Water-ballasted multihulls

Discussion in 'Boat Design' started by xarax, Feb 11, 2005.

  1. xarax

    xarax Previous Member

    Is there a theoretical reason against the use of water ballast in an outrigger hull ? Such a ballast would permit a larger sail area, wouldnt it?
     
  2. Skippy
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    Skippy Senior Member

    If you're talking about shifting the water to the windward hull or ama when you tack, it should be fine. In fact, a proa outrigger NEEDS ballast, because any time it's flying, the ballast (including human ballast) is the only thing keeping the boat upright. If you mean static ballast sitting in both sides of a cat or tri, that serves no purpose, except adding inertia, both linear and rotational. Mostly it just makes the boat heavier. You can carry more sail as long as the ballast is kept windward of the flotation.
     
  3. Doug Lord

    Doug Lord Guest

    foils ,of course

    Waterballast in the windward hull of any multi will add to the power to carry sail(beneficially) IF the boat is designed for the extra weight.
    A better system at least for fast boats may be using foils to create RM. As an example, the little Rave foiler generates all it's righting moment from hydrofoils-so much so that the boat will structurally fail before the foils quit producing power to carry sail. That just means you have to reduce sail before the structure fails-and it is really fast.
    A guy by the name of Bruce came up with the idea of a foil strictly for the purpose of creating righting moment,not to lift the boat; I think you could find mention of it on proa sites.
    And there was some discussion under a couple of the trimaran topics under "Sailboats" on this forum.
     
  4. dionysis
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    dionysis Senior Member

    If it is a cruiser / racer, then the freshwater can be shifted from side to side with advantage. The only problem is that water will have to be "lifted" across when tacking, unlike a mono where the water will "fall" across.

    For example say you have about 800 lbs of water to shift before a tack - in a mono, you just open the tap, and most of the water will drain from the high windward side to the low leeward side by gravity. The rest can be pumped. If the pipes are big enough, this will happen pretty fast.

    In a cat say, since the tanks situated low down in the hulls the water level will be lower than the crossbeams, so you will need to pump up to them ( this is assuming there is no heel), then across the beams. The speed of transfer will be slower than a mono, since there is just that much more pipe to flow through.

    So it can be done but there are limitations as skippy has said.

    If on the other hand the boat is a racer, then I would go along with Lorsail, and investigate bruce foils.

    Here is the starting point: www.foils.org
     
  5. xarax

    xarax Previous Member

    Ther must be something fundamentally wrong with the concept of the water-ballasted outrigger that is not obvious to me. If catamarans and trimarans could gain sail area that way, why there aren t any around?
     
  6. casavecchia
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    casavecchia Senior Member

    Hydroptere

    Have a look at WWW.hydroptere.com

    This boat sometimes make use of water inside the outriggers to gain stability.

    Marco.
     
  7. tspeer
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    tspeer Senior Member

    The Spitfire hydrofoil catamaran uses water ballast. When I asked Mark Pivac why he went with water ballast instead of using hydrofoils to create all the righting moment (ala Rave), he said, "Why lift the same load twice?" And, of course, he makes perfect sense.

    Water ballast isn't needed until there's enough heeling moment that the lee hull or foil is supporting the whole craft. After that point, one can use down-force on the windward hydrofoil or an equivalent amount of water ballast in the windward hull. If you elect to use water ballast, the lift on the lee hydrofoil has to increase by an equal amount, which will incur a drag penalty. If you elect to use hydrodynamic down-force, the lee hydrofoil still has to oppose the down-force and incur the same drag penalty. But in addition, the windward hydrofoil has its own drag penalty. Hence, Pivac's comment about lifting the same load twice - once for the leeward foil and once for the windward foil.

    However, for a high-speed craft more sail area is not necessarily a good thing. So it may be better to back off on the heeling moment than to add weight so as to sustain a greater moment. For example, I have two different sized sails for my landyacht. Provided there's enough wind to move the yacht, the small sail is invariably faster than the large sail. It's only in the light winds that the large sail pays off.

    I think the real question regarding water ballast for a multihull is, "Is the extra sail area worth it, or would a lighter boat with less sail area be faster?" In principle you can get any righting moment you want from a mulithull just by making the platform wider. But you have to think of stability in two-dimensional terms with a multihull instead of just roll stability. So when the beam is great enough, pitch or diagonal stability become the limiting factor. Of course, water ballast can be used aft as well as forward. But you're submerging the lee hull even more so you need reserve buoyancy to cover the water ballast, too. You get into a vicious circle that may not be buying you much additional performance.

    The real problem with multihull stabilty is the downwind trap. And the most promising solution for that issue is new types of sails, not water ballast.
     
  8. Skippy
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    Skippy Senior Member

    Oops, I didn't get my original comment quite right. The center of flotation will always be leeward of the centerline anytime there's a heeling moment from the sail, and evenly distributed ballast will be centered on the centerline, providing some righting moment. So it's more a question of how much more power you get from the righting moment vs. the additional drag from the weight. For a catamaran (or tri) flying the windward (or main) hull, centered ballast will create only half as much righting moment as an equal weight of ballast in the windward hull. For any less extreme (and more common) situation, it will be less than half, sometimes much less.

    And Tom makes an excellent point that a windward foil pulling downward creates at least as much drag as windward ballast. Not to mention that the foil will fail catastrophically if it ever plows through a big wave and becomes airborne in the trough. :( :)
     
  9. rob denney
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    rob denney Senior Member

    G'day,

    Water ballast is a pain to move around each time you tack, plus it takes up a lot of space with tanks, and costs a lot in terms of valves etc. Generally, it is easier just to go wider on a multi.

    However, another option is a proa with the accommodation in the windward hull. These boats do not tack, they shunt. The rig and rudders rotate through 180 degrees, the bow and stern swap jobs and you sail off in the other direction. In this situation, all the weight in the boat is always to windward, so there is no need to move anything.

    Shunting is easier and safer than tacking and gybing, but maybe not as fast. However, it never results in getting in irons, backing headsails, or mainsails slamming across the boat. There is no quicker way to return to a man overboard than a shunt.

    For a shunting diagram, see http://www.harryproa.com/shuntinganimated.htm

    For proa information see http://www.harryproa.com

    Regards,

    Rob
     
  10. Skippy
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    Skippy Senior Member

    Rob, have you ever done any deep-water cruising in proas? It seems like they wouldn't be very pleasant in a knockdown. No leeward ama to prevent it, and no heavy keel to undo it. And if you shift the weight toward the ama, you can overturn the other way if you get backwinded, not to mention that it won't do as well in lighter air. One advantage would be that if you can move the center of gravity between the hull and ama, you can adjust for changing wind conditions. As long as the windspeed doesn't change too rapidly, the ballast doesn't have to move when shunting. Although I do wonder about fore-aft trim.
     
  11. rob denney
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    rob denney Senior Member

    Skippy,

    Yet to do any extended ocean cruising, but have doen enough local stuff to know your fears are ungrounded. Harryproas have the weight in the windward hull. In terms of athwartships balance, they are like catamarans, except that all the weight of the accommodation is always in the windward hull. I

    The sheets are attached to the boom and the windward hull. If you are caught aback, there is nothing to stop the sail weathercocking. A capsize the wrong way is impossible.

    In light air, the fact that the entire boat can be built 20-30% lighter than a cat or tri negates any disadvantage of the windward hull being heavier. In fact, a short fat windward hull performs better in light air (when wetted surface is more important than length) than a long skinny one.

    If you did capsize, the buoyant masts in the leeward hull prevent the boat going past 90 degrees. From here righting is a relatively painless exercise. With a righting pole, it could be done without getting wet.

    Regards,

    Rob
     
  12. Skippy
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    Skippy Senior Member

    rob denney: Harryproas have the weight in the windward hull. In terms of athwartships balance, they are like catamarans,
    That sounds like an Atlantic proa with the rig on the ama.

    rob denney: A capsize the wrong way is impossible.
    "Famous last words" :) I guess between the rig and the payload, the CG is evenly balanced. Thanks for responding Rob.
     
  13. rob denney
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    rob denney Senior Member

    Skippy,

    Just like an Atlantic proa with the rigs to leeward (and a shorter, lighter, less loaded, more spacious windward hull). Also just like a Pacific proa, except with the accommodation in the windward hull. Also just like a double ended cat with the rig in the lee hull and the accommodation in the (shorter, wider, more spacious) windward hull. Also like a double ended tri with the rig on the lee hull, the middle hull shortened and the windward one removed. The flak I got from the self appointed proa experts when I first proposed my boats was intense, hence the need for a type specific name.

    Famous last words, indeed. Right up there with monos don't lose their keels, capsize or sink. But ignoring human frailities (people attaching the sheet to the lee hull, then deliberately sailing aback), it would be pretty hard to capsize the wrong way.
     
  14. Tim B
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    Tim B Senior Member

    just a point with this "shunting manouvre" is it just me or does it seem like an insane waste of energy?

    Consider the tack, where the boat's current energy is used to move the boat through the 'no power' zone. it may slow down, but it certainly never stops. when gybing (if done properly) the power can be retained to a large degree throughout the manouvre.

    In contrast, when "shunting" the boat has to stop and start off in the opposite direction. It also seems to take more room. Now perhaps that is the only way of sailing a proa. But I might suggest that the origenal idea of pumping ballast water, even if it means easing the power off as you cross-pump before and after the manouvre is far more efficient. as a designer, the one thing that really irritates me is a needless waste of energy. For the case we started discussing (about 10 - 15 degrees heel , assume calm conditions) we only have one hull immeresed, so we can say that the surface area is similar, but our ballasted catarmaran can carry more sail area, and tack/gyb faster. I think you all see what I'm getting at here in terms of efficiency.

    Cheers,

    Tim B.
     

  15. Skippy
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    Skippy Senior Member

    Tim, Phil Bolger points out in "Boats with an Open Mind" that in a well-executed shunt, you don't really fall off. Instead, the boat continues into the wind until it stops of its own accord. Then when you sheet in on the other side, you accelerate downwind, and head up on the other side of the wind as soon as you can. I would agree that some time and energy are lost, but I don't think it's excessive outside of a race. Especially in open water, where you don't come about very often. And the proa has the advantage that it can't be caught in irons ever. Another advantage is that you don't have to keep moving the ballast from side to side. It's always on the same side (the windward side). You might move it from end to end inside the windward hull to keep the bow up, but my guess is that's easier than transferring between hulls.
     
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