FYI: Propellers

By Andrew Spaulding

Propellers are last in your boat’s drive line and often forgotten. This never made much sense to me, since without the props the rest of the boat makes a nice downtown apartment for the summer, but doesn't get you out on the water. Knowing the ins and outs of propellers will help you identify performance and vibration issues that may entice you to have a prop expert take a closer look at your “wheels”. Considering your propellers are the engine’s interface with the water, I want to spend a few sentences describing how they actually work without getting too technical.

The propeller blade acts much like an airplane wing by developing areas of low pressure in front of the prop and a high pressure area behind the prop. This difference in pressure is what pushes an airplane up and your boat forward. How much pressure differential there is depends on the pitch of the propeller and the rpm.

There are many designs of propellers that use different aspects of the prop to achieve their designed performance. The most efficient propeller according to a physicist is one with a single blade. Having a single blade allows the prop to operate in the least disturbed water making it the most efficient. Unfortunately, a single bladed prop would be impossible to balance so prop designers use multiple blades to reduce vibration. By this logic, we would expect boats to only have two bladed props, except that blade area affects the amount of power the propeller can transfer to the water at any given time. Due to the need to transfer large forces to the water larger power boats that are heavy with powerful engines will see 4 and even 5 bladed propellers.

So how do you tell if you have a propeller problem? The first indicator of a propeller problem is variations in a wide open throttle (WOT) test. If your engine goes over the manufacturer’s recommended top operating rpm at WOT, then your propellers are too small in pitch or diameter or both. This situation is commonly referred to as “not having enough prop.” If your engine won’t reach that top operating rpm at WOT you have the opposite problem which is “too much prop.” You should conduct this test once a year and note the engine rpm, boat speed and environmental conditions (waves, air temp, water temp, etc.) at WOT. This is important information to record since if there is ever a change either your props have changed or your engines have changed. If nothing has changed with your props, you just discovered a potential engine problem that you should investigate.

What else to look for? You don’t need visible damage for the propeller to cause unnecessary vibration. Many times you won’t hear or feel anything in the boat when you run over a half-sunk log or other debris, but you will notice a slight vibration that wasn’t there before. Perhaps it comes and goes with different rpms or you notice it more on one side of the boat than the other. These are signs that you need to get the props to the prop shop. Ignoring a prop vibration can lead to all sorts of long term problems including damage to cutlass bearings, struts and engine mounts.

Other signs that your props are due for a tune are an increase in fuel consumption, cavitation, and difficulty synchronizing your engines. At Crowley’s, we have all of our prop work done by Airmarine right here in Chicago. Click here for Airmarine’s web site. Please contact us if you have propeller related questions. The closer the boating season gets, the busier Airmarine will be, so now is the time to get your wheels in the shop.