Frequently asked questions about boat propellers

While people have asked me a lot of questions about propellers over the years, there are eight really important ones that keep getting asked over and over again. I can tell you exactly what they are and what I tell people when they ask them.

Keep the the answers to these eight questions in mind the next time you are in the market for a propeller. It should not only help with your selection, but could save you money. Much can be gained through experimentation or by reading recent prop tests conducted by professionals.

Good luck and happy boating!
Bill Jennings
Technical editor for 2 performance magazines.

Tech terms

Pitch – the theoretical distance in inches that a propeller moves forward in one revolution. Think of a screw in wood. If you increase the angle of the threads, it will move further into the wood in one turn.

Diameter – the diameter in inches of the circle that the blade tips will cut. Or, the distance in inches from a blade tip to the center of the prop, multiplied by two.

Propeller Size – props are named by the diameter inch number, followed by pitch inch number (i.e. 14 x 19 or 14″ diameter x 19″ pitch).

Blade – the part of the prop that acts as a paddle.

Elliptical Blade – one that’s elliptical in shape.

Tapered Blade – a blade that is tapered in shape. (As opposed to round ear or elliptical.)

Cupped Blade – a blade that has a lip built into its trailing edge. (Most do) The lip generally helps a prop hold water. It also adds the equivalent of about 1/2-inch to one-inch of pitch.

Polished – a prop that has had the steel polished from a dull to a shiny finish.

Hub – the center part of the propeller. The hub, as an actual part, is the piece that fits inside the barrel of a prop and what the shaft slides into. If you strike something, the hub breaks free of the barrel and spins so that you don’t damage the drive-train. The word hub is also often referred to as the size of the barrel. (i.e. 4.5″, 4″, 3″, 2.75″)

Skew – a blade whose shape sweeps in a curve that follows its rotation is said to have a skew.

Rake – if a blade sticks straight out of a hub, in other words, is perpendicular to it, that prop has no rake, or zero rake. If the blade leans back more towards the trailing edge of the prop, it is said to have rake. If it leans way back, it is a high rake prop. Rake can be measured in degrees.

Should I go from a three blade to a four blade prop?

If you look at a propeller from behind, and visualize a circle that the blade tips would draw, you will see that the blade of a 3 blade prop covers about 50 -55% of the circle. (This is called diameter area ratio, or DAR). You can add thrust within this drive circle by increasing that percent (to say 60 – 65%) by adding a 4th blade. When you look at a four blade prop you will notice that more of this area is covered because of the extra blade. That means more push contact with the water. Think of it as going to wider tires on your car. More traction for your car and more traction for your boat. Just as wider tires on your car usually do not alone increase top speed and can actually reduce it, the same usually holds true for a four blade prop. One big advantage, however, is that the added thrust from a four blade prop can keep your boat on a better planing angle in mid-range and lower RPM’s, which as you can imagine translates to better fuel economy.

In specifying propellers, there are exceptions to just about everything. As a general rule, the above benefits of four blades are not as noticeable on boats under 24 feet.

Do composite and aluminum props flex?

Of course! All propellers flex. What is often missed is that flex is a good characteristic. For example, if a prop flexed under the load of acceleration, it would actually be reducing its pitch, which in turn would produce a faster acceleration. A problem would occur only if the prop failed to return to its specified pitch once up to speed with the load reduced. The measurement of this two-way propeller movement is called “yield.”

The question people should ask is: “should I be concerned with a low yield number in a composite and aluminum prop?” In this regard, the yield numbers for aluminum and composite are about the same and high enough to handle the loads designed for. Stainless (being much stiffer) has a higher yield number and is well above basic requirements.

What about composite props?

When propellers made of composite, or plastic, first came on the market over 14 years ago, I was very skeptical that they could match an aluminum prop in strength and performance. Today, while not all composites are created equal, a couple of manufacturers use space age composites to exceed the strength of the old traditional props. One uses a long-fiber nylon and carbon composite to deliver outstanding strength and performance. I suppose this should not surprise us as many commercial turbo-prop (propeller powered by a jet engine) aircraft now use composite props and ‘Formula I’ race plane rules specify composite props over aluminum, because “aluminum props are subject to catastrophic failure.”

So far, composite manufacturers have held their prices down to about the same as aluminum, making them an excellent value.

My pick of the composites is a prop called “Piranha.” This is not simply due to superior performance, but because the hub is guaranteed for life and you can easily and inexpensively replace individual blades should you break one. Applications range from six to 300 horsepower.

What prop can I buy that will improve my hole shot and top speed?

Probably none! These are two separate objectives that require a separate specialty prop to accomplish each one. For example, while a five bladed propeller could get you onto plane quicker, it will usually knock three to five mph off your top speed.

Should I spend the extra money for a stainless propeller?

In most cases, by simply making a direct conversion to stainless in the same size as your aluminum prop, you will notice an improvement in both acceleration and top speed. The reason for this is very simple, but frequently misunderstood. Stainless is seven times stronger than aluminum. Propellers don’t have to be that much stronger so manufacturers can make them thinner and still be two or three times as strong. Thinner props cut through the water easier and this of course translates to better performance.

Stainless propellers generally cost a couple of hundred dollars more than an aluminum or composite propeller.

There are two situations where we don’t recommend stainless over aluminum or composite: smaller outboards in pleasure applications, (because the prop can be stronger than the gear case parts) and for very occasional boaters who are happy with the existing performance of their boat.