The weight of a keel

Discussion in 'Boat Design' started by timwithoutaboat, Apr 19, 2008.

  1. timwithoutaboat
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    timwithoutaboat New Member

    can anyone tell me how to work out how heavy a keel should be?
     
  2. alan white
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    alan white Senior Member

    For what?
     
  3. Nordic Cat
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    Nordic Cat Senior Member

    Enough to carry the desired sail area in a given set of conditions:idea: :idea:

    Regards

    Alan
     
  4. Pericles
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    Pericles Senior Member

    Catamarans don't need keels, just dagger boards.
     

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  5. Nordic Cat
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    Nordic Cat Senior Member

    What's that thing in the picture?
    Oh, a daggerboard with a method for setting the depth. Count the bumps??:D :D

    Regards

    Alan
     

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  6. timwithoutaboat
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    timwithoutaboat New Member

    hi and thanks for any replys so far.
    the reason i ask about the weight of a keel, is that i could buy the hull of a 60foot yacht for restoration, unfortunately the original old lead keel was removed sometime in the past (possibly stolen) and has been replaced with a concrete one which also now needs to be replaced.
     
  7. PAR
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    PAR Yacht Designer/Builder

    You can calculate the area with in the dimensions of the current appendage and have a new one made of the material you desire. If using a denser material (a very good idea) then you'll have room to spare in the appendage, which can be "built up" from one of several materials, depending on the yacht, construction type, etc.

    Considering the basic nature of the questions, it would be my strong recommendation you contact a qualified designer and have a new appendage calculated up for you. You don't want this things to break off, or affect sailing performance. The location of CG will be moved with a change of ballast materials. With the concrete one in place, I would suspect she's more tender then she was (compared to the "old lead" one) or the appendage was redesigned to accommodate the dramatically lower density of a concrete fin.

    What's the worst that could happen right? Well . . . if a new fin was set up for the lower weight material and you just make a new one with a lead filled bottom, then the strains on your rig will increase dramatically, very possibly bringing it down, the first time you have a rough jibe or encounter stiff winds. Call a pro or walk quickly away from this yacht.
     
  8. Guest625101138

    Guest625101138 Previous Member

    Tim
    Hope you are single. I am guessing that if you are looking at a 60ft yacht that had a lead keel replaced with concrete then you are not wealthy so you are intending to do the restoration work yourself. So if you are not single now, I expect you will be if you take the project on.

    OK - So in case you are seriously looking to do this a photo or three would be useful.

    Concrete is not a good replacement for lead because it has such a low SG. In fact I cannot imagine how you could replace lead with concrete and have the yacht perform anything like it was intended to.

    A 60ft yacht might need 7 tonne of lead or more. At current prices that would set you back about USD20,000.

    Restoring a 60ft yacht that is in poor shape is a significant project that will require expensive and time consuming effort.

    Rick W.
     
  9. Landlubber
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    Landlubber Senior Member

    Well said PAR and Rick, hope he listens, we all see sooooo many people loose what little they have into dreams that will never materialise.

    "The poor man pays twice"
     
  10. Guest625101138

    Guest625101138 Previous Member

    The largest yacht I have sailed on was 70ft. This was 30 years ago now and the yacht was maybe 50s vintage. It had been confiscated by customs and three hard working tradesmen went into to partnership in its purchase.

    At AUD10,000 they thought they were getting a bargain. In those days you could buy a house for AUD30k. The main sail was a bit tatty so they had it repaired as they could not afford the AUD7k to replace it. Even then it cost AUD2000. They slaved on the yacht for about a year to get it up to sailing condition and spent about another AUD5000 on incidentals like rigging and engine parts.

    Two guys eventually pulled out leaving the third guy to hock everything to pay them out and chase his dream. His marriage had failed over the time and money he put into the yacht. Fortunately no kids and his wife just left him for a fellow who thought more of her than a damn yacht - so not cost split.

    I sailed a couple of harbour races on the yacht and we clocked 14 knots on a reach. Not bad for a heavy displacement yacht.

    He always struggled to find enough crew to sail it. He managed to get enough people interested to sail away for a week but on the first night they struck heavy weather and the planking started to leak badly. They called for help and were towed back by a tug with all effort to keep it afloat.

    I am not sure what happened after that but it did stay afloat and sat in the harbour for a long time. I left before the yacht so have no idea where it ended up.

    No matter how little you pay for a big boat it is still going to be expensive to operate and maintain. What does it cost to slip a 60ft boat for a week? What does it cost to antifoul a 60ft yacht? What does a mooring cost for a 60ft yacht?

    Rick W.
     
  11. ted655
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    ted655 Senior Member

    :confused: We went from what size keel to not being able to afford one. :D
    Actually, a poor man can't afford to keep a 60' boat!
    .
    As for dreams, any worthwhile dream always seems unobtainable, That's the definition of a dream. Dreams & an opposing thumb are what separates us from the apes.
    Go for it! build that keel...., just as soon as you learn how big it needs to be. :)
     
  12. PAR
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    PAR Yacht Designer/Builder

    Ted, you don't think an old ape doesn't dream about a sweet young female to replace his nagging old lady? Please . . . and they have opposing thumbs too . . .
     
  13. ted655
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    ted655 Senior Member

    This will get a sharp response, but is there a rule of thumb? Say, 1# of keel for every 30 sq. ft. of sail? I've always wondered about keels myself. Bilge keels, bolt keels, box keels, twin keels, swing keels, shoe keels, lotsa keels!!!.
    When I look at a sailboat like a "Spray" design, iy has a power boat keel.
     

  14. amolitor
    Joined: Jan 2005
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    amolitor Junior Member

    You could write down a potentially useful rule of thumb for righting moment per square foot of sail, but that doesn't translate directly into keel weight. It depends on the hull form (for the form-stability part of things) the vessel size (again, form stability) and where the center of gravity is (this is the only part where keel weight figures).

    The keel weight contribution to the center of gravity depends on the weight of the keel, and the configuration.
     
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