Ballast/Displacement Ratio-minimum for self-righting

Discussion in 'Sailboats' started by Doug Lord, Sep 17, 2009.

  1. sharpii2
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    sharpii2 Senior Member

    Hi, Doug.

    I don't think a canting keel would work well as the majority of the weight oh the boat would be high above the waterline, magifying the effects of any heeling.

    What I tried to do with my sketch was to use the hull itself asshifting ballast.

    I do sympathize with your critisism that the foils may be too large.

    Perhaps the same size crossbeam could be used with smaller foils canted at some angle at each end of the beam.

    Another possible critisism of my idea is that the boat would have to heel a lot (30 deg) before the Center of Lift moved to leeward. This was intended, as to use the ballast bulb itself as shifting ballast, with the added effect that it would be automatic.

    This has the bad effect of creating a downward force conponent from the rig on the leeward side. Perhaps the angles of the foils could be reduced to 20 or even 15 deg.

    Also, the rig could be designed to cant to windward. This should not be any more difficult to design than a canting the ballast keel.

    Running my own formula with your question, I came up with about 3.35 seconds, with no friction and no dampening, do to foils and other appendages, with a 25% increase in the ballast bulb. With friction and dampening, it could take three to five times as long.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Bob, thanks for your calculation and the configuration ideas.
    Below are some very rough sketches showing a small and large buoyancy assisted self-righting foiler similar to the one "B" comented on-both with 110 degree canting keels. The third sketch is one of three versions of a boat I am building now-and the motivation for this thread. It is an experimental configuration where lead slides side to side inside a sealed "wing" supported by trapeze wires- and the wing slides as well. It is the "turbo" version of this concept specifically designed to be self-righting from a capsize or pitchpole. In either case the ballast bulb will be the prime mover in getting the boat back to a position where the sliding "on-deck" ballast would be effective again. It is possible that in the capsize case the buoyancy in the sealed wing could be brought into play. But the worst case capsize scenario is with the wing to windward so there would be little wing buoyancy in the water and the wing would be sticking straight up with the boat at 90 degrees. Again, I may find, experimentally, that in that situation the wing could be centered and the buoyancy in the wing would then add to RM. But in a pitchpole situation ballast is the only way it will right.

    (red canting keel sketch NFlutter-60' Moth)
     

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  3. Gary Baigent
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    Gary Baigent Senior Member

    Doug, forget the bloody lead, go to beam for stability and power, and have spread foils off the floats, amas. Maybe I have an unhealthy fixation against carrying ballast (unless it is water and can be quickly dumped, or taken on for the rare occasions when it is needed) - and I still think Cheezilla is wrong in carrying heavy weight on an extremely wide and super stable platform - they don't need it IMO, do some calculations: 1 - to get equivalent power from sails on Alinghy type, weighted down with ballast to windward .... and then forced to carry extra sail to compensate for this weight, higher rig, much higher loads, more weight, more windage, more drag, heavier gear to operate big rig, motor weight in fixed position, this is keelboat mentality with resulting lower Bruce Number..... and 2 - a pared down Cheeze, still the super wide platform, no ballast, less gear, less weight, less rig height (but still pretty damned high of course) less sail area, less windage, less loads ..... and higher Bruce Number - as Uffa Fox said, "The only place where weight is good is in a steam roller."
    Number 1 is for brute power and complexity ..... and 2 is for refinement and efficiency. Who is right, bludgeon or rapier?
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ===============
    Hey,Gary-I think you're wrong about ballast-look at Hydroptere(fastest sailboat on the planet)-she carries a ton+ of ballast. Look at the Tornado-50% of its sailing weight in movable ballast. The 40' Aussie foiler Spitfire carries 30+% of her weight in ballast.
    ------
    But all that is a different subject: what I'm working on are ideas to produce(for experimental purposes, initially) extremely fast boats/foilers that can also be self-righting and very easy(if terrifying) to sail. Ballast(traditional and on-deck; water and lead) can help achieve this if extremely carefully designed.
    The boat with the sliding wing/inside sliding lead ballast creates more RM than a single man on a trapeze ,yet does it in a much more aerodynamically friendly enclosure that is bound to significantly reduce drag. It seems to me it is worth investigateing the potential of this application of movable ballast on a self-righting platform. One of my definitions of "having your cake and eating it too" is a monohull boat as fast or faster than a cat which is also self-righting. It is-to me- a worthwhile pursuit.
     
  5. Gary Baigent
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    Gary Baigent Senior Member

    Doug, I have nothing against water, (or movable and necessary human ballast) and with water you could perhaps take on or dump it with your mono tip truck, but have an aversion to lead, (and more so to wacko lethal chemicals of course) which you will be forced to carry all the time. Equals real slow in some conditions.
    On the point of Hydroptere, yes, she cranks a tonne of water out to windward - but look at her low rig, (and can you feel those monstrous loads? - because they couldn't carry anymore sail with out the thing turning to confetti flotsam) and look at the fresh conditions she requires to achieve her (fantastic) speeds. She is purposely set up for high speed runs and I believe she is right on the ragged edge of top performance, sail carrying area ability and structural soundness - and congratulations to the team for succeeding on this dangerous tight rope. But what is she like in lighter conditions? Even dumped water, can't carry much more sail.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    ================
    A couple weeks ago: 40 knots boat speed in 15 knots: 2.5 times windspeed!
     
  7. Gary Baigent
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    Gary Baigent Senior Member

    Yes, definitely incredible performance - but she achieved that because the boat is so efficient, namely because she is light, super wide and has very clean air foil rig (but could still be better IMO) and creates an apparent wind that has to be .... say around 25 knots - plus I bet they didn't have their full load of water aboard.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    Yeah, I doubt they had any ballast on board that day. The bottom line is that while there are lots of ways to do high performance sailboats I think the use of ballast is pretty well required in a self-righting high performance boat-particularly if it is a monohull that one wants to sail at multihull speeds. The question is how little ballast can be used- and it seems to me you have to separate out the functions ballast is applied to:
    1) sailing RM
    2) righting from approx. 90 degrees to the point where #1 takes over again....
     
  9. Brent Swain
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    Brent Swain Member

    The buoyancy in deck structures has a huge effect on ultimate stability. A beachball can be 100% self righting with a tiny ballast ratio, but a raft can remain inverted indefinitely with a 75% ballast ratio.
     
  10. sharpii2
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    sharpii2 Senior Member


    Hi, Doug.

    From your third sketch, I can see that your idea is probably workable. At least in theory.

    The only problem I see is the slow reaction time.

    Lulls in the wind can be just as deadly as gusts.
    Racing dinghies have capsized to windward.

    I have great skeptisism that you will be able to kant the ballast bulb the correct direction (windward for a gust, Leeward for a lull) quick enough to keep things under control. It will take an enormous amound of force to kant this keel (dozens of tons, by my reconing). Especially as far as you want to, 90 deg to either side.

    I think it would be instructive to try to sketch the hydraulic system to do this as well as all the pumps and valves. Hydraulic systems trade force for time. The more force needed for a given power imput, the greater time that is needed to accomplish the job. This creates slow acting systems.

    To speed things up, you need greater power imput. This applies to movable water ballast as well. And this greater power imput will inevitatably increase weight. And all this weight will be where it will be contributing little or no righting moment.

    The big problem of trying to imitate a Moth foiler is that the huge movable ballast, the crew, which is as much as one and a half to two times the weight of the craft, can move about quite quickly.

    The dinghy sailing people I have known are quite nimble and quick. This is almost impossible to imitate with robots.
     
  11. bistros

    bistros Previous Member

    I've been trying to get this point through to Doug for a long time, and you've missed a critical component.

    Dinghy sailors, especially those of us who sail high performance boats have to be well AHEAD of ambient conditions - reacting to conditions results in capsizes - you have to ANTICIPATE and prepare in advance of problems to prevent trouble. This is where feedback loop based control systems fail. The Moth wand systems that function are placed as far ahead of the mainfoil as possible to reduce hysteresis as much as possible.

    Part of the evolution of control systems for high speed boats is simplification where possible, reducing interaction required to allow the operator to keep his head out of the boat as much as possible. Cat rigs, wands, pitch adjustment by crew weight etc. all contribute to simplifying the potentially complex into a manageable amount of variables.

    Optimizing the number of complex things that can be handled autonomically by crew weight is critical. By accumulating time in the boat, many control functions become automatic and unthought - someone like Bora Gulari does not have to consciously think about pitch plane adjustments while optimizing roll plane windward heel - it happens without conscious effort.

    I know this discussion started as a poorly expressed desire to isolate roll axis self righting from righting moment, but in my opinion the whole topic is moot. Small boats handle both righting moment and capsize recovery by crew weight manipulation - and it is the most flexible and optimum use of that weight which enables the performance these boats are capable of.

    Adding lead (or water, or batteries or whatever) is 180 degrees out of phase with optimizing performance in small boats where the crew makes up more than 60-70% of the sailing weight of the boat. As boats scale up in size, and crew's effect on these issues is moderated, these other methods of addressing the issues make more sense.

    --
    Bill
     

  12. Doug Lord
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    Doug Lord Flight Ready

    ------------------
    Thanks, Bob. Reaction time is interesting-as the boat gets bigger the time you have to react generally increases. I've used the on-deck movable ballast on numerous monohull models and boy that is tough(and fun). It's the RC helicopter of RC sailing and takes plenty of time to get the hang of. On both the versions in the 22' range(the two top photos) the weight is designed to move faster than a crew could move. That fact
    and the model experience leads me to believe that on-deck movable ballast(coupled in some cases with a fixed or in one larger version,a canting keel) will work well in a range of dinghies from 12' to 22'. My new boat will experiment with this system and I'm going to convert a smaller two person dinghy and see what happens. Couple this with my firm conviction(backed up by lots of numbers) that a self-righting foiler is possible things get interesting.
    The bottom illustration above (by NFlutter) is of a 60' Moth described in detail elsewhere in this forum. The 110 degree canting keel in that boat would
    move at least as fast as a Volvo 70 keel since the loads are similar. The on-deck movable water ballast would move very fast. Both myself and a friend who is a well respected NA and ME( and has looked at the concept) believe it could work but that it is right on the edge of what is possible. Another individual looked at the concept and found that it had the potential to be at least as fast as an ORMA 60 tri flying a hull with 70% of the tri supported by a foil. So this technology could work but requires lots of experimental testing,re-testing and testing again.
    Julian Bethwaite,no less, believes that an on-deck movable ballast boat in the 60' range could work as well.
    As for me, I'm concentrating on small boats and will have results as soon as I can.
    Bob, thanks again for your thoughtful and in-depth responses.
     
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