Big sails on a small boat

Discussion in 'Boat Design' started by sighmoon, Nov 25, 2008.

  1. sighmoon
    Joined: Oct 2007
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    sighmoon Junior Member

    In my ignorance, it seems to me the best way of making a sailing yacht go fast would simply be to have a huge sail plan. Nevertheless, you seldom see this taken to the extreme. I'm guessing this means that in practice, there are limits.

    What problems would be encountered, having a mast many times the length of the boat?

    Would the pitching moment of a huge mast mean that the bow gets buried every time a gust hit? Could a full and buoyant bow mitigate this?

    As the boat heels, Would the turning moment of the rig tend to skew the boat into the wind uncontrollably?

    It would obviously need a substantial amount of ballast, and a lot of strength generally, so it would probably be heavier than a boat with smaller sails. Could this get to a point where additional sail area no longer provides an advantage?

    Are there any other issues? If so, how might they be addressed?

    Thanks,
    Simon
     
  2. sailfish25
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    sailfish25 Sailing the Great Lakes!

    Sail area can help (I'll let others fill in here) but you need to remember that your hull design (waterline length) will limit your speed. The only way to increase speed past theroetical limits is to get the hull to start planeing, not an easy feat for a sailboat.

    I'm sure others will have much better info for you, I'm no expert here.
     
  3. Stumble
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    Stumble Senior Member

    I think the issue you are going to run into here is that for any given rig dimension there is going to be a certain amount of righting moment to keep the boat upright. If you take the rig off of a Andrews 70 for instance and put it on a flying scott the scott is obviously going to need the same keel as the Andrews to keep the thing from flipping over. The problem is that the scott doesn't have enough flotation to keep 18,000lbs of lead off the bottom so either the boat sinks, or the hull has to get bigger. To add to this is the fact that a smaller boat has almost no initial stability, requiring an even bigger keel to make up for the lost form stability between the two different hulls.

    To some degree of course (and I picked a rediculous example) putting a larger rig on a boat will increase the speed of the boat, but it also forces you to reef the boat sooner in order to keep the boat under control. The extreme of this is some of the small planeing dinghys that actually switch rigs depending on the wind so they don't have to carry the non productive extra weight of the bare pole.
     
  4. sighmoon
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    sighmoon Junior Member

    Stumble,

    Alternatively, it could have a keel half the weight of the flying Scott, and twice as deep.... I'm planning to have a bulbed centreboard, so the draft won't limit the sailing area too much.

    When you say we'd need to reef sooner to stay under control, are you referring to heeling or something else?

    mbrandt1,
    I'm planning on a planing hull form. By that, I mean a fat, flat transom, a fine entry, and as light as feasible. Are there other factors I need to consider?

    My initial sketch has a draft of 3m, a length of 6m, and a beam of 2.4m (the maximum you can tow behind a car in the UK).
     
    Last edited: Nov 25, 2008
  5. rednev
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    rednev Junior Member

    from the history books zephry was an 8 footer skiff she carried an18ft mast,
    16ft boom a 10ft6inch gaff and a 10 bowsprit.
    britannia an 18 footerof 1919 had a 21ft bowsprit her boom with ringtail spar extended measured 42 ft her 3 piece spinaker pole was 45ft overall into the wind she carried about 1000sqft of sail. running she carried a main ,balloon jib spinnaker,watersail and a ringtail for about 2800sq ft.

    its not a question of how much sail can be carried its more how much is practical for the use you have in mind.
     
  6. sighmoon
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    sighmoon Junior Member

    Hi Rednev,

    Are there any reports of what those two boats were like to sail? Did they have steering problems, for instance?

    Thanks,
    Simon
     
  7. rednev
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    rednev Junior Member

    zephry won 9 out of eleven starts with irene pritchard on the tiller,later she wonjohnstone bay s.c championship in heavy weather leading home second paced inez by over 6 minutes.
    brittania launched 1919 was raced most weekends was still winning in1945 retired in 1950.
    by the stanards of the day it would appaer they had no particulary nasty habits
    if you want to fast ie at times only the center board in the water have you considered a modern 18 fter as sailed on sydney harbour ? fast but expect to get wet.
     
  8. Stumble
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    Stumble Senior Member

    Sighmoon,

    In my example I was working with a 15 foot deep keel already. Theoretically a 20' boat with a 100' deep keel could carry a rig significantly outsized for her length, but that isn't the same thing as it being practical.

    Now a 6m boat with a 3m keel while a little out of proportion compared to most boats in that size range it not rediculous. I have two real concerns that could probably be solved by proper design however. The first is that to keep wetted area down on a keel this long you will need to go to a very high aspect wing with a thin cord and a short run for and aft.

    This implies a problem in that you are reducing the amount of material available to handle the high loads of a bulb hung this far below the hull. So you are going to need to do some pretty good enginearing to prevent the foil from deflecting and to keep it from bowing between the two hard points of the bulb and the keel. I don't really know the proper material you would need, but I am thinking carbon fiber or some other exotic just to give you a chance.

    The problem is that if you go to a larger keel that is the same length you will definatly hurt your speed, particularly in light air, since you will be stuck with additional drag that isn't adding to stability.

    You may also want to look at going to a lifting keel design similar to what the Melgus 24/30/32 have. This allowes a lot easier trailoring and launching though it does add to the complexity of the boat.
     

  9. Cheesy
    Joined: Aug 2008
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    Cheesy Senior Member

    What sort of wind speeds are you anticipating? A Shaw 6.5 or even a Shaw 6.5 turbo will be very close..... unless you dont want to hike and want to stay dry
     
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