Steve Hiss of North Kingston, Rhode Island, asks:
“Ive installed a rebuilt M3-20 Universal diesel on my 29-foot Ericson, and Im 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 its not too tight. Whats 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.