Propellers versus Engines

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.

So you have a new engine?

CONGRATULATIONS!! You have a brand new engine and it purrrrs like a kitten and it’s all shiny and bright. Here are a few pointers that come from frequently asked questions by new engine owners after we install it.

1. What is the break-in period and how do I run the engine? We normally consider 50 hrs. to be the “break-in period” on most engines. The engine really continues to “wear in” long after that, but as far as the owner/operator is concerned, 50 hrs. is it. During this time, we recommend that you avoid running the engine at high outputs for extended periods of time. If you want to give the engine a brief WOT (wide open throttle) for less than a minute, fine. It’s actually recommended to do that at least once each time the boat is used. This way you can make sure the engine will develop full speed and you can determine if anything has changed since the last time you used it. We like to have our customers “mix up” the rpms during the break-in period. This means don’t run the engine at one speed for the whole day. Every hour or so, change the engine speed, as much as practical. This changes the load on the engine and allows more complete break-in.

The first engine service, usually at 50hrs or so, is probably the most important service that your engine will have. This is where the oil and filters is changed which gets rid of all the particles of metal from the engine “wearing in”. In addition, you get remove any dirt or contaminants that might have been in the engine from the factory. Just like people under going an operation, engines get contaminated the worst from being taken apart and/or being put together.

At this point we also want to set the valve lash. Since the pushrods, rocker arms, and valves have all gotten to know each other better, they are not usually at the same setting as when they left the factory. Once we set the valves at 50 hrs., you can usually go for a couple of hundred hrs. before doing it again, but that will vary with each engine. If your 50 hr. check calls for re-torqueing the cylinder head bolts, then a valve adjustment is mandatory.

Engine alignment should also be checked at this point. The internal engine parts are not the only things that have gotten to know their neighbors better. The engine and the boat itself have figured out where they are going to be and that may not be the best thing for the propeller shaft. Engine mounts will tend to “settle” with vibration and pressure, finding where they want to be. They may allow the engine to fall lower in the boat, bringing the prop shaft out of alignment. As with the valve adjustment, once you align the engine now, you will not have to do it for several hundred hours.