# Propeller Puzzles

Steve Hiss of North Kingston, Rhode Island, asks:

“I’ve installed a rebuilt M3-20 Universal diesel on my 29-foot Ericson, and I’m wondering whether the new three-blade prop, which looks a bit too big, is correctly sized. When I put the engine in gear just above idle speed the boat moves forward quietly, but any increase in rpm produces a pulsing vibration under the back of the boat. When I checked it last spring, the prop shaft felt snug in the cutless bearing. I also repacked the stuffing box and am getting five to six drops of water a minute from the box, which tells me it’s not too tight. What’s the best way to determine the optimal prop size and pitch for my engine and boat?”

Nigel Calder replies:

A general rule of thumb is that an engine should achieve maximum rpm when the throttle is wide open and the boat is running in smooth water. Some sailors like their engine to be able to “over-speed” by up to 5 percent. This provides a margin of excess power that can be used to compensate for a fouled hull or other adverse conditions. Further, most modern marine diesels can handle a marginally oversized prop. It prevents the engine from achieving maximum rpm, and that helps fuel economy when the boat is running at cruising speeds.

Another relevant detail is tip clearance, which should be around 15 percent of the propeller diameter. Blade loading, which relates to the surface area of the blades, is another important factor. Any good propeller shop can help you with all these details. Sizing a prop is complicated, so you should always seek out a specialist, preferably one who has experience with your type of boat.

## Also Popular on Sail

2 comments on “Propeller Puzzles

1. CapnAl

Once upon a time I've fought this problem myself. In marine literature I've run upon another rule of a thumb (sorry for metric numerals):

1).Hull speed of a saiboat may be roughly calculated as:

2.43 x sqrt(Lwl)       /Lwl in meters/

2). Engine HP should be about 3HP per 1 ton of displacement;

3). Optimal engine rpm is 200 per knot of hull speed;

4). Propeller diameter (D): 42mm per 1 m of Lwl;

5). Pitch should be 0.6-0.8D for a 3-blade propeller

It worked for me.

2. antoine

Hello,

I have a Sailbaot 30 foot Pearson 303 with a diesel yanmar 2gmf. it's 30 year old

My mechanic has tried to fix the engine but there is still black smoke/liquid by the exhaust when I push it over 3000 ( I beleive this  engine would go at 3500rpm).

The smoke doesn't come when we run the engine on on neutral.

He told me that he doesn't understand. We saw the bottom, it's clean. He told me naybe the propeler is the cause of the problem.It's a 3 blade and the notice of the boat talks about a two blade 16/10/1

I have done a oil test after 30 hours and the oil doesn't show anything wrong. The air filter is clean.

Any clue?