Daggerboard Rake

Discussion in 'Hydrodynamics and Aerodynamics' started by Cleffyboy, Jan 21, 2012.

  1. Cleffyboy
    Joined: Jan 2012
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    Cleffyboy Junior Member

    Hi all, quick question from a non yacht designer or physicist!

    If in both situations I have a well balanced helm, is it preferable to have daggerboards raked back (as many in my class seem to do) and thereby increase effective centreboard chord length and decrease depth, or to have the daggerboard more upright, effectively narrower and deeper? I understand that increasing rake increases wetter area, and decreasing rake and increasing depth is better for lift? Most people in the class seem to rake in order to adjust helm balance, but I think it may be better to simply shift the centreboards back as a way of achieving helm balance, and at the same time maximize their depth.

    Am I on the money here?

    I sail a paper tiger - a 13ft catamaran, centreboards say 250mm wide, 600mm below bottom of hulls.

    Check out www.papertigercatamaran.org if interested...

    PS don't mind if you all get all technical on me, I've enjoyed some of the earlier posts here :)


    Regards, Greg.
     
    1 person likes this.
  2. Doug Lord
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    Doug Lord Flight Ready

  3. Cleffyboy
    Joined: Jan 2012
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    Cleffyboy Junior Member

    Thanks for redirecting me Doug.

    So I think I understand that a more upright centreboard has a deeper chord depth in relation to its length, so therefore has improved left, and a raked back centreboard has a shallower chord depth in relation to its length and therefore creates more drag.

    Sounds to me though that it would be a relatively minor difference in performance either way.

    Thanks for your help Doug.
     
  4. Petros
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    Petros Senior Member

    The higher the aspect ratio of the surface, the better the performance (best lift to drag ratio). Sweep reduces the aspect ratio, so sweep is bad. It just looks cool, but no reason to have it.
     
  5. Cleffyboy
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    Cleffyboy Junior Member

    Thanks Petros, you said what I tried to, but you did it so much better!

    Second question, when I'm setting up for a degree of weather helm, i understand the correction of which results in steering to leeward a little bit thereby changing the rudder angle of attack by x degrees - is there a desired AoA that optimizes lift for the rudder?

    Thanks!
     
  6. Silver Raven
    Joined: Oct 2011
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    Silver Raven Senior Member

    Rudder & C/B angles, shapes etc etc.

    'Cleffy - Possibly - there's "a" (being sever factors) AoA that optimizes lift for the rudder - however there is a heck of a lot more to the whole subject than you are seeming to understand.

    PT's are 'huge' in K1-land. I'm sure there are 100's of people - leading the chase - to go better - than they currently are. Doug & Gary are great starting points & Gary is 'just up the road from you' - Why not start there? I could make a lot of comments about - size vs shape ve section vs angle's (both of them) - however I think there are smarter minds in here than I'll ever be - so keep trying them & I may 'chip-in' with a little throw-away comment now & then. I'm sure you could improve on Ray's original design - this far down the track if the rules will let you do it. Ciao, james - from just across the 'puddle'. PS You might try the same question in 'crew.org'nz' but be careful there - there are many 'very-smart' people in there & they get to be a 'bit-much' a lot of the time. You figure it out. jj
     
  7. Petros
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    Petros Senior Member

    There is an optimum AoA for max Lift to drag ratio (L/D), but that is a complex issue. On a drag polar of CL vs. Cd of the foil shape, the best L/D occurs at the tangent point of a line from the graph origin (zero, zero point). The problem in a boat is that the amount of lateral force necessary (CL on the rudder) would vary under various sailing conditions and points of sail.

    On an aircraft the best L/D is designed in based on design payload, cruise altitude, cruise airspeed, etc. So the foil shape and wing size (wing loading) are chosen based on these design conditions for a range dominated design (like a passage or freighter aircraft).

    So I guess it would be possible to choose a rudder AoA to achieve the best drag performance, but that means you would have to choose a hull speed, airspeed, wind heading and condition of trim to optimize it around. Does not seem very practical. You could have some flexibility in these factors if you make the rudder variable area (slip it up and down in a slot) so you will always have the same CL on the rudder. this would allow you variable rudder side forces (for various sailing and wind conditions) while still maintaining the optimum CL at the best L/D.
     
  8. Cleffyboy
    Joined: Jan 2012
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    Cleffyboy Junior Member

    Cheers mate from oz, I won't pretend I understand the whole deal, just a little bit at a time is ok with me. The plan is to try a few things, spend some time on the water, try some other things, more time on the water etc etc.

    I just started thinking about this after raking my mast back somewhat, then thinking about the effect of balance on the rake, then wondering why a number of other PT's having a reasonable aft rake on the centreboards, when by shifting the CB's back you can presumably achieve the same effect on helm balance while have deeper CB's.

    Who knows I may destroy the whole damn setup, then its back to square one!

    Thanks Folks
     

  9. Silver Raven
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    Silver Raven Senior Member

    G'day 'puddle-jumper' (might make great name for a cat).

    I'm sure if you go looking for the right blokes - you'll find that there's more balance to be gained by using the best foil sections for rudders & c/b's than most people could possibly imagine. Bit like smarter cut sails beat the hell out of bigger fuller, fatter sails. See if you can get Gary to make a comment. Try PM'ing him direct. He really truely is in the know at the leading edge of all this stuff. Ciao, older 'puddle-jumper' - james
     
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