With counter rotating props,* ..* indeed there would be as much prop walk as a single screw, since both screws will be rotating the same direction.*Is that how a twin screw boat can leave a dock such that the bow and stern leave the dock at the same rate and no way is created in either*direction? The bow is levered and the stern is prop walked .
Eric---* To your question*"is the bow levered out and stern is prop walked to make the bow and stern leave the dock at the same rate?" the short answer*is "no."
The long answer is this:
I have never seen anyone move a twin-engine, no-thruster boat directly sideways to or from a dock.* I have read numerous explanations of how this can be done IN THEORY, but I've never seen it done, and when I've tried it, even under the direction of a very experienced twin-engine, ex-USCG skipper, neither one of us could make any of the theories work.* At least not on our boat, which has a conventional semi-planing-hull-with-keel configuration with the props pretty close-in to the keel.
Putting one transmission in forward and the other in reverse causes both props to rotate in the same direction, so their prop walk will move the stern sideways.* We have 23" diameter, 4-bladed props so the prop walk force is pretty strong.**
In a conventional twin-engine configuration with both transmissions in forward*the port prop rotates counterclockwise and the starboard prop rotates clockwise when viewed from behind.* If the dock is against your starboard side and you want to move the stern out to port away from the dock, you would put the port transmission in forward and the starboard transmisison in reverse.* This will cause both props to turn counterclockwise and thus "walk" the stern to port.
BUT...... putting the port transmission in forward and the starboard transmission in reverse will also pivot the boat to starboard, or if*viewed from above, clockwise.* This will lever the bow to starboard, or into the dock in our example.* This is why, as we did at Ganges last year, my wife dangled a large fender between the starboard*bow and the dock as I moved the stern out to port*away from the dock and into the wind.
Of course, prop walk alone is not enough to walk the stern out against a wind or current.* The main force moving the stern out in this case was the fact I put the rudders hard over to starboard.* The thrust from the port prop in forward against the rudder was doing most of the work of moving the stern to port.
You could ask "Why put the starboard engine in reverse, then, since it's not contributing much to pushing the stern out?"* More about that in a moment.
With the exception of the one-in-forward-one-in-reverse prop setup, we would have used the same technique if the boat had been a single.* We'd have been using the prop's wash in forward against the hard-over rudder to move the stern to port away from the dock, and we would have used a line from the bow to the dock to prevent the boat from moving forward along the dock.** And we still would have needed the fender between the bow and the dock.
As to why we put the starboard engine in reverse to help move the stern away from the dock.......** The water*being thrust*by*a turning prop has a bit of a spiral to it, and this spiral will turn or spin the same direction as the blades.
Remember that in most installations when viewed from behind, the port prop turns counterclockwise (left-hand) and the starboard prop turns clockwise (right hand).* This means the ouboard blades of each prop are descending when the transmissions are in forward.
When the transmissions are shifted into reverse, the outboard blades of each prop are now ascending.* This causes the*spiraling movement of the water coming off the prop to*turn in and push against the side of the boat and keel forward of the prop.
With both transmissions in reverse, the sideways force of the water hiting one side of the hull is countered by the force of the water from the other prop*hitting the other side of the hull.* Same as the propwalk force*from one prop is negated by the propwalk force from the other prop.
But when you put one transmission in forward and the other in reverse, not only will both props be "propwalking" in the same direction, but the prop in reverse will be spiraling water*toward the front of the boat against the hull and keel which will push the hull in the same direction that the props are trying to "walk it."
So when moving the stern of a twin-engine boat out away from a dock on your starboard side you actually have three forces helping you do it.* 1.* With both props turning counterclockwise, their combined propwalk will move the stern to port.* 2.* With the rudder*over to starboard, the propwash from the port prop in forward will move the stern to*port.** 3.* The water spiraling forward from the starboard prop (in reverse) will push against the starboard side of the hull and keel and try to move the hull to port.* Of these three forces, the strongest one is the thrust of the port (forward) prop against the deflected rudder.*
Note that none of these forces will move the bow out from the dock.
This has all been about*what happens at a dock.* But our boat will pivot 360 degrees*quite quickly in open water simply by putting one transmission in forward and the other in reverse and leaving the rudders at zero degrees.* It does this partly*because of*propwalk and the spiraling water against the hull.* But the main force pivoting the boat is exactly*what you mentioned in your previous post--- the leverage from one prop thrusting backwards and the other prop thrusting forward.* In open water, the boat pivots around.* With a dock next to the boat, this leverage is still being generated but the dock prevents the boat from pivoting*and the bow is simply*forced against the dock.*
Of course Newton's action-reaction law means that pushing the bow against the dock will move the stern in the other direction, so this, too, will contribute to the forces moving the stern out.* So I guess there are really four forces moving the stern out, not just three.
The only way I have experienced to move our twin-engine boat sideways is not much different from what you would do to move your single-engine boat sideways.* You have to "walk" it in.* In the case of a single-engine boat you use inertia, momentum, and alternating the direction of thrust (rudder) to move the boat to the side.* In a twin, you use the same forces although you have the additional "pivoting" force from putting one transmisison in forward and one in reverse to "twist" the boat.* But this alone will not move it sideways.
As I said earlier, I've read*and tried a number of techniques to move a twin directly sideways up to a dock.* Outboard engine in forward, inboard engine in reverse, rudders toward the dock, rudders away from the dock, you* name it, none of them have worked as their proponents said they would, at least not in our boat.* The only way has been to alternate the forces acting on the boat to keep it walking over to the dock.* But there is no "set the controls like this and it will move straight sideways"* that I have found to work under actual docking conditions.
There is also the additional tool of diferential power between the engines on a twin*that can be used to affect how the boat is moving but then it starts to get complicated.* The "one-in-reverse-one-in-forward-use-the-rudders" technique is pretty basic and quickly becomes instinctive to the point where you don't even have to think about it to do it.
Of course you can get a bow thruster and all problems are solved....
Sorry for the long answer but you asked a good question and this is my understanding of the issue.
-- Edited by Marin at 04:30, 2009-03-01