|
|
Post by davidso451 on Jul 21, 2020 0:39:44 GMT
In 12 to 15 foot swells, I had lost autopilot so was hand steering at night along the California coast heading south last December. I ran down below to fill my cup of tea with hot water, seemed like it only took 15 or 20 seconds max. I ran back up top and everything looked fine, about 3 minutes later I realized the coast was on the wrong side. I had spun a full 180 in not much time, the swells were rocking the boat so much I didn't even feel it turn. My boat will quickly turn, I was motoring about 2500 RPM with no sail, big seas but very little wind.
|
|
|
|
Post by johannes on Jul 21, 2020 4:56:28 GMT
We experience the same effect. It is caused by the combination of a balanced rudder and prop wash. When sailing, the rudder is slightly feathering and the steering will tend to keep centered. When motoring, prop wash will cause more force on the foward edge of the rudder than the aft part, since the propeller is closest to the foward edge. This means the rudder is no longer stable and a small disturbance may force the foward edge of the rudder to be deflected to either side. Since the prop wash is not symmetrical (right or left rotation) in relation to rudder and water depth, one side may be more favoured.
|
|
|
|
Post by moonshadow on Jul 21, 2020 14:49:05 GMT
Yes I think you are correct about the balanced rudder effect in prop wash. To gather more information after my usual turn to Stb when releasing the wheel I tried something new. I started with 1 or 2 degrees wheel to port and released the wheel. And the boat began a slow but increasing turn to port. So I am convinced that it is simply prop wash on the leading edge of the rudder overcoming the natural tendency to remain straight down the majority of the rudder behind the pivot point.
Now I am still wondering about the few messages I received that said they don’t see this tendency.
Thank you all for the help figuring this out and I am glad that there is nothing amiss.
|
|
|
|
Post by ForGrinsToo on Jul 21, 2020 19:13:36 GMT
Asymmetric thrust occurs when the angle of attack differs between the rising blade and the falling blade of the propeller. If you do not have a saildrive, your propshaft is at probably 15 degrees relative to the forward motion of the boat, so there will be a 30 degree difference between the two. There is also a spiraling slipstream which provides more wash along one side of the rudder than the other, which will tend to turn the rudder. We tend not to think of propellers as waterfoils, but they are, just as an airplane propeller is an airfoil.
Powerboats shoot for 10 degree shaft angle, more efficient at speed, but for most of us that would put the engine over the keel, in the middle of the salon.
I think a lot of shaft drive people don't notice because there is a good bit of friction in their steering system.
I'll also note that a balanced rudder has about 40% of the chord forward of the rudder post. The center of the rudder post on my 36i is maybe 3 cm behind the leading edge, so this spade rudder is semi-balanced - as I understand it.
A friend owned a Cape Dory with a full keel, an unbalanced rudder, and a tiller which required a lot of effort.
Geoff
|
|
|
|
Post by Bora on Jul 24, 2020 11:26:44 GMT
Unfortunately not been sailing too much in croatia so far with the wind either gone or against us so lots of motoring!
According to my rudder indicator I’m using 2-3 degrees or starboard rudder to go straight, it may be that the indicator isn’t centred, I’ll have to dive down and see.
But even with that 2-3 to stbd it doesn’t induce a turn.
Looking at the posts above I’d agree it’s probably not something to worry about, just something to be aware of.
|
|
|
|
Post by zaphod on Jul 26, 2020 22:16:55 GMT
After all this discussion I was paying attention to how our 39i behaves under power. Motoring at full cruising speed I can let go of the wheel and the boat will continue on pretty much dead straight in flat water. Certainly nothing like the forces the PO described. Perhaps it is a problem peculiar to sail drive setups.
|
|
|
|
Post by johannes on Jul 27, 2020 2:21:44 GMT
After all this discussion I was paying attention to how our 39i behaves under power. Motoring at full cruising speed I can let go of the wheel and the boat will continue on pretty much dead straight in flat water. Certainly nothing like the forces the PO described. Perhaps it is a problem peculiar to sail drive setups. No, we don’t have a saildrive. |
|
|
|
Post by zaphod on Jul 28, 2020 3:59:54 GMT
After all this discussion I was paying attention to how our 39i behaves under power. Motoring at full cruising speed I can let go of the wheel and the boat will continue on pretty much dead straight in flat water. Certainly nothing like the forces the PO described. Perhaps it is a problem peculiar to sail drive setups. No, we don’t have a saildrive. Ok, the OP does have sail drive, and you experience the same thing with shaft drive. I know on some boats they offset the prop shaft slightly to facilitate removal of the prop shaft without removing the rudder. Perhaps that is the case with your shaft drive, but I suspect it is a different reason for the OP and his saildrive. |
|
|
|
Post by johannes on Jul 28, 2020 5:53:15 GMT
No, we don’t have a saildrive. Ok, the OP does have sail drive, and you experience the same thing with shaft drive. I know on some boats they offset the prop shaft slightly to facilitate removal of the prop shaft without removing the rudder. Perhaps that is the case with your shaft drive, but I suspect it is a different reason for the OP and his saildrive. Our shaft drive is not offset. In any case, the reason has to with the physics of balanced rudders and it has not to do with saildrive or shaft drive, as I tried to explain in an earlier post. |
|
|
|
Post by zaphod on Aug 2, 2020 7:40:47 GMT
Ok, the OP does have sail drive, and you experience the same thing with shaft drive. I know on some boats they offset the prop shaft slightly to facilitate removal of the prop shaft without removing the rudder. Perhaps that is the case with your shaft drive, but I suspect it is a different reason for the OP and his saildrive. Our shaft drive is not offset. In any case, the reason has to with the physics of balanced rudders and it has not to do with saildrive or shaft drive, as I tried to explain in an earlier post. I understand the theory, I am just saying that I do not suffer the effects you describe on my boat with a very similar balanced rudder. Perhaps it it just the increased friction of my twin wheel system that negates the effect, although that would suggest that the effect is not very strong. |
|
|
|
Post by jbrelin on Nov 27, 2023 21:55:09 GMT
This is how I understand this phenomenon. There are several things going on here.
1. When a right handed prop spins (think of your thumb pointing forward, and your fingers curling clockwise to a fist), there is a port thrust lower on the rudder, and a starboard thrust higher on the rudder.
2. The rudder post in most sailboats with balanced rudders, is behind the leading edge.
3. Because of that fact, the port thrust will try to move the rudder in a counter-clockwise fashion (when looking down), and the starboard thrust in a clockwise fashion.
Then why does the boat move to starboard when the rudder should be balanced by opposing forces?
4. It is because the water pressure below where the port thrust is applied to the rudder has a higher pressure than the water pressure above where the starboard thrust is applied, because it is deeper. Also there is a larger area of the rudder that is being applied by the port thrust than the starboard thrust because some of the starboard thrust is deflected by the hull.
So depending the the physical distances between the prop and the rudder and the hull will determine the effect of the pull.
|
|
