rudder design

Discussion in 'Sailboats' started by josch, Nov 5, 2006.

  1. josch
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    josch Junior Member

    Hello all,

    I am looking for a report and/or Graph about rudder design and rudder area estimation for sailing yachts.
    It shall be used within a diploma thesis
    I know that there are some articles and good graphs published for instance by "Dalzell" and A.F. Molland in 1978, (both having lectured and/or attended the Southampton Institute`s Yacht and Powercraft Design Course) but they can all not be accessed by internet.
    Does anyone have something reliable perhaps scanned??

    Any link or advise would be helpfull
    Thanx

    josch:confused:
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Dave Gerr just wrote a series of three articles for Professional Boatbuilder magazine on steering and rudders. They appear in the issues: Dec/Jan 2006, Feb/Mar 2006, and April/May 2006. You can see these issues online at www.proboat.com for free. This is probably the most concentrated material on rudders ever written, and it should answer your questions.

    Eric
     
  3. Guillermo
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    Guillermo Ingeniero Naval

  4. JPC
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    JPC Junior Member

  5. josch
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    josch Junior Member

    Thank´s all for the replies,

    I am using XFoil already and have though designed a pretty well section and I found some reasonable information about estimating the rudder area of modern sailboats in the professional boatbuilder link mentioned above as well, but that was more a rule of thumb formulation.

    As far as I know the rudder planform and rudder area has much more effect on the rudder behaviour as the section profile, due to frictional resistance effects and so on.

    So the aim of that post was more to get information about estimating the rudder area and planform. Especially in terms of rudder area I find no good data, graphs or formulation other than rules of thumb, which is not very sophisticated......

    But anyway thanks a lot for the answers
     
  6. RHough
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    RHough Retro Dude

    If you know the force the rudder has to create, it should be very easy to design a foil to create that force.

    For a sailing yacht, with rudder area as the only variable you should be able to resolve the force that the rudder needs to create. There is a yaw moment due to heel from the lateral displacement of the sail effort that creates a moment that acts to turn the bow into the wind. There may be a windward or leeward yaw moment from the hull due to heel. There is a windward or leeward moment created by the longitudinal displacement between the sail effort and lateral resistance.

    The relationship between these forces is not constant. The force from the sails varies as the square of the wind speed with a wide range. For a displacement hull, there is an upper limit to the velocity of the water. You may find that a rudder that is large enough to work at low boat speeds is too large at high boat speeds, thus creating excess drag. You may find that the "perfect" rudder area for 20 knots wind and 8 knots boat speed is too small to work well at 6 knots wind and 2 knots boat speed.

    Or you could use a rule of thumb. :)
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    You can also try two other articles in Professional Boatbuilder in issues 95 (June/July 2005) and 96 (August/Sept 2005) which David Vacanti and I wrote, respectively. The first one is "Keel and Rudder Design" and the second one is "Keels and Rudders: Engineering and Construction". I believe these articles can also be viewed on line, the first issues in the on-line series.

    Principles of Yacht Design by Larsson and Eliasson also has a whole chapter devoted to keels and rudders. Generally, they say that rudder area is typically around 1-2% of sail area. Another guideline is that rudder area should be about 5% of the underwater hull profile. There really are no other guidelines than that, at least for the amount of area. The shape of the area is something else, and the articles and book mentioned above also discuss that.

    Finally, read C.A. Marchaj's book, "Aero-Hydrodynamics of Sailing" where he discusses sail plans and keel shapes. He has a whole section devoted to 3-D shapes for lifting foils. What works for keels generally works for rudders, but without the ballast feature.

    I hope that helps.

    Eric
     
  8. josch
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    josch Junior Member

    thank you very much those articles help indeed ;-))
     
  9. Crag Cay
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    Crag Cay Senior Member

    Now they are beginning to haul the boats for the winter, have a walk around Hamble Point / Mercury / Swannick marinas, etc with a tape measure, camera and note book. A days measuring will get you you loads of data. It's not that much effort to get boat type, draft, keel area, rudder type, area and planform. Rudder section is harder, but with a little 'jig' thickness and position of maximum thickness are both easy. A search on the internet will usually find the other details (displacement, Sail Area) of production boats.

    You will see trends but will also be amazed at the variety, which suggests either 'the' perfect answer has not been found or there are a number of trade offs to be made at the design area.

    You are sitting close to some of the answers you want.
     
  10. Karsten
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    Karsten Senior Member

    I have seen a rudder with almost twice the area. Both were for 42' racing yachts and the yachts were designed to the same class rules just by two different designers. With the large rudder you only need half the rudder angle to produce the same force assuming you also travel at the same speed. Therefore the boat might feel quite twitchy. With the small rudder and the larger rudder angles the rudder will also more act like a brake. The friction will be smaller but if you need a large rudder angle to keep the boat on course the total drag might actually be larger.
    I'm sure you could figure out the optimum quite easily with a spreadsheet but it will depend on the boat speed and course as mentioned before.
     
  11. JEM
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    JEM Senior Member

    To be clear, you mean 5% of the hull wetted surface area?
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    No. The profile area of the rudder should be at least 5% of the profile area of the underwater profile of the hull. That is, these are the projected areas of the rudder and hull as seen in a side-on view.

    Eric
     
  13. JEM
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    JEM Senior Member

    ok that's better. ;)

    Didn't think I was reading that right so I wanted to be sure.
     
  14. teoman
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    teoman Junior Member

    if you use two rudder, would that be 5% + 5% or 2.5% + 2.5% ??

    thanks, just working on mine, but I will have 2 rudder for dual engine...
     

  15. Eric Sponberg
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    Eric Sponberg Senior Member

    For total turning power, you want a total 5% of area. If both rudders are working simultaneously, that would be 2.5% each. If one of the rudders is retractable so that only one rudder is working, as on some sailing yachts, then that's 5% per rudder. Again, these are minimums.

    Eric
     
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