Keel : Rudder Length

Discussion in 'Sailboats' started by Alixander Beck, Oct 3, 2005.

  1. Alixander Beck
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    Alixander Beck Junior Member

    What is the reason for the Keel being longer than the rudder on a boat?

    What would happen if the rudder was longer than the keel? would this produce adverse effects?

    Practically speaking I have a dinghy that draws 32" of draft from the centreboard. What would happen if the rudder also had a depth of 32"

    I would think that this would be alright as they would both produce some good CLR and make the boat track well and possible head to windward better.
     
  2. Seafarer24
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    Seafarer24 Sunset Chaser

    Rudder size is determined by how much you need in order to turn the boat. It really doesn't take much, and if you have too big a rudder you're causing undue drag and could have a very twitchy helm.

    Check out the Dotan Marine website. Their Sampson series should have a great rudder for your dingy.
     
  3. Skippy
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    Skippy Senior Member

    The rudder is also weaker than the keel, so you want the keel to be deeper than the rudder just in case you bash into some rocks. That way the rudder is protected and hopefully the keel will survive.
     
  4. yokebutt
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    yokebutt Boatbuilder

    Some boats do, but you get a fairly high aspect ratio, and that can be tricky to reattach flow to if you stall it. Another thing is that most boats have some degree of rocker, wich leads to the top of the keel being deeper than the top of the rudder. Also, as previously said, the keel being deeper helps protect the rudder from unanticipated load conditions.

    Yoke
     
  5. Alixander Beck
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    Alixander Beck Junior Member

    So aside from occasional grounding there is no performance reason to not have a rudder at equal depth with the keel?

    The Dotan Series Rudder 20 is similar to my design but with more area for the same given boat length, So I think mine should work out fine.

    I notice that all the Dotan rudders and to a large degree many rudders are unbalanced.

    What difference in performance could be expected from a balanced rudder over an un-balanced one? with say the leading edge being 1" forward of the pivots.
     
  6. Seafarer24
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    Seafarer24 Sunset Chaser

    The Dotan rudders are not exactly unbalanced. There is an adjustment in the way the rudder pivots so that the blade can be raked further forwards for more balance. You have to tune the rudders to your boat, mast rake, etc.
     
  7. Alixander Beck
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    Alixander Beck Junior Member

    thanks Seafarer,

    I guess you could consider forward rake to be "balanced" .
    I am using the term "balance" to explain the position forward of the pivot of the leading edge of the rudder on a plumb vertical to the transom.

    While the Dotan ones look versatile and adjustable - it would defeat the exercise of building one myself and exacting the beauty of fine woods being used.
     
  8. Doug Lord

    Doug Lord Guest

    Rudders

    CBTF boats use a fwd foil and aft foil of equal size to develop lateral resistance and turn the boat.
    The ARC 21 catamaran uses a concept called "shared lift" where the daggerboard is forward of the cross beam and the rudder is the same size or slightly bigger than the board.
    On most dinghy's the rudder is somewhere near 50% of the area of the board; if you just arbitrarily made the rudder larger w/o also moving the daggerboard forward the boat would probably develop lee helm because, in effect,you have moved the center of lift of the foils(sometimes called CLR-center of lateral resistance) aft keeping the CE(center of effort of the sails ) in the same place.
    Having 15-19% of the rudder area forward of the pivot point simply reduces the load you feel on the end of the tiller-while still allowing you to "feel" weather helm(or lee helm!); raking the rudder blade fore and aft allows an adjustable amount of area for doing the exact same thing.
     
  9. Alixander Beck
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    Alixander Beck Junior Member

    I guess there is no mathematical way to calculate whether the new rudder will produce lee helm without actually getting it on the water?

    while both foils will pull equal draft. the rudder will be ~60% area of the Board with ~190 insq and ~320 insq respectively.

    Having used roughly the same area as the original rudder but in a higher aspect design I hope that there will be a slight decrease in weather helm, but that more can be added if neccessary by raking the rig.

    I will post a picture of the pre-epoxied rudder if anyone is interested.
     
  10. yokebutt
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    yokebutt Boatbuilder

    Alixander,

    No, you won't get any lee helm from merely inreasing the area of the rudder, the balance of forces doesn't change. The only real downside of a larger rudder (within reason) is skin friction.

    Yoke.
     
  11. Doug Lord

    Doug Lord Guest

    rudder area/ balance

    Bad advice, Yoke. If you increase the rudder area on a previously well balanced boat you are moving the effective CLR of the boat aft in relationship to the sails. You will- at the very least- substantially decrease weather helm and could ,in fact, create lee helm. To be damn sure of my 45 plus years of experience I double checked by calling Eric Sponberg, a friend and a naval architect. He concurred with my previous advice.(Thanks Eric!)
    You can visualize this by considering the balance of the Arc 21 previously described: by manipulation of the areas of the rudder foil and daggerboard Bill Roberts was able to place the daggerboard in a position that with a "normal" rudder would have caused unacceptably high weather helm. His concept proves rather emphatically that changing rudder AREA can have a profound effect on BALANCE. By visualizing a CBTF boat with the twin equal size foils it may also make it clearer: if you keep those two foils the same distance apart and move them forward relative to the rig the boat will have excessive weather helm; if you move them aft the boat will have lee helm. The point is that the "Center" of lateral resistance lies between the two foils.
    Another way to look at it is to visualize a"normal" well balanced(I pressume) boat such as yours with the rudder held at zero degrees(parallel to the cl of the boat). Assume that the daggerboard is twice the size of the rudder. With the rudder held on the cl upwind the boat should want to turn into the wind and would begin to do so slowly. If you then doubled the rudder area the tendency of the boat to turn into the wind would be significantly reduced and could result in neutral or lee helm. That is all true because we know the boat is developing leeway and so it is obvious(I hope) that with the rudder held on the centerline both the daggerboard and rudder are developing lateral resistance proportional to their areas. If the weather helm was correct BEFORE the area change it will be less positive with the area change even ,possibly ,to the point of being neutral or lee helm.
    If you not only change the area but change the efficiency of the area(aspect ratio) from what you previously had the effect could be magnified.
     
  12. yokebutt
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    yokebutt Boatbuilder

    Okay, thought experiment here. Sail along in a boat, sails trimmed to give zero weather-helm. Now, increase area of rudder, do we get weather-helm all of a sudden with that?

    I'd argue that the boat doesn't know how big the rudder is, but then again, few boats have 45 years of experience.

    Yoke.
     
  13. Skippy
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    Skippy Senior Member

    Lorsail: Bad advice, Yoke. ... my 45 plus years of experience ... Eric Sponberg, a friend and a naval architect. ....(Thanks Eric!)

    That's wonderful Doug.

    Lorsail: You can visualize this by considering the balance of the Arc 21 previously described: by manipulation of the areas of the rudder foil and daggerboard Bill Roberts was able to place the daggerboard in a position that with a "normal" rudder would have caused unacceptably high weather helm. His concept proves rather emphatically that changing rudder AREA can have a profound effect on BALANCE."

    Doug, that proves nothing without a description and explanation. It sounds like the larger rudder made it easier to compensate for a greater weather helm. That would be profoundly unremarkable.

    By visualizing a CBTF boat with the twin equal size foils it may also make it clearer: if you keep those two foils the same distance apart and move them forward relative to the rig the boat will have excessive weather helm; if you move them aft the boat will have lee helm. The point is that the "Center" of lateral resistance lies between the two foils.

    By visualizing the tiller or wheel left unattended, one can quickly determine that freely rotating foils have no effect on helm balance at all.

    Another way to look at it is to visualize a"normal" well balanced(I pressume) boat such as yours with the rudder held at zero degrees(parallel to the cl of the boat).

    If you're holding the rudder in any position, even straight ahead, I would not call that the boat's natural tendency. The helm balance is what the boat does when the rudder is free.

    I seem to recall people including half the rudder area in finding the CLR. This must be a compromise between the boat's natural behavior unattended and its response to the tiller being held to weather under normal sailing conditions. Sounds reasonable from a practical point of view. Doug, why don't you ask Eric what he thinks about that?
     
  14. Alixander Beck
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    Alixander Beck Junior Member

    Lorsail, good explanation.

    In this particular case it is not that I am increasing the area a great deal from the original rudder. The original rudder was a disgusting piece of iron bolted to a rough stock with a tiller. There was absolutely no shape to the rudder what-so-ever, just a flat board of iron.

    I have taken the original area of 160 in2 up to 192 in2 roughly adding 17% more area and put it on a higher aspect balanced (16%) design. My original questions regarding helm force and balanced were answered and the previous discussions have explained a lot (albeit, a lot that I already know) but good explanations regardless.

    I would have to agree with Lorsail on what Skippy has just questioned him on:

    I would argue that holding the rudder at 0 degree is the same as a "free" rudder. either way you slice it, the boat will reveal its helm balance with the new rudder if the foil is held at CL.

    incidentally I think I will be ok with the helm balance as the old rudder could produce a lot of weather and pull at speed and frequently went totally dead when surfing down waves. (I am not 100% sure why still)

    but if there are any educated opinions as to why......
     

  15. Doug Lord

    Doug Lord Guest

    rudder area

    Yoke,in your thought experiment if you suddenly increased the rudder area you would definitely not increase weather helm but you might create lee helm since you are moving the area of the underwater lift AFT as compared to the CE(center of lift) of the sails.You set up the conditions as the boat having zero weather helm. So the rudder is on the centerline. Now we know that the boat is developing lateral resistance and it should be clear that both the rudder and daggerboard would be contributing lateral resistance roughly in proportion to their areas,right? Then if you all of a sudden double the rudder area the boat would have to "know" that it had more rudder area -45years of experience or not, right?
    -------
    Skippy,regarding the Arc 21 I thought I had given a description but I'll try to be clearer. Roberts moved the daggerboard forward and, as compared with "normal" cats, both decreased the daggerboard size and increased the rudder size. As best I can tell he did this to improve the boats' handling to weather particularly tacking since with the "shared lift" system the boat now has a longer moment arm between the foils.Both foils develop lateral resistance("shared lift") and because the rudder is larger and further away from the "new" daggerboard the turning force is also bound to be improved as compared to a "normal" set up.
    The illustration of the boat with the rudder held on the centerline was an attempt to illustrate the fact that both foils develop lateral resistance and that substantially changing the rudder area of an already well balanced boat would be very likely to change the balance of the boat.
    Many designers, including Eric, consider the whole rudder area in looking at both lateral resistance and balance particularly in small light boats where the hull plays less of a role in both.
    PS-Yoke and Skippy- I apologize for sounding like a boorish ******* but I was trying to get Alixander the most accurate information possible....
     
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