Wing Keel Balast Question

Discussion in 'Boat Design' started by ahmetb, Sep 22, 2008.

  1. ahmetb
    Joined: Sep 2008
    Posts: 1
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    Location: Boston, ma

    ahmetb New Member

    Hello,
    I am purchasing an Irwin 43 with a wing keel. Looking at the specs, the earlier models with the fin keel had 8000 lbs of balast, whereas the wing keel has 7000 lbs.
    I presume that is because the wing keel has more weight at the bottom of the keel.
    Nonetheless, 7000 lbs on a 26000 lbs boat sounds awfully low to me.
    Is there any way additional balast could be added to the wing without stressing the keel bolts, without major surgery, and without increasing the drag of the keel too much ?
    It seems to be that 1000 lbs of lead is about 1.4 cubic ft
    Could it be bolted on ?
    Thanks
    Ahmet
     
  2. diwebb
    Joined: Jun 2008
    Posts: 122
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    Location: New Zealand

    diwebb Senior Member

    Hi,
    it is not just the weight in the ballast keel that counts but the depth below the waterline of the c of g of the ballast times the weight, i.e. the righting moment. I recently converted an IOR type ballast keel to a drop keel on a 27 foot boat. The c of g of the original keel was 17 inches below the waterline. The C of g of the new keel is over six feet below the waterline and the weight is only 400 pounds compared with 1400 for the original keel, but the righting moment of the ballast is 15% greater, and she lost 1000 pounds in to the bargain!!
    So you need to look at all of the factors involved. What was the depth of the c of g of the old keel? What is the depth of the c of g of the wing keel? Are the righting moments the same or is one greater than the other? These also need to be considered at various angles of heel as they will vary, and offshore cruising boats may need the greater ballast for ultimate self righting ability, or a higher c of g for a more comfortable motion.
    Until you know the answers to these questions then I would not recommend altering the keel on the boat.
    David
     
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 488, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Rig considerations come into play with ballasting changes as well. Consult with a professional.
     
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