Why a Yawl or Ketch instead of a sloop

Discussion in 'Boat Design' started by saltydog123, Apr 29, 2009.

  1. Paul B

    Paul B Previous Member

    Rick, to set up your foils correctly you should keep in mind the sheeting angles of modern rigs.

    A modern sloop rig with non-overlapping headsails would sheet (upwind) at most 8 or 9 degrees from the CL. The mainsail would sheet with the boom on centerline, until overpowered and the boom drops to depower. Of course high end racing boats (ACC type) sheet at as low as 6 degrees, but let's be fair and use something closer to 8.

    With the main boom sheeted on CL, that doesn't give any mizzen sail much of a chance. It is always "gassed" by the main.

    You mention the interaction of a 3rd sail increases the pointing ability, but in practice ketches never point as well as sloops. This is because the main boom has to be dropped to allow the mizzen to breathe. So the ketch rig is really reaching through life compared to the sloop in an upwind leg.

    All anyone has to do is look back at the change made to the great Windward Passage back in the late 70s. The ketch rig was eliminated in favor of a new masthead sloop rig. The boat was transformed in upwind conditions. It was easier to sail. It was faster. It sailed closer to the wind.

    Not only was the sailplan more efficient, they also removed "the weight of a volkswagen" from the back end of the boat, 20 feet above the deck.

    Even if you take the mizzen down while beating you are still carrying around a huge amount of drag from the 2nd mast and all the rigging. Just two America's Cups ago the Kiwis were taking off their topmast running backs on the beats when not required. They calculated the exact HP loss for the drag of just that one piece of rigging. Just think of the HP loss a split rig sees when dragging around an extra spar/rigging that is not providing any drive.

    The split rig in modern times is mostly an aesthetic choice for people who think this is how boats are supposed to look.
  2. BeauVrolyk
    Joined: Apr 2009
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    BeauVrolyk Sailor


    I played a bit with JavaFoil and I must admit it is interesting to see what the model produces. I have a question.

    What do you do to model the different wind angles and velocities at different altitudes? Typically, there is more velocity up high, also the local wind direction at the bottom of the rig is quite different than at the top. Some of this is caused by the effects of the hull and waves on the local wind, and for the main a lot of it is dependent upon the effect of the jib, particularly in a fractional rig where the bottom of the main is in the draft of the jib and the top part isn't at all.

    Finally, I did try moving the mizzen away from the main and then started to think a lot about the gap between the sails. At the deck, because the main is a triangle, the mizzen is a lot closer to the main than it is at the mast head. That combined with the differences in velocity and direction makes this a really complex thing to model. Just wondering if you'd done any thinking on this?

  3. BeauVrolyk
    Joined: Apr 2009
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    BeauVrolyk Sailor


    Your first point above is why I was thinking of taking the mizzen _mast_ down, not just the sail. You'd still have the weight, but it would be on deck.

    On your second point, about the drag of rigging and the Kiwi's backstays, I have found a couple of sources that indicate that a round line has a drag that is about the same as an aerodynamic shape six times as wide as the diameter of the line. It seems that a line, like a loose backstay or a spinnaker halyard vibrating in the wind, causes substantially more drag. The things that need to come down off of most rigs include:
    1) flag halyards and flags
    2) lazy jacks
    3) topping lifts
    4) spare halyards (I even run a small piece of monofilament when we're not using the spinnaker halyard on a long beat, 1/10th the diameter)
    5) radar antenna
    6) radio antenna
    7) anemometer and wind direction sender
    8) mast head lights
    9) spreader lights
    10) radar reflector
    11) anything else that can be removed without the rig falling down

    This drag stuff is why almost all modern boats have internal halyards to reduce drag, along with various ways of reducing the number of lines and wires aloft. Various skippers I crew for are always surprised when I take all their spare halyards and other stuff down.

    Finally, I remain stunned at the junk that people put on mast heads. Lights, direction indicators, radio antenna, and all sorts of other junk. But, as a skipper I crew for once said: "What the hell is the good of yachting if you can't fly a really big flag?" I suppose some of us go "yachting" and some go "racing" and things are little different for the two groups.

  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Ay ay ;)
  5. Paul B

    Paul B Previous Member

    Yes, that would be better. On the other hand, just sailing with a modern fractional sloop rig would be just as fast or faster off the wind, so why even bother with the second rig at all?

    We are always very vocal with the bow guys after leeward mark roundings to clean up the halyards and topping lift. Sometimes in the drop things can catch behind the spreaders, so you have the line loose in the slot. When you have someone new doing mid bow they say things like, "it's not hurting anything", but you can feel it and it is a lot of windage, drag, and turbulence.

    It's nice to have the windspeed and direction when doing tactics on a big boat. Especially newer boats that sail quickly downhill, the apparent wind is significantly less, so it is nice to know what the true windspeed is for selecting between the heavy 1 or the 2 on the next beat.

    From a tactical point of view, the true wind direction is probably the most important thing to know, so I would keep that.
  6. BeauVrolyk
    Joined: Apr 2009
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    BeauVrolyk Sailor


    I completely agree. But...

    I race an IOD which was built in 1947 to a 1936 design, a lot like a miniature 33' long 12 meter from the '40s, in addition to a lot of modern techie boats. What I find quite refreshing is that the IOD fleet has outlawed all electronic instruments whatsoever. No tactical compass (only an old fashioned magnetic one), no wind point, no knot meter, no anemometer, no GPS, nothing. At first, it took a bit of getting used to. Like you, I had become used to knowing the wind speed, angle, and (particularly important here in San Francisco Bay) the current speed and direction. But, after a year or so it because much more fun sailing. You are forced to use your senses and REALLY feel what the boat is telling you.

    To your point, this is particularly critical during a run when you're trying to sort out what to do on the next weather leg. Jib car placement, mast bend, which side of the course to go to, all sorts of things that are best set up prior to the leeward mark rounding - you simply have to feel it. What's stunning is that most of the sailors in these boats get so they can accurately call the true wind speed within a couple of knots without any instruments at all.

    My point is, while they're is certainly no doubt that one can sail the boat better with instruments, it's not as interesting or as much fun. Stan Honey built an autopilot that included the local wind angel and velocity, the pitch and yaw of the boat from a 3 axis gyro, the compass course, and who knows what else (maybe the pucker factor in the skipper), and it can steer his Cal-40 down wind as good or better than most sailors I've sailed with. But..... is that actually better fun? Of course, Stan used it to set the single handed TransPac record and numerous other records, so clearly it works for short handed sailing and it may make many races "possible" that wouldn't be otherwise. But, wouldn't it be better if we simply eliminated the electronic arms race entirely with respect to performance instruments.

    I find myself sitting on the weather rail during a long beat watching how each helmsperson is performing by reading out the VMG in my Velocitek. When we slow down I start looking at sail tune and at how the helm is being managed. It's certainly helpful. But, on the IOD I know when we're going slow, and I've no instruments at all.

    I think we all may be missing something as we peer into dimly lit instruments to be instructed on what percentage of our target polars we're actually hitting, we're missing something rather important, fun.

    I'd strike the instruments and just sail.

  7. Guest625101138

    Guest625101138 Previous Member

    It takes quite a bit of fiddling to get the orientation correct. If you work with 100% foils then the modify screen is not large enough to display 3 foils.

    It is a very complex model. You can only work on one horizontal slice of the sail at any time. To get more meaningful results you would need about four horizontal sections and work out for this. A thin foil can only work over a very narrow range so ideally the sail twist will match the different angle of apparent wind as you move higher in the sail. This means the lift coefficient is the same but the force vectors are rotating slightly more favourably as you move up the sail.

    When I design propellers I use regressions for the various parameters for a particular foil and segment the blade into 20 sections but this is a single foil.
  8. Guest625101138

    Guest625101138 Previous Member

    With 10% foil camber the maximum L/D is achieved around 8 degrees if you have two identical foils. The maximum lift is around 28 degrees so, depending on how easily the hull is driven will depend on the optimum sheeting angle for VMG. At least this is what JavaFoil produces.

    Of course none of the data I provided allows for the rigging drag and it is substantial.

    Rick W
  9. bntii
    Joined: Jun 2006
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    bntii Senior Member


    I don't race but really enjoy just sailing the boat. By seat of the pants, wit and will.. what have you.

    A bit of yarn on the rig and knowing when the boat is happy & driving well.
  10. Guest625101138

    Guest625101138 Previous Member

    Yes - but the more you learn about the fundamentals the more you think about what is actually going on. May not make the sailing faster but it keeps the mind busy.

    Rick W
  11. Paul B

    Paul B Previous Member

    In a real sense, sailing upwind in a modern fractional sloop rig and efficent hull form and foils, you sheet at say 8 degrees, but you are sailing at maybe 35 - 40 degrees to the true wind (mid 20s in apparent wind angle).

    I have to assume you are saying max lift is at a sheeting angle of 28 degrees? Of course this is not possible on most boats, since they are not wide enough to sheet that far out. So sail shape changes dramatically from the nice upwind foil shape to something a bit fuller and maybe "cupped". I'll bet that is difficult to model in the program.
  12. Guest625101138

    Guest625101138 Previous Member

    The 28 degrees is related to the orientation of the sails. You cannot angle a 10% camber foil with that much angle without losing streamline flow. The attached image shows what I mean. This is near to the maximum lift that I could get from two identical foils having AR of 4.

    The best L/D is achieved with a much narrower slot. So the centreline of the boat is closer to the wind. The actual angle of the foils to the air flow does not change very much.

    Rick W

    Attached Files:

  13. Paul B

    Paul B Previous Member

    I've sailed, raced, and been part of winning efforts in every type of boat from beach cats to maxi sleds, so I have raced with nothing but telltales to the latest electronics. All can be equally "fun".

    I have an electronic compass on my boat, a knotmeter that works intermittently, and no motor. For that type of sportboat sailing it is fine. I would not put wind instruments on it. It doesn't have the sail selection that a big boat had, so it wouldn't be too helpful anyway.

    On the other hand, I would rather not race against a fleet of TP52s and the like without all the aid I can get. When doing tactics and navigation in a fleet like that you can have a bit on at any given time. Believe me, we don't sit and stare at numbers, these are tools and not crutches. With your head constantly out of the boat it is nice to be able to take a quick look and see where things are when you have to make a "now" decision.

    I do agree some people focus on numbers instead of sailing, but the good sailors sail and use the numbers to help back up what they already know. I once did a race on an IOR boat, and every time the helmsman/owner mentioned a number on one of the many readouts he had on deck our headsail trimmer would duct tape over that readout. Best thing for that guy.
  14. Paul B

    Paul B Previous Member

    Understood. I think.

    However, as drawn the "boat" is pointing at 29 degrees to the True Wind as I read you sketch. No one can sail that high, except maybe something like an ACC boat in pinch mode.

    If we assume this to be the Apparent Wind angle then the leech of the forward foil would be much closer to the leading edge of the trailing foil. As drawn the forward sail would be luffing. Both sails actually, unless I am misunderstanding your document.

  15. Guest625101138

    Guest625101138 Previous Member

    JavaFoil is not difficult to use and can give useful data related to sails. I have verified its output against wind tunnel data for a number of thin foils and it is so close I do not bother with the measured data anymore. The money I paid for the Selig volume is a waste.

    Rick W
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