Calculating the Torque required for Rudder

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David Hope

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Dec 11, 2022
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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.
rudder.jpg
 
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
 
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.
 
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.
 
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
 
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
Should you desire to reduce the rudder loss by 2-5% fuel saving, I would suggest you have the front counterbalance on the prop side of the rudder stock modified so that the prop wash which hits your prop at an angle, should be angled into the prop wash at top and bottom,
Look at your prop and note that the prop pushes the water at an angle as it leaves the prop.
So, the prop wash hits your rudder at an angle.
Note carefully: That prop sends the water at an angle in one direction for the top of the prop blade, and in the opposite direction for the bottom blade.
Simply angle the front counterbalance of the rudder to hatch the upper water flow from the prop so the water flow hits the rudder front piece head on, rather than hitting the counterbalance at an angle.the water flow from the water flow coming at the rudder from the lower prop flow.
Ask your friends to read this to make sure you do it correctly.
That rudder counterbalance when offset about 20 degrees, will make your rudder turn the hull better, and stop wasting that power which hits your rudder sideways, wasting fuel.


Do the same for the
 
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.
View attachment 159493
I replaced my aircraft wire steering with hydraulic. Dometic did the engineering once I gave them the specs. They then gave me the specs on which equipment to buy. Everything when smoothly. I'd be concerned to rely on electric steering in a boat. Something about electricity and salt water.
 
I think if you contact the vendor of the steering system you are contemplating they can run the specs for you and design something that should work. Curious whether you are trying to solve a handling problem. I snapped this photo of a 66 on the hard and was a bit puzzled.
 

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I think if you contact the vendor of the steering system you are contemplating they can run the specs for you and design something that should work. Curious whether you are trying to solve a handling problem. I snapped this photo of a 66 on the hard and was a bit puzzled.
I think the offset rudder was to make it easier to pull the shaft out.
 
This is not an intractable calculation. I have a PhD in hydraulics, hydrodynamics. and hydrology. If anyone needs assistance with these calculations fell free to contact me. That being said, it can only be calculated on an Ideal basis assuming the rudder is flat. That will priduce a worst-case calculation since any curvature would reduce the results.
 
Should you desire to reduce the rudder loss by 2-5% fuel saving, I would suggest you have the front counterbalance on the prop side of the rudder stock modified so that the prop wash which hits your prop at an angle, should be angled into the prop wash at top and bottom,
Look at your prop and note that the prop pushes the water at an angle as it leaves the prop.
So, the prop wash hits your rudder at an angle.
Note carefully: That prop sends the water at an angle in one direction for the top of the prop blade, and in the opposite direction for the bottom blade.
Simply angle the front counterbalance of the rudder to hatch the upper water flow from the prop so the water flow hits the rudder front piece head on, rather than hitting the counterbalance at an angle.the water flow from the water flow coming at the rudder from the lower prop flow.
Ask your friends to read this to make sure you do it correctly.
That rudder counterbalance when offset about 20 degrees, will make your rudder turn the hull better, and stop wasting that power which hits your rudder sideways, wasting fuel.


Do the same for the
Look up "Contra Guide Rudders" they were used on Liberty ships advertising the same savings you quote. Except that the forward part of the rudder was turned the opposite way you call for thus the name ContraGuide. These rudders were split at the prop shaft line and each portion was aligned opposite the propwash in an effort to straighten it and add to thrust aft. Your method seems to make some sense too. But neither works to any measurable degree.. My experiments show that the upper and lower propwash diverge almost 20 degrees from each other so it would seem that a rudder with that divergence would offer less resistance but it doesn't offer more thrust b/c the wash isn't going straight aft and there is a loss there.
 

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