Looking for Advice on Water Ballast

Discussion in 'Boat Design' started by ancient kayaker, Mar 14, 2012.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    I have run into a problem on the the stability of a small sailboat I am trying to design. It is a downsized Haven 12-1/2 for lake use, without the Vee bottom and lead ballast but the KM comes out too low. I can tighten the fairly slack bilges a bit but not a lot at its current displacement.

    At present it has a scaled down version of the Haven's transom and I'm not prepared to level it off at the waterline as it would then look entirely different. I want to keep the Haven look in this smaller boat with the pretty transom but I am sure a lot of stability is being lost there.

    The Haven gets away with it by having lots of lead ballast: I don't want to use lead as it is meant to be a beach-launched car-topper.

    Question: will water ballast help? The bottom can be changed to a vee to provide the volume. Better still I can lower it which will also increase the BWL. I find it difficult to understand how water ballast works in practice as it's the same density as the water the boat floats in which seems counter-intuitive . . .
     

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  2. tunnels

    tunnels Previous Member

    Ballast is just weight (anything at all thats heavy ) inside the boat !! right!! the placement and amount is up to you !! it needs to be unmovable maybe !! Needs to be central in the lowest place and if you want it can be simply a bladder filled with water placed in a box where you want it to be !
    At the end of the day just pull the plug ,empty the bladder or tank or what ever and drain it out . simple is nothing new and has bee round for long long time . Sailboat use movable water ballast from one side to the other , ships use lots water for ballast . some inflateable boats have flooding bottoms that when you stop automaticly fill with water ! when you take off again they darin in seconds so dont have to carry the weight till you stop again .Sea planes have a simular concept for stability when they land . Off shore powerboats have bultin water tanks that fill inseconds in differant parts of the boats hull for stability and smooth the riding in waves and bg swells
    We are building a long skinny boat and stability is going to be a issue i am sure so i am jumping the gun and have been looking at building 6 large water tanks ! all auto filling to go in the bottom of the boat !!
    Will be interested to hear some of the answers that will apear here .
     
  3. alan white
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    alan white Senior Member

    I almost said something when you wrote about a downsized Haven. The problem with downsizing those slack bilges is that initial stability (and well into heeling) is very poor.
    The solution isn't water ballast though. What you said about the slack bilges gaining needed displacement isn't really valid since you could do so many other things to increase displacement such as increasing the waterline length and beam (something you naturally need to do as a boat design gets shorter).
    The dynamics of a boat's interaction with the water change very quickly as a boat changes length. What makes a great 16 footer like the Haven can't be easily transferred to a smaller length. In fact, doing so even to a 12 1/2 foot LOA boat creates an abberation of sorts.
    There are a few very short heavy displacement sailboats. McNaughten assoc. (boat designers) have designed such a boat. This would be a very slow and heavy boat, one designed specifically to journey far offshore safely. I don't think you want a boat like that.
    If you want to build a smaller Haven, you will have to make quite a few non-linear changes. Reduce overhangs quite a bit. Widen the beam waterline. Square up the bilges a bit. Finally, recognize that a reduction by 20% means a much greater reduction in displacement. At 12 1/2 feet LOA, a displacement of maybe a half of the 16 ft Haven would probably be about right (I'm too lazy to calculate it right now).
    Advice: Build a boat that LOOKS very Havenesque without adhering to the same proportions. If you check out some of the larger Herreshoff daysailers that are based on the 12 1/2 footer concept, you'll see that as they get bigger, their displacements and general shapes are customized to the new length even while they belong to the same design family.
     
  4. jehardiman
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    jehardiman Senior Member

    Are you trying to match the original KM or the original GM?..not that GM is all that useful for a sailing vessel. What you should try to match is the original cross curve of stability at design displacement. As Alan pointed out, scaling the vessel changes the proportions all out of whack for some things. So with a linear ratio of X, KB scales to X, but displacement scales to the X^3, and Iwp scales to X^4.....which is why scale pond yachts need foot long fins with a big lead bulb on them. Normally if you shorten a hull you need to widen it and lower the CG to keep the same stability.
     
  5. philSweet
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    philSweet Senior Member

    Obviously, water ballast only works if the water is restricted, ie. no free surface. The mid section should have a bit of a vee, or at least develop some deadrise by .2 beam. Flooding angle over sheer should be about 12-14 degrees. Figure your water tank needed to bring her down to that point. A firmer bilge would help a water ballasted boat. For maximum righting moment, the dead rise at 1/2 beam should be about 2/3 of the angle that submerges the sheer.(not maximum area, maximum moment at sheer). It takes quite a bit of waterballast to do much good, and downsizing and a conversion to waterballast doesn't sound like a great combination. Usually, downsizing is compensated for with a higher percentage of moveable ballast and a proportionate increase in beam at the sheer to support those butts.

    I wrote a crap code spreadsheet a while back that solved for the maximum righting moment section shape for a given perimeter assuming water ballast. Shape, heel, and ballast were independently optimised. It still used some small angle aproximations, so it wasn't all that accurate, but I did learn a heap from it. A better mathmetitian than me might be able to produce something like an exact solution. It appears to involve some nasty eliptical integrals though.
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    All good advice and the quick replies are much appreciated! To summarize, use water ballast, square up bilges, increase waterline length and increase beam. It's going to need a combination of small changes to avoid a big change to the appearance, which I rather like.

    I'll comment on each idea, leaving ballast for later:-


    I can likely increase the beam by maybe 10% if it doesn’t start to look paunchy.

    Although I can square up the bilges that may change the nice smooth transition of the cross-curves and may make it more difficult to build using the radius chine method that I want to try - I’ll have to take a look at those potential problems and perhaps run some more ply-bending tests.

    There’s not a lot I can do to increase the waterline length without increasing LOA; the stem profile tries to emulate the Haven’s appearance and the transom has already been straightened up by 15 deg compared with the Haven.

    Increasing the displacement with water ballast will add weight afloat but not on land, and also lower the CoG. It means a double bottom and low-down buoyancy chambers to make it self-draining, so there will be some dry weight penalty. To avoid raising the floor, since internal height is already limited, it will also mean a deeper hull. That’s not a bad thing if the extra depth is added above the curve of the bilge since that will increase both the waterline beam and length slightly. The transom will have to be raised to preserve the sheer profile and keep the transom from dragging.

    In addition, although I don’t want to change the transom above the waterline I may be able to fiddle with the bottom shape ahead of the transom to add a little more RM; I’ll take a look at the waterlines.

    Of course there’s always the option to reduce sail area if the above changes plus the greater effect of crew weight turns out to be insufficient in practice. The original Herreshoff had a gaff rig, fairly tall, but it could be lowered discretely.

    That gives me a lot to look at; thanks again.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    I got more KM by tightening the bilges, broadening the forefoot and modifying the transom profile. This despite the fact that I significantly reduced the beam which also seems to have slightly increased the flooding angle to a very adequate 42.5 (crew of two) to 44 (solo) degrees. I think this will do the job without the water ballast, which is always an inconvenience especially for a car-topped short-trip beach-launched day sailer. Many thanks for the suggestions and support!
     

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  8. kvsgkvng
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    kvsgkvng Senior Member

    I think KM has very little to do with water balast. If used, the water ballast would bring your boat down, raising the waterline, and thus increasing a bit the displaced volume. This should in theory raise KM a little. But the cost would be not justified. Increasing the volume of displaced water sideways helps much better, that is what you did.

    Water ballast starts working when lifted above the water table. Then you have full gravity righting moment. If you place two submerged water chambers at each end of your boat, it will do very well for the righting moment. When the water chamber is submerged, it practically do nothing except resisting rolling.

    Just my opinion.
     

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  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    I fear I made things a bit confusing in my earlier post. The water ballast is not intended to increase KM but to increase the righting moment RM. Adding any ballast raises the waterline as you said and actually lowers KM slightly. I'm not very familiar with boating math but I think the following is correct -

    RM = GZ x Displacement
    GZ = Righting Arm = GM x sin(H)
    Where H is the heeling angle; let H be Pi/2 radian to take it out of the equation so that GZ = GM
    GM = is the metacentric height or the height of the metacenter above the center of gravity
    GM = KM - KG (I think some authorities use KN for this term but that gets confused easily with KM)
    KM is the height of the metacenter above the keel
    KG is the height of the center of gravity above the keel

    Now we can have an equation for righting moment containing KM -
    RM = (KM - GM) x Displacement

    What happens when we add ballast low in the boat is, KM and GM both go down and Displacement goes up with the overall effect of increasing RM.

    Adding water ballast outside the hull below the waterline has no effect of course until heeling raises it above the surface. Adding any kind of ballast inside the hull instead of at the bottom of a deep keel is not very efficient but it will work provided it is kept low to keep GM low.

    Adding ballast higher than necessary raises GM and of course as the equation shows that will reduce RM, although it doesn't really need math to understand that. The only reason I know of to do that is to reduce the roll rate of a boat that is an uncomfortable ride in heavy weather.
     
  10. DCockey
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    DCockey Senior Member

    The validity of this equation depends on the definition of M used, which was discussed in another thread. M is frequently defined as representing the metacenter as the heel angle approaches zero. In that case this equation is only valid for "small" heel angles. Using this equation for large heel angles, such as Pi/2 radians (90 degrees), requires that M be a function of heel angle.
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    Why not flair the sides more so you use crew weight for power and windward RM. Excuse my Southern Antipodean ignorance but a 12.5 foot version of a 16 foot Haven sounds a small boat to me and unless it carries a cloud of sail, sound just like another dinghy - which wouldn't require ballast. How about a profile and sail plan drawing?
    Greg Elliott claims water ballast just dumbs a boat down and on his Jim Young water ballasted 18, he left the water out and relied on intelligent? ballast - and won all the class races ... until they found him out.
    However there would be nothing wrong in fitting a central WB tank below the cockpit floor with a water venturi in (and out) if you want to quieten the boat down.
    On my 18 foot Cox's Bay Skimmer this works well in strong winds when single handing, or anchoring (the wing masts are problematic - without taking on ballast).
     

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  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    Flaring the sides would change the hull shape too much and invalidate the exercise. It must be a small boat to be a car-topper. I am still trying to avoid ballast but wanted a better understanding of how water ballast behaved and was used; as you noted it could be added retroactively if I found it necessary. I'm still thinking about the rig - may adapt one I'm developing for another (existing) small boat.

    Link to Kempenfelt Lines
     
  13. philSweet
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    philSweet Senior Member

    Update-

    earlier I posted

    This thread sent me back to revisit the old spreadsheet and I plugged in explicit formulas for RM instead of the approximate ones I had been using. I also fiddled with the whole rasterization arrangement. Got some different results- mostly do to the elimination of small angle approximations. The new routine showed a much flatter bottom. No deadrise at all at .2 and nearly none at .5 beam. The program finds the shape that gives maximum RM for a given total perimeter (chine to chine). Default is water ballast, which is used in any amount to optimize RM.
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    - that makes a great deal of sense and is more intuitive than your previous post which I struggled to understand.

    The design I am working on started with a flat bottom over half the beam with circular arcs with radius = 25% of beam around the bilges. That creates problems joining the planks at the centerline and morphing straight sections into the circular arcs. To simplify construction I am modifying it slightly to add a small amount of deadrise - enough to avoid the risk of a hollow resulting from a minor building error - and the circular arcs are being softened into a more natural shape that will not require as much clamping.

    The effect on RM will be minimal. I believe I no longer need to use water ballast to achieve sufficient stability, but at least I have a better understanding of it.
     

  15. jehardiman
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    jehardiman Senior Member

    Of course it should show flat floors if optimized for max RM. Max RM occurs with the maximum shift in displacement, which means that for a fixed volume you move all the volume out to (or beyond as in bulges or Popov hull) the edge of the waterplane. This is classic pie pan stability.

    FWIW, you need to be careful when using water ballast in something like this. Only the non-waterballast shape matters for stability until the water ballast is lifted above the water plane as kvsgkvng pointed out. If it never gets lifted above, it has no effect and is only wasted volume and some small CG effect from having the plating lower.
     
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