Rudder Design

Discussion in 'Boat Design' started by JohnnyC, Mar 3, 2010.

  1. mark775

    mark775 Guest

    A few clamps to prevent copper tubing from rattling around. Some cleaning, lots of piddling stuff. I'm gonna' splash maybe tomorrow. No, not much else done. All this is more time consuming for me than most.

    Joined: Oct 2002
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    FAST FRED Senior Member

    "All this is more time consuming for me than most."

    You are not alone,

    Most folks use the 90/90 Rule.

    90% of the work takes about 90% of the build time.

    The remaining 10% of the work ALSO , takes 90% of your time.

  3. mark775

    mark775 Guest

    Finally launched. Smoother than mag-lev. Gained speed but don't know how much better as I am still hitting against the rev limiter even with more pitch. 15% less rudder with this profile is MORE rudder than flat plate with trailing wedge! Not too much balanced, even at 21%.
  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Great news, Mark. :)
    The fish tail has moved the center of pressure somewhat aft, hence the balance issue. It would be interesting to see the final gain in speed and in maneuvrability when compared to the flat plate version - once you find out the correct prop pitch, of course.
  5. mark775

    mark775 Guest

    Hard to say just how good, daiquiri, but we took it out for a series of tests yesterday and it is nothing short of phenominal - every apect is an improvement. At eight knots, I can turn a diameter not much more than a boat length with no vibration, no stall (my rudder hangs four inches behind the transom edge so I can see what is happening to flow. There is a slight propensity to wander more when not actively steering but still less than similar boats I know. A knot more top speed and, I'm still trying to figure this out, 1.5 knots more at cruise. My guess is that I am getting on a bit of a plane earlier, I was at an inefficient cruise before, and fuel consumption will improve accordingly. I am just going to use this prop for the time being but am looking for a larger one to borrow - This is a revolution - the smoothness, the efficiency, the removability (of the rudder) have exceeded my highest expectations. I was hoping for a quarter knot to a half knot and got three times that. If it's not insufficiently built and falls apart, I will pay for these changes in one year. Thanks for your help and encouragement.
  6. mark775

    mark775 Guest

    Slavi, a friend's rudder - a problem. Aluminum, aluminum shaft. This thing feels bound (difficult to steer) at anything above 14 knts. I calculated 21% balance. I asked him if it was just difficult to one side and no, it's not. I don't know who came up with the shape or why but could this be part of the problem? Could aluminum in a UHMW bearing cause this? It would seem better to attach the bearing to the shaft and run against a SS sleave.

    lee's rudder.jpg
  7. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Hi Mark,

    From time to time some new and fancy rudder forms can be seen around. :)
    I believe the problem here is that balance area is not suitable to the foil shape.

    This rudder appears to follow the same design philosophy which has produced the fish-tail (MacLear Thistle) rudder which you have mentioned in post #3. It adds an additional deflection to the waterflow at the rear portion of the pressure side of the foil. The mechanism is similar to that used by split-type flaps seen (rarely) on some simple airplane wings, so I can use the flap data to explain what happens there.

    A clean, symmetric airfoil has the center of pressure placed somewhere around 25% of the chord (can be somewhat more abaft, due to trailing edge thickness and when the aspect ratio is low). For cambered airfoils it may arrive to 30%. That is the immaginary point on which the lift force acts.
    When you apply a split flap to the foil, the center of pressure moves aft. how much aft, depends on the flap deflection. For example, a split-flap with a length of 40% of the foil chord (which is similar to your case), when deflected 10°-15° will move the center of pressure backwards to 35% of the chord. It means that the lever arm through which the lift force acts will increase by 10% of the chord, and the corresponding steering moment can become up to 4-5 times higher (even up to 10 times higher, if you account both the increase in Cl and the movement of C.P. with flaps!).
    I'm enclosing a pic which illustrates the concept.

    A similar thing happens with your friend's rudder. The main differences are in the aspect ratio (AR = 0.95 in this case), and in the fact that the foil is a flat plate (plus that stuff behind). Since it is a low AR rudder, the center of pressure will be somewhat different (more forward) than the 2D airfoil case. For a flat-plate foil of such small AR it will be inside a range between 15% at small angles of attack and 30% at stall.
    With the addition of that flap-like shape aft, these values will grow and can reasonably (based on previously exposed data for 2D foils) be expected to become something like 25% and 40%, respectively for low and high angles of attack. An analysis with a Vortex Panel Method would give a more accurate result but it is not worth the effort in this case, imho.

    So, reassuming, the center of pressure for this shape is more abaft but the rudder balance was not increased accordingly. Normally, a balance is comprised between 16%-20% for a rudder with the center of pressure at 20%-25% of the chord (depending on AR). In the case of your friend's rudder, I believe it should have been increased to somewhere between 25% and 30%, to compensate for the new shape (if I remember well, D. Gerr wrote about up to 35%-40% compensation necessary for a Thistle rudder, in a PB magazine article).

    That's how the things are now, in my opinion. In order to understand what can be done, a bit more info would be necessary. First of all, is it possible now to increase the balance of this rudder? If not, then I believe it might be possible to modify the leading edge by other means, but this question needs to be answered first: how much clearance is available right now between the rudder and the prop? Also, is it a left-hand or right-hand prop? A photo would be very helpful.


    Attached Files:

    Last edited: Jun 18, 2010
    1 person likes this.
  8. mark775

    mark775 Guest

    Wow, I am going to be a rudder expert by the time you are done with me! I'll try to get a picture tomorrow, thx. - Mark
  9. tunnels

    tunnels Previous Member

    If you hit something it is possible that you could part your rudder sides and loose the blade panels straight down . The steel tangs you have welded to the shaft need to be completely glassed over and to one side of the rudder panel THEN the otherside fitted and glassed together and filled solid :D

    Have made long rudders for racing yachts simular to what you have and during early times they simply parte and the bades went to the bottom and left a nice clean shaft with no blade . So if you have the moulds for each half keep them !!:confused:
  10. mark775

    mark775 Guest

    Tunnels, you're talking about my personal rudder now? This rudder?


    It's these 3/4" x 2" welded tangs that will break?


    It is a 5/8" thick epoxy laminate held by the epoxy core friction fit and four 1/2" bolts tapped into the 2 1/2" post. I designed it to shear the bottom half of the rudder in hard impact leaving the top half and undamaged shaft/hull/steering gear to get home on. I am always open to informed, critical suggestions.


    Weren't the rudders you built like this a tad heavy for racing craft? I have never seen or heard tell of a similar design - perhaps the impending failure is the reason why. Do you have any pics of similar rudders? Knowing how to build a lightweight rudder, it was a struggle for me to make this thing perhaps 100 pounds heavier than it needed to be for the sake of getting home after an impact. I don't think that matters as much with a race boat.
  11. tunnels

    tunnels Previous Member

    My email is send me and i will find the pictures when i find them The shafts were quite long and the tangs were drilled and welded on the otherside . One half was glassed to the shaft really well so if it split one half would still be there to get back home on . Plus the top and bottom Bearings were self alining to counteract The bend in the rudderstock when it would bend under stress of loadings . The system was a french design and all the 8 boats have work absolutely faultlessly since 2007 and the boats get a lot of use since then at a sailing school for the ast 3 years

  12. mark775

    mark775 Guest

    You are not following me. You are talking about a different type of rudder, probably with an aluminum shaft. It's all okay - thanks for your input.
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