Catamaran Point Angle

Discussion in 'Boat Design' started by Iridian, Jun 6, 2021.

  1. Iridian
    Joined: Jan 2020
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    Location: MD

    Iridian Junior Member

    All over the internet, I see it constantly repeated that Catamarans have a worse upwind performance than monohulls, and that they point lower.

    My thought is that this is likely due to the types of catamarans most people are familiar with (either no daggerboard, or low aspect dagger boards), such as the Hobie 16, Hobie 18, Hobie Getaways, or larger charter party boats like those made by Fountaine Pajot or Lagoon.

    In the case of the smaller beach cats, these boats have either asymmetric hulls or low aspect daggerboards/centerboards. In the case of the larger cruising catamarans, they have a massive amount of weight, freeboard, and also small low aspect keels.

    For performance, but non-foiling catamarans, is there a reason why a catamaran would point worse? Assuming similar high aspect daggerboards to the equivalent high aspect monohull keel.

    The only thing I can think of is that since catamarans are faster, the apparent wind will shift forward, making the lower point angle more optimal, but this shouldn't reduce the possible point angles.
     
  2. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    You have answered nicely yourself to your question. Catamarans with a good rigging and sails with battens plus profiled daggerboards and rudders can point largely as well as a monohull. But it's not always interesting to do so as it's not the fastest way for going upwind from the point A to the point B.
    I mean that for example a good competition 18m2 with top sails and daggerboards can point very close to the wind less than 30 degrees, so often better than most dinghies, but you have to "slow" the cata if you want to point as the apparent wind goes very soon in the nose, and the main sail will stall at the first acceleration.
    It's a Velocity Made Good (VMG) problem, you can point to 30 degrees but it's far faster to point less at 45 or 60 for making the distance A to B, "laisser courir" we say in French (let it run), so you optimize the apparent wind for the capacity of your sails.
    A good catamaran is far faster than even very good monos, so the apparent wind is most of the time in the nose in a regatta. For example the true wind is crosswind, but you're in such an apparent upwind that you're close to stall your sails because you're going well faster than the wind. Often twice the speed of the wind and with a very good 18m2 or a Class C or a high level Tornado for example you can get not far from 3 times the speed of the wind, 2.5 is common.
    The Class C Yellow Pages Edge3 (design Lindsay Cunningham) 1991 was able of more than 12 knots in a 4 knots wind...So it was sailing most of the time with an apparent wind in the nose. This catamaran creates its own wind.
     
  3. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    I used to out point all the monohulls in local club races in a ragged out Tornado.
    Still sailing faster than them, and not pinching to get the last degree of pointing.
    And I was not better than mediocre as a sailor.

    Of course this was an Olympic class, not a heavy crusing barge of a catamaran.

    +1 for everything Ilan said.
     
  4. CT249
    Joined: May 2003
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    CT249 Senior Member

    Is the speed advantage of a cat over a good mono so great that the reason for the lower pointing angles comes down to apparent wind angles? According to the British yardsticks, for example, an F18. Spitfire, Hurricane 5.9 Spi etc is only 7% quicker than a 49er and most of that is probably obtained in stronger winds. In very light conditions even the very efficient cats don't go much faster than a good mono and still point lower when getting their best VMG.

    Surely the size of the centreboard and rudder has an impact? For good reasons, fast cats have fairly small foils and rigs designed for high apparent windspeeds. In light airs it can be hard to open the leach enough while keeping the draft far enough aft. The small foils also seem to need to be driven fast (ie lower) to achieve enough lift. For example, even Classic A Class centreboards would be smaller and shallower, compared to rig size and height, than Laser foils so surely the A CLass will have to sail lower to develop sufficient lift.

    Around our small (400 acre/180 hectare) lake the A Class Classic cats, which are just about the best of the breed, are sailing about 15% slower than their yardstick indicates against the small monos. A lot of the time, when for example short tacking in very light airs when the Lasers can roll tack and not lose speed through tacks, the upwind boatspeed difference between the top Lasers and the top As often doesn't seem to be enough to explain the Laser's pointing advantage.

    Similarly, longboard windsurfers like Raceboards and Windsurfer LTs can foot about faster or about as fast as a Laser in light winds but probably because of their small centreboards they have to point lower. Because the upwind speed of the boards is not much different to the Laser (or not different at all, in the case of the LT)
    the difference in pointing can't put down to apparent wind. Even the Lechner/D2 style windsurfers, which have round boat-shaped bottoms and are extremely fast in light airs, don't quite point with a Laser or similar boat in the light stuff. At the other end of the scale, try pointing with a 2.4 Metre in 7 knots of breeze......
     
  5. Iridian
    Joined: Jan 2020
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    Iridian Junior Member

    I think in light winds there isn't much wave drag, and it's mostly friction drag, in which monohulls and catamarans are pretty well matched, assuming equal weight.
     

  6. CT249
    Joined: May 2003
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    CT249 Senior Member

    Sure, there's not much wave drag.

    Yes, it's mostly friction drag, but as far as I can see that doesn't relate to the fact that the cats (and long windsurfers) seem to point lower than the simple apparent wind/boatspeed relationship would indicate, at least in light winds.

    Another thing that's interesting is that the best cat guy (and he's REALLY good) has since switched to sailing a singlehanded dinghy of about Laser speed at the club. It's got a fully-battened roachy main and despite his skill, when the wind gets very light that big roach seems to stall and cause more drag than the Laser sail. The Laser's very tight leach means that its head must also stall, but perhaps what's happening is that the smaller head of the Laser sail causes less drag, when stalled, than the bigger roach of the fully battened roachy main.

    While we are dealing with very different boats than fast cats here, the point is that while fully-battened roachy sails work beautifully in many conditions, especially the sort of conditions in which we sail championships and major races that really count, in other conditions they can close off and stall even when well handled. For very good reasons linked to their high performance in most winds, most cats have roachy battened mains and flat sails which may stall more and develop less power than the typical deeper mono sail. And for the same very good reasons, fast cats have fairly low-area centreboards which may mean that they can't use deeper sails in light winds without "overloading" the foils.

    A while ago I was on rescue boat duty and watching our best cat sailor go downwind in about 5 knots. His main had 19% draft at the middle and about 20 degrees or more twist, which is a really deep, twisted shape - but his results indicate that it really, really works. Other fast cats do the same thing, and I've never been able to find a good theoretical explanation. I don't think it's just wind shear or anything. This is one of those things that makes one wonder whether the aerodynamic theory we know of, which is so fantastic in many ways, properly accounts for very low apparent windspeeds and high-aspect rigs.

    These are just ideas on a subject that interests me; I'm not claiming to know the truth but I do sail a wide range of craft and so may have an unusual perspective.
     
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