Sailboat Daggerboard Attack Angle

Discussion in 'Boat Design' started by BobBill, Jan 13, 2012.

  1. BobBill
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    BobBill Senior Member

    I did a search and there were a couple items on the topic but they did not really answer my question.

    Consider a boat with a daggerboard that drops straight down and is assumed to be placed in optimal position re CE, etc, to be perpendicular to the center or stem line of boat's length...

    Now, what if one made the slot a bit longer, to allow the board to angle a bit when inserted, so the board's bottom would be slanted aft underway?

    Would that change alter anything?

    Would its length need to be increase?

    Would it make a difference on a single, multihull or outrigger?
     
  2. tom28571
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    tom28571 Senior Member

    If the DB is angled aft, the CLR will move aft and you will help balance out weather helm which usually increases in stronger wind. Very common among racing sailors. Centerboards are often handled the same way. I often do this on large centerboard boats when racing in high wind. Some will angle the DB forward in light wind to gain some weather helm. Much depends on how much the boat is heeled, crew weight, how much they like to hike, how full the sails are, etc, etc.

    Because multihulls don't heel much, the situation is different there and balance of CE/CLR does not change as much.
     
  3. DCockey
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    DCockey Senior Member

    For clarification BobBill is asking about changing the sweep angle, not the angle of attack. Sweep is the angle of the board from vertical in side-view. Angle of attack is the angle of the mid-plane of the board relative to the water flow when viewed from the top.

    As Tom responded this can be an effective method for tuning the helm balance. There isn't a single position which is optimum for all conditions and points of sail.

    Sweep angle can also affect the separation characteristics at high angles of attack. This usually only matters if the board is small and heavily loaded, or possibly during tacking. In theory at least sweep will also affect induced drag but the effects would not be significant unless the sweep angle is relatively large.

    A downside is the modified, larger slot will have higher drag, particularly if the unmodified slot is shaped to match the section profile of the board.
     
  4. BobBill
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    BobBill Senior Member

    First of all, thanks. I to tom and DCockey. Am getting idea.

    D...I must infer, re slot opening, that if the trunk (daggerboard) were lengthened at top only, the drag would not change. as the opening along keel line is same. Board rocks forward at top as drag pulls exposed area rearward. Make sense?

    Similarly, it seems, drag on a centerboard opening remains the same, with the board being lifted a bit to increase the aft "sweep as it pivots. That make sense as well?

    Am trying to figure out why on my little boat, the dagger board pivots and if I build a new boat, if the board should be constructed similarly, instead of fixed vertical box/board.
     
  5. DCockey
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    DCockey Senior Member

    Yes, if the trunk was lengthened and the slot only lengthened a slight amount to allow it to rock the drag wouldn't change. Good idea.
     
  6. BobBill
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    BobBill Senior Member

    Thank-you, very helpful. I think a daggerboard trunk could be built that allows for both sweep and perpendicular positions via use of a sort of insert or forward wedge at top or removable part of the board. I like it. Again, much thanks...experimenting is my major flaw.
     
  7. messabout
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    messabout Senior Member

    Bob; A common method for affecting dagger rake is as follows. Let the slot in the bottom of the boat be just a wee tad longer than the chord of the board. Think of the part of the board that is inside the case when the board is full down, Just cut a tapered piece out of the forward part of the board that lies inside the case. The chord at very top of the board is now less than the chord at the bottom exit. All you have done is saw off a small triangle. Now you can rake the board when it is appropriate and have it vertical whenever you choose. Some class boats use this simple ploy to advantage. Windmill class for one. That board detail is on the official plans.

    A rubber wedge, like the ones used as a door stop, will hold the board at your chosen angle. Alternatively, some people line the board case with carpet. The carpet puts some friction on the board so that it will stay where you put it. The carpet stunt also holds the board up at whatever level you choose, such as adjustments for sailing on the run.
     
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  8. BobBill
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    BobBill Senior Member

    Daggerboard Sweep Angle

    Yep. Had the Windmill in mind...thanks, you reinforced what I had in mind. Little bit low and a bit more topside, with wedgie topside to gauge.
     
  9. tom28571
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    tom28571 Senior Member

    I use felt inside the DB trunk to hold the DB in place on Windmills, where ever you may want it. Works better than wedges or shock cords and no loose pieces to keep up with. It also allows the DB to be moved while under load, like on a beat.
     
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  10. Silver Raven
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    Silver Raven Senior Member

    G'day bloke. Your - para 3, sentence 3. In who's theory? After all area = area, same be same? If the angle of attack is the same - please explain why the 'induced drag' is greater - thank-you. Ciao, james
     
  11. Silver Raven
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    Silver Raven Senior Member

    G'day to you. IMHO - of course. If you are sailing fast enough - ie 10 kts & + - especially to windward - then 'tack' the db to windward at the front & you will gain much higher pointing ability. 2 to 4 degrees will do the trick. Many international racing monohulls used this to great advantage before the 'rules' blokes banned it. Banque Populaire uses a db with a trailing edge flap - also to great advantage but that's much more complicated & costly. The ever so slightly bigger exit hole at the bottom of the cb case is a highly over rated resistance that is hard to prove & mostly in the minds of the people with pencil & paper that aren't out there sailing fast. As well as the racking of the db either fore or aft to balance the helm. Ciao, james
     
  12. tom28571
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    tom28571 Senior Member

    Generation of tip vortices would be affected by the angle of sweep of a CB. Tipping a CB/DB forward would reduce tip vortices, or so the aerodynamics folk tell us.

    Jibing boards have been popular from time to time but have not been proven to reduce leeway of the CB. They may cause the hull to point more in the direction of travel thereby causing a small reduction in hull drag. Sail trim may also be affected positively by a minor amount. Flaps and other devices may allow a smaller area CB but drag may be increased, reducing the advantage somewhat. A CB of the same shape, area and swept angle is going to have the same angle of leeway, no matter whether you move the angle of incidence toward windward or not. Neglecting the hull and other effects above the water, a jibing board has zero effect on pointing angle.

    I don't follow your argument on the swept CB angle having no effect on helm balance, which it clearly does.
     
  13. DCockey
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    DCockey Senior Member

    There was a lengthy thread (which I started) somewhat related to this last spring. Basically induced drag, as in the drag due to trailing vorticity, for a given "lift" depends on the span, not the area, and also on the sweep angle. If you increase the sweep angle the span will decrases, though only slightly at first (cosine effect). Induced drag increases with increasing (tip-aft) sweep angle for a constant span which is one of the reasons why low speed aircraft generally don't have swept wings. Note that span measured perpendicular to the flow, not along the board/wing.
     
  14. DCockey
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    DCockey Senior Member

    Is that pointing as measured by the angle between apparent wind direction and centerline of the hull, or as measured by the angle between apparent wind direction and course made good?

    One is considerably easier to determine and what most people mean by pointing. The other is what matters when going to windward. My understanding is changing the angle of attack of the board relative to the hull has a much stronger influence on the first then the second.
     

  15. BobBill
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    BobBill Senior Member

    Daggerboard Sweep Angle

    Gentlemen, wonderful discussion and information. Thanks. I had no idea and it gets more fun. I am just a dodger who likes to tinker with these kinds of sailing tweaks.

    If the DB on a Windmill sweeps aft, say 40 degrees, would it alter helm to point of being dangerous, ie lee? (Using that class as example, following above.)

    What about healing?

    And, I must assume the effects are less on a multihull like a cat or outrigger.
     
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