Skeg To Main Keel Area Ratio ???

Discussion in 'Boat Design' started by viking north, Sep 28, 2011.

  1. viking north
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    viking north VINLAND

    This recent multistar rated possible parallel universe post on the boat forum got me thinking about a little (two) partical quark on my boat project here on earth. :) In a foil keel/foil skeg tandum set up we are dealing with two individual and seperate foils and their associated individual lifts. The main foil generates lift that has effect from say,aft of midship forward giving us good pointing ability in the desired direction to windward. However that little devil of a foil skeg also generates lift but in this case controlling say the last 1/4 of the hull aft. This in effect is acting as a rudder driving the stern to windward offsetting the pointing ability and direction of the bow. To counteract this whole senerio one has to bring into play the rudder creating drag. Thus my question on area ratios between the two.

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  2. jehardiman
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    jehardiman Senior Member

    Area ratio between the keel and skeg/rudder will depend on the directional stability of the hull throughout the range of heel and seperation between keel and skeg/rudder. The ratio between skeg size and moveable rudder area is more a structural consideration than a hydrodynamic one, modern thought generally comming down on the side of more moveable area. Have you read Marchaj's Aero-Hydrodynamics of Sailing? Or looked at the Antelope tests? Both touch on this issue.
     
  3. viking north
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    viking north VINLAND

    Thanks Jehardiman i'm just amazed someone hasn't posted: "Where did you come up with that theory"? I have read Marchaj's but can't recall this being discussed, then again alot of it is above my design education and i may have missed it. I will have another look. The Antelope tests is new info to me but will certainly have a read there. Meanwhile would really appreciate some forum discussion on this aspect.(pun intended) Geo.
     
  4. jehardiman
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    jehardiman Senior Member

    Your thoughts are not new, and you really need to read the first 12 sections of Chapter IX; Controllability in PNA. A forum is not the place to get into the theory of controllability derivatives and the ilk. Sufice to say that the AoA of the rudder is not the same as the keel due to rigid body machanics all wraped up in the placement and L/D ratio of the sail and that is the way it has always been and also the way you want it.
     
  5. viking north
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    viking north VINLAND

    Will do but as I have said on many ocassions prev. alot of higher teck vessel design theory is far above my head and I mostly just want to get a rough idea so as not to make a really big BO BO on this build.I have spent alot of time educating myself on basic foil theory and didn't want it to be all in vain because of an oversize skeg. One important point you posted that I questioned myself on was the skeg to rudder ratio --In my case skeg 1.5 sq. ft verses rudder approx. 4.5 sq. ft. (> 10% sail area) so i should be safe there. The skeg foil to keel foil is in around 1.5 to 8 sq. ft and looking at other designs --making rough measurements I seem to be in the ballpark. Before building my skeg. mold I will do as you recommend re read those sections tonight . Tnx. Geo
     
  6. jehardiman
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    jehardiman Senior Member

    At issue here is that the actual answer revolves around an understanding of the stability derivatives of foil theory. A fixed symetric foil, as well as most hull shapes, are not directionaly stable, and will impart a moment on the free-free center of mass that varies with foil location, speed, yaw, and roll. The rudder is placed and sized so as to counter this moment on the rigid body with as little drag as possible. This can mean anything from a flap on the back of a full keel to no keel and bow and stern rudders. I just pulled out Marchaj and noticed that while he touches on the subject in part C of the introduction, he assumes that the target audience already understands this. Perhaps it is better to go read Hoerner Fluid Dynamic Lift where he discusses the vertical stability of aircraft and horizontal stablizers/elevators.
     

  7. viking north
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    viking north VINLAND

    Far too technical for this old boatbuilder-- guess there's no quick and dirty ratios to work with here :). Since my design is more of a Brewer Bite than a modern skeg and the ratio rudder to skeg area is 3 to 1 so on second thought the only concern I should have had was the rudder to skeg and i'm well in the ball park there so it's on to building the skeg mold tomorrow. It will be based on a 63 series foil with the rudder on the 00 series.I thank you for your time and return posts, sorry i couldn't engage in a more technical exchange but there's a limit to my broadbanding.:D Thanks again G

    A yacht is not defined by the vessel but by the care and love of her owner.
     
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