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I Form Drag on Sailboat Hull

  1. Sep 26, 2016 #1
    I have been around Sailboats all my life. For the last 25 years I have built and raced r/c model sailboats.

    My current question, I hope, can be more of a discussion than just a simple question and answer. Since I am into the design of all aspects of boat and sails. I have a question about the hull and form drag. Most tests of this require the hull in a static position with water running around the shape or computer sim. This is fine for a ship or powerboat that have a straight line propulsion system. This cannot be optimal for sail propulsion.

    The Sails provide lift and propulsion in a wide angle of directions. This is the reason you would see a sailboat leaning to one side or the other while making headway. The amount of forward propulsion is exceeded by the amount of side force generated by the sail foil. The boat does not sail in an honest straight forward direction. It slips to the side as well as traveling forward. So there cannot be the same optimal hull shape value or calculation of both.

    It is easy to see the drag effects or flow, of healing moment, in a static position by just rotating the hull over ten, twenty, or thirty degrees. This still does not satisfy my question. I want to be able to design a mono hull that will have a better performance area in relation to side slip or not sailing true to direction. Its like a car rounding a turn and having the tires lose traction and send the car out of position in relation to direction. This would change the effect of airflow around the car to something less than optimal.

    Well, in a sailboat, this is the dominant relationship between direction and actual position of the boat. The water direction and actual hull direction are not in a direct bow to stearn relationship. I hope someone or two can jump in and provide me more insight and clarity. Thanks.
    Last edited: Sep 26, 2016
  2. jcsd
  3. Sep 26, 2016 #2


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    There are a lot of questions raised in the post. So, where do you want to start?

    As for hulls that are efficient when heeled over and proceeding with an angle of attack… various approaches to "beating the rules" have long been tried, such as a shape which lengthens the waterline when the boat is heeled over. A fundamental constraint is that the boat needs to be symmetric if it must be equally able to sail on either tack. For multihulls (see: Hobie) it is possible to have hulls which are optimized for their respective tacks. However, the progression of boat design suggests that high aspect keels/foils provide better lift/drag than asymmetric hulls.
  4. Sep 27, 2016 #3


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    For sail craft you want to maximize the two lift/drag ratios at both interfaces: with the air and with the surface. The are both equally important for maximal speed:
  5. Sep 27, 2016 #4
    Thank you both. @olivermsun, yes, thank you, I am familiar with hull symmetry, length of waterline, etc. I should have stated that its the Angle of Attack of the water flow over the boat.
    Mr. A.T. has provided a very nice link to what I was looking for. I guess this exactly what I was looking for as it provides a math foundation to work from. I am not sure what the exact angle of attack of flow over the hull is at this time. I can experiment to find an flow advantages within the "Course Theorem" "The pointing angle or sailing angle, beta b, is equal to the sum of these two drag angles e(H) and e(A). Thank you so much.
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