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Old 07-06-2013, 05:51   #1
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Sailboat Rudder (ROM) Range of Motion

(1974 Tartan 34) I am reinstalling my rudder, autopilot, and all associated hardware (quadrant, cables and pullies etc.) and am curious about the specified or typical range of the rudder movement in degrees. Here is what I have read so far (valid or not): Beyond 45 deg. the rudder will cavitate much like a boat or aircraft propeller and be ineffective in controlling the direction of the vessel. Is this true? A further description is that it will "stall" like an aircraft wing. Certainly makes sense taken to it's logical extreme (a rudder turned to 90 deg. would be ineffective). Given the range of old cruiser sailing velocities (say, 0-8kts). where is the "sweet spot area" on the range of movement and should rudder stops be placed at the point beyond which the rudder ceases to work? Should the effective control range be expected between 0 deg. (straight ahead) and say, 40 deg. starboard and port, or is that level of attempted control excessive for a small to mid size cruiser?
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Old 07-06-2013, 07:48   #2
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Re: Sailboat Rudder (ROM) Range of Motion

More of a bump than anything, as I'm not the most experienced with this one...

I do know however the "stalling" is indeed fact... I have adjusted the stops on one of my boats because hard over was a little too much...

Another boat, J120, I experienced it at the helm, and labeled it as downright dangerous...

Why not make the range +/- 50, and you can always reduce your range?

The boat I adjusted had a sweet setup that was beyond simple... Machined block threaded with a bolt and locknut... changing the range took 5 minutes...
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Old 07-06-2013, 08:11   #3
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Re: Sailboat Rudder (ROM) Range of Motion

The rudder stop angle will be determined by your autopilot drive unit. You do not want your AP drive becoming the limiting stop for the rudder, as that will damage it. Typically, this angle will be between 30-40* for most drives.

Check the installation manual for your AP drive for the recommended rudder stop angles.

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Old 07-06-2013, 14:15   #4
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Re: Sailboat Rudder (ROM) Range of Motion

Cavitation consists of a liquid turning into vapour at ambiant temperature, due to a drop in pressure. Then, an aircraft propeller, working in air, can't cavitate.

The stall angle of a rudder depends on the aspect ratio (height/chord): high aspect ratio -> stall at about 15-20 angle of attack. But when turning, the water already comes at an agle to the rudder, reducing the AoA. For this reason, rudders on ships generally go to 35. On sailing boats, 45 is useful for turning tight at low speed, e.g. in port.

On a sailing boat, the main reason for losing rudder effectiveness is ventilation: air from the surface is sucked on the suction side of the rudder. This can happen when sailing fast (suction is proportional to the speed squared), as soon as the rudder top is out of the water: very common on transom-hung rudders, slightly less on under-hull ones (needs some heeling).

When the rudder is ventilating, you can see a cloud of white bubbles of air in the wake.

To reduce ventilation, there are 3 possibilites:
- reduce the AoA: reduce rudder angle
- reduce the speed
- put the hull at the rudder top back in the water: reduce the heel

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Old 07-06-2013, 14:32   #5
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Re: Sailboat Rudder (ROM) Range of Motion

Hydra; But air is a liquid. What you see when a propeller or rotor blade causes a big drop in pressure in the air, is the moisture comming out in the form of vapor.
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Old 07-06-2013, 14:49   #6
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Re: Sailboat Rudder (ROM) Range of Motion

In normal conditions of pressure and temperature, air is a gas with some water vapor dissolved in it. In fact, vapor or steam is the name of water in gas form. It's possible to have some of this water released as fog, rain, snow, etc.

To make liquid air, you just have to cool in below -140C / -220F. Gasses and liquids are fluids.

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Old 07-06-2013, 17:04   #7
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Re: Sailboat Rudder (ROM) Range of Motion

Problem dealt with. The Orion steering quadrant, when properly mounted with bracket, acts as a stop at @ 40 deg. Orion, no longer manufactured, is similar to Edison Marine. Thanks all for the input!
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Old 08-06-2013, 05:28   #8
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I had a race boat on which one could rotate the rudder all the way around. Poster above is correct that the spin of the boat reduces the angle of attack and makes the rudder useful at any angle to the hull. When turned past 90 degrees the boat would begin to go backwards. Kinda cool for stupid tricks.

45 is plenty for cruising. 70 is handy for a fin keel in a tight spot. But not necessary. And the A/P will limit the angle too.
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