setting twin rudders

[Antpsi]

I have a boat with twin rudders and was wandering if there was a way to set the rudders parallel, other than by the eye ball method.

[green-boat]

Tape measure.

[Captain Bill]

You can measure the distance between the front leading edge of the rudders and measure the distance between the rear edge of both rudders, when the distance of the front edge and the rear edge is the same, they should be straight.

[420TEE]

Why would you care if they are precisely parallel? If they look good, that's good. :rolleyes:

[Antpsi]

Thanks ver much, your advise is good and practical. I will try using external calipers, now that I have the idea.
Thanks again.

[meechingman]

Just to muddy the waters, what about the effects of toe in or toe out?
Andy

[CG Bob]

You're more likely to see vessel with rudders that toe out. The USCG 41' UTB has "toe out" rudders, they are set perpendicular to the bottom hull plating. The prototype USCG 47' MLB and had toe-out similar to the 41" UTB.

The WWII German S-boats were fitted with triple rudders. To get the "Lurssen effect", the outboard rudders could be adjusted outward; so the outboard rudders were not parallel with the center main rudder. The Lurssen effect gave the boats a little more speed.

[Jim Duda]

Bob - when you refer to rudder "toe out" do you mean the dimension between the Trailing Edge of the rudders in greater or less than between their leading edges?

[Phoneman2005]

CG Bob

I had the job of replacing all the hydraulic steering system lines on the first 47'er at Cape Disappointment. This was when the rudders were perpendicular with the bottom. Steering was very scarry at 27 knots when a hard turn was preformed. the vessel would veir out of the turn and roll the outside rail under water. They had to change the rudders so they were vert with respect to the water line. Nice boat other than that one small problem.

Dave

[Phoneman2005]

deleted

[CG Bob]

Bob - when you refer to rudder "toe out" do you mean the dimension between the Trailing Edge of the rudders in greater or less than between their leading edges?Jim - look at the picture of the 41467 - its rudders are toed out. On this class of boats, the rudders are set perpendicular to the hull bottom; they are not vertically parallel with the keel. The rudders are longitudinally (fore & aft) parallel with the keel. The center-to-center distance of the top of the rudder is smaller than the center-tp-center distance of the rudder bottom. If you were to connect the dots at the corners of the rudders, you would have a trapezoid.

[Jim Duda]

Bob - understood...thanks. I was thinking in automobile suspension terms where wheels are aligned "toe in / toe out" and CAMBER would be used to describe the rudders in the 41467 image. I guess automobile and nautical descriptions don't always agree...

Thanks again!

[meechingman]

Ah, terminology! Yeah, what Jim said is what I meant. I have the twin rudders on my tug set so the trailing edges are slightly closer together than the leading edges. That way the outboard rudder in an ahead turn is working slightly harder - or so I think. But hey, maybe I'm wrong!! :)

Andy G

[pkboo]

CG I found this on the Lurssen effect:

A PT boat roaring through the sea with the forward third of its hull suspended above the surface of the ocean and churning out an impressive wake, is a joy to watch. But it is not the most efficient means to move a boat through the water. The Lurssen Effect is created when two, small Lurssen rudders, mounted to either side of the main rudder and turned outboard, lowers the wake height, which, according to Trent “requires less energy, allowing the vessel to go faster.”

Do you know anybody who tried this on a PT109 or any boat with the same type/kind of hulls (like some of the CG-boats)?

[pkboo]

I'm answering my own questions, I guess! And I think, while my PT is being refitted, that I'm going to implement this on it! Here's what I found on a forum over E(S)-boats.

About the Luerssen Effect:
The two outboard rudders were ‘trim rudders’, designed not to assist with steering, but to affect the angle of the hull as it goes through the water.

At low speed, the two outer rudders would remain parallel to the line of travel. Once the vessel got up to about 25-26 knots, they would be turned outward, away from each other, until they were pointed 30-degrees out from the line of travel. This caused the water to be pushed ahead of them, instead of passing cleanly around them. Because this zone of water traveled along with the hull, the screws were kept in a zone of water that was moving more slowly past them than the actual speed of the ship. The propulsive force of the screws is increased, allowing them to push the vessel along at a slightly higher speed (about 1 knot) without increasing RPMs; basically, instead of spinning the screws faster to keep up with the water rushing by, you have slowed the water down to allow the screw to push against it at the same RPM.
The biggest benefit is that the stern of the vessel gots pushed upward, leveling the boat, which eliminates the ‘rooster tail’ stern-wave and also cuts down on drag. The rising stern forces the bow back down, and the ‘white water’ bow wave disappears. So now you are going 27 knots instead of 26, making less noise, and showing very little wake.

Of course, you want to go faster than 27 knots, but the drag of the trim rudders angled out 30 degrees prevents this. But here is the cool thing...once the Lürssen effect kicks in, you can slowly bring the trim rudders back towards the centerline as you increase speed, without disrupting the effect, until they are at just a 17-degree outward angle. Now the speed increase is on the order of 5%, so you can go 42-43 knots on the RPM for 40 knots, and you are still making very little ‘white water’ wake. So you have no white-water wake to betray your presence at night, and you are making less noise than you enemy...does it get any better for a night attack?

[green-boat]

It would make sense that if the rudders were angled outward, that would pull more water in under the hull/ stern causing a high pressure area lifting the stern.