Deck sweeping sails

Discussion in 'Sailboats' started by DSmith, Apr 19, 2005.

  1. DSmith
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    DSmith Junior Member

    I would like to hear peoples thoughts on whether it would be beneficial to have the boom effectively 'sweeping' the trampoline on an A Class catamaran (mainsail only). This would theoretically reduce the amount of air flowing under the sail from the high pressure to low pressure side of the sail and hence the induced drag.

    I believe that I should be able to tack and gybe by passing behind the sail and therefore not under the boom. Some other issues which may effect this design choice may be as follows.

    1. The wind at the level of the trampoline is very light due to the wind gradient. The apparent wind will therefore be from very close to the bow.
    2. The wind at the level of the trampoline will be very disturbed due to the wind flowing over the hulls and the front cross beam.
    2. It will be difficult to get an adequate seal between the sail, boom and the trampoline to substantially reduce the flow under the sail


  2. Tim B
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    Tim B Senior Member

    You'll gain a bit, certainly, I seem to remember that there is a good bit about this in "principles of yacht design" by Larsson & Eliasson. However, the trick is usually used an monohulls for the jib, as it can be almost completely sealed with the deck along it's length. Naturally, the more 'permeable' the deck is, the less improvement you will see.


    Tim B.
  3. Doug Lord

    Doug Lord Guest

    deck sweeping main

    I don't think you can lose unless what you do causes handling problems. I can't find it nowbut I recently ran across a website site where Steve Dashew discusses just such a thing he rigged on a large cruising boat. He gained enough to convince himself of it's worth..
    On the A class you'd have to find out if the extra area would be considered sail area in which case it might not be worth it.
    Heres the link to Dashews comments; this site is behaving in a weird manner-kind of blinking on and off.It may be something peculiar to my equipment or it might be that web site:
  4. John Perry
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    John Perry Senior Member

    My bifoil hydrofoil sailing dinghy (probably the first such boat to be built) has a mainsail with the foot right down on the deck, indeed the end of the lowest sail batten rubbing on the deck has taken the paint off the deck! When tacking this boat the crew has to get right aft and move round behind the sail, just as you propose for your A class catamaran. I don't know how much aerodynamic benefit this gives but I agree with other people here that at worst it is unlikely to have an adverse aerodynamic effect and it must lower the center of effort compared with a similar shaped sail set higher up. Of course, with a hydrofoil boat the benefit is lost or much reduced after lift off because the wind can then blow right under the hull itself.

    My boat has a narrow hull and two very small floats on outriggers mounted well aft. The design intention was that these floats are normally clear of the water but when the crew moves right aft to tack the stern sinks bringing the floats down onto the water and giving the boat extra lateral stability just when it is most needed (lateral stability is less of a problem when sailing normally since you have the wind to lean against, it is tacking and other manouvres which seem to cause the most problems).

    I have found a problem with this idea of moving aft behind the sail to tack and that is that it depresses the stern so moving the centre of lateral resistance aft and making it difficult to complete the tack. The rudder has to fight against the lateral resistance of the deeply immersed stern. You may find the same problem if you apply this idea to your A class catamaran. Clearly, I am talking here about tacking non-foilborne, so far I havent managed to complete a tack foilborne, perhaps because I just have never got around to practicing sailing this boat for any length of time. Foilborne tacking (as the Moth sailors can do) would eliminate the problem, but even so I expect you are going to sometimes need to tack without being foilborne, eg in very light winds.

  5. Doug Lord

    Doug Lord Guest


    John, would love to hear more detail(length,weight sail area, foil sections) about your boat!Maybe you could post some details under the foiler design thread? I have a and I'd like to include your boat there if you can supply some description of the boat and the date it was first sailed and a picture. You can e-mail me at:
    I found Johns hydrofoil story here:
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    FAST FRED Senior Member

    The other way to get an end plate effect for the main is the "Park Avenue Boom",

    which is a simple quite wide plank on the top of the boom with (usually) short pieces of sail track athwartships and sail slides to allow the foot of the main to take a good shape.

    Seemed to work well 100+ years ago,.,

  7. dionysis
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    dionysis Senior Member

    Yes, most texts say that a deck sweeping main gains by increasing the aspect ratio of the sail. In effect this halves the induced drag of the sail. This is a substancial gain in boat speed.

    On your netting, try sewing a triangular piece of sailcloth, large enough to act as an effective endplate for the sail in the closehauled and close reaching conditions. That way you do not lose the netting function either.

    You will get little endplate effect from the netting alone.
  8. yokebutt
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    yokebutt Boatbuilder

    According to the very esteemed Mr. Marchaj, the increase in efficiency by having the foot of jib closed off against the deck is in the neighborhood of five percent.

  9. usa2
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    usa2 Senior Member

    Ive seen the efficiency of having a low sweeping foot of a sail, as I sailed on CCA-era boats that carried huge genoas low to the deck. One thing I have noticed is when the boat heels over, the foot of the sail is in the windshadow of the heeled deck, therefore doing nothing. That sail area isnt being used at that point than. Is there any solution for that?
  10. Doug Lord

    Doug Lord Guest


    I don't think any solution is necessary since regardless of the effect of the "windshadow" you refer to the high and low presure sides of the sail are still separated by an effective seal.
  11. usa2
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    usa2 Senior Member

    i guess "solution" wouldnt be the right word to use in that case. The sail area down low there isnt being used though, and in most cases the majority of the pressure against the sail will deflect upwards as the boat is heeled over. Its not a problem obviously as most racers have gone with the low deck sweepers because they are extremely effective, but does one suppose that there is a way to be able to use that sail area effectively all the time, rather when the boat is not heeled as much?
  12. Doug Lord

    Doug Lord Guest

    deck sweeper

    It is already effective all the time-at the very least as a vertical "endplate"....
  13. usa2
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    usa2 Senior Member

    yes, but not as "effective sail area"
  14. usa2
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    usa2 Senior Member

    in terms of driving force

  15. Doug Lord

    Doug Lord Guest


    You've already said that deck sweeper sails are "extremely effective"; thats because the seal between the sail and deck improves the performance of the sail(driving force) by reducing induced drag-- regardless of heel.
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