Interaction between hull and transom-hung rudders

Discussion in 'Hydrodynamics and Aerodynamics' started by erdben, Nov 27, 2016.

  1. erdben
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    erdben Junior Member

    Some transom-hung rudders have a lip under the hull to get the rudder balanced. My question is: besides balancing the rudder, is there any additional beneficial effect from having a small portion of the rudder blade under the hull in close contact?

    On my boat, the lip is about 10% of the cord. I guess the contact with the hull could help to prevent ventilation, but would it really make any difference on such a short portion with all the turbulence there and maybe even some air mixed in when the boat heels.

    The reason I'm asking is that I'm planning to replace my kick-up rudder with a cassette-type rudder. I'd balance the rudder using a forward sweep, but I would need to omit the lip under the hull. So the question is, would there be any difference in performance given everything else (area, foil section) stays the same? BTW the boat is a Santana 2023R.

    Thanks!
     
  2. gonzo
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    gonzo Senior Member

    The main difference you will see is increased effort to steer.
     
  3. erdben
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    erdben Junior Member

    If the rudder is not balanced. But I plan to balance the rudder with a forward sweep, so that about the same area of the rudder would be in front of the axis of rotation.

    The question is whether the flow around the rudder would be affected in any way if the lip and the close contact with the hull is gone. In other words, would the rudder be more prone to ventilation or would the effective aspect ratio change.
     
  4. mydauphin
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    mydauphin Senior Member

    Can you post some pics?
     
  5. PAR
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    PAR Yacht Designer/Builder

    Your boat is performance oriented, so you might notice a very slight decrease in rudder effectiveness, but generally transom hung rudders, as yous currently is, doesn't offer a lot in terms of the slight overlap in the balanced portion. To improve this, you might consider a fence at the top of the blade to keep pressure bleed off reduced.

    Simply put, your forward canting rudder will still feel similarly, given similar area in the balance, but the fully vented top portion of the blade will detract a slight amount of blade efficiency, without the hull acting as a fence. Speed wise, I doubt you'll notice any, that can't also be explained with sail or boat trim. Frankly this would be a tough modification to justify, particularly with the forward cant. I'd just put a fence on the cassette and let the blade fall where it does.
     
  6. Steve W
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    Steve W Senior Member

    You will never notice a difference performance wise. Forward canting the rudder blade is a fairly common way of achieving balance on a cassette rudder. As Par indicated, you could add a fence to the bottom of the cassette and I have seen this done although again, you would never be able to know if it actually made a difference it would be so small.

    Steve.
     
  7. erdben
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    erdben Junior Member

    Thanks for all your comments.

    Here are some pics. First the side view of the boat, then 3 versions of the rudder.

    On the left is what I have now. Rudder is between two aluminum cheek plates held together by a bunch of SS bolts. It's big, clumsy, yet, as it turned out, not strong enough. I had a big broach under spinnaker this summer and the cheek plates got bent and the 3/8 in SS bolt holding the blade broke. So now I either make this even bigger/clumsier or build a new rudder that's stronger and looks nicer. Another problem with the current rudder is that it's useless in shallow water, since it's a pretty deep rudder, and when it's partially raised, the forces are enormous. It would be nice to have some steering when getting to the ramp for example.

    So, the middle drawing is the usual cassette type with the blade raked forward for balance, and the one on the right is a hybrid, where the cassette is open to the back. This type (example on the last pic) would maintain the ability to release the blade if it hits something, and could even accommodate a rudder blade that has the lip that could slip under the hull when lowered completely. It just seems to be a lot more difficult to build (both the cassette and the blade) and may be more difficult to use as well.

    As for the fence on the cassette? Is there any rule on how big the fence should be? Was this ever studied in any way? Thanks again!
     

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  8. Steve W
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    Steve W Senior Member

    I built a cassette rudder years ago where I built the blade first and then waxed it up and laminated a fiberglass cassette around it I split it up the back edge and held it closed with bungee wrapped around it, if the blade hit something the cassette could spread open and let it angle back and being a perfect fit the blade stayed in whatever position you put it. It was much like your middle drawing with a vertical ss strap joining the top and bottom gudgeons at their aft ends so they could be well glassed to the cassette. Quite easy to build.
     
  9. erdben
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    erdben Junior Member

    Thanks. That's kind of my plan, too. So did you glass in the SS strap joining the gudgeons embedded between layers of fiberglass or did you attached it to the cassette once that was built?
     
  10. Steve W
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    Steve W Senior Member

    After the cassette was built. To be honest I cant remember if I used VE or epoxy, probably VE as that's what I built the boat with. This was 30 years ago so the memory is a bit hazy.

    Steve.
     
  11. bruceb
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    bruceb Senior Member

    Rudder replacement

    Ben, I happen to have a rudder blade and cast alloy housing that was removed from a friend's 23. It was replaced for the same issues as yours. This one was the "improved" version with a vertical lifting blade, but still a piece of junk. Heavy, too thick of blade, and prone to "auto" turning, enough to snatch the tiller out of your hand.
    We built a blade and housing about the same as what you are describing, with the forward rake and a flange on bottom and it worked great.
    Some Corsair tri's have a nice strong rudder system that would be a good guide to model yours on ;)
    Good luck!
    B
     
  12. erdben
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    erdben Junior Member

    Thanks!

    Now, an interesting question about using these lifting rudder blades:

    Let's say I'm sailing upwind in straight line in steady conditions. There is a certain amount of lift the rudder needs to generate.

    What is the optimum rudder planform area and angle of attack?

    Let's assume that the planform shape is close to a rectangle so that the cord stays the same regardless of blade position and that the max area is larger than optimal for the given conditions.

    As I pull the rudder up and reduce the area, weather helm increases until the angle of attack gives maximum lift/drag ratio for the rudder. However, in the meantime, the aspect ratio decreases, too.

    How would you find the optimal AOA - AR combination? I kind of got stuck trying to put everything in one equation.
     
  13. Erwan
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    Erwan Senior Member

    Your rudder optimum AoA will depend on the L/D ratio of your centerboard.

    If the rudder shares some lift with the centerboard, the induced drag of the centerboard will decrease more than the increase in induced drag from the lift generated by the rudder AoA.

    Assume your centerboard carries 100% of the lift @ 4° AoA , the Cl= 0.4
    The centerboard's induced drag will be proportional to 0.4^2=0.16

    To make things simple, imagine your rudder is similar to your centerboard

    If your rudder carries 25% of the lift then the centerboard Cl=0.3
    and its induced drag proportional to 0.3^2=0.09

    And for the rudder to provide the appropriate lift its Cl=0.1
    and its induced drag will be prop to 0.1^2=0.01

    So you can compare total induced drag in both cases:

    0.16 vs 0.10 =(0.09+0.01)

    Hope it can help

    Regards

    EK
     
  14. erdben
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    erdben Junior Member


    Hi Erwan,

    thanks for your post, it’s very interesting you brought this up, because I've been trying to come up with some numbers for the lateral forces on my boat and analyze the sizes and profiles of my centerboard and rudder. I'll get back to this, but first I'd like to explain what I meant in my previous post.

    With my current kick-up rudder, I always have to sail with the rudder fully down, but with a cassette-type rudder, I have the option to adjust how much of the blade is in the water. I was wondering how I could decide what the optimal up-down position of the rudder blade is for a given condition.

    So this is what I did, I'd be interested to hear whether it's correct:

    Using methods from Principles of Yacht Design, I've first calculated the side forces on the sails for a few wind speeds and sailing angles with the speeds I usually get with my boat. I also compared these to J24 polars, since my boat has the same rating, although I'm usually slower upwind and faster downwind. Then, I calculated lift forces for the centerboard and rudder using Cl = Cl(1deg) x leeway angle / (1+2/AR) and adjusted for the effects of hull and heel as suggested in the book to find the leeway angles for the different side forces.

    Then, I took the lift on the rudder as a fixed number from these calculations for each sailing angle and speed, and plotted how the AOA on the rudder needs to change if I change the area of the rudder by pulling up the blade (reducing depth, while cord stays the same).

    For each of these angles, I calculated viscous drag and induced drag. This where I'm not sure I did everything correctly. For viscous drag, it seems I can get the same numbers I'm getting for a NACA0012 from Xfoil with 0.0077+(AOA x 0.0086)^2.

    For induced drag I used Cl^2/(Pi x AR). I've adjusted AR and area based on the varying depth of the immersed blade and plotted the sum of the drag forces vs AOA of the rudder and vs depth of the blade.

    I got figures like the ones below. Not surprisingly, it showed that in light winds, sailing on a reach for example, it’s best to raise the blade so that only 0.6m remains in the water. In light upwind, the rudder seems to be sized perfectly as the optimum depth is 1.1m which equals the depth of my current rudder. Stronger upwind on the other hand would require deeper rudder for minimum drag.

    What’s interesting is that the optimal rudder AOAs I got for upwind sailing are very close or in some cases even smaller than the leeway angles I got for the same points of sail meaning that I wouldn’t have the usually recommended few degrees of weather helm. While I’m not really sure about my calculations above, it does seem to agree with what I experience while sailing.

    I think the problem is that this boat had several versions throughout the years. The same hull, deck and rig were sold as a dagger board version that seems to have a ~40% larger lateral area than the centerboard on my boat. It seems from my calculations that fully powered up, I could have 7 – 8 degrees of leeway upwind while sailing at target speed, which seems rather high. The leeway could go as high as 10-11 degrees while accelerating after a tack or if slowed down by a bad set of waves.

    I read it in some “how to sail fast” article that the optimum weather helm on a given boat depends on the leeway, and that boats that have higher leeway don’t need to angle their rudders as much (and still get the optimum lift on the rudder to share the loads with the keel or centerboard). I already sail with a pretty significant mast rake and was always bothered why I can’t induce some weather helm. Maybe it's fine for my boat then.

    Anyway, I’d be interested to hear whether all these calculations and guesstimations make any sense. :)
    I’ve attached a pic comparing the daggerboard version of this boat to mine (indicated by red) showing how the lateral area got reduced and how the board got shifted aft as well. On top of this, my centerboard seems to have a NACA64-012 profile, which doesn’t seem to be the best if the leeway is really this high – but that’s a different topic.
     

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  15. Doug Lord
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    Doug Lord Flight Ready

    Rudder endplate-could be adapted to a cassette rudder by putting the endplate on the bottom of the trunk:

    illustration from a book by Pierre Gutelle-

    [​IMG]
     
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