The second third of our turning ability co*es from the ‘paddle wheel’ effect of the large propellers. As well as driving the boat forward or astern, at slow speeds the shape of the blades also paddle the stern of the boat sideways.
In this case both propellers are paddling in the same direction as the offset – outwards in ahead and inwards in astern – helping the boat to turn to starboard when the port engine is in ahead or to port when the starboard engine is in ahead.
The last third co*es from the rudders. Motor boat rudders are relatively small (when co*pared to sailing yachts’) as they are optimised for high-speed cruising efficiency rather than low-speed manoeuvring.
One way to increase the turning effect is by accelerating the flow of water past the rudder with the aid of the propeller in front of it; by throwing water at the rudder in this manner it dramatically increases the rate of turn – a technique that is often forgotten by the person at the helm.
Forget it at your peril, as on a windy day you will struggle to get the bow round and through the wind without using the rudder and the propeller in unison – or a lot more power! Lots of power can work but it also greatly increases the risk of things going horribly wrong.
Just remember that this technique only works in forward gear – when going astern the effect is negligible. The final tool in your locker is a bow thruster, which when used at slow speed (less than 2 knots) helps move the bow in the direction it is pushed but don’t forget that it also makes the stern move the opposite way.
First published in the December 2023 issue of MBY.
Source: How to manoeuvre a shaft-drive boat: Expert skipper's tips with Jon Mendez (http://ht**://www.mby.c**/video/how-to-manoeuvre-shaft-drive-boat-129894)