AS SEEN IN THE MAY/JUNE 2006 ISSUE

If you are not getting the performance you think you should from your boat, it could be you have the wrong propeller(s). One size does not fit all in the world of boat props. There are two critical questions you should ask yourself with a performance shortcoming – what size and type of prop do you have and what type and size do you actually need? The answer to the first question is straightforward, but the second takes a little thought and a bit of head scratching. Let’s start with the first question. What do you have?

“I have a three bladed, stainless steel, size 14x17, whatever in the world that means.” Let’s start with the numbers. By convention, propellers are named by their diameter and their pitch. The first number is the diameter of the prop and the second is the prop’s pitch. That’s easy to remember because “D” comes before “P” in the alphabet – diameter comes before pitch. In this case, the rotating propeller makes a circle with a 14-inch diameter. Diameter is straightforward, but pitch is not universally understood. The ‘17’ in our example represents the propeller pitch. “Okay … so what exactly does that mean to me?”

Simply put, propeller pitch is a measure of how far the propeller would travel through a semi-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 theoretically progress 17-inches forward with each complete revolution. It would, that is, if it were not for propeller slip. A propeller does not progress through the water perfectly, and the slip effect reduces the 17-inch value a bit. The amount of slippage varies greatly with the number of blades, blade design, and boat speed. Slip is a good thing though, as it eliminates the need for a clutch mechanism when the drive is engaged.

“So, if I trade that 17-inch pitch propeller for the same 19-inch pitch prop, then I can go faster, right?” That may be true, but it’ll take you longer to get up to speed. A low pitch prop can be compared to low gear in your vehicle, and a higher pitch is just like a higher gear. Imagine trying to pull away from a stop sign in your car and starting in third gear. You can do it with care, but it’s going to take a whole lot longer to get going. If you are pulling a trailer, forget about it! With a lower pitch propeller, you’ll get better acceleration and less slippage when you start off. That will mean getting on plane faster and reducing the drag of the hull sooner. With low pitch props you’ll get out of the hole a lot quicker, but the trade off is a limit in overall top speed.

“What pitch is best for me?”

The answer is an unequivocal “It depends.” It depends on exactly how you use your boat. If you are a long range fisherman and it typically takes you more than an hour to reach your favorite fishing spot, a higher pitch prop may be what you need. It’ll take you a bit longer to get up to cruising speed, but once you are there, you’ll make good time and have good fuel economy too. You don’t start and stop a lot, so the tradeoff is a good one. Just like in your car, you get best mileage at highway speeds in a high gear such as overdrive. Your boat will too. What you gave up was acceleration.

Sometimes, you may even use your craft to pull one or two lightweight beauties on their slalom skis. What you need in this case is good acceleration. A medium pitch prop delivers a fair top speed but still gives you good hole shot performance. Your slender beauty is up and out of the water before long and looking oh, so good. Both the boat and the skier are on plane and small, precise speed changes are no problem. But wait … now it’s time for your friend Bubba to try his new wake board. Bubba is big, I mean really big, and his wake board is like pulling a sea-anchor. More weight and more drag means you drag Bubba a lot longer than Bubba wants to be dragged. Bubba is not happy and threatens not to chip-in for the refreshing beverages or the fuel. A low pitch prop will help you effectively transfer more power from engine to propeller with less slippage, and before long, Bubba is up and away. Once he’s up, the lower speed is no problem with that wide, heavy wake board – you’re on plane and stay there with small, precise power and speed adjustments. Unfortunately, your bank account just won’t support two boats. What to do?

A propeller that changes pitch as you need it would be ideal. It would adjust from low pitch for getting out of the hole, to high pitch once on plane. Unfortunately, this represents a whole new level of complexity, with a tremendous increase in initial price and maintenance. Adjustable pitch propellers are used on aircraft, and if you thought your boat prop was expensive…how does ten grand or more sound to you? A simpler solution might be to have several different props with different pitch specs. You can use the low pitch prop when you need the power, but switch to the medium or high pitch propeller when you are traveling. We are on to something here - but why change the entire prop when all you need is to change the pitch of the blades? Thanks to modern technology, this type propeller is available, and it takes just a few minutes to change pitch. Two, or perhaps even three compact and lightweight blade sets can fill all your needs without the expense and storage requirements of multiple props. The cost is also much less than buying several different propellers. If you are interested, the good news is the folks over at Piranha Propellers can fill you in on your options and help you choose the right prop for your particular application. Give them a call at 800.235.7767 or visit www.GreatProp.com.

A word of caution is appropriate if you are considering changing props to get better performance from your boat. Your vessel’s manufacturer has probably specified a range of diameter and pitch options for your particular 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 one inch of pitch change will result in a 175-225 RPM change at WOT. Increase pitch, and RPM will drop. Decrease pitch, and RPM will increase. If you wish to maintain the same RPM with a change in pitch you can also change the prop diameter. Generally, as the pitch increases, diameter is reduced; however, small changes of plus or minus a half inch in diameter have little impact on total performance because the blade design parameters are adjusted to compensate. The simple point is before making any changes; you need to know the range your engine can handle without undue stress.

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 three blades to four may provide you better acceleration, better handling, and vibration reduction, though that’s an entirely different story. Nonetheless, I hope you now have a better understanding of what size propeller your engine(s) is currently equipped with and what you might need to improve your boat’s overall performance. Before this is all over, we’ll make a propeller expert out of you yet.

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