Extra lead in bottom of boat

Discussion in 'Stability' started by tamas, Apr 15, 2010.

  1. tamas
    Joined: May 2009
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    tamas Junior Member

    Hi, I hope someone can help. My bulb is about 230kg and sits about 1.8m below the water line. I have read my boat was originally designed to have another 80kg in the bottom of the boat as ballast. But it does not. I have about 27m2 of working sail area on a 7.3m trailer sailer.

    My question is, how much difference would an extra 80kg in the boat make. Yould it give me an extra 1 knot to windward or an extra 5 knots of breeze before I reef. Is it better to get another crew on the rail perhaps (we have 3 now)

    If there was a benifit to have the extra 80kg would it be better to increase the bulb by say 35kg. Would that equal about 80kg in the boat at about 450mm below the waterline?

    Not a lot to go on I know but there must be a rule of thumb to work with. EG: 1/3 needed at 1800mm below the waterline tha if in the boat at 450mm below wl. Extra righting = extra speed???
     
  2. Paul Kotzebue

    Paul Kotzebue Previous Member

    There is no rule of thumb. Righting moment = righting arm * displacement. Righting arm is the horizontal distance from the heeled center of bouyancy to the center of gravity. Lowering the center of gravity of the entire boat (not just the ballast) will increase the righting arm by the distance the center of gravity is lowered * the sine of the heel angle (all else being equal).

    A naval architect can make some reasonable assumptions and estimate the effects of adding or moving ballast without doing complete stability calculations, but there is no reliable rule of thumb.
     
  3. alan white
    Joined: Mar 2007
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    alan white Senior Member

    By your description of the boat, it sounds like asking a friend to lie down on the cabin sole (assuming you have one--- a cabin sole, I mean), you'd have your answer. I'm guessing you'd have difficulty feeling the difference.
     
  4. willfox
    Joined: Oct 2008
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    willfox Junior Member

    I have a simple programme which is a simplified version of win vpp but could estimate the difference roughly. Do you have a rough idea of the VCG? I will also need the dimentions of the keel, displacement, Lwl, Bwl etc? what is the boat?
     
  5. tamas
    Joined: May 2009
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    Location: Brisbane

    tamas Junior Member

    Thanks Willrox,
    Not sure how to work out the VCG but will try if you let me know or maybe these measurements below will help. The boat is a Spyder 24. (7.3m)
    Keel details: 230kg bulb at 1.8m when down. Bulb is 980mm long. Keel is about 500mm wide. Draft keel up is 430mm.

    Displace is 920kg.

    LWL is 6.95m

    BWL is 1.9m - beam is 2.5m

    Mast ht is 9m but starts about 1.1 above the water line.

    If lead was added in the bottom of the boat it would be abou 400mm below the water line. Hope this is enough but let me know if not. Many thanks.
    Cheers Mike
     
  6. alan white
    Joined: Mar 2007
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    alan white Senior Member

    Your question was whether you might get another knot to windward or handle 5 kts more breeze before reefing if you added 80 kg of ballast.
    While this can all be analyzed to the Nth degree, Your question isn't hard to answer.
    The answer is, you probably won't see any measurable difference, and definitely not anywhere near one knot or five more knots of wind before reefing!
    Maybe, as a boat design exercise the question has value, but from any practical standpoint, your boat will also be heavier enough to slow it slightly when running and probably reaching, but you'd need instruments to analyze the difference, it's so minor.
    There would probably be some small fraction of a knot advantage sailing to windward, for what it's worth. Not a knot. Maybe not even a tenth of a knot.
    If your boat is currently slow to windward, the problem lies elsewhere, and is likely your sails or rig. The indication would be a strong weather helm and this would be corrected by learning about what could be done to improve the sails and rigging.
    So it's not a bad idea to learn about how your boat's design effects its heeling angle and so forth, but as said, its just an exercise, and if you have a problem, it lies elsewhere.
     
  7. tamas
    Joined: May 2009
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    Location: Brisbane

    tamas Junior Member

    Many thanks for your reply's. The thing is I am sailing 80kg under the design weight and wondered if it is a penalty. From what I gather, "NOT MUCH". I don't think I am slow to windward but I know sports boats sail with extra crew and less ballast to increase performance.

    I was thinking out loud and wondered if I cant have extra crew then the extra ballast may help tenderness. My boat seems tender but I do not want to increase the ballast to the design amount to penalize performance and would rather it helped, but if it did nothing but makes the boat less tender than I should do it as long as it was reasonably noticeable.

    Many thanks again.
     
  8. alan white
    Joined: Mar 2007
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    alan white Senior Member

    Extra crew is movable ballast, which is ideal, and makes a big difference to windward performance.
    Unless you are seriously racing, don't overthink this. If you don't see any problem at this point, you'd be much better off putting money and effort into something else.
    Your boat's tenderness has far more to do with hullform, specifically waterline beam and deadrise amidships. The ballast, even a deep bulb, has almost no effect as you walk about the boat at rest except to damp motion. Keel ballast has (increasingly) more effect when you're heeled well over.
     

  9. stmbtwle
    Joined: Jul 2010
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    Location: Florida

    stmbtwle Junior Member

    In most circumstances I think the extra weight would just slow you down, but it MIGHT help under extremes. Adding a smaller amount of weight to the bulb would have more effect and you wouldn't have the weight penalty. However it might not be allowed by class rules.

    My suggestion would be go to the home store and get about 4 bags of gravel or stones (they're cheap) and set them in the bottom of the boat; then go sailing. If you like the results you can get more sophisticated, if you don't like it you take them out and put them in your garden.
     
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