Water Ballast vs Keel Ballast

Discussion in 'Boat Design' started by jag459, Sep 6, 2005.

  1. jag459
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    jag459 Junior Member

    The lead portion of the keel on our 78' ILC Maxi was destroyed. This is presenting us with the opportunity to replace the keel with a newer shallow draft design for use in shallower coastal waters. We have also been considering the installation of a movable water ballast system with a 4000 to 8000lb capacity which would allow us to further reduce the draft and increase righting moment We are just in the research stages at this time and would appreciate any input or advice we can get. The boat is 78'oa, 18' beam, 28Tons, 13.5' draft (current keel configuration)
     
  2. mackid068
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    mackid068 Semi-Newbie Posts Often

    Keel is certainly a better idea than water ballast. (Personal opinion) 78' boat!!! Wow! Sounds nice. But you might want to check out the TP52 thread. There's some comments on water ballast, or so I've heard.
     
  3. Tim B
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    Tim B Senior Member

    The problem with water-ballast is that it has to be pumped, and thus the quantity of water ballast is largely dependant on how big the pump is, and how long you want to wait. Personally, I'd leave all the ballast in the keel. It's one less thing to go wrong, and on 78 footers, if it goes, it's serious.

    Tim B.
     
  4. jag459
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    jag459 Junior Member

    Thanks for the input.
    We don't plan on replacing the keel entirely with water ballast but we are trying to reduce the draft to a minimum. We are conferring with the marine architect who designed the boat however, we like to gather information on it from as many sources as possible before making a decision. We have designed a water ballast system that will be able to move over 4000lbs (500gals) in 4 minutes using two 3700GPH pumps.
     
  5. mackid068
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    mackid068 Semi-Newbie Posts Often

    Oh, by the way, the boats that "die" on the Hobart are often water-ballasted, or so I've heard. Honestly, even a bit of water ballast is a BIT too much. Stick with lead in your keel. Make it canting or whatever you wish, but avoid water ballast.
     
  6. D'ARTOIS
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    D'ARTOIS Senior Member

    The efficiency of the boat would dramatically reduced by omitting any of it's designed draft, specifically when it concerns a "racer".
    Once, Ed Dubois told me that the performance of the sailing yacht depends largely on the draft. Take it, or leave it, when such an expert speaks.

    I agree with MacKid, who probably follows his intuition, that the waterballast concept houses no good.

    For the actual sailor, waterballast is a nuisance, the boat should have designed for it and when not, it wiill not lead to the road of success.

    I only can tell that for increasing boat's performances, one was increasing the draft of the keel as a first measure.

    Just my one penny
     
  7. mackid068
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    mackid068 Semi-Newbie Posts Often

    How about this: Water in a boat is generally not good.
     
  8. sharpii2
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    sharpii2 Senior Member

    Is Water Ballast all wet?

    Dear Jag:

    There are two issues with moveable water ballast.

    1.) Will it actually improve your performance?
    2.) Will it be safe?

    To actually improve performance it has to significantly add to your initial stability. This can be tricky. If your hull has flaring sides, the water ballast will end up either high in the hull or discouragingly close to the centerline. If it is high in the hull, it only adds initial (sail carrying) stability over a very narrow range of heel, say 0 to 45deg. Once the boat has heeled further than that, the water ballast may actually subtract from stability, even though its on the high side :eek:
    If the water ballast ends up close to the center line, it doesn't end up being very effective because of lack of leverage.
    Also. I would hate to be in a tacking duel where it took me two minutes, the time needed to move half the ballast, to change tacks.

    The question of whether it would be safe or not is, in my opinion, relatively easy to answer. My test would be: could the boat actually sail, including up wind, with the water ballast on the low side. If it can, even if poorly, the system is not only probably safe, but it can probably dispence, in an emergency, with pumps all together. The proceedure could then be to drain the ballast from the high side to the low side, Then change tacks. This way you would be limited by your pipe capacity rather than your pump capacity. Also, to enhance safety, you would need pipes running along the deckline of your boat, so the tanks can drain into one another if the boat ever turtles (ends up upside down). This way, the tanks can help right the boat by shifting the CG over to the low side, making the boat less stable in that condition.

    As I hope you can see, it is not a simple yes or no answer. More likely, it's a probably not but maybe.

    How much draft/fixed ballast you can safely dispence with for how much water ballast,with all its pipes, pumps and valves, you will have to add to provide the same initial stability, becomes the operant question. I would suggest you take this, with your boats blue prints in hand, to an established NA. And preferably the one who actually designed your boat.

    Bob
     
  9. MikeJohns
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    MikeJohns Senior Member

    If you do the stability analysis of movable ballast the numbers look very seductive in a beamy boat, you can generate a high righting moment by pumping water to a off-centre tank, (or by shifting bags of sand).

    In big seas movable internal ballast is an accident waiting to happen. The problem being the proclivity of the racing skipper/team to use every advantage over safety concerns. If the boat is broad off in a big sea and she broaches to, then suddenly your water ballast is adding to your roll moment instead of subtracting from it. As the sandbaggers used to find to their peril.

    Sailboats would be safer more reliable and cheaper without the complexity and it would be good to see this banned under the rules.

    Mind you I feel the same way about cantable keels. Your full stability curve should pass muster with the movable ballast in any of the possible positions . ie the stability should meet the minimum criteria with the ballast at max disadvantage.
     
  10. Karsten
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    Karsten Senior Member

    I don't know what you guys have agains water ballast. I once raced on a 30' yacht with water ballast in very windy conditions on the North Sea. We only had 4 guys on the boat plus 400 litres of water ballast. The water replaced 4 to 5 people and can be dumped overboard if not needed. Try that with your sailing mates.
    We sailed circles around much bigger boats and just opened a valve before each tack to move the water from the high side to the low side. No pumps required.
    Water ballast is great for cruising and long distance racing. It's useless in tacking duells.

    The water ballast is most effective when the boat is sailed upright. Beamy boats have to be sailed upright anyway and therefore it adds to the initial stability a great deal. Down wind you can dump it and therefore reduce weight.

    Reducing the draft will also reduce the aspect ratio of your keel and therefore make it less efficient. The only solution to avoid that is a lifting keel or some sort of additional centreboard.
     
  11. Skippy
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    Skippy Senior Member

    What would be so bad about a bulb keel with the same depth and righting moment as the original ballast, but less weight? Draft would be reduced just from the lower displacement. If you don't like the boat being more tender, then a longer shoal keel with a bulb, same weight and same CG as the original. If that's too slow, I'm not sure how much more you can do without trading to something else like a multihull.
     
  12. PAR
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    PAR Yacht Designer/Builder

    A 78' ILC maxi that draws over 13 feet of water folks.

    From Cape Coral (nice area) I guess you really need to reduce your draft if trying to enjoy her in your home waters. Getting down to a respectable draft for west coast sailing will be quite difficult. Even 6' of draft is too much for most waters, within a mile of land in your neck of the woods, without being restricted to shipping channels, as I'm sure you are now. Reducing your draft somewhat would be reasonable easy, but getting below 9 feet would be asking a lot of your boat. How much draft reduction are you looking to achieve? You are also arranging for a new sail plan, correct?
     
  13. D'ARTOIS
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    D'ARTOIS Senior Member

    Van Oossanen experimented in the late '80's with the double wing keel configuration. I would rather go for such a solution than the application of waterballast in aboat that is not designed for it. Waterballast is not practical nor desirable. I understand that a draft of over 13 ' gives a bit of headache in shallow waters, on the other hand, cutting in one of the most vital parts of the boat's design - the keel, is a kind of surgery one should execute with care.
    In this case, I would opt for Van Oossanen's Wingkeel - where you can reduce draft considerably whilst maintaining a bit of performance and stability.
    You have to cast e new keel, so it might cost something, but this is better than to spoil the boat with provisional solutions.
     

  14. jag459
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    jag459 Junior Member

    We greatly appreciate everyones thoughts, opinions, & suggestions. Keep them coming! It's giving us plenty of food for thought.
    Our goal would be a draft of 9' which we understand may or may not be achievable without drastically affecting the boats performance and stability. We have been in contact with the designer, Nelson/Marek, who has given us several options which we are looking at with both performance and finances in mind.
     
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