Speed while motoring

[nrx500]

Sounds like your engine is underpropped. Without a working tachometer, it's hard to tell. I had a 13 inch diameter by 8 inch prop on a 10 foot wide by 22 foot long catboat with the Yanmar 2GM20 diesel. That prop drove my boat at 4.5 knots. I overpropped to 13 x 14 and improved speed to 6 knots, my theoretical hull speed. But my engine could not reach its 36000 rpm maximum rotation speed, so I changed to a 13 x 10 prop. Next time you haul, pull your prop and take it to a prop shop. Also shake the shaft to see if the cutlass bearing is worn, and check your engine mount bolts. If you think there's a problem, pull the shaft and shoot a laser through the stern tube to the center of the coupling to check alignment. If you're out of alignment, use the nuts on the engine mounts to raise or lower the engine until the lase shows it is lined up with the stern tube.