When it comes to propellers there is quite a bit of mystery regarding which propeller is best for a particular engine and vessel. Although there are no sure fire answers, there are some guidelines. The proper propeller is instrumental in getting the maximum performance from your engine, regardless whether yours is gas or diesel, sail or power.
Each engine has a specific rpm where its maximum power is developed. The amount of power developed is a combination of rpm and torque. The faster the engine spins, the more power pulses are made and the more fuel is burned. Hence, the faster the engine turns the more power is developed. If the engine turns faster than it’s governor set for, then the governor will reduce the amount of fuel delivered and reduce the torque, reducing the fuel being burned . Inversely, if the propeller keeps the rpms below rated rpm, the lower rpm reduces the amount of fuel burned and therefore horsepower. A propeller that causes the engine to turn too fast or too slow will reduce the available power.
To determine what the proper rpm is for you particular engine, you need to consult your operator’s manual and look for it’s maximum rated power rpm. This is the rpm your engine should develop when underway in calm water. Some specifications provide “intermittent” power and “continuous” power. The intermittent rpm is the maximum power and is what the propeller should be sized for. Continuous power is the rating that you can cruise the engine at all day long. Take your boat out and get the engine warmed up. Open the throttle slowly until wide open throttle is achieved. With the boat operating at maximum speed, what is your maximum rpm? Is it less, or more, than the “intermittent” rpm in the manual?
Now comes the fun part. What to do with your prop? We’ll give you the simplified version by saying: Rpms are low, reduce pitch: Rpms are high, increase pitch. This is a huge simplification, but that is the basic idea. Most sail boats with a 2:1 transmission will respond by a change of approx. 300rpm for each inch of pitch change on the prop. Power boats can change more or less depending on type of boat, propeller, and gear.
Many owners are apprehensive about running their engines at wide open throttle as they don’t want to damage their engines. Running at rated rpm and throttle should be part of your diesel ( or gas) engines operating procedure. Not for extended periods of time, but just enough to make sure that your engine can still maintain full rpm. Conditions change, fuel load, people and gear load, fouling on the bottom and propellers, all contribute to slowing the boat (and engine) down. If your engine can only achieve 200 rpms less than it used to, then you need to reduce your cruising rpm as well. Otherwise, you will be operating the engine at a higher percentage output than you think.