7/8 rig un-tapered

Discussion in 'Boat Design' started by kenbker, Aug 15, 2011.

  1. kenbker
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    kenbker Junior Member

    hello, any big deal about rigging a half tonner 7/8 fractional without tapering the spar?
     
  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    The general idea for tapered mast is to reduce weight aloft. Every little bit counts, particularly on a small boat. Think of the aloft weight as if it were a very long hammer. As little as one pound of weight 25 feet above the center of bouyancy has a profound influence on the behavior of the boat.
     
  3. kenbker
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    kenbker Junior Member

    thanks messbout. really do wonder how much diff 1lb would make at the top of a 40' stick. I notice a lot of comparable euro production boats 7/8 rigs have a funny little taper about 2' long which I assume must be purely cosmetic since a taper of that size would be lucky to remove 1/2 a pound of metal. other thing is a know a guy with a similar boat who put up a stick 6' oversize and went from single to double spreader, so he not only has the xtra rig wight but also extra rigging cause now he's got inters. made the boat a bit more tender in 20knts + but he fixed that buy fitting a shoe to the fin
     
  4. Stumble
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    Stumble Senior Member

    Ken,

    Very simplistically to find the loss of righting moment from weight aloft, multiply the weight by the height above water. This is a gross simplification, but as a rough number it works (the real number requires knowing the Metacenter location of the boat, the heel angel, and uses a more complicated formula, but this gets you in the range). So that one pound weight reduces your max rigging moment by 40'*lb=40foot pounds.

    To offset this weight from mass in the keel, you take its distance below the waterline, let's assume a 5' deep keel bulb you would need M(keel)=40footlbs/5'. So M=8.

    So to offset that one pound at the top of the mast you use 8lbs in the keel bulb. This is why it is so critical to reduce weight aloft, as well as maximize mass deep.



    And for the N/A's, and Engineers, yes I know this is a gross simplification, but it is demonstrative.
     
  5. Paul B

    Paul B Previous Member

    What sort of HT are you talking about?

    Many boats with decent performance were built with non-tapered masts, especially under the old MORC rule. MORC gave a credit based on the rig, so some boats opted to go untapered.

    You'll probably have to have a different luff curve, as the top of the rig will bend less. Your sailmaker should be able to take the measurements once the rig is in the boat.

    The good news is modern sail materials need less mast bend than the sails from the old HT days.

    As for weight, a 10 foot taper (2" at the top to zero at the bottom, so 1" average) takes out about 120 sq inches of material per side. With a wall of 0.125" you are talking about 15 cubic inches pers side, or 1.5 pounds per side, total 3 pounds.
     
  6. kenbker
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    kenbker Junior Member

    thanks guys. I will chew on all that. boat is a vintage HT...whiting 32 built to cheat the IO rule.
     

  7. Paul B

    Paul B Previous Member

    On an old Whiting boat I would prefer to see a tapered spar.

    I'm not sure why you don't want to do it. It can't add much to the cost of a new rig. I'm assuming you are replacing the existing rig due to breakage or other problem.
     
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