Rudder Question

Discussion in 'Sailboats' started by saildog, Apr 10, 2006.

  1. saildog
    Joined: Dec 2005
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    saildog Junior Member

    A question please, that I need some help on. We know that sailboats can typically want to "round up" or head into the wind as they sail towards towards the direction of the wind (some designs more so than others). The process as you know is referred to as "weather helm". To counter this effect, sail can be reduced, sail form flattened and the helmsmans applies opposite rudder to keep the sailboat on course.

    If a boat has a lot of weather-helm (even after sails and rigging have been appropriately adjusted..i.e. mast angle forward etc...etc..) that requires a great deal of opposite rudder, is it correct to say that an INCREASE in the LENGTH OF THE RUDDER would help this condition...(anotherwords minimize the amount of rudder correction needed???) My theory is that if the rudder is an element that counters weather helm and the deflection of water (thrust) provided by opposite rudder counters this...wouldn't an increase in rudder LENGTH IMPROVE the amount of rudder needed to correct and overabundance of weather helm? I realize that increasing length at some point would have diminishing returns, but wouldn't this help?

    Any help would be appreciated...
     
  2. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    If you increase the DEPTH of the rudder, then it will exert more force to correct weather helm, but probably not enough unless you really do something drastic. Which you may need to if the designer flubbed :)
    If you increase the CHORD of the rudder (length along the rudder at the waterline in simple terms) then you can increase its force as well, but unless you alter the balance you may have to pull harder to get the same force as before. Again, counterproductive, probably
    It is like "curing" your car's smoky exhaust by putting a filter on it. It looks better, but the root cause is still there - in your case the boat is badly balanced. If the keel (or centreboard) and rudder are in proportion to each other, and to the sail area, then there is little point playing with the sizes, you have to move something.
    Do you have any drawings or photos you can post of the offending bathtub?
    Steve
     
  3. gypsiemarine
    Joined: Apr 2006
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    gypsiemarine Gypsie Marine

    gypsiemarine

    First and formost you have to look at the rig and play with it more. If you sail with just headsails and still have a lot of weather helm then you have to look more at the rig. Was she a yawl before? Has the rig been replaced?
    There are some things you can do to the rudder. One is to make sure it has the right hydradynamic qualities, ie proper shape. Of course by lengthening the rudder you apply more lateral resistance and therfore have a more efficient rudder but also you have to have a look at the rudder balance ie "leading edge". If you like I can provide you with a rudder design that will work, send me the length and width of rudder and keel/skeg configeration.
    Kind regards
    Randall
     
  4. saildog
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    saildog Junior Member

    Thanks to all....I greatly appreciate your time to answer these questions. I don't have photos or drawings, but this helps immensely. I'm in the re-design phase of an old homemade boat...thanks again.
     
  5. Windvang
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    Windvang Yacht Designer

    Adding stability usually als helps a lot when curing weather helm. I recently added 40 cm. lead to the keel of a 43' and it completely solved the problem, dramatically increasing upwind performance in the process. If it is a real old shape rudder (low aspect ratio) building a new one can also solve a big part of the problem.
     
  6. tspeer
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    tspeer Senior Member

    I would think that a longer rudder would help. It would shift the center of lateral resistance aft and balance the boat better. Making the rudder deeper will not only make it more efficient, there will be less load on the tiller than if the chord is increased, too.
     
  7. Skippy
    Joined: Nov 2004
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    Skippy Senior Member

    Uh oh! We had a heck of a discussion about this a while ago.

    Saildog, there are a couple issues here. The first one is that, if you let go of the tiller, weather helm won't really depend on the size of the rudder, since it's not doing much of anything.

    When you're underway, you won't need to pull a larger rudder to weather as far, but you'll have to hold it pretty much just as hard if all you do is lengthen it. When SailDesign says a larger rudder exerts a larger force, that's only true with the tiller at the same angle as it was with the smaller rudder, and you will have to bear that heavier force on the tiller. It may be more efficient if your old rudder was undersized, but if it wasn't, the tiller will not be any easier to hold in place. It's not like you get more force out of the rudder with you exerting the same force as before on the tiller.
     
  8. FAST FRED
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    FAST FRED Senior Member

    The weather helm is not only a safty factor it makes the boat more efficent (in moderation) going to windward.

    A boat hard on the wind will generate much of the "lift" from the hull as well as the keel. Both need an angle of attach to develop hydronamic force .

    The angle of the rudder will give the hull about a 5deg side load , and that IS why the boat goes to windward.

    Reducing the "drag" of the rudder by a higher aspect ratio, as it can develop the force steering with less indiced drag.

    Making the rudder longer will only increase surface drag .

    FAST FRED
     
  9. kenJ
    Joined: Jul 2005
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    kenJ Senior Member

    RE: Rudder

    If you increase the length of your rudder, having it deeper than the keel is usually not a good idea. In a grounding situation, the keel is much stronger, want it to hit first. You probably dont' have to increase the cord by much. The original design of the Catalina 34 rudder was too small, rounds up badly in winds/gusts over about 18kts. The factory fix was to add to the trailing edge. The shape is a triangle with a slightly buldging hypotenuse, point at the top, expanding to about 4'' midway finishing at the bottom at about 4.5". Total length was not changed, about 38". Rudder feel is a little heaver, the round up problem is helped, and the boat backs much better.
     

  10. Cliff Pope
    Joined: Nov 2005
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    Cliff Pope Junior Member

    The easiest way to adjust weather helm, assuming the mast position and sail plan is given, is to alter the trim by shifting internal ballast aft.
     
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