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Thread: Rudder alignment

  1. #1
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    Default Rudder alignment

    I just (funally)got my Woods 36 Scylla out of water. A good surprise is that the beaching keels have a hefty 1/8 inch thick stainless steel plate screwed on the bottom... A curious thing is that the rudders have a couple degrees "toe -in" Is this a builder error or should rudders be "toed -in"? Im thinking of insulation for the thin wood cabintops any suggestions- re weighed boat -came in at 6550 with oysters scraoed off,,,,-Im sure I will be putting AC in one hull here in the Florida summer--Im also raising the boom,have ordered a t-top full batten main from mack sails --getting a 8ft long bimini -will leave pathways clear----and new forward tramp from Sunshine- going to foam beams....--let er rip! LOL fromThanks.Georgetheleo

  2. #2
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    Default Re: Rudder alignment

    George,

    If your rudders have toe-in when you try to center them, you definitely have an issue. It is common practice to implement "Ackerman" steering on cats whereas the tillers are angled in 5 degrees (before they are attached to the crossbar) when the rudders are aligned with the centerline of the hulls. This results in the "inside" rudder turning more than the rudder on the outside of the turn, seeing as the turning radii are different due to the separation of the hulls. It may be that either the builder or previous owners didn't quite grasp the concept and bungled it.

    lincoln
    Zbiggy2

  3. #3
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    Default Re: Rudder alignment

    Your statement is kind of puzzing--Should or should not rudders be set straight or as you say angled -in and I say toed in ???? The built alignment is "angled- in" about 3-4 degrees going straight and Im sure induces some drag and puts a slight force on both rudders which is not felt at the helm as force on the rudders being opposite counteracts the other.

  4. #4
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    Default Re: Rudder alignment

    I suggest you ask Richard Woods.
    Nothing works on an old boat, except the skipper.

  5. #5
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    Default Re: Rudder alignment

    Sorry if my answer was fuzzy. When you have the helm centered, your rudders should be parallel, but each of the tillers (if ackerman was implemented properly) will be angled inboard 5 degrees. There will be no axtra grag, since the rudders are parallel. When you turn the helm, the rudders are no longer parallel, one turns more than the other. This actually reduces rudder drag, because the hull on the indside of the turn travels a smaller arc than the one on the outside of the turn. Without ackerman, your rudders would stay parallel and as you turned neither rudder would match the arc of the turn as closely as they would with ackerman geometry.

    Draw 2 concentric circles on a piece of paper. The inner one having a 5" radius, the outer having a 10" radius. Draw your 2 hulls onto the drawing, one hull on the inner circle, one on the outer. Your boat now has a 5" beam on center (hul centerline to hull centerline). Now draw the rudders on there too, and make the rudders tangent to the circles. Notice thatAs your boat turns around the center of the circles, the inside hull's turing radius is 5", while the outer is 10". Big difference. So if your rudders always stay parallel, there will be extra drag. If you use ackerman geometry, the drag will be less. It is not perfect for every turning situation, but the 5 degree number is generally accepted as optimal.

    Here is a couple of sketches showing the difference in angles between parallel rudders and rudders tangentto their respective hull's turning radii.

    Lincoln
    Attached Images Attached Images

  6. #6

    Default Re: Rudder alignment


  7. #7
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    Default Re: Rudder alignment

    yep, I could have saved some time there, eh? Same principle, adapted to cats.

  8. #8
    Join Date
    Feb 2009
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    Totnes
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    Default Re: Rudder alignment

    Re: insulation.

    In the Canary Islands, a plywood monohull we were living on got so hot in the afternoon, even with the hatches open and a breeze passing through, that we'd have to leave and go and sit under a palm tree till it cooled.

    I lined the coachroof with thin ply, and between the ply and the coachroof, I put a layer of bubble wrap that had aluminium on one side. It was cheap, and available, but I didn't have too much hope for it.

    It was fantastic - made the boat cooler than under the palm tree. Reflecting the heat of the sun back did the trick. Not sure how good such a solution would be for where it is hot and humid though, or through a different type of coachroof.

  9. #9
    Join Date
    Oct 2008
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    in BC Canada, the UK
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    Default Re: Rudder alignment

    Sorry I didn't answer this one earlier, that's a drawback of sailing.

    However bax and tim have answered it as well as I could (thanks guys)

    The rudders should both line up with the hull CLs. But the tillers should both angle in about 5deg (from memory)

    Richard Woods on board a Transit 38 (for sale) heading south and currently in SC

    www.sailingcatamarans.com

  10. #10
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    Default Re: Rudder alignment

    Thank you Richard -I will make it that way. George Myers

  11. #11
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    Default Re: Rudder alignment

    Before spending a lot of time and effort on extra insulation try a canvas cover over you coachroof and extending to keep the windows in the shade. Provided it is open to the ends and sides you should get a cooling draft over the top of the coachroof. It doesnt seem to matter what colour the canvas is either ..... although I have been told that bees are attracted to the usual blue canvas.

    You probably wont need the extra insulation whilst sailing either.

  12. #12
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    Default Re: Rudder alignment

    Thanks-

  13. #13
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    kefalonia
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    Default Re: Rudder alignment

    This is very interesting ,. When my cat was launched the builder set the rudders with "toe in " . I dont know how the amount was calculated , but he stated it was neccessary for keeping a straight course ?. Ever since , whenever the rudders are dropped out and greased , I continue with the original settings of toe in .
    From these recent posts it would appear to be the wrong setting ?
    Another point , if the helm is on one side of the boat giving different length cables ( 2.4m port , 7.2m stbd) would the ackerman angle be different ? I always seem to get better performance on a stbd tack !.....
    I didn,t get where I am today by being somewhere else !

  14. #14
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    Default Re: Rudder alignment

    Quote Originally Posted by gramos View Post
    Another point , if the helm is on one side of the boat giving different length cables ( 2.4m port , 7.2m stbd) would the ackerman angle be different ? I always seem to get better performance on a stbd tack !.....
    Would it not depend on the length of the tiller arm to which the cable is attached, and then the clamp point for the cable sleeve, rather than the length of the cable itself?

    Mike
    Last edited by dmmbruce; 21st November 2010 at 04:01 PM.
    Nothing works on an old boat, except the skipper.

  15. #15

    Default Re: Rudder alignment

    Having just gone through the Ackermann Angle determinations for the boat we are building, you can calculate a nominal Ackermann Angle using your specific beam, center of lateral resistance, and turning radius. The Ackermann Angle does change somewhat with different rudder angles - the Ackermann Angle increases with increasing angle. For this reason, we used a 30 degree angle for our calculations. The resulting Ackermann Angle for our boat was more like 19 degrees (vs 5 degrees). Many people have noted that if you use small angles it is not worth messing with - and indeed many cruising cats do not apply Ackermann Angles to their steering geometry. Take a look at a Hobie Cat and you can clearly see the "toe-in" of the tiller bar (greater than 5 degrees), and the Ackermann steering concept.

  16. #16

    Default Re: Rudder alignment

    I just replaced a rudder on my Fountaine-Pajot Belize43 with a factory fresh rudder. (Long story - don't ask.) the design is such that there is no adjustment or alignment possible. The tiller (which does have several degrees of bend toward center) is attached to the rudder with two set screws that fit into machined grooves in the shaft. The two tillers are joined by an aluminum tube with no adjustment. With the wheel centered, I can measure ~1" difference between leading and trailing edges of my rudders. Does anyone know if this is the way FP designed it to be? I don't really see how it could be otherwise as there is just no way to change things except by replacing (or re drilling) the connecting tube

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