Canted mast for proa

Discussion in 'Sailboats' started by pchenrrt, Mar 25, 2009.

  1. pchenrrt
    Joined: Mar 2009
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    pchenrrt Junior Member

    I came up with what I thought was an interesting idea for a mast specific to shunting multihulls using the crabclaw rig and was wondering if anyone has seen it/thought of it before and was also looking for comments regarding its pros and cons. I felt that since, according to Marchaj, the crabclaw was an excellent rig it would be nice to simplify its use so that I could sail it single/shorthanded.

    The rigging I thought of is fairly simple really. Rather than having a conventional upright mast that a crabclaw sail has to be moved around whenever a shunt is performed (read: needs large crew to manhandle rig) why not step the mast on the windward hull and have it lean out over toward the leeward hull. The sail would then be suspended under tension between the hull and the tip of the mast. The angle of the canted (note not canting but canted, it is fixed) mast would be limited to be less than or equal to the angle of the sprit/gaff/upper boom, whatever it is called (I'll just call them upper and lower spar). The upper spar would be tensioned upwards to the tip of the mast by two halyards set apart on that spar. The lower spar would have two downhauls also set apart on the spar and secured to or through a point where a traditionally stepped mast would be. These would all be kept under tension, basically replacing a conventional mast by determining the pivot point of the rig. The two halyards and downhauls are used to be able to tune the angle of the spars for different conditions (if I understood what I've read, especially Marchaj a more vertical angle is more efficient to windward for the crabclaw, and more horizontal for reaching and running). Having the 2 top as well as 2 bottom sheets would allow you to control how hollow/flat the sail is. The reason for having the downhauls secured to the same spot (where the base of the mast would be on a traditional rig) is that the tension on them would be equal regardless of which direction the boat is moving; this point would become the pivot point as explained above. These four sheets would not need to be adjusted with each shunt.

    The mainsheet arrangement would then be like the jibsheet arrangement found on sailboats whereby you would have two sheets, one to tension on each tack (if you can call them tacks on a shunting boat) To shunt you would release tension on the currently used mainsheet and the crabclaw sail's leech would naturally be pushed to leeward by the wind and the equivalent of the clew would swing through the triangle bounded by the mast, the akas and the pivot created by the halyard/downhaul combo - and the boat would slow. You could then tension the opposite mainsheet and as you do you would start to move in the opposite direction. And voila, the shunt is complete without leaving the ama and by controlling only two lines. I feel that this would make it practical to use even singlehandedly and very simple even on small boats.

    One of the reasons why I feel this would benefit is that the mast would be contributing to the righting moment as opposed to trying to flip the boat (though the upright masts are short so aren't to bad at heeling the boat they are not actively contributing to stability). You can with this rig easily move the whole sail to windward (i.e. shorten the mast so that the tip hangs over the amas and not the hull/vaka), further increasing righting moment - as long as there is enough room for the forward part of the sail to sweep under the canted mast. For a pacific proa it would also be easy to arrange for the sail to tilt slightly to windward so that it is vertical just as the ama lifts out of the water so that you have maximum thrust right when you have minimum drag in the water and maximum righting moment (although this can be done with any rig).

    In its basic form this seems like it could easily be built at home using low tech. equipment.

    For those who want lighter boats, since this mast will inevitably be a little longer than the conventional mast on an equivalent proa, and also need to be stronger as there is a side force place on it, a half marconi type rigging can be applied on the upper edge of the mast (just like what you see on the fishing trawlers poles) as the load is always pulling in the same general direction.

    Also, you could make the spar in an inverted airfoil shape to try and contribute to righting the boat, probably not by much, but by some at least.

    This to me can be a very simple arrangement, even though it has been rather complicated to describe (I'll try to make and add diagrams later if it seems from a lack of responses that I have not been clear). You would have one sail and only 6 lines, only lines 2 needing to be tended regularly and those only one at a time. All lines could be led to the "cockpit" on the windward hull. There can be many refinements, just some that came to mind I included above. You would still need to sort out the infernal how to steer on a push-me-pull-me boat problem, but that has nothing to do with the rig and will be a problem on any proa with various ways it has been imperfectly solved.

    I'd love input into the inevitable oversights I've made and new problems this rig creates. I'd also love to hear if anyone has any ideas on how to further improve this rig. Any input welcome!
     
    Last edited: Mar 25, 2009
  2. keith66
    Joined: Sep 2007
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    keith66 Senior Member

    Having mucked about with three full size proas i have a little experience though only one had a crab claw rig. This last one was built by a friend of mine and had her mast stepped on the windward gunwale suported by a strut from halfway out to the ama, dipping the sail was relatively easy and we experimented with many sail setting combinations. I think what we tend to forget is that we as westerners tend to regard the rig as something that should be controlled & pivoted around a vertical axis, this is i beleive a fundamental mistake in how the rig works.
    If you look at a drawing of a classic micronesian flying proa the one thing that tends to stay constant is that the top yard is nearly always locked to the centreline of the main hull. The Lower yard is free to swing outwards & upwards and this increases the lift from the rig markedly as the proa bears away downwind until it is functioning more like a delta winged kite than a sail. of course when running downwind like this the rig tends to generate fearsome weather helm which can be exciting for the crew!
    It is not neccesary to step the mast far to windward as how do you stay it? just to windward of the gunwale would be a good place to start as there is enough room to dip the sail through the shunts.
    What we did experiment with with some success was a jack strut between the upper & lower yards, this allowed considerable tension to be put into the sail flattening it without heavy tension on the standing rigging. the mainsheet was a single piece of line and there was no real weight on it but the proa did fairly fly. Bugger its been five years i got to build another one!
     
  3. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    please add sketches?

    The rig has everything to do with steering the trad. rudderless proas.
    With big enough balanced rudders you don't need to take any care where the center of sail sideforce is so some more rig options are available.

    Conversely some rig options make rudders less necessary.
     
  4. yipster
    Joined: Oct 2002
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    yipster designer

    [​IMG]
    couple of things i learned of the crabclaw by Marchaj, the net and other sources: amazingly it does almost double lift on most courses
    and at an amazing stall angle, mainly its vortex lift like a concorde uses, that lift keeps rising and dont flat out at a point
    contradictionairy is that vorrtex lift has a big drag component that somehow at low speed and with claw set proper seems to be acceptable
    what else did i learn.. o here and here some more reading
     
  5. pchenrrt
    Joined: Mar 2009
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    pchenrrt Junior Member

    pics.

    Thank you for your input

    sigurd: you're absolutely right and I should have been more precise - what I meant was more along the lines of: 'The type of rig has nothing to do with the steering since balancing and tuning of the individual sailplan is what matters and I don't think that 'my' rig has any inate characteristics that make it any harder than any other type to balance. (you can have sloops, schooners, and everything in between that are well balanced, but there have been examples of each type that are poorly balanced as well) Even though those were my original thoughts, in making up the diagrams of the concept I realized just how much the centre of effort moves around on this plan (although I doubt it's more than on a traditional crab-claw proa) so I guess it does matter. I was under the impression that the polynesians always used oars/paddles to steer - are there some proas that did not need this?

    keith66: its nice to get input from someone who has experience with these boats as any info I've got is what I've read in books/internet. My intent with this was to simplify sail control and I felt that the best way to do this was to have each sheet control one variable. This then meant that I would need to create a pivot point, but one of the things I liked about this rig was that, unlike a conventional mast, you had considerable lattitude in determining where that pivot was both laterally and the angle across the sail at which it pivoted (and looking at it now by moving the point to which the bottom yard is downhauled to port or starboard, maybe on a track, you could also control the angle of heel of the sail) . I did not realize that with the crab claw the top yard was maintained at the centreline. This could easily be done on this rig with a couple of sheets leading to the ends of this yard at the cost of a bit more complexity, but maybe the traditional sail is just a better design for this. Could you take a look at the pics below and tell me if you think the design would be adaptible to the way a crab claw it traditionally sailed.

    The pics:

    Legend
    Black: support member (mast, aka, spreader)
    Blue: Yards
    Pink: Mainsheet
    Teal: shroud
    Green/Red: halyards and dowhauls

    I know that these diagrams are pretty basic, but they are just to try and convey my thoughts and are not study plans. Please disregard the obvious errors such as not being semetrical/the ama being to high (it was so that the sheets were more visible)/changes is sail shape.

    The first pic is looking from stern to bow with the sail perpendicular to the boat, such as when in mid shunt.
    View attachment proarearview.doc

    The second is a side view
    View attachment proasideprofilenormal.doc

    The third is to show some of the lattitude this arrangement allows. By tightening the red halyard relatively more than the green halyard and then by tightening the green downhaul more than the red downhaul, the pivot point would change and the sail would be inclined more like in the following
    View attachment proasideviewpointing.doc

    Anyways, my idea was to match the performance of the crab claw with more control and simplicity, but maybe I just need to gain some experience with the traditional rig to understand it and the way you control it better
     

  6. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    Nice sketches, thanks. Is the lower yard always the same yard or is the sail flipping rather than tacking?

    "I was under the impression that the polynesians always used oars/paddles to steer - are there some proas that did not need this?"

    Yes that is my understanding from literature - the rigs on some traditional polynesian boats, at least for some courses, were able to be put in such a position as to balance the yaw against the long keel of the vaka. Model proas seem to bear it out as well. Ofcourse one would always bring along an oar anyway. Low aspect rigs as well as keels tend to move the center of sideforce around a lot longitudally.

    Modern proas most often use high aspect water foils instead of deep V keel so the self steering issue is solved differently.
     
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