When you run a boat under power, the prop size and pitch needs to be matched to the maximum speed of the boat as well as the maximum rated RPM of the engine. In other words, at wide open throttle (WOT), the boat should reach its maximum (comfortable) cruising speed and the engine should be operating at its rated maximum RPM.
Our PDQ 32 LRC has two Yanmar 2GM20f Diesel engines, each rated at 18hp at a maximum RPM of 3600. So, at WOT, the engines should be able to operate at about 3600 RPM and the boat should reach a speed of about 7 knots.
When we bought the boat, she had upgraded 3 blade props (considered more efficient and smoother than the stock two bladed props). However, when motoring on smooth water, the engines would only achieve 2800 RPM at WOT. In addition, there was smoke in the exhaust indicating the engines were laboring too hard. The analogy would be driving a car up a hill in 4th gear. The car would go, but the engine would be lugging and operating at a suboptimal RPM. If you downshift, the RPM goes up, you have more horsepower, and the car runs more smoothly. The term for this in boats is being over-propped. This is due to the pitch of the prop being too steep.
Fortunately, there are propeller specialist shops that can re-pitch props and they have sophisticated computer programs to determine the optimum prop pitch based on your boat, transmission and engine. I used Coastal Props in Fort Meyers. The existing props were 15” in diameter with a pitch of 12. After consulting with Coastal Props they ran the info through their program and decided that the pitch needed to be reduced to 9. I removed the props when the boat was on the hard, sent them to Coastal Props and they re-pitched, balanced and polished the props for $135.00 each, a bargain.
The improvement in engine performance was dramatic. I can now get almost 3600 RPM at WOT and comfortably cruise at 3200 rpm, much closer to the peak HP of the engines. The engines run much more smoothly and no more smoke! Diesel engines also need to periodically be run at their max rated RPM to burn off carbon from the valves and exhaust. The re-pitched props now allow me to do this. Thus, for a small investment, I believe I have greatly increased the life of the diesels on Terrapin.