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)
    100.0%
  2. Strategy 2

    0 vote(s)
    0.0%
  3. Strategy 3

    0 vote(s)
    0.0%
  1. AJB
    Joined: Jul 2021
    Posts: 56
    Likes: 8, Points: 8
    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.

    CT,
    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
    Posts: 1,449
    Likes: 191, Points: 63, Legacy Rep: 215
    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: 234
    Likes: 97, Points: 28, Legacy Rep: 10
    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: 56
    Likes: 8, Points: 8
    Location: 31 42S 152 04 E

    AJB Junior Member

    CT,
    Concentrate!

    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: 166
    Likes: 87, Points: 38
    Location: Trondheim

    Robert Biegler Senior 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.
     
  6. CT249
    Joined: May 2003
    Posts: 1,449
    Likes: 191, Points: 63, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    Don't patronise!

    1. Yes

    2. A cat jib without an endplate can't be all that ineffective, considering it allows a boat/crew combo that weighs about 40% more to go about 3% faster than the cat rigged version.

    3. Yes, so....?

    4. Yes, but my point is that if the same effect occurs with rotating rigs then why apparently assume that it has a different cause?

    5. Possibly, but why assume your explanation is correct and mine is not? And since when did the move to squaretops make such a difference? I can't see any general difference in the way my squaretops act or twist to the way my pinheads and other sails act, as far as the bubble goes.

    6. Depends on the boat. Nationals have been won against Olympians and AC sailors on keelboats that had a "bubble" a lot of the time, and on keelboats that didn't. One difference in my experience was the width of the slot.

    7. Yes, so....?

    According to your sig, you don't live too far from me so we may have raced together in northern NSW regattas. Given that, I'd much rather get on with you than feel patronised by you, so please don't do that.
     
  7. Robert Biegler
    Joined: Jun 2017
    Posts: 166
    Likes: 87, Points: 38
    Location: Trondheim

    Robert Biegler Senior Member

    I found a tentative answer to my question here: ttps://aviation.stackexchange.com/questions/30960/how-does-the-slotted-flap-work

    The pressure distributions for a plain and a slotted flap look like this:
    [​IMG]
    Without also seeing the velocity distribution, I can't be sure that this is the Coanda effect that sharpii2 is interested in, but it sounds like it.

    I also think this is irrelevant to most sailboats, on the grounds that to get the faster air flow needed, both sides of the constriction have to compress the air, and a conventional mainsail will not do that. It would bulge out to windward until the slot is open enough not to compress the air within the slot. The possible exceptions are slotted wings, and perhaps sloops with large wing masts. And even then, I don't think this has anything to do with the rig being weatherly, on the grounds that the accelerated air stream helps when flow over that part of the would otherwise stall, and that is more likely to happen when trimming for maximum lift, rather than best lift to drag ratio, and that should apply more if the flap is a larger proportion of the total chord. (Taking into account L/D of hull and rig instead of rig alone might complicate this a bit, as pointed out by CT249 a while ago.)
     

  8. Howlandwoodworks
    Joined: Sep 2018
    Posts: 223
    Likes: 80, Points: 38
    Location: USA MO

    Howlandwoodworks Member

    Looking at some of the pictures of "Bluenose" they shows it with large spaces between sails of less than 100%. Most photos have overlapping sails.
    I once read that Caption Angus Walters had so many sets of sails he had to dragging some of them behind him.
     
    Last edited: Sep 25, 2021
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.