Most Weatherly Schooner

Discussion in 'Hydrodynamics and Aerodynamics' started by sharpii2, Aug 26, 2021.


Which of these three design strategies is going to be most effective?

  1. Strategy 1

    1 vote(s)
  2. Strategy 2

    0 vote(s)
  3. Strategy 3

    0 vote(s)
  1. AJB
    Joined: Jul 2021
    Posts: 14
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    Location: 31 42S 152 04 E

    AJB Junior Member

    There's quite a few boats that go faster with the mainsail luffing along the mast, even in fairly light winds. It's not hard to see why; you may lose 5% of effective area but the wider sheeting angle etc can increase the drive by more than the amount that is lost. Actual lift may not be increased, but the vector seems to be more efficient.

    It seems probable that the small bubble represents good aero "scrubbing" and flow re-attachment after the mast generated turbulence; so the drag is reduced.

    It is assuredly not, not anything to do with the wider sheeting angle. Gentry, as Doug H pointed was correct a very long time ago: the main job of the mainsail sailing upwind is to improve the onset flow at the jib luff.
  2. CT249
    Joined: May 2003
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    Location: Sydney Australia

    CT249 Senior Member

    Dunno, AJB. Gentry refers to the bubble as being caused by the mainsail "just reacting to an increase in leeside pressure resulting from the genoa's slowing down of the air in the slot", which appears to be a very different issue to re-attachment.

    I note that the bubble appears to be very similar in effect and operation when one is sailing a conventional boat (ie J/24) or a wingmasted boat with a very small jib, which presumably has less mast generated turbulence.

    Finally, in such craft the mainsail must have a MUCH bigger impact than mainly improving the onset flow at the jib luff. For example, one of the craft we've seen the effect in is a 16' wingmasted cat sailed either as a two-up sloop with 4.2m jib and 17.5m mainsail, or a singlehander with just the same main. The doublehander is faster upwind most of the time despite having an all-up displacement of about 50% more. It seems obvious that in the doublehanded version the little jib is not providing most of the drive, because there's nothing in the world that can go that fast in those conditions with anything like such a small sail.

    The other thing is that in many small-boat fractional rigs (and the cat mentioned above has an unusually low foretriangle) about 30% of the main is well above the jib, and it seems odd that such a large area wouldn't be having a major effect on its own. That seems particularly so, when we consider how important the trim of that area of the mainsail is.

    A further clue on the cause of the "bubble" I'm referring to is that it doesn't exist above the jib, despite the fact that in some boats (such as the wingmasted ones mentioned) the mast is just as large physically as it is further down, and larger in proportion to chord length. If the bubble was caused by the mast, surely it would extend all the way up the span?

    Finally, the luff bubble on the mainsail can also normally be cured by moving the jib lead outboard. If the bubble was caused by mast turbulence, surely it would not be so very dependent on the width of the slot?
  3. rangebowdrie
    Joined: Nov 2009
    Posts: 102
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    Location: Oregon

    rangebowdrie Senior Member

    I remember watching the 1987 Americas Cup finals in Australia, when Dennis Conner was skippering Stars and Stripes.
    In the upwind leg the upper part of the main was doing this "lazy" luffing action.
    I figured that the boat was trimmed about as good as it was going to get.
    'Course that was a fractional rig, but it was sure powered-up.
  4. AJB
    Joined: Jul 2021
    Posts: 14
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    Location: 31 42S 152 04 E

    AJB Junior Member


    1. Gentry gave us the cause of the bubble. My points are about the effects, which are many.

    2. Without a (deck) endplate, a small jib on a catamaran might not be all that effective, compared to the similar proportions on a keelboat. See IMOCA 60 heavy air setup for comparison.

    3. The lift is generally assumed to be proportional to the span squared, span being from the lowest point of the jib to the masthead, although somewhat subject to Reynolds number issues.

    4. My comments, to be clear, were intended for conventional, not rotating rigs.

    5. The reason the bubble does not generally extend above the jib is, in part that the top of the main is not in downwash from the jib; and structurally (until square tops came along), it was difficult to get the main twist to match the discontinuity in the downwash.

    6. Experience suggest that some degree of inversion in the main ('bubble') is desirable upwind in conventional keelboats and skiffs in virtually all conditions.

    7. Review a modern skiff rig (18, 49er)going upwind in say 14 knots. The highest part of the main square top is flattened to around 4% depth and is just lying at zero AWA to the onset flow. It operates just as the upturned wingtip on a jet. The skiffs still use hound heights around 70%, as well.

    8. There is no evidence that mast turbulence can make a LE bubble!

  5. Robert Biegler
    Joined: Jun 2017
    Posts: 54
    Likes: 22, Points: 8
    Location: Trondheim

    Robert Biegler Junior Member

    In the funnel, of course. The only way I can see for the Coanda effect to do anything would be if the air coming out of the constriction moved faster relative to the sail than the free airstream. So what I believe to be sharpii2's theory (he didn't explain beyond mentioning the Coanda effect, so the rest is my inference) is a bit different from the classical. Does your knowledge of CFD and empirical measurements include what happens to the air coming out of the constriction?

    Also, I think the best candidate for such an effect would be a slotted wing, like those the AC 50 catamarans used. So when a two element wing has a slot rather than a flap with an airtight joint, does the air coming through the slot improve the pressure distribution more on the forward or the aft element? If (my interpretation of) sharpii2 is correct, it should be the aft element, though even if it is, it might not be for the reason he proposed.
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