wrong way counter rotate

[yotphix]

Help! Can anyone tell me why on a $180,000 centre console sport fish the outboards would be installed with a LH prop and gearbox on the stbd side and RH on the prt? It requires that in docking the 31' boat is treated like a single engine since the prop walk counter acts the effect of reversing one with the other in fwd. Is this thought or proven to have better performance or efficiency? Could it just be a mistake? I have certainly never seen it before but man do I hate parking this boat! (Not mine so I don't have to very often! )

[Verytricky]

Speed vs Comfort.

If the props rotate ( as seen from the rear ) LHS clockwise, RHS anti clock, then you have the boat pulled down slightly, giving better grip, and handling. The opposite gives a more 'skittish' hadling of the boat, but as th boat is slightly pushed out the water, it goes faster.

[jehardiman]

Simple answer as given by Verytrick above is better efficency due to flow interferences between the wheels. See also the comments under this thread http://boatdesign.net/forums/showthread.php?t=12192

[yotphix]

Thank you both, for the answer and for the thread link. I figured that there must be a reason for it. In my experience on large yachts (100'+ LOA and 140+ tons displacement 4000-5000 hp) it has been universally true that the props spin outwards in forward. This is critical to the close quarters handling of the vessels given that we often med moor or park in slips with under 2' clearance on our beam. We need prop walk!

[yotphix]

On the topic of this boat, the other thing I noticed about it is that the outboards are placed as close together as they can be without touching when turned. Would this have been done to maximize the desired effect of "pulling the boat down slightly" as tricky put it?

Last, has anyone got experience with just how dramatic the effect of switching the rotations and spreading the motors a few inches would be? I know that it would definitely improve the close quarters handling, wide set and other way rotaion boats I have driven are quite manoueverable. Would it likely make the boat unmanageable at 35-40knots?

[Verytricky]

This depends very much on the boat weight.....

a 1.5 ton planning boat would have significant differences with a change of gearboxes. I have seen a 3 mile an hour improvement in top end, but skittish handling.

If you have a huge 150 ton boat I think this is irrelevant.

[Frosty]

I tried both ways on my boat because I asked the same question. The only difference was that the effect on the rudders was reversed wich meant that the hydraulic tie bar was in suction rather than under pressure. This resulted in the pressure in the tie bar to be so low as to boil the oil (It took me a long time to work that one out). The rudders then mis aligned and oil spewed from the upper station. I had to reverse it to normal --left is left - and right is right.

[Willallison]

Another reason for mounting the engines as close together as possible is because as you move away from the centreline, the engines must be mounted higher, meaning the props are likely to come out of the water more often

[yotphix]

I suppose you are talking about inboards getting higher as they move out on account of deadrise. These are outboards mounted on one of those transom extensions. (I have never understood those either but that is probably another topic.)

[Willallison]

No, I was talking about o/b's (though it applies to inboards too, but also for the reason you suggest)
The ventilation plate (usually, but incorrectly called the cavitation plate) is set a certain distance above the bottom running surface that it is behind. (This distance varies according to how far back from the tranom the motor is mounted)
So, as you move out, the motors must be mounted higher.