Are You Over-Propped?

Robin Urguhart's Dufour 35 Monark being hauled out allowed easy access to the propeller.

- Robin Urquhart

Before heading out on a long multi-year trip, you might want to consider taking a good look at your propeller. Overpitched and oversized propellers (over prop) are common problems that often go undiagnosed. Unresolved, this issue can significantly reduce the performance and longevity of your engine. It can also cause untold frustration as you try to figure out why your engine is overheating.

The issue affects boats both old and new. Boats that come straight from the yard are often configured with propellers suited to clean hulls, empty tanks and only one person on board. As soon as the tanks are filled and extra weight has been added for comfort and long-distance cruising, the propeller and engine are likely no longer well matched. The same is true for an older boat, as was the case for us.

We purchased our 1979 Dufour 35 three years ago in the frigid waters of the Pacific Northwest. The previous owner had told us that the Kubota engine, adapted for marine use, would overheat above 1,800 rpm in the summer. As relative neophytes to the world of marine diesels, we didn’t think much of it, especially as we could easily cruise at 6 knots under power. We hauled out for a three month stint in the yard in preparation for a multi-year trip to Australia. We added a water tank, solar arch and panels, and a literal ton of other equipment and supplies. The boatyard owner, upon seeing our boat, remarked on the large size of our propeller. I’m compensating, I joked. The warning signs were all there, but we were unheeding. Then we headed south…