Hull Shape as Keel

Discussion in 'Multihulls' started by Fanie, May 27, 2008.

  1. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 303, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    This is not correct. You don't control the lift on a daggerboard by changing how the board is mounted in the hull. You control the lift on a daggerboard by trimming the sails.

    Pointing the daggerboard inward does not steer the boat to windward. Pointing the daggerboard inward points the bow to leeward by the same amount. It's the daggerboard that stays fixed, and the boat that rotates.

    Let's say it takes 5 degrees of angle of attack on the board to produce enough lift to counter the sideforce from the sails, and the lift from the round-bottomed hull is insignificant. If the board produces any more lift than required to counter the sails, the boat will be turning to windward. If the boat produces any less lift, the course through the water will be curving to leeward. The skipper steers the boat so as to ensure this is so.

    If you have the boards mounted at zero degrees, the hull will be aligned with the boards and the leeway will be 5 deg. If you mount the board toed in at 5 degrees, the board still needs 5 degrees of angle of attack to produce the required lift. The board will be angled at 5 degrees to the direction of travel through the water. But in this case, the hull will be aligned with the direction of travel and the leeway angle will be zero. But the heading will be different.

    If the boat goes to windward at a 45 degree angle (Vmg is 70% of boat speed), then boat with toed-in boards will tack through 90 degrees of heading change. But the boat with straight boards will tack through 80 degrees of heading change. To sailors on another boat, it will look like the boat with straight boards is pointing higher, but slipping to leeward. They will see the boat with toed in boards as tracking straight, but sailing freer. However, provided there's no difference in drag, they will not see either boat gaining on the other.

    The issue of whether to mount the boards straight or toed in, or whether to use symmetrical sections vs cambered sections, is all about drag, not lift. The boat's course through the water depends on the lift/drag ratio of everything above the waterline, and the lift/drag ratio of everything below the waterline. It does not matter a bit which way the hull points, except in so far as it affects the drag.

    If generating a small amount of lift with the hull effectively extends the span of the board, thereby reducing induced drag, then it may be effective if it doesn't generate too much additional form drag on the hull. If the form drag penalty is too high, then it would be better to toe in the boards to reduce the leeway angle on the hull. But the point is to reduce hull drag, not to "lift the boat to windward" - the board will already be doing that, regardless of how it's mounted.

    When considering two boards vs one, if they are parallel, then the lift will be divided roughly equally between them. This will reduce the induced drag because the wake from one board is farther away from the other board and doesn't have as big an impact. If one board could produce the required lift with a 5 degree angle of attack, two boards will produce the same total lift with a 2.5 degree angle of attack. The leeway angle will be halved to 2.5 degrees, but that just means the bow will be pointed down by 2.5 degrees compared to the single board case.

    Toe-in shifts the load from the windward board to the leeward board. If we take the example before, where a 5 degree angle of attack was required to produce the necessary lift from a board, then if the boards are toed in at 2.5 degrees, the boat will be operating at a 2.5 degree leeway angle, the leeward board will be operating at a 5 degree angle of attack, and the windward board will be operating at zero degrees angle of attack. The boat still has 2.5 degrees of leeway, so the heading is unchanged from the parallel board case. But the lift has been shifted from the windward board to the leeward board, and you might as well pull up the windward board because it's not doing anything for you at the moment. That will reduce the drag by reducing wetted area, not by changing the drag due to lift.

    To summarize:
    - You don't get more lift by changing the angle a board is mounted in the hull.
    - Instead, you change the angle at which the hull goes through the water.
    - Two boards down and parallel will be more efficient if the sails are trimmed in tight and the boat speed is slow.
    - One board down and toed in will be more efficient if it allows you to reduce the wetted area by retracting the windward board, especially when sailing fast.
    - Two boards down and toed in doesn't necessarily have much of an advantage over two boards down and parallel
     
  2. Bryan 44c
    Joined: Apr 2008
    Posts: 5
    Likes: 1, Points: 0, Legacy Rep: 12
    Location: Brisbane

    Bryan 44c Junior Member

    I like the kiss principle.
    If it is a cruising cat with performance your after, put one dagger board in one hull. The boat will still go to windward very well on either tack ( you shouldn't be flying hulls in a cruising boat), you won't have to worry about pulling one board up, and putting the other down and you will have a lot more room in the one hull to place the galley.

    If you break the board, you will discover that most cats will still go to windward without a board but with greatly increased slip. Mine slips between 8 and 15 degrees extra without the board, depending on sea state, i know this as I had to sail 500nm to windward last year to get home.

    This happened on my way home form a race (I got 3rd), and it was a break from building my new 45' cat. I had already built the hulls and joined them, had built two dagger boards (symetrical). On return home, the experience racing and with the one broken board changed my thinking and I have know built and fitted only one case to the starboard hull for the new boat..


    The windward performance with a board has my 38' cat performance around 10% faster then most 45' cruiser/racer mono's in my club on vmg.
    Interestingly with out a board in strong winds and poor sea state, i still caught and passed two 45' cruising mono's after 18 hours sailing, that had left an hour before me.

    Food for though.

    Bryan
     
    1 person likes this.
  3. Fanie
    Joined: Oct 2007
    Posts: 4,604
    Likes: 177, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    I'm speechless.

    I need some time figuring out when the board attacks the angle and when the angle attacks the board :D

    Let me get my mind around this.

    I'll be back.
     
    1 person likes this.
  4. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 44, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    Tom.

    Thank you for that very clear and precise evaluation of this subject. :D
     
  5. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 303, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Here's something from your own experience that will help understand why board design does not determine the lift, but sail trim does.

    Imagine you're sailing to windward. What's the lift on the board? It's high.
    Imagine you're sailing dead downwind. What's the lift on the board? Zero.
    Imagine you're sailing to windward, and you luff all the sails. What's the lift on the board? Very low - only enough to oppose the sideways windage.

    Same board design, different values of lift. What's the difference? The forces on the sail.

    Would things have changed if you'd had a different board? No. The purpose of the board is to oppose the sideforce from the sails, so it's the rig that dictates the load on the board. We all know this, but it's easy to lose sight of it with all the talk about hydrodynamic phenomena.

    All of this assumes that the board is operating in its normal range - what we call the linear lift range. Where design of the board with regard to lift comes into it is when you start stalling the board, so it can no longer do its job of opposing the sail forces. Then the leeway angle increases dramatically and drag (the force component parallel to the direction of travel) gets recruited to help oppose the force from the sail because now the boat is traveling sideways.

    The side load necessary to stall the board depends overwhelmingly on the area of the board and to a lesser degree on the choice of section for the board. Technique comes into it, too, because if you keep the speed up the angle of attack required to produce the lift is less and you avoid stall. This is where two boards can help. You can have both boards down for extra area and more stall resistance when tacking, then bring up the windward board for less drag on the beat.

    Similarly, you can make the leeway as low as you want by adding more area to the board(s). But that adds drag due to excessive wetted area. From a designer's standpoint, more leeway is good - up to a point - because that means the board area is being reduced (achieved by reducing the chord) and the wetted area is going down. But this process is limited by stalling the board, which increases the drag far more than adding wetted area would do.

    So board design does matter, but you're working on the aspects of secondary importance to the hydrodynamics and practical matters that have little to do with hydrodynamics. The fundamental aspects are to give it enough area to avoid stalling, and make it as deep as possible to minimize drag due to lift.
     
  6. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 44, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    Which is where my rule of thumb comes in for board design.

    3:1 aspect ratio and 2% of the projected sail area. Some racers use 6:1 AR or even more, but this puts extra stress on the boards and cases which IMHO is not for amateur builders.

    I also like a NACA 0008 section for the board and 0009 for the rudder on high performance multis wIth efficient rigs and perhaps 0009 for the boards and 0012 for the rudders on cruising boats, as these have a more forgiving L/D ratio curve.

    Incidently what Tom says brings up the subject of tiller/wheel handling.
    Just slamming the rudder over to tack causes immense drag just when you don't need it. Easing the rudder on so that the changing angle of the water flow as the boat starts to turn hopefully keeps the L/D ratio in that linear portion of the drag curve. Experience by the helmsman in relation to his own boat is very important here.
     
  7. bobg3723
    Joined: Aug 2005
    Posts: 278
    Likes: 7, Points: 18, Legacy Rep: 95
    Location: Crystal, MN - USA

    bobg3723 Senior Member

    Good topic discussion here!. :)
    Much appreciated from everybody. Please keep the questions and answers coming.

    Here's a dramatic picture I found of exactly what Tom described in the above quotes.

    Enjoy,
    BobG
     

    Attached Files:

  8. bobg3723
    Joined: Aug 2005
    Posts: 278
    Likes: 7, Points: 18, Legacy Rep: 95
    Location: Crystal, MN - USA

    bobg3723 Senior Member

    Hi Brian,
    Please tell us, if you would, what displacement your 38' cat is at LWL, LWL length, hull beam and hull to hull c/l beam, rig type, sails carried and sail areas, skeg area and lead percentage.

    Much appreciated,
    BobG
     
  9. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 44, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    Bob.

    It's interesting that the Cat in that photo has hulls like seaplane floats.

    Fine in flat water and planing conditions, but must be very draggy in light airs don't you think.?
     
  10. bobg3723
    Joined: Aug 2005
    Posts: 278
    Likes: 7, Points: 18, Legacy Rep: 95
    Location: Crystal, MN - USA

    bobg3723 Senior Member

    Ahoy OldSailor7
    Actually, its not just the seaplane floats hull "step" that are unusual. The bows chines are "fluted"! The builders did tank testing in France and this "hydroplaning" hull was the result. With all the immersed wetted area in slow winds, one would expect it to be draggy, I would think.

    I think the thing only knows how to go fast.:cool:

    As the personal injury attorney sayz in his commerical..."I only do one thing, but I do it well". :D
     
  11. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 44, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    Incidently I measured off that daggerboard in the raised position in the pic---and it is about 6:1 aspect ratio. High Efficiency. :D
     
  12. Meanz Beanz
    Joined: Jun 2007
    Posts: 2,280
    Likes: 33, Points: 0, Legacy Rep: 585
    Location: Lower East ?

    Meanz Beanz Boom Doom Gloom Boom

    Sorry about the delay, preoccupied at the moment.

    [​IMG]

    That help?

    I'd say vertical is probably much better...
     
  13. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 44, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    Yes MB. That Seawind 24 design is late 70s early 80s technology.

    At that time the designer was much more concerned about capsizing than they are today. The very angled boards on the SW 24 were a compromise between leeway reduction and lift. The Seawinds answer to pitchpoling was very full bows and a wide beam for its time. It was a very good boat in its day, but heavy and too slow for competitive racing now.

    I sailed one extensively and loved it, despite it being a wet boat in a chop.
     
  14. Meanz Beanz
    Joined: Jun 2007
    Posts: 2,280
    Likes: 33, Points: 0, Legacy Rep: 585
    Location: Lower East ?

    Meanz Beanz Boom Doom Gloom Boom

    Just showing Fannie what my boards look like, in reference to a comment I made in an earlier post.

    The boat weighs about 900kg light ship, similar to Mr Woods Sango but in effect smaller accommodation, I suspect that the Sango is faster. Yes the bows are full but full at height, the flare in the bows is what directs the spray far and wide, sometimes shes OK but mostly she is wet. You probably could not design a better shape to throw spray IMO.

    Chris Williams designed the center boards so that they didn't impinge on cabin space and claimed that they helped stability with the windward board "digging in" and the leeward board "sliding away" to quote him... this I suspect is a complete nonsense, Chris was more an experienced enthusiast than a qualified designer from what I read (none the less he did pretty well). I know from my windsurfing days that as soon as you take a board off vertical at speed it generates lift. A windsurfer could get uncontrollable at speed as the big center boards literally attempted to plane and flip the board over, hence the introduction of storm center boards. I suspect that the only saving grace of the idea is that the boards generate equal lift on both sides so the effect is most probably largely neutral. One thing it does achieve is the boards lock very firmly into place when the boat is moving.

    They are fun I must agree, hell of a big bang for the buck and the handling short comings can be dealt with easily with simple things like raking the rig and adding a dolphin striker. I'm going to experiment with spray rails in the vain hope that they will achieve something, jury is out on that one. Anyway a fun boat to bash around in that cost about the same as a new Hobie 16 and still has a good turn of speed :D
     

  15. Fanie
    Joined: Oct 2007
    Posts: 4,604
    Likes: 177, Points: 63, Legacy Rep: 2484
    Location: Colonial "Sick Africa"

    Fanie Fanie

    Blimey, I should never have gotten involved with you guys :D Now I have to start calculating things...

    The sail I planned (nothing fixed in stone yet) is drawed at 50m^2
    If the daggerboard is 2 % of that then it is a 1m^2 board, 3:1 aspect ratio and and I'll have a board 3m long and only 330mm wide...

    Did I understand this right ?
     
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.