Offset single prop - which direction?

[Ben Lindenberg]

Old wooden sailoats often have their single prop offset to on side, to avoid a big hole in the keel wood.

Now, assuming we look at the boat from astern, and the prop turning clockwise in forward gear, which side is the correct one for the prop? Port or starboard? And why?

Or, reformulated, if the prop is to port, does one need a prop turning clockwise or anti-clockwise in forward gear?

Thanks for your input,

Ben L. (Bergen, Norway)

[Corpus Skipper]

I'm fairly certain you want the top of the prop turning outboard from the keel. Clockwise on the starboard, counter to port.

[Eric Sponberg]

Here is an easy way to answer your question. When the engine is idling and the propeller is turning slowly, a clockwise-turning propeller will "walk" the stern in the direction it is turning. You can imagine the propeller as a wheel that is rolling, and a clockwise-turning prop will "roll" to starboard, taking the stern of the vessel with it. That necessarily points the bow to port, and you want to counteract that tendency. Therefore, you want the thrust from the propeller to create a moment in the opposite direction. A moment is always a force x distance. In this case, force is propeller thrust, and distance is the distance of the shaft line offset from the hull centerline. A port moment (shaft line to port) will cause a turning moment to starboard. Therefore, a clockwise propeller should be offset to port, and a counter-clockwise propeller should be offset to starboard.

Eric

[Michael Kablitz]

There you have it: A simple either/or question and two diametrically different answers by senior members!

Who's right? Well, both: Because -surprise- it all depends. It depends, namely. on whether your prop shaft is parallel to the centreline (that is, the engine is offset in the same way the prop is), or not (engine on centreline, but prop offset).

Here's my take (prop clockwise in fwd):

parallel shaft: prop (and engine) to port
angled shaft: prop to starboard. (engine on CL, at an angle)

The first case was explained by Mr. Sponberg: You want to counter the turning moment of the wheel effect with the opposite turning moment that results from the prop being off-centre.

When your shaft is angled, you introduce a third, and very powerful, moment: The effect of the prop wash being at an angle to centreline. Since this moment is quite large, is must counter BOTH aformentioned moments for this setup to behave right.

Since you talk about wanting to avoid to bore a weakening hole in the wooden keel, and this would really only be an issue with small vessels, I assume both engine and boat are quite small? This may force the engine to CL, for space reasons. In this case, the setup with the angled shaft is ok under power, and beneficial under sail, because the prop cutout in the keel is avoided and the rudder runs in smoother water.

With a big engine (for the boat size), the third moment will be too much for good behaviour, and the angled setup is out.

[Danielsan]

Hi,

Does these things also aply for powerboats?

Greetz,

Daniel

[Eric Sponberg]

Yes they do. As a matter of fact, in a twin screw powerboat, the propellers should turn top going outboard. This means that viewed from the stern, the starboard prop should turn clockwise, and the port prop should turn counter-clockwise.

Eric

[Danielsan]

Does it need to be offset in a single prop configuration?

Greetz,

Daniel Peeters

[Eric Sponberg]

No. You can set a single prop on the hull centerline if you want. You will necessarily get a slight turning moment to one side or the other, depending on prop rotation. The reason for putting a prop off centerline is to counteract that turning tendency. But the vast majority of single-screw boats have the prop on the hull centerline.

Eric