patriktegelberg

For the right length rudder the torque from the prop on the hull can be brought to zero. The rudder length under the prop shaft level needed to accomplish this is approximately the same as the propeller diameter plus a little more.

For a boat that runs with half the propeller over the surface and the “rudder water line” level with the prop shaft the prop torque is eliminated if wet rudder is the same as prop dia. For boats with wetter prop and/or rudder water line above the prop level a little more length is needed.

The reason this is possible is that a surface-piercing prop exerts a lateral force as well as a torque. The lateral force is countered by the rudder (or there is prop walk), The rudders counter force is acting below the prop and is thus exerting a counter torque. If these torques are equal there will be no list from prop torque.

Calculations are approximate and some details are unspoken.

edmkills

Interesting Patrik I wonder if increasing the rudder length would play havoc with turnability... Im thinking high speed flips?

edmkills

Would the torque not be different for different motors, say stock vs full mod, there by requiring different length rudders even with the same prop?

DaveMarles

Patrik, Are you saying that a rudder with zero deflection and a fixed length below the prop will counteract the effect of propwalk?

patriktegelberg

Edited: Note, this is the answer to the #2 post, not Daves post.

I do not know. There are offcourse many more aspects than force equlibrium that is important to make a rudder work good. I described a little about one of the parameters. I find it helps me in aquiring a feel for how a boat works/handles to know information of the kind mentioned above. Less trial and error is needed if the physics is known.

patriktegelberg

Post #5 is response to the edmkill post #2.

No Dave, that I am not.

patriktegelberg

Answer to post #3. No the full mod will have both more prop torque and more lateral force and it will work out to yield the same result.

MrMikeG

Yeah, No

patriktegelberg

I am saying prop torque causing the hull to roll can be countered with the right length rudder, if a surface piercing prop is used.

edmkills

come on Mike you got to have more to add than .

Flabum

The torque roll and propwalk would vary greatly from boat to boat. A tall, narrow deep V boat would exhibit a lot more torque roll than a wide, flat bottom boat would. At the same time the prop walk would be similar. I do not see how a rudder will help prevent torque roll unless it is directly behind the prop. The best method of controlling torque roll is by using trim tabs. Hydros use turn fins for turning, but the turn fin also prevensts propwalk, so rudder length is not needed fo the prevention of propwalk.

patriktegelberg

What do you mean Mike?

glennb2006

Hi Patrick,

I can kind of see how a longer rudder will eliminate an amount of both propwalk and torque roll, proportional to effective length, but I am thinking the rudder will then almost be a deep keel. Which is how the stability is achieved. Am I on the correct train of thought - or at least the same one as you?

Not quite thought through the rest yet.

Glenn

patriktegelberg

Hi Glenn.

Subtle, about my train maybe not the correct one .
I am not talkong about prop walk, it was just written because I thought more would recognize that there is a sideways force from the prop. About the keel thing I don't really see how you mean, and I am not saying stability I am saying equlibrium. To see it try to exagerate (called taking the limit in fancy language) and imagine a one foot rudder. The prop is trying to move the stern forward and to to left. Can you now see that in this exagerated case the boat will list to the left. So the normal is prop torque is rolling the boat to the right and a too long rudder makes the boat roll to the left. I calculated approximately at what length rudder the boat is not rolling at all.

glennb2006

n

glennb2006

OK Patrik,

nearly with you, I am assuming this scenario is with the rudder adjacent to the prop.?

No offence intended by the way on the train of thought comment.

Glenn

patriktegelberg

So far I have only been concerned with the roll degree of freedom. If one wiches to mix in the yaw freedom the rudder force center must be at the same length coordinate as the prop force center. Which maybe can be called adjacent. Will work for offset rudder yes.

MrMikeG

Rudder is not a keel in any form, a weighted keel (like a sail boat) would help torque roll. A longer rudder would help the momentary force of ENGINE torque roll but Prop torque is constant load which is lifting one side of the hull. Prop torque lifting one side of the hull will not be effected in any way by rudder length. Think of prop torque as an unbalanced hull (it is lifting one side) if you put a brick in one side of your hull is the rudder going to straighten it? The rudder will level it out in a turn but not going straight.

patriktegelberg

I will argue that it is the other way around. The rudder does not counter the momentary engine torque originating from accelerating the flywheel. But does counter the constant torque from turning the propeller. About the brick, to many unspecified variables. If I aswer I fear I will walk into a trap. But yes it will. You say it will level it out in a turn, to some extent the rudder is turning a little all the time to counter the prop side force.

Flabum

Prop side force is propwalk. Prop walk is not corrected by turning the rudder. If propwalk were corrected by turning the rudder, then you have a boat running dog-tracked. The way teh rudder corrects propwalk is by providing side resistance. The deeper you run the rudder, the less propwalk you will have. You can also put more rudder in the water by adding to it's length. Either option prduces more drag, a touchier steering system and overall reduced performance. One other way to fight propealk would be add a turn fin or skeg to the back of the boat. This also would create more drag. the best we can do is find the best set-up for each of our particular boats.

patriktegelberg

Flabum, I disagre with you on almost all points. In particular the dog-tracked thing is not correct.

Flabum

Well, if you have a prop pushing the back of the boat to the left putting you into a right turn and you steer left to compensate, then you will be dogtracking. I have seen it, especially on shallow V's and flatbottoms.

MrMikeG

I agree with FlaBum, again the difference between running boats and theory. I do like that Patrick is thinking about the better mouse trap though.
Prop torque and prop walk are cured by the right prop depth and a sharp prop. You can off set the drive as well as angle the drive too. Any turning of the rudder to try and correct ether is a loss of speed.
You don't have to use a brick, any unbalancing of the hull will work. Chines will flatten an unbalanced hull too!
Oh one more way to stop prop torque and prop walk,,,,,,,submerged drive!

Flabum

Don't you mean "Hard Chines"? The chine is where the side of the hull meets the bottom of the hull. Putting a 90* angle there inwards an inch or so or less will create a "Hard chine" , a flat surface to run on. [8D] I agree with you on all the other points as well.

DaveMarles

Mike, Thats not correct. With submerged drrive you have a massive increase in prop torque on the hull. Take a look at any modern FSRV hull to see what is done to counteract it.

Patrik , I still dont understand. Are you saying a deeper rudder with no angle on it i.e. no turning at all, will will correct prop torque/walk?