Calculating the Torque required for Rudder

[David Hope]

I need to know the torque required to operate the rudder's (2) on my boat. It is a Cheoy Lee 66LR (45 ton's, cruise at 10 knots). Rudder size is approx 42"H x 24"W. I've gone on line and reviewed the formulas to determine the max torque required but am looking for verification. I've included a pic: This is not my boat but I have the same layout. I'm considering going to electric drive but am still researching. General thoughts on max time required from center line to port/starboard would also be appreciated. I"m upgrading the controls and will be able to tie the auto pilots signal to the electric drive unit if I decide to go this route.

 

[O C Diver]

While I can't tell you the torque, it will substantially increase the farther you go from zero rudder. Also it will substantially increase with speed.

As an interesting side note, if you make a rectangular rudder. The leading edge (what's in front of the rudder shaft) needs to be about 28% of what's behind the shaft. This is for boats traveling up to 20 knots. I made one for my charter boat. With manual steering (not hydraulic) you could steer the boat with one finger to maybe 8 degrees of rudder. After that, speed (water pressure) puts too much pressure on the trailing edge of the rudder.

Ted

 

[sean9c]

Buy Dave Gerr's book on boat mechanical systems, everyone should have it. There are easy to understand formulas to calculate everything you need to know.
Autopilots shoot for 10 second hard over to hard over. That is too slow for steering while maneuvering. It seems like forever.

 

[DDW]

The dynamic pressure of a fluid can be found online, 1/2*v^2*d is generally the formula. The center of pressure of a symmetric foil (including a flat plate) is classically 25% of chord (up until near stall). From the planform and pivot point those allow you to calculate the hinge moment.

One of the big variables in a power boat is, what is the fluid velocity? Since the rudders are directly behind the props, it is something greater than boat speed and perhaps significantly greater.

 

[DavidM]

If your formulas include these parameters you are probably all right. Otherwise?

Rudder parameters
Rudder angle
Rudder shaft position at or behind leading edge
Rudder area

Boat parameters
Length
Hull type
Boat speed

It might be easier to temporarily disconnect the rudder cable while underway (yes I know, it sounds sketchy) and pull the rudder shaft over with a fish weighing scale and see what the real world force is at 10 degrees, 20 degrees and 30 degrees.

David