Spade rudder design for 40 foot sailboat

Discussion in 'Hydrodynamics and Aerodynamics' started by DennisRB, Dec 15, 2013.

  1. DennisRB
    Joined: Sep 2004
    Posts: 1,265
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    Location: Brisbane

    DennisRB Senior Member

    Hello guys. I have already made a thread regarding the construction of my new rudder here:


    We have discused some hydrodynamics but I thought I would make a thread on it in this section to talk about shapes specifically.

    The rudder is draft limited as the boat has a shallow draft keel. This fact also makes the rudder a very important lifting surface as its area is about the same as the keel! So upwind performance is not as good a deep keel boat, and I wish to make sure the new rudder has the best possible planform and foil sections to help in this area.

    I plan on getting say 10 plywood foil sections laser cut which I will join up to the armature then add foam between using the plywood formers as a accurate guide for fairing. So I should be able to get an accurate shape. The foil needs to be around 14% due to a thick hollow rudder stock.

    I will probably go for a planform that looks something like this as suggested by PAR.


    I am interested in using a more modern foil section. I have read many posts on the subject and have seen a few posts suggesting the 63 series is a poor choice and that there are many more modern sections such as eppler or wortman sections. I would like to learn more about these foils and how to best select them for my purpouse. I have downloaded profili which has a xfoil plugin and am trying to learn to use the predictions etc.

    See the attached pics. It seems the original foil is a 63 series, which appears to not really be the best section to use here? The middle pic is an outline of the rudder section at the top, I believe it is the same all the way down. The other drawing is the original rudder.


    Attached Files:

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