Determining rudder torque

Discussion in 'Inboards' started by Landlubber, Jun 25, 2007.

  1. Landlubber
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    Landlubber Senior Member

    Could some kind soul please direct me to a site that has a calculator to determine the torque on a rudder on a motorboat. The boat is 60 feet long with 450hp engines (two), I cannot find anything on the web, but need to know for hydraulic cylinder calculations. Ta, Landlubber
  2. Guest625101138

    Guest625101138 Previous Member

    You can determine the forces on a rudder using something like JavaFoil:
    It is reasonably intuitive. It allows you to specify a foil section and adjust it or you can load your own. You can also set the blade aspect in the 'Options' tab. It will provide Polar Plots for Cl, Cd and Cm. The Cm is based on the foil lift centre being at 0.25 chord. You will find it is often negative. Meaning if you placed the rudder pivot at the 0.25 chord point the helm would be self-centring.

    If you are designing the rudders I have read (somewhere on the net) that a fat section like a NACA0020 is good. This ia a powerful section that will operate at a high angle of attack. I use this section and keep the blade area small to reduce drag in my low power applications.

    Using JavaFoil to determine the coefficients you can then calculate the forces using boat maximum speed with a velocity that allows for the additional flow past the blade with prop slip. However you are likely to find that going astern will set the design case if you have close to a balanced prop. A really conservative approach for astern would be to assume a Cd of 1 over the whole area of the blade with the force at the geometric centre. You need to select the likely maximum speed astern. (A good reason to install stops so it cannot steer beyond about 30 degrees). You could get a more detailed analysis using JavaFoil and a wedge shaped foil. You need to consider lift, drag and moment in this case as they are all significant. However I would question going into this detail unless you have some way of limiting the maximum speed astern.

    This paper discusses the forces involved with a rudder going forward.
    You can see that it is possible to balance the forces so control forces is very low. You like to have the pivot located far enough forward so the helm has the tendecy to centre when going ahead.

    To say again. The most important design condition from a control force perspective is likely to be going astern or backing into something solid. With tiller steering on a decent size yacht it easy to get hauled across the cockpit if you go hard astern.

    Rick W.
  3. Olav
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    Olav naval architect


    the problem is that it's often hard to determine a realistic rudder angle at a given speed from which you could go on and calculate rudder force and torque.

    Therefore the classification societies have developed some semi-empirical formulae to evaluate this. You might want to check out the GL Rules and Guidelines which I prefer to use as they're quite straightforward. In contrast, with the ABS rules one is often forced to jump back and forth to calculate some coefficients needed which quickly becomes very tedious to do.
  4. Landlubber
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    Landlubber Senior Member

    Teleflex hynautic

    I have been on the Teleflex site, it is useless for Hynautic steering cylinders, and every other site I visit does not have any simple calculators. One would expect the manufacturers to have a site that included their majot heavy duty products, but they only seem to care about the recreational boat market, especially the runabout with outboard, I guess that is where the money is!
    Thanks to those aboue that have tried to help, I do not need to purchase GL rules, just need to know what size ram I need for the rudder fitted.
  5. CGN
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    CGN Senior Member

  6. Landlubber
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    Landlubber Senior Member

    Thanks CGN.

    Thanks CGN for that web, it was just what I was looking for.
  7. mbourgeois
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    mbourgeois New Member

    I am also interested in learning how to calculate rudder torques. I am a young engineer (BSEE) new to the marine industry. Are there any Naval Architecture text books that discuss rudder torques?


    Mike Bourgeois
  8. Fanie
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    Fanie Fanie

    I have read somewhere that the max usable rudder angle is 30 deg... seems small though
  9. Guest625101138

    Guest625101138 Previous Member

    If you use a thin section it could even be lower if you are making way. Typically it is less than 30 degrees in reverse.

    A fat section like NACA 0020 makes a powerful rudder that has a high angle of stall. Some designs have a rounded trailing edge to improve operating range in reverse.

    You can use JavaFoil to test the operating range of various sections. It is very instructive to understand how different sections behave.

    Rick W.
  10. Çemberci
    Joined: Apr 2007
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    Çemberci Senior Member

    rudder torgue

    I worked for rudder torgue.three attached files is added.
    OKtay Cemberci

    Attached Files:

  11. mbourgeois
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    mbourgeois New Member


    This will keep me busy for awhile.

    Mike Bourgeois
  12. mydauphin
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    mydauphin Senior Member

    Great thread and to the point....

  13. Ad Hoc
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    Ad Hoc Naval Architect

    To establish the torque needed to turn a rudder it is necessary to find the position on the rudder at which the rudder force acts. That position is the centre of pressure. Also sounds like the boat is high speed from your description. A thick flat plate rudder will suffice, or a simple symmetrical section.
    For a flat plate there is a standard empirical formulae by Joessel, for the proportion of the breadth of the plate that the centre of pressure is abaft of the leading edge:
    0.195 + 0.305 sin A

    For a flat plate center of pressure is roughly 0.3 chord length, as a rule of thumb

    Stall angles are typically up to 35 ~ 45 degrees, that's why 30 degree is used as a limit.
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