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Propeller Performance Primer

By Don Garrison

First in a series of propeller performance articles: May, 2003
Don't Pitch a Fit,
Fit your Pitch

It's true, size matters. But bigger is not always better in boat propellers. Do you know what size propeller you have? Do you know what you need? The answer to the first question is straightforward. The second answer is not so easy. Let's start with the first question. What do you have?

You know it's a three bladed 14 X 17, but haven't a clue what those numbers mean. By convention, propellers are named by their diameter and their pitch. The first number is the diameter and the second is the pitch. If you are like me and have trouble remembering which is which, just remember that alphabetically, "D" comes before "P" - diameter comes before pitch. In the example above, the rotating propeller makes a circle with a 14-inch diameter. That's pretty simple to understand, but the second number is a bit more difficult. The ‘17' in our example represents the propeller pitch. So what is that exactly?

Simply put, propeller pitch is a measure of how far the propeller would travel through a solid material during one revolution. Think of a propeller as a helical shape, much like a wood screw. A propeller with a 17-inch pitch would ideally progress 17 inches with one revolution. It would; that is, if it were not for propeller slip (see image on opposite page). The propeller does not progress through the water perfectly and the slip effect reduces the 17- inch value a bit. The amount of slip varies greatly with the number of blades, blade design, and boat speed. Slip is not necessarily a bad thing, though. If not for slip, you'd need a clutch when you engaged your drive.

So now you are thinking that bigger is better. If I trade that 17-inch pitch prop for a 19-inch pitch prop, then I can go faster! That's true, but it'll take you a lot longer to get up to speed. A low pitch prop is just like a low gear in your car. Imagine trying to pull away from that stop sign and starting in third gear. You can do it, but it's going to take a while. If you are pulling a trailer, forget it. With a lower pitch propeller, you'll get better acceleration and less slip when you start off. That will mean getting on plane faster and reducing the drag of the hull sooner. With a 15-inch pitch prop you'll get out of the hole a lot quicker, but the trade off is a limit in top speed. What pitch is best for you? The answer is an unequivocal "It depends". It depends on how you use your boat.

Are you a bass boater? Does it typically take more than an hour to get to that fishing spot? A higher pitch prop may be what you need. It'll take longer to get up to speed, but once you are there, you'll make good time and have pretty good fuel economy too. You don't start and stop a lot so the tradeoff is a good one. Your car gets best mileage at highway speeds in high gear and in overdrive. Your boat will too (the tradeoff is in the acceleration). Sometimes you use your craft to pull your better half on her slalom ski and need better acceleration. A medium pitch prop still delivers a fair top speed and gives good hole shot performance. It's not long before your 110 pound beauty is up and looking good, both of you are on plane, and small, precise speed changes are no problem. Life is good … until her 275-pound brother Tiny wants to try out his wake board. More weight and more drag means you are dragging Tiny much longer than Tiny wants to be dragged. Neither Tiny nor his sister are happy.

With a lower pitch prop, more power is effectively transferred from engine to propeller with less slippage, and before long Tiny is up and away. Once he's up the lower speed is no problem with that wide wake board - you are on plane and stay there with small, precise power and speed changes. Unfortunately, your bank account just won't support a bass boat and a ski boat, let alone three boats. What to do?

A propeller that changes pitch - as you need it - would be ideal. It would change from low pitch for getting out of the hole, and to high pitch once on plane. Unfortunately this represents a whole new level of complexity, with a tremendous increase in price. A simple solution might be to have several different props with different pitch specs. We are on to something here -- but why change the entire prop when all you need is to change the pitch of the blades? Piranha Propellers offers this, and it takes just a few minutes to change pitch. For example, at 5,000 ft altitude your engine operates at only 75% of what it did at sea level. You would need to drop 2 inches of pitch to get some of your acceleration back. That's a quick change with this type of prop. Two, or perhaps even three compact and lightweight blade sets can fit all your needs at 1/3 the cost.

A word of caution is appropriate if you are considering changing props to get better performance from your boat. Your boat manufacturer has probably specified a range of diameter and pitch options for your power plant based upon boat and equipment weight. Too low a diameter or pitch and you may "over rev" your engine. Too high a pitch and your engine may lug at wide-open throttle (WOT). A general rule of thumb is that a one inch pitch change will result in a 175-225 RPM change at WOT. Increase pitch and RPM will drop, decrease pitch and RPM will increase.

I've concentrated mostly on the effects of pitch in this article but there are many more factors that affect overall prop performance. For example, switching from 3 blades to 4 may give you better acceleration, better handling, and reduce vibration. Changing the prop diameter also changes the boat's RPM, but is significantly less pronounced than a change in pitch. Generally, as the pitch increases, diameter is reduced. Small changes of +/- 1/2 inch in diameter change the RPM a little, and have a small impact on total performance. Before making changes, you need to consider the range your engine can handle without undue stress.

If I did my job I answered the question of what size prop you have. As for the second question, I hope you are on your way to understanding the options and tradeoffs of pitch choice. At least now you can better understand what your retailer is talking about. Look for future articles on topics such as blade types, prop materials, maintenance and repair costs. We'll make a prop expert out of you yet.

Thanks to all the staff at Piranha Propellers for sharing their expertise for this article. I recommend giving them a call at 800 235-7767 or visiting their web site at www.great prop.com if you have more questions about propellers.

When not in front of a boat propeller, Don Garrison can also be found behind the prop of an airplane as a flight instructor and charter pilot.