Your yacht is floating in her slip, securely tied to the pier and ready to go sailing. You need to reverse you yacht out of the slip, but which way will she walk?
A very simple test to see which way and how severely your yacht will walk is to put the boat in reverse under full throttle and evaluate the prop wash.
The side with more prop wash will be the side you walk away from. If you see prop wash on your port side, you will walk to starboard. If you see prop wash on your starboard side, you will walk to port.
While most of the factors that compound to create prop walk are beyond your control right before you go sailing, one key factor is: engine speed.
With the engine off, there is no prop wash and the water around the boat will be still.
With a little bit of throttle, the engine is consuming 32.1 amps and the propeller is spinning at 697 RPM. There is very little prop wash on either side, as there is little thrust being produced.
With the engine drawing 61.2 amps, and the propeller spinning at 889 RPM, there is considerably more prop wash and prop walk. You can see from this picture looking at the bow how the docklines are holding the boat steady as well as the difference in the amounts of prop wash on each side of the boat. The port side is very disturbed, while the starboard side is very calm. This illustrates why the boat will walk to starboard.
With the engine drawing 89.4 amps, and the propeller spinning at 1012 RPM, the prop wash is considerably strong on the port side and only beginning to disturb the water on the starboard side.
With the engine drawing 178 amps, and the propeller spinning at 1255 RPM, we are creating considerable trust and prop wash on the port side.There is a very slight amount of prop wash showing up on the starboard side.
The final test was performed at full throttle, with the electric motor drawing 302 amps, and spinning the propeller 1480 RPM. At this pace, our very small battery bank will only last for 0.4 hours, or 24 minutes, which illustrates why we sail everywhere instead of motoring everywhere. With a larger battery bank and a generator, we could have a significantly longer range under power, but we chose a simpler and less costly installation and opted for a small battery bank with a full suit of sails.
At full throttle astern, the prop wash on the starboard side is starting to be more evident, but nowhere near the level of disturbed water on the port side. Under full throttle, the boat will walk to starboard as all of the thrust is being directed towards the port side of the hull.
As the RPMs increase so does the bias in the in thrust, leading to an increase in prop walk to one side while moving astern.
The prop wash on the side the boat walks towards in reverse is about the same under light and full throttle.
On the contrary, the prop wash on the side the boat walks away from in reverse is vastly different. Under light throttle, the prop wash is very light and can be somewhat combated by the opposing sides prop wash. Under full throttle, the prop is very powerful and will push the boat over, causing the phenomenon known as prop walk.
Next time you need to reverse into a slip, try using a light amount of throttle. If you are reversing out of the slip and have significant prop walk, maybe you should consider walking the boat out of the slip by hand and then motoring away once clear of the slip.