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

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.

Xsbank said:

Rudders are frequently toed in a bit to compensate for torque.

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And to put a little water pressure on them to avoid the prop wash vibration.

David

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.

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.

Comodave,
Why dosn't my single screw ever flutter?

Nomad Willy said:

Comodave,
Why dosn't my single screw ever flutter?

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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.

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.

I am not sure why a single doesn't flutter, maybe less connections and no tie rod?

My single flutters under certain conditions...maybe a worn bearing in the keel shoe, but cant duplicate by hand every hauout.

No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.

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.

markpierce said:

No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.

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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 ?

That looks like a left hand wheel to me. Counter clockwise rotation, will back to starboard.

Correct, it backs to starboard and turns to starboard going forward under power without a three-degree port rudder.

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.

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 ?

markpierce said:

Correct, it backs to starboard and turns to starboard going forward under power without a three-degree port rudder.

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Eric, maybe no play in the rudder tube, or tight bushing in the drag link would be why you have no play or "chatter".

Well it dosn't chatter and I'm not upset about it.

markpierce said:

No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.

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Me too, but it's 3 degrees to stbd

On a twin, the wash from the props gets some rotation to to them, and twin rudders usually being smaller, are mostly in the upper part of the helix. The "toe in" (leading edges closer than trailing edges) is to avoid having rudders fight each other and increase drag.

One redneck trick is to get the boat underway and then disconnect one rudder from the tie bar. So one rudder is steering, the other finding its natural position in the flow. Adjust tie bar link so clevis pin just drops in. Afterward, you find the rudders are "toed in" a bit.

It gets more complicated on high performance craft as tweaks of the toe in can have major changes in sea keeping. End up experimenting with different settings.

Rudders also create LOTS of drag.

Ski wrote;
"Rudders create LOTS of drag."

I wonder how much more they "create" with the propwash flowing or blasting over them?

ANY rudder angle is drag , if its turning the boat its fine .

"Back in the day" it was common to angle the engine and shaft 2-3deg and have the rudder not produce drag going ahead.

But FF most all the time it's just there causing drag.

"But FF most all the time it's just there causing drag."

Sure but the skin friction can only be reduced by shrinking the rudder.

The induced drag from its being required to keep the boat going straight can be totally eliminated.

FF wrote;
"Sure but the skin friction can only be reduced by shrinking the rudder."

Or the velocity of the water going over it.
Can you imagine a cruiser going 18 knots w a sailboat rudder? Too much drag because there is too much velocity of the water flowing over it.
Also if you doubled the size of the rudder you'd double the drag. But if you doubled the velocity (speed) I think you'd increase the drag by four times.

Just another idea of mine ......
One could conceivably reduce rudder drag on a slow trawler by having two rudders .. one to each side of the propwash but outside of it. The propwash innitially cones inward so the two rudders need'nt be that far apart.

Does having the propeller at an angle of direction create a drag too because the hull is being pushed sideways?

"One could conceivably reduce rudder drag on a slow trawler by having two rudders"

Our USN Utility does have 2 rudders, one is called a backing rudder.

The launch can go 12+K depending on load so the ruder is power boat size , not sailboat size.

The hassle is docking , and since the Utility prime job was taking 150 sailors ashore , a backing rudder is the Navy solution.

It is a cone like a megaphone , installed in front of the prop with a connection to the rudder.

The concept is (I think) that the cone would take prop wash when in reverse and it would provide thrust to the boat, so it could be stopped straight into a slip,with no prop walk. .

Might have worked , but my installing a 32x32 prop totally out walks any thrust from the device.

Internally it does provide a great spot to mount the AP rudder angle unit.

FF,
I remember talking about that in early TF daze. But I don't remember seeing them on the boats we made at Uniflite. Seems to me it would be quite noticable and my inquiring mind would not have missed that.

When did they start making these boats? Perhaps it was as far back as WWll. Could have been that the backing rudders wern't needed over time. Those Utility helmsmen were very good at boat handling. Didn't know much except full throttle. Don't remember any imperfect landings. I was on shore patrol in SanDeigo and was stationed at the floats where the Utilities brought the sailors in for "liberty" so witnessed hundreds of landings.

FF,
Got a pic of your "backing rudder"?]

markpierce said:

Does having the propeller at an angle of direction create a drag too because the hull is being pushed sideways?

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Wouldn't think so Mark .. the way you put it.
The rudder creates x amount of drag when aligned w the flow of water and more and more drag deployed to either side.
But being in the propwash the drag is much higher cause the propwash is trying to push the rudder backwards .. and hence the boat. And the boat has x amount of drag going dead ahead and any degree of sideways drift would cause extra drag. But the sideways drift of the boat is corrected by the rudder so the "extra" drag is at the rudder.

In this way I believe twin engined boats are more efficient than twins. No propwalk to compensate for while going ahead w both engines unless the props don't counter rotate.