hull has. Seems like prospective owners should know what this angle is-and it's effect on the performance of the boat in stronger wind.

    PS- to try to make it clearer: you can have the exact same ama clearance(or lack thereof) on two different boats but if the main hulls have different rocker or the amas have different buoyancy, the angle of heel at takeoff will be different. Angle of heel at takeoff and ama clearance are two different things. If a design feature of the boat is "flying the main hull" then angle of heel at takeoff is important to know.

     
  4. waynemarlow
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    waynemarlow Senior Member

    No misunderstanding at all Doug, there are just too many variables on these plan built boats other than to hazard a guess at somewhere between 10 -26 degrees of heel.

    One set of Amas are going to give different bouyancy levels than another. One only has to change the amount of rocker on the Ama and that will change the answer. A builder builds a very heavy weight boat with lots of on board goodies, thats going to change things. Do the downsides of having lots of dihedral ( poor lightwind and mooring performance ) overcome the upsides of fast heavy air performance ( or maybe thats not the case here ), as stated before we are giving the owner a bit of adjustment so they can decide what dihedral angle is best for them.

    At the end of the day, both Tim ( who has access to some pretty good design software ) and I agree that to get a boat on the water, will give far more information, than any further computer analysis. Once we get Hull No 1 up and sailing with a few miles under the bow, then we can start to give better guestimates on what angles of heel are faster than others in what winds and can start to advise better other future owners from practical experiance rather than computer predictions.

    One thing is for sure though, it won't be exactly as the computer predicted and it is something that perhaps some haven't grasped on this forum, computers don't always know the answers and sometimes full size models are needed to get the full answer.
     
  5. rcnesneg
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    rcnesneg Senior Member

  6. rcnesneg
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    rcnesneg Senior Member

  7. markstrimaran
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    markstrimaran Senior Member

    How much of a risk is pitch poling at slower speeds. Seams like the weight of the boat. Would need a enough momentum. To capsize
     
  8. upchurchmr
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    upchurchmr Senior Member

    I've never pitchpoled a tri, but a beach catamaran generally only happens at high wind/ high speed when the bow goes under - stops, the sails are not released and the entire sailforce is pitching the boat over.

    If you are quick on releasing the sail, it depends upon how much momentum you have, where your weight is on the boat (far aft is better) and lots of luck. Of course when the person on the trapeeze cannot anchor himself, he flies forward, helping to pull you over.

    I'd say minimal risk of pitch poling at slow speeds, but you still need to be quick to release the main sheet (first) and the jib.
    A very strong quick gust can pin you into the water, if you are sitting forward (as you want in light wind) you may not have much to stop you going over. But that is mostly over the side.

    When you are tacking if your weight is too far aft you can be blown over backwards and nothing you do with the sheets matters.

    I bet that was more than you needed. :D
     
  9. rcnesneg
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    rcnesneg Senior Member

    I've been on a beach cat that has stuffed the leeward bow on a broad reach in 35 mph of wind, while I was on the trapeze. I went flying into the water infront of the boat, but the skipper let go of the sheet, so the boat came up on it's own and the trap lifted me right back onto the trampoline, as the hook didn't fall out.

    Here are two videos of large trimarans, both under spinnaker running fast downwind, nearly pitchpoling. Notice how in each case, letting out the spin stopped it.

    https://www.youtube.com/watch?v=k2vaJGsyRvM


    https://www.youtube.com/watch?v=85mIDnNRptg
     
  10. waynemarlow
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    waynemarlow Senior Member

    As much as I agree on the mechanical forces of a airborne crew out on the trapeze pulling you over in the classic pitch pole, one has to ask why he/she was there in the first place.

    The classic reaction of most uninformed skippers is to let out the main sheet, that's just wrong on so many fronts. The main sheet is your mast back stay, that's the thing that stops your mast breaking in the slowing and de acceleration when you stuff a bow in.

    Now let's think about releasing a handful of sheet. As you slow the wind changes from aparrant wind to wind coming from the beam to almost the rear. Release the sheet a handful or so and all that does is open up the head of the sail to more wind and to make things even worse, the depth of the sail is greater making an even more powerful top part of the sail, the more power up there, the more likely you are going to pitchpole.

    So keep that main sheet firmly on and travellor more to the middle guys and gals, that's not the sail creating the problem.

    The jib unless really large has little effect as its largely shielded by the main at beam and rear winds.

    So what about the spinny, over the years, certainly in the F16's and more recently the F18's, I'm involved with, they have become more and more like a screecher or large Genoa. Typically quite powerful and if set up right with the correct length and height of the pole tip, create quite a lot of uplift as well as foward dynamic. So to just dump the Spinny sheets will inevitably slow the boat allowing the wind to come more from the rear and probably the least desirable, lose all that nice uplift which is going to pull your bow up out of the water.

    It does take some cajones but if it does go wrong and you start to feel the bow start to submerge, then release the spinny just a small arms length, it will alter the sail shape enough to give more uplift at the pole tip. Remember though on later well designed beach cats you can get the bow and hull completely submerged well back past the front beam, before you really have to be concerned. Many will say that's way beyond redemption but with a slight release on the spinny sheet, the boat will simply power through the wave rather than want to pitchpole.

    As with all sailing, being ahead of the problem will normally prevent most pitchpoles, with good helming skills of turning more downwind in the gusts or if seeing a wave coming at you, getting the crew further to the rear with their back leg locked against the rear beam perhaps or even putting the spinny away as in unskilled hands it maybe faster without the spinny, one can normally deal with the issue of pitchpoles before getting into that situation .

    If you do tip it over as we all will, then do remember to fully release the down haul before trying to right the boat, you will be surprised just how much easier it makes to right the boat.
     

  11. upchurchmr
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    upchurchmr Senior Member

    Wayne,

    You have identified a weakness in my experience. I have never sailed a spinny on a beach cat.

    So just let me say that what I said applies to a main and jib.

    Staying ahead of the situation is certainly best. That is so silly it really doesn't merit much discussion. Staying ahead could mean staying on the beach if you don't want to run up against your limit in heavy winds.

    So what are you going to do if your spinny is packed away? Just hold on to the main sheet since it is your "main backstay"?
    I would have assumed you have helped at least a few beginners learn to sail?
    Heaven help them.
    Your boat is either designed well enough to be out or not.

    If you have the nose under and you relax whatever sheet a bit and hope the boat will come up is always an option. But don't fool anyone that you are taking a risk with going over. The amount of risk depends upon a lots of factors, including hull design, wind variability, skipper and crew experience/ judgement, and at least a little luck.

    I'm going to defer to you on the spinny situation.

    Playing with the edge of disaster is the greatest part of the fun of racing so I don't want to make it sound like you should always be safe. On the other hand not everyone here wants to sail that way.
     
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