Rudder Angle Geometry

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Rudder control steering geometry is one of those things which looks simple, but is actually quite difficult to get right. Loads can be high, space tight, and when you are shooting for maximum rudder deflection, it can be a challenge.

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Shown here the rudder is turned 40 degrees to starboard.

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And here all the way to port.

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In this view we see both centered and the two extremes. The red blocks represent the rudder stops. The hydraulic cylinder has a 12”/300mm stroke, and this has to work with a “tiller” that gets us the 80 degree hard over to hard over range. It has to do so without binding. And the cylinder base must be connected to something extremely strong.

With the geometry established, Kelly and the engineers can get on with the detailing.

Posted by Steve Dashew  (October 31, 2013)

9 Responses to “Rudder Angle Geometry”

  1. Evan Thompson Says:

    I’m curious as to why over the years you have stayed away from “trunnion mount” hydraulic cylinders. It seems they would allow the cylinder to pivot through an arc as the rudder is turned through its range, as well as providing a more robust double shear anchor point. Maybe ease of access/serviceability?

    This type of cylinder mount is common in heavy equipment, canting keel systems (admittedly not the best marine system example in regard to ultimate reliability – but that’s likely due to factors other than the cylinder mount), and even found their way into the Alinghi 5 mainsheet system:

    Thanks again for sharing all of these details of your design process. It’s fascinating and inspiring.

  2. Steve Dashew Says:

    I think this is a matter of terminology, Evan:
    We would call all of the cylinders we use trunnion mounts. Previously they have had their “trunnions” at the end, where in this case they are in the middle of the cylinder.
    The key factor is we want a large bolting base (foot), so that the reverse loading to which these are subject does not result in the connection becoming wiggly.

  3. Roger Says:

    Is ±40° all you need? Aircraft nose steering gear is hydraulically actuated to ±70° or thereabouts.

  4. Steve Dashew Says:

    Under way five to ten degrees of AOA is the max we ever see. MAneuvering, at minimum speed, 40 degrees, with twi engines and rudders, creates the maximum usable side force.

  5. Don Joyce Says:


    Which (whose) steering cylinder are you proposing to use?

  6. Steve Dashew Says:

    We are using Lecomb and Schmitt hydaulic steering cylinders. Very heavy and expensive, but steering is a system where one does not want nagging doubts…

  7. Don Joyce Says:

    Thanks. I have 2 L&S cylinders on Cats Meow quadrant. Only complaints are that the rod seals leak a bit. Luckily parts are now available from PYI.

  8. Steve Dashew Says:

    How much leakage, Don, what size units, and how many hours of operation?

  9. Don Joyce Says:

    Sorry about the slow response. I missed your questions Steve when we left for parts south. 2X VHM 60 DT cylinders. Leakage is about 5 drops per day. Last rebuild 3 years ago. Hours probably upwards of 5000. Leakage is constant…same before and after rebuild. My complaint is the rod seals are from the stone age. Paper seals cut to fit on installation. Sigh