34 ft asymmetrical catamaran/tacking outrigger/cataproa

Discussion in 'Multihulls' started by dsigned, Feb 27, 2019.

  1. rwatson
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    rwatson Senior Member

    Well, no. In that video they have a helm station in the cabin, so it's NOT just for sleeping.

    These two examples you quote actually support my point.
    Go through every Wharram for sale, of an ocean crossing size, and see how many have put a dodger or protected helm station on the boat. James designs centre pods as standard on his larger models, and builders make up their own to survive long trips.
    The Farrier 32 isn't a valid example either. You steer from the protected centre cockpit at all times. You are isolated from the leeward "splash" by the two outer hulls, and are well protected behind the cabin.

    You AREN'T building for yourself ? Who is the boat designed for then ?

    "it seems perfectly functional" - well it would, in videos. he isn't going to say he suffered from exposure because he got caught on a lee shore in the dark and had to beat his way to open ocean for hours. BUT, anyone with any experience knows that even a supposed overnight trip can become a three day nightmare.

    The question is, if you can build something that can help you survive the inevitable bad experience, why wouldn't you ?

    I don't believe the "asymmetric catamaran" concept offers any build savings, or any performance benefits, and runs the risk of being and uninsurable oddity that can only be used comfortably in the rare "season of assured warm weather".
     
  2. dsigned
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    dsigned O.R.C. Hunter

    First off, welcome. Don't let the old retired codgers scare you off. There's a bunch of retirees on here who have nothing better to do than troll the forums necroing anything that departs from the received wisdom. While a few of the trolls are engineers most of those spent the last few decades of their careers in management, and I have encountered none who are naval architects. As always, try to avoid feeding the trolls (this is advice I don't always take myself).

    Now, a little bit of history and/or explanation of the different hull configurations:

    Pacific proa: the OG proa. This has the ama perpetually to windward. Instead of tacking, it shunts, with a few theoretical advantages. Firstly, since the ama is always on the same side of the wind, you can theoretically make the boat optimized to that tack (since it only has one tack). It's lighter and longer for the same sail area than nearly any other configuration. A pacific proa is always wanting to fly its ama (the smaller hull). In practice however, shunting is slower than tacking, so if you are doing any kind of sailing where you need to change tacks, the hypothetical speed advantages of a pacific proa somewhat disappear. There is also a cost factor. For really small boats, shunting isn't so big a deal. You can use cheap sails, and moving the whole rig is relatively easy. For bigger boats, shunting requires a little more creativity.

    EDIT: I realized that the "cost factor" makes it sound like pacific proas are expensive. For little boats especially, they're really fast reaching. Quite possibly the cheapest speed you can get as far as rolling your own. Check out the Balkan Shipyards guy to get a notion of just how low-rent you can make these things. Or look at just about any video of the traditional boats. You'll have like ten guys hanging out on the ama, just shredding, until it's time to shunt. Here's a video that ought to give you an idea of just how cheaply these things can be made and still be crazy fast:


    Atlantic proa: same as above, except that the ama is perpetually to leeward. Think of this as a trimaran that has had its windward ama cut off and is constantly reversing directions to keep from putting weight on it's missing ama. The advantage here is that you potentially have greater righting moment than a pacific proa, because the heavier hull is the one being lifted. The disadvantage is that there is potential for pitchpoling like in a trimaran, and the shunting problems still exist.

    Tacking outrigger: basically a proa that switches between atlantic and pacific proa configurations to avoid tacking. It functions kind of like a catamaran that has one hull either significantly overloaded or underloaded. Part of the reason I think this will work is because atlantic and pacific proas are both really fast and stable, even though in both cases the ama is lighter and has the smaller volume.

    I need to do a free body diagram to illustrate the physics behind why I'm reasonably confident that the idea will work just fine. But in short, what matters for righting moment are volume that's likely to be below the water (i.e. below waterline and a little bit above) and the center of mass. The key here is that the volume above the waterline can be just as asymmetrical as you please (as far as righting moment is concerned, windage is another story), it's the mass that matters. Specifically, it's how far the center of mass is from the center of rotation. In a catamaran that's perfectly symmetrical, the center of mass will stay in the center on either tack. But even if one hull is half the mass of the other (and the structure between the two hulls is assumed to have no mass), the center will be between the two of them (at one third the distance from the heavier hull). So in that circumstance, the center of mass will either be 1/3 the beam between hulls away from the center of rotation, or 2/3 the beam between hulls. That sounds like a lot, but rather than simply blowing over on one tack, we simply design for the weaker righting moment, and allow for more righting moment than necessary on the other tack. We can further optimize this by distributing the mass that's movable as evenly between the two hulls as is practical.

    But here's where it gets interesting. Remember that volume that I said didn't matter for righting moment? Well it does matter for material cost and usable living space. Set aside the considerations for a moment of the material for the hull itself, and think about the living space inside the hull. Think of diminishing marginal returns. One house is much better than no house. But is having two houses that much better than having one house? A normal catamaran is like having two houses. There's the one you use, and the one that's just there (e.g. for guests). On a boat, any space that you don't need is just a waste. It's freeboard that you don't need to build. It's cushions that you don't need to sew. It's a head that you don't have to maintain. So even if the ama is 90% the displacement of the vaka, it's still a hull you don't need to build to house people. Which means the deck can be extended over the area where headroom used to be needed. Which means (in theory) the bridgedeck can be moved over, both for the sake of balancing mass, and also to free up space between the vaka and the bridgedeck.
     
    Last edited: Mar 8, 2019
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  3. Dejay
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    Dejay Senior Newbie

    Thanks :) And yeah that makes sense. If you're not racing and the ama is comparatively large it makes little difference compared to a catamaran.

    I think I understand the harry proa concept better now, ideally you'd want the longer vaka with less wave resistance lift up and partially "carry" a relatively heavily loaded aka with crew acomodations.

    I can also see that there is real value in the asymmetry because you don't need to spread out but can be more centralized and compact. Basically you save a lot of "corridor" and don't need a center pod for cruising like catamarans do.

    I think the video is sped up btw to intensify the action haha.
     
  4. gonzo
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    gonzo Senior Member

    Many of us who are engineers and design boats are, unlike you, very active on the water. What experience do you have? I think you should show some respect for your betters and maybe learn something.
     
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  5. fastsailing
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    fastsailing Junior Member

    This may be a language thing, but can you explain your comment about pacific proa having only one tack?
    I thought that if any sailing boat (with sails or wing, but not a wind propelled rotorship) (proa or not, tacking or shunting design) is going NW while true wind is coming from N, the boat is in starboard tack, and if it's going NE it's on port tack. So they all have both tacks if intended for general usage, some speed record machines being the exception.

    And then the last statement quoted above. If you compare a light air optimised racing boat to an ocean going cruising boat, then sail areas will be different even if the boats were originally identical. Ie, sail area does not have to depend on boat type (proa, tacking outrigger, symmetrical multihull) at all.
    But if you limit the discussion on boats for similar usage on similar conditions, then sail area should depend on righting moment, and your statement would become equivalent to: "It's lighter and longer for the same righting moment than nearly any other configuration".
    If this is what you meant (not claiming it is), then that claim seems factually incorrect to me. Large RM for same displacement (or weight or mass) requires large leverage between center of mass and C of Buoyancy, which the pacific proa with little weight on windward side no not have if comparing it with other type of multihulls like trimarans and atlantic proas. I'm not even sure it can compete with cats regarding the claim (not talking about the speed potential at all).
    Now having said that it's certainly reasonable to think pacific type proa can be made longer overall while having less RM and less sails than trimaran with same weight.

    Seems like a typo to me. ie.to avoid shunting, not tacking.

    Are you assuming flying all but one hull all the time?
    If not the center of rotation is not going to be where you seem to think it is and as a consequence you can't calculate amount of RM using center of rotation, only how quickly Rm changes with changing heeling angle depend on it. But the value of RM must be calculated using center of buoyancy instead of center of rotation to get the correct result. As an example, the center of rotation for a symmetrical cat with zero heeling angle is midway between the hulls, initially one hull lifts up the very same amount the other goes down. But the center of rotation moves toward the center line of the leeward hull as heeling angle becomes larger, and gets there at the time the windward hull lifts off the water.
     
  6. fastsailing
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    fastsailing Junior Member

    Perhaps it would, perhaps not. It's not proven because wave making resistance is not the only thing having an effect on speed of a sailing boat.
    It's possible the catamaran might have greater leverage and thus greater righting moment (RM) for the same displacement allowing it to use bigger sail area in a taller rig than a pacific type proa. Having less wave making resistance due to a longer hull, results also having greater viscous resistance.
    It's possible that at some speed range the longer hull has more resistance due to that, especially at lower boat speeds.
    Overall it's far more likely a trimaran would benefit the most for both mentioned issues in light air, but uncertain which is faster in light air, a cat or a proa. Either can be faster or slower, depending on other details, but most of the cases a cat with conservative size rig is probably the slower one, since equally distributed displacement on 2 equal hulls for a cat results more wetted area regardless. If both types are light air racers flying a hull all the time, I wouldn't bet on either one.
     
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  7. gonzo
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    gonzo Senior Member

    I always thought of proas as a car with wheels of different sizes on each side. It will behave asymmetrically, which is in my opinion, a bad thing. Since the hulls have different resistance the boat will tend to round up, particularly when running which is dangerous.
     
  8. rwatson
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    rwatson Senior Member

    The "rounding up" thing is a valid concern, but in practice, I have never seen it reported.

    Given a long slender powered Leeward hull , with two control surfaces, it appears to NOT have any problems with directional control.

    I have actually experienced more problem with "rounding up" with Trimarans, where the leeward hull is suddenly pushed into the water when tacking, creating a definite pressure on the steering.

    Perhaps the lightly loaded "always to windward" Ama setup, just doesn't come anywhere close to matching the directional power of the main Vaka.

    Now, if you want to experience "rounding up", pull the mizzen sheet on tight on a Yawl......
     
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  9. Timothy
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    Timothy Senior Member

    One thing to consider with the current trend towards electrification is the of the weight and density of the batteries. These if placed always to windward on a longer arm ,allow for a windward ama with less freeboard and less aerodynamic drag (think canting keel). As a consequence the over all weight of the boat can also be reduced without reducing RM.
     
  10. Rumars
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    Rumars Senior Member

    Arguably the best proa is a catamaran, as evidenced by countless sport cats that routinely fly a hull. Why did the islanders invent outriggers? They were limited by the available technology. Not having to build another hull is very atractive when your tools are stone and bone. Even today after a certain size it is simpler to build a trimaran than a catamaran of equivalent power.

    Back to tacking outriggers, when designing one you have to think trimaran missing a float. First you have to decide between low volume ama and high volume ama then about movable ballast or not. This drives the structure of the beams and how the boat has to be sailed.

    Most traditional outriggers (tacking or shunting) use low volume amas and movable weight. The ama can have some form of suspension, or be fixed, can be as long as the vaka or very short. There are pros and cons for every configuration.
     
  11. JamesG123
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    JamesG123 Senior Member

    Not technology. Materials. They had far more time, than materials. Or rather their time was cheaper and more plentiful than good clear wood with which to build hulls with. Today that is usually to opposite, even with home builders.
     
  12. Rumars
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    Rumars Senior Member

    When I say technology I mean everything related to crafting a boat. They were in the stone age, so tools are limited to stone, bone and wood. The only fastener available is coir rope. The textile manufacturing was limited to bast fibres so the sails were woven out of palm leaves as mats. Wood availability varies considerably from islands with really big trees to islands where only driftwood is suitable for canoes. They don't have pack animals to transport wood from the forest. Even the human resource is limited, the islands can not support an unlimited workforce. They basicly followed the path of least resistance, it is simpler to shape a log as an outrigger then to split and carve planks for a double canoe. It's enough to do it for the main hull only. And they got creative in solving the deficiencies of outriggers using more or less independent suspension systems for the ama and having the ability to move the CE awarthship. And lastly we must not forget that this boats always had auxiliary propulsion in the form of paddles, even the big ones.

    Looking only at the building materials is misleading. Around the Indian ocean people buildt big seagoing monohulls fastened only with coir rope.
     
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  13. Dejay
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    Dejay Senior Newbie

    Thanks fastsailing, I guess I really shouldn't start making assumptions. I didn't realize that viscious resistance increases exponential and wave making isn't the largest contributor with thin multihulls. I would still think for a cruising proa/catamaran, e.g. something that doesn't have really thin hulls, this holds true, less resistance for the same total hull length. But I have to learn to use michlet. Not really interested in a sailboat, more like a "motor proa".

    Normally I dislike asymmetry, but somehow I just love the proa designs like the harry proa. They are so wonderfully weird and futuristic. You seem to get more deck out of them.

    And whether the tools are stone and bone or cutting knife and vacuum bags, building less hull sounds good to me!
     
  14. JamesG123
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    JamesG123 Senior Member

    True, but they had unlimited time to do so (make reed sails, burn and hammer out log hulls, etc.) with, not to mention a thousand years of accumulated knowledge of what works and what didn't.
     
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  15. dsigned
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    dsigned O.R.C. Hunter

    They have one tack in that they always keep the same side towards the wind, so instead of tacking, they go backwards.

    Than a trimaran of the same weight. No, I think this is a mistaken assumption based on how most proas and trimarans are optimized. The speed machines (as you call them) are proas largely because they have no weight that's not serving some purpose. A trimaran is symmetrical so that it can have the same righting moment on both tacks. But if you only had one tack, you could eliminate one of the amas, and push the main hull all the way out. Let's assume for a moment that your hull weight was small and you were going to add ballast to the boat to get the desired displacement and righting moment. Let's further assume that you're reaching one direction, but you don't have to go the other direction. Clearly you would want all your ballast as far to windward of the hull carrying the majority of the displacement as possible. Now, whether you could optimize a trimaran like a proa to where there were two hulls with the displacement distributed between them and a third "flying" hull seems like an interesting question. Traditional pacific proas just use movable ballast (people) on the righting arm, whereas atlantic proas are designed to assume no moveable ballast and put the heavier hull (and accomodations) to windward.

    You're correct, it's a typo.

    For the sake of the illustration, assuming the hull is just barely flying is fine. The bouyancy windward hull is going to detract from the righting moment up until the hull is flying. If we're trying to calculate the righting moment curve, then it's not an assumption that we would want to maintain, but for the sake of explaining how multihull righting moment works, it's going to simplify the free body diagram.
     
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