Proa design

Discussion in 'Multihulls' started by Tcubed, Sep 13, 2008.

  1. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    Terminator^3 in gale
     

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  2. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    We used a technique like this to stiffen the axle of a landyacht. The axle was an aluminum teardrop extrusion, and it proved to be flexible enough to all the bottom of the landyacht to contact the ground when the rig was highly loaded and the yacht hit a bump. Once, it hit hard enough to put a small permanent set in the axle, so it had flexed to its yield point.

    We took a piece of 2"x2" lumber and tapered it toward the ends, giving it a truncated spindle shape (but still rectangular in cross section). Unidirectional carbon fiber was then laminated on the top and bottom of the tapered wood. Then it was forced down the inside of the axle to the center to make the axle a nonlinear spring. The length of the insert was on the order of a quarter to a third of the length of the axle.

    For small deflections, the axle only contacted the insert at the middle, and there was negligible influence on the flexibility of the axle. However, as the load increased, more and more of the insert contacted the inside of the axle, and the stiffness increased. Eventually, the entire insert was in contact with the axle, and it basically flexed outboard of the end of the insert.

    This successfully stiffened the axle and we had no more problems with it contacting the ground.
     
  3. Tcubed
    Joined: Sep 2008
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    Tcubed Boat Designer

    That's pretty funny (#106).

    But in fact it wouldn't do that since there is a stay from the mast to the ama.

    When it could be subjected to bending is if caught aback when essentially you have a monohull with lee buoyancy pod, and this is the situation that is difficult to engineer, as it should never happen (basically the frequency of it happening should be very low) yet it needs to be accounted for and with a minimum of cons for the 99.999% of the time.

    Maybe it should have one under aka stay just for this purpose, and lift the akas up the side of the main hull as much as possible to improve the angle...?
     
  4. sigurd
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    sigurd Pompuous Pangolin

    yeah, either a completely depowering, feathering rig, or, as you suggest, flexing the beam ends upwards, to let the mast bend towards the ama, would also depower it when caught aback.

    I have been pondering a bit more on the possibilities of flexible beams.
    The following sketch is a bit far from the subject boat, but I thought I'd include it anyway.
    If the ama can slide back and forth, and is fitted with a leeboard, and the rig is balanced, then rudders could possibly be omitted.
    I read about a french "parallellogram" proa - it died when the skipper let go of the bridle line, the ama crashing into vaka.

    Further, it would be possible to shape the leeboard like a J (or inverted T - kelp catcher), and adjust its pitch according to necessary downforce (or upforce). A sensor wand (or a hydraulic piston connected to a water port on the leeboard) could actuate this pitch change, possibly along with altering sail trim.
    A further feature would be to use the leeboard's own force to power up the boat - since, when the leeboard is out of the water, it is time to effect generous depowering. In the sketch this "safety release" is done by letting the rig cant to lee, as well as letting out remaining tautness from the rig trim line. This could reset itself when the ama landed again.

    Just some rough ideas, intended to show some variation on the "set and forget" rigging possible with proas.
     

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  5. sigurd
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    sigurd Pompuous Pangolin

    I think the rig you have proposed looks good. But I like windsurf rigs, especially balanced ones. Here is a variation from the keel stepped, free standing sort.

    If it is attached close to the boom/mast joint and to aka, the shroud could be able to take compression, so that no leeward shroud is needed. As shown, two S-shaped shrouds trades length between each other. This moves the rotation axis of the boom further back, making the balancing job easier on the tail.

    Finally it should be possible to devise a way for the fore and aft stays, from the S shrouds or boom, to move away when caught aback, allowing +-180' boom rotation. For instance, a stay from leeward side of the rig would take over the job from a windward one, when the boom went past 90'.

    Some things about such a rig. You can rake it fore aft and tilt it sideways, its flex is unimpeded for gust eating, no sheet loads, boom end can be tall, so can be depowered fully when heeling, meanwhile a batten can lock down the sail against the deck, closing the gap. Also, you can lock a sheet and use the tail for steering the boat. Works the same with canard but the windsurf boom end is already far aft, so an air rudder in that vicinity gets good leverage.
     

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  6. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    I would like to draw attention to this discussion, load paths in a catamaran. My conclusion, a single beam is less stiff in torsion than two beams the same weight, but it would give more bending stiffness and strength. I thought, since your plans involve torsion flex, but not neceserily bending flex, that a single beam may be more favorable. In particular, I think that bending backwards, lifting the vaka off the water, is going to be the strongest load on your aka(s).

    Reinforcing the vaka for beam/rigging loads may also be lighter when using a single beam?

    edit: post #804 in the foiler design thread is about a shock damped foil for proas.
     
  7. peterAustralia
    Joined: Mar 2006
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    peterAustralia Senior Member

    hi T cubed

    First I would suggest moving your discussion/questions to the yahoo group proa_file. On this Yahoo group you will find many many people that have actually built proas. I know of three that have built very large pacific proas, and are on the yahoo group.

    At least three people in this thread are on that group, Tehro, Gary and Rob. I think that in the yahoo group the quality of your replies and information will be higher. Secondly, do you have money? or is your proa sketch, just something you want to do one day.. when you get money?

    In terms of dagger-boards, on a large boat the weight and mass of boards becomes an issue. If the sail plan could be balanced so as to only require small boards for steering, then you make shunting and raising and lowering of board much easier, as the boards will be smaller. If you notice Russ Brown's boats, they are very deep and appear to obtain lateral resistance from the hull as opposed to foils, this seems a rugged and simple approach, definitely less susceptible to damage. Downside may/may-not be does not point as high upwind (guess here, no hard data)

    I see you have a board on the outrigger, I am personally not keen on that, but is not my decision to make. A little thing, a proa can allow for the outrigger to flex independently of the main hull so as to go over waves. This is not a bad thing and does not really have a downside. The equilibre proa, as best as I can tell, has the crossbeams going out a fair way and be rigid at that point, but later on they are allowed to flex. At this junction there is a torsion box, beams on main hull side are rigid, on outrigger side, can flex,

    Rob does not like Russ Brown boats, that is OK. I humbly suggest taking his points with a grain of salt. Firstly he says that at 2 tonnes for Jzerro it is unnecessarily heavy. Well, I guess it is technically possible to build lighter, but is it really that that bad, I mean a 2 tonne 37ft boat, (is that overly heavy? In other places he has said that if the Jzerro Proa gets back-winded the mast falls down (I am going from memory and may possibly be mistaken) however this falling down claim has been disputed by some that have actually been on the boat.

    I think if you want good advice, go to yahoo group,, and forget about boatdesign.net. Also some other russ brown boats, may be for sail, be that be Cimba or Kauri (there are 3 similar boats.. Jzerro, Cimba and Kauri)

    regards
    N Peter Evans
     

  8. rob denney
    Joined: Feb 2005
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    rob denney Senior Member

    G'day,

    Couple of points:
    1) Russ's boats have a dagger board in the ww hull. They also have daggerboards incorporated with the rudders, same as Cheers did. They go upwind very well. All three are susceptible to damage, but have Newick type crash boxes.
    2) Take my views with as much salt as you like, but get them right, please. Russ is a consumate seaman and a builder par excellence. I have not sailed on any of his boats, have only quoted from articles in Cruising World and Wooden Boat magazines, both by reputable authors who have sailed on his boats. I suggest you read these (I can send you copies if you like), before making any judgements. The weight comparison, and all my other comments were in relation to harryproas, where 2 tonnes for a boat with a single double bunk is heavy. The remark about the mast falling down when caught aback is in the Cruising World article. You can read the author's contradictory statements about it and the rest of his article by searching for Steve Callahan on the Yahoo proa group.


    regards,

    Rob
     
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