Prop rotation vs helm position

[baeckmo]

Originally Posted by gonzo

........There is no such thing as "vertical torque", it is the turning force. Maybe we are loosing something in the translation.

Sigh..., you still don't get it! With the geometry of this drive design there is a resultant torque along the so called "rudder shaft" (A in the pic.), that is twisting the drive leg to turn along this line (in relation to the hull). In other words, the steering is not stable at zero rudder angle, but strives to give SB rudder with a LH propeller, and strongly to P with a RH prop.

Normally this tendency will be dealt with by the trim tab and the selflocking qualities of the steering. The effect of the instability is the greatest during acceleration, when the engine is passing through its range of maximum torque, not max power. Floppy steering Bowden wire or air in a hydraulic steering will result in a selfinduced turn as any elasticity in the system is taken up. Some boats have hull shape properties that increase this tendency while passing hump speed, hence my questions regarding the vessel behaviour. I will dwell on these hull properties when my questions have been answered.

[gonzo]

Baeckmo: Amazingly, this boat is capable of being operated at other rudder angles than zero. The owner/operator has the uncanny ability to turn the wheel. Slack in the steering will be taken up by any side force from the outdrive. That means the operator will be turning the wheel against this force. By the way, I am factory trained by VolvoPenta.

[baeckmo]

Beeing factory trained by Volvo apparently is no guarantee against ignorance... I am fully aware of this boatowners options regarding turning his wheel either direction on his own initiative, don't give me ******** when I am in all decency trying to explain a technical matter to readers of this forum! You have questioned the effects of a certain design issue, and I have, so far failed to make you understand and that's it. Here my final comment on the steering unbalance inherent in the SP drives:

Visit a boatyard where there is a Volvo single prop drive, Aq 250, 270, 280 or 290 hanging on a transom in its normal working position. All SP drives, except for the 275 are factory set up for LH rotation, due to the instable selfsteering effect. Select one with a LH propeller, and ask for permission to get onboard and, without starting the engine, put the gear lever into "forward". Now you step down to ground and apply a clockwise torque to the propeller; this reflects the water reaction to forward propeller rotation. When applying this torque, watch the drive unit trying to swing to a SB rudder position.

Then put the lever into reverse (which would be forward for a RH prop), apply a counterclockwise torque to the propshaft and, alas, the drive is climbing around its "king pin" trying to reach a Port rudder position. In an old boat there is often a lot of play from the bushings in the yoke holding the drive, via steering "helmet" all the way to the wheel. A "budget" refit may, for instance include a cheap, undersize Bowden wire. It steers ok as long as the feedback force from the drive is low, but when the engine torque is increasing in order to pass the hump resistance, the drive may flex away to SB rather suddenly.

Most steering systems have a limited feedback to the helm; hydraulic systems practically always have a set of block valves so there is no feedback at all. This means that there is little or no feedback to the helmsman when the deflection of the drive is initiated, so the only signal for him to react upon, is that the boat is already into a more or less uncontrolled turn.

[TollyWally]

Quite interesting. Just me personally, but I've never cared for inboard outboards. Seems like too much monkey motion and too fragile. I know there are plenty of good boats running them with lots of horsepower. I have my prejudices and that is one of them. I certainly don't mean to denigrate anyone else's choice for propulsion. Boats are what we hold in common and they are all good. (as long as they float! )

[baeckmo]

Originally Posted by TollyWally

......Just me personally, but I've never cared for inboard outboards. Seems like too much monkey motion and too fragile. Boats are what we hold in common and they are all good. (as long as they float! )

Fully agreed; as a designer and builder of special workboats, they are an unwanted part of my working day! If at all possible, I avoid them for commercial use.