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Thread: Bolger rudder plates

  1. #1
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    Default Bolger rudder plates

    I remember reading Bolger suggesting plates on the bottom of smaller rudders make the rudders as affective as if they were larger.

    Has anyone here tried this?

    Did it work as well as he suggested?
    How do we form a mutiny? Our new captain is navigating poorly.

  2. #2
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    Default Re: Bolger rudder plates

    I put one on my Nimble 20 rudder after the drop down part broke; it worked well and I would do it again. Tony Dias uses them as well. If IIRC he used on Small http://boatbldr.com/html/boats/small.html Rick

  3. #3
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    Default Re: Bolger rudder plates

    They are called "splates" and are used to control airflow ove wings to increase lift over control surfaces/ailerons.......water is 800 times as dense as air so the size can be considerably smaller.
    Wakan Tanka Kici Un
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    Fighting Illegal immigration since 1492....
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  4. #4
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    Default Re: Bolger rudder plates

    Especially for low aspect-ratio foils (the sharpie rudder is the pathological case), the fluid wants to flow from the high pressure side of the foil around the tip to the low pressure side. This causes loss of lift, early stalling, vortices and drag. Endplates reduce this.
    I added an endplate of 1/4" G-10 to Damfino's rudder last year. It seems to help in low-speed maneuvering. It extends a bit aft of the rudders trailing edge and serves as a get-back-on-board step.

  5. #5
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    Default Re: Bolger rudder plates

    I replaced a conventional rudder on a 25' Searunner trimaran with a shorter and wider one with an end plate. The feel and performance was unchanged, and I had now a boat with about a foot less draft when the centerboard was up. I more recently shortened the rudder on my 16' Whisp skiff with one with an end plate with the same finding. I think it was Bolger who gave the rule of thumb that the total underwater area needs to be preserved, so that if you chop off 6" then you'll want a 6-inch wide plate, or say a 3-inch wide plate with a few more square inches width in the rudder. In my two experiments, that's shown to be true.

  6. #6
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    Default Re: Bolger rudder plates

    Quote Originally Posted by Woxbox View Post
    .... I think it was Bolger who gave the rule of thumb that the total underwater area needs to be preserved, so that if you chop off 6" then you'll want a 6-inch wide plate, or say a 3-inch wide plate with a few more square inches width in the rudder. In my two experiments, that's shown to be true.
    For boats that heel much, I can see that model.

    On multihulls that don't heel very much, maybe you only get benefit from enough area to prevent tip losses.

    I recollect that the idea of the (in)famous Lexcen keel on Australia II in 1983 was that the wings became effective foils laterally when heeled. I expect that Tony Dias' wings on the rudder of the Arey's Pond boat were pursuing this idea. Getting the right angle of attack is a tricky problem.

    Calling Doctor Marchaj!
    Last edited by JimConlin; 08-22-2010 at 04:40 PM.

  7. #7
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    Default Re: Bolger rudder plates

    Couldn't find a pic of the real thing but here's a model of a Bolger Bobcat showing rudder plate. Doesn't look like the intended purpose has as much to do with lift as it does grabbing some additional H2O while heeling with a shallow rudder




  8. #8
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    Default Re: Bolger rudder plates

    I have the plates on the keel, bilge keels and the rudder of my Bolger sloop. Phil told me that he expected it would help with lateral planing. They also help out when grounding by keeping you from sinking too much and offering some protection to the keels and rudder. Do they help with sailing ability? I don't know because I've never sailed her without them.
    "If a man speaks at sea where no woman can hear, is he still wrong?"

  9. #9
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    Default Re: Bolger rudder plates

    I had them on a 26' Wharram cat - I couldnt tell if they imroved steering because they were there when I bought the boat. The boat steered good, but they are good sized rudders anyway. They made great boarding steps - no way in without them. Makes them winners on that point alone.
    I reckon they provided some pitch damping too.
    Cheers Peter

  10. #10
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    Default Re: Bolger rudder plates

    Quote Originally Posted by imported_Peter K View Post
    I had them on a 26' Wharram cat - I couldnt tell if they imroved steering because they were there when I bought the boat. The boat steered good, but they are good sized rudders anyway. They made great boarding steps - no way in without them. Makes them winners on that point alone.
    I reckon they provided some pitch damping too.
    Cheers Peter
    My specific problem is that I have twin rudders, both very large. Boat sails with no noticeable lee or weather helm. Problem is launching. Removing the rudders is not too hard, but getting them back on in the water is challenging. At many ramps, they reach out so far behind the boat




    I can tilt the motor, but I cannot tilt the rudders

    I'd like to trim them. Thought is to cut some of the aft end, and put a plate on the outside of both outside rudders. This would prevent the rudders from hitting at launch ramps, and not lose much in steering ability. I don't have a shot from the side, but the aft edge of the rudders slant back pretty far.
    How do we form a mutiny? Our new captain is navigating poorly.

  11. #11
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    Default Re: Bolger rudder plates

    Another approach to adding foils to a rudder;as used in National 12's.


    I know it wasn't designed by Bolger.I also know that the angle of the entire rudder assembly can be varied while the boat is sailing,to adjust the lift generated by the wings and that no lead is used to keep the rudder in its intended position.

  12. #12
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    Default Re: Bolger rudder plates

    I added such a plate to our 30' gaff sharpie ketch. Attached it to the bottom of the barndoor rudder. It has helped greatly in stabilizing the boat, particularly under power when prior it would nose up. It had the same effect as adding those black rubber wings to and outboard motor.

    It also became a swim platform for our Lab. Will try to post some pictures when I get home
    Last edited by Anthony Zucker; 08-23-2010 at 04:44 PM. Reason: spelling

  13. #13
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    Default Re: Bolger rudder plates

    Here's a better picture of my rudders



    My plan is to make a vertical cut, cutting off 25 - 30 % from the aft point, and adding a plate.
    How do we form a mutiny? Our new captain is navigating poorly.

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