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Thread: "Toe In" On Rudders?

  1. #1

    Default "Toe In" On Rudders?

    I am about to replace the bar connecting my rudders. I recall seeing references to benefits from rudders toeing in slightly-is this correct and if so how much toe in?

  2. #2
    Join Date
    Jul 2009
    Location
    currently Columbia/Panama
    Posts
    352

    Default Re: "Toe In" On Rudders?

    No, the rudders should be parallel to each other when amidship. You may be thinking of setting an Ackerman angle, where the tiller arms are toed in so that the rudders attain different angles when turning. If you are shooting for Ackerman angles, then somewhere between 10-20* will be close. You can calculate this. Be sure that you don't have the cross link so short that you can't turn fully.

    Mark
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

  3. #3

    Default Re: "Toe In" On Rudders?

    Quote Originally Posted by colemj View Post
    No, the rudders should be parallel to each other when amidship. You may be thinking of setting an Ackerman angle, where the tiller arms are toed in so that the rudders attain different angles when turning. If you are shooting for Ackerman angles, then somewhere between 10-20* will be close. You can calculate this. Be sure that you don't have the cross link so short that you can't turn fully.

    Mark
    Thanks Mark
    Parallel it is but as a matter of interest how would you set up an Ackerman angle if you wished to?

  4. #4
    Join Date
    Jul 2009
    Location
    currently Columbia/Panama
    Posts
    352

    Default Re: "Toe In" On Rudders?

    That is set by the connecting bar. The tiller arms that the bar is connected to are set with a toe-in angle. In other words, the connecting bar is shorter than the distance between the rudders. So the rudders will be parallel when amidships, but have increasing difference in angle as they are turned.

    Mark
    Mark Cole
    Manta 40 "Reach"
    www.svreach.com

  5. #5

    Default Re: "Toe In" On Rudders?

    Quote Originally Posted by colemj View Post
    That is set by the connecting bar. The tiller arms that the bar is connected to are set with a toe-in angle. In other words, the connecting bar is shorter than the distance between the rudders. So the rudders will be parallel when amidships, but have increasing difference in angle as they are turned.

    Mark
    Thanks for reply mark but I do not see how if the connecting bar is shorter than the distance between the rudders that there cannot be toe in at mid ships position. Perhaps I am being a bit dense but i just do not see it?

  6. #6

    Default Re: "Toe In" On Rudders?

    Assuming a connecting bar linked directly to the tillers, and tillers forward of the rudders, the tillers would normally be parallel with the rudders, the connecting bar perpendicular to the tillers.

    Looking at the straight ahead position, for Ackerman geometry the tillers wouldn't be parallel, but would be angled inboard. So the connecting link is shorter, and isn't perpendicular to the tillers although the rudders are still parallel. Both tillers will be at an equal and obtuse angle to the connecting bar.

    As you turn the rudders, say to starboard, the port tiller becomes closer to perpendicular to the connecting bar, while the starboard one becomes more obtuse. This causes the starboard rudder to move more quickly than the port one, resulting in a tighter turn for the inside rudder.

  7. #7

    Default Re: "Toe In" On Rudders?

    Quote Originally Posted by 44C View Post
    Assuming a connecting bar linked directly to the tillers, and tillers forward of the rudders, the tillers would normally be parallel with the rudders, the connecting bar perpendicular to the tillers.

    Looking at the straight ahead position, for Ackerman geometry the tillers wouldn't be parallel, but would be angled inboard. So the connecting link is shorter, and isn't perpendicular to the tillers although the rudders are still parallel. Both tillers will be at an equal and obtuse angle to the connecting bar.

    As you turn the rudders, say to starboard, the port tiller becomes closer to perpendicular to the connecting bar, while the starboard one becomes more obtuse. This causes the starboard rudder to move more quickly than the port one, resulting in a tighter turn for the inside rudder.
    Thanks for the explanation I understand now.

  8. #8
    Join Date
    Aug 2009
    Location
    Southern Chesapeake
    Posts
    189

    Default Re: "Toe In" On Rudders?

    This shows the geometry to determine the angle to be used for the steering of a car. I would imagine for a catamaran the middle of the keels, or perhaps a bit forward of the middle would be used as compared to the middle of the rear axle.

    ackermann steering geometry

    And the tillers don't necessarily need to be angled in. It's the point at which the tiller bar is connected that is the reference point for this geometry. Angling in is probably easier if that point needs to be brought inward very much

    Our previous cat had the tillers angled OUT to give better clearance to the sliding locker hatch just under the tiller. A fellow who owned the same model boat claimed that he had read that under some circumstances (high speeds, surfing??), reverse Akermann was superior, but I couldn't understand that.

  9. #9

    Default Re: "Toe In" On Rudders?

    It's necessary on a car, where (at low speeds at least) there should be zero slip. And the pivot point is easy - it's the centre of the rear axle.

    But IMO Ackermann geometry is of dubious value on a cat'. Where is the boat's pivot point? Is it always the same? How about with daggerboards up or down? Or using one daggerboard?

    And when turning, boat rudders need to slip. They need an angle of attack to generate lift. But is the amount of slip always the same?

  10. #10

    Default Re: "Toe In" On Rudders?

    Quote Originally Posted by ggray View Post
    This shows the geometry to determine the angle to be used for the steering of a car. I would imagine for a catamaran the middle of the keels, or perhaps a bit forward of the middle would be used as compared to the middle of the rear axle.

    ackermann steering geometry

    And the tillers don't necessarily need to be angled in. It's the point at which the tiller bar is connected that is the reference point for this geometry. Angling in is probably easier if that point needs to be brought inward very much

    Our previous cat had the tillers angled OUT to give better clearance to the sliding locker hatch just under the tiller. A fellow who owned the same model boat claimed that he had read that under some circumstances (high speeds, surfing??), reverse Akermann was superior, but I couldn't understand that.
    If you tillers were forward of the rudder shaft, and angled outward, you would have reverse ackermann geometry. However if they're behind the rudder angling outward would give correct geometry.

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