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Old 05-26-2017, 09:51 AM   #1
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rudder angles

I noticed yesterday for the first time that my rudders are not exactly parallel. When the rudder connected to the rudder sensor is parallel to the prop shaft, the other rudder is out by a few degrees. I would have thought both would be parallel, but this is my first two engine-boat.

A friend told me that if both are parallel, I will get vibration from the rudder, because of disturbed water going by the rudders. Does not seem intuitive to me.

Thanks in advance.

Gordon
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Old 05-26-2017, 10:14 AM   #2
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Rudders are frequently toed in a bit to compensate for torque.

I find it interesting that if you look carefully at a twin engine prop airplane, the vertical tail is mounted with the vertical tail offset, it's not oriented "square." Same principle.
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Old 05-26-2017, 10:18 AM   #3
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Rudders are frequently toed in a bit to compensate for torque.
And to put a little water pressure on them to avoid the prop wash vibration.

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Old 05-26-2017, 10:20 AM   #4
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Xsbank,
That would not be best. In a way.
But in reality it will come to pass that one rudder will compensate for the propwalk (not torque) and together they will keep the boat going straight.

Interesting part pf this is that the single screw vessel will not require nearly as much rudder deflection to compensate for the propwash .. the rudder being so much larger. A detail that would make the single more efficient. But there are other factors. Fly stuff of course.
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Old 05-26-2017, 11:07 AM   #5
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If they are truly parallel you may get some flutter in the rudders. With some toe in, there is some pressure to hold the rudders steady.
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Old 05-26-2017, 02:57 PM   #6
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Comodave,
Why dosn't my single screw ever flutter?
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Old 05-26-2017, 03:19 PM   #7
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Comodave,
Why dosn't my single screw ever flutter?
Maybe the alignment of your rudder or your bearings are extremely tight? I have no idea. I get a flutter occasionally at certain RPMs/sea conditions but its so infrequent I don't worry about it. I have to be by the rudder post to hear/feel it.
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Old 05-26-2017, 05:01 PM   #8
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Larry,
Rudder not that tight at the shoe.
With my small prop (18") and small power (40hp) the turbulence is probably low. Also the rudder is large so LE to TE distance probably drops some turbulence too.
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Old 05-26-2017, 05:51 PM   #9
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I am not sure why a single doesn't flutter, maybe less connections and no tie rod?
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Old 05-26-2017, 06:20 PM   #10
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My single flutters under certain conditions...maybe a worn bearing in the keel shoe, but cant duplicate by hand every hauout.
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Old 05-26-2017, 07:09 PM   #11
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No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.

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Old 05-26-2017, 07:36 PM   #12
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If your rudder flutters, it's more likely to be the linkage. Mine started vibrating and I needed a new gudgeon pin. The new Wagner pins have grease nipples.

I bet dollars to donuts the rudders on a twin are no more likely to flutter unless something is worn. It makes no sense.
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Old 05-26-2017, 07:45 PM   #13
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No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.



Please educate me on that port rudder/ prop walk. Is that not a Rt. hand screw? Would that not create a prop walk in the same direction rather than an opposed direction ?
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Old 05-26-2017, 07:58 PM   #14
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That looks like a left hand wheel to me. Counter clockwise rotation, will back to starboard.
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Old 05-26-2017, 08:11 PM   #15
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Correct, it backs to starboard and turns to starboard going forward under power without a three-degree port rudder.
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Old 05-26-2017, 09:28 PM   #16
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Same w my boat Mark.
Left hand wheel, back to stbd and pull to stud going fwd.

LarryM just the mass of my big rudder probably keeps it from fluttering.
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Old 05-26-2017, 09:39 PM   #17
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could the difference be that single screw rudders are often attached top and bottom, while twin screw rudders are often only attached at the top ?
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Old 05-26-2017, 09:40 PM   #18
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Correct, it backs to starboard and turns to starboard going forward under power without a three-degree port rudder.
Eric, maybe no play in the rudder tube, or tight bushing in the drag link would be why you have no play or "chatter".
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Old 05-26-2017, 10:42 PM   #19
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Well it dosn't chatter and I'm not upset about it.
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Old 05-26-2017, 11:59 PM   #20
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No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.

Me too, but it's 3 degrees to stbd
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