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Understanding Propellers

Author: Stuart Buckingham   Date Posted:18 September 2017 

Propellers, every outboard has one. Whether it's stainless steel, aluminum or plastic they all have the same task, which is to push your boat forward or backward. But, the wrong propeller on your outboard can cause under performance in the way of a lack of top-end speed, hole shot or could possibly cause excessive load on your outboard resulting in powerhead failure.

Pitch
Propeller pitch is defined as "the distance the tip of the propeller blade travels in one revolution". So, for a 17 pitch propeller it would travel 17 inches, for a 19 pitch it would travel 19 inches and so on and so forth. As a basic guide, the smaller the pitch the less distance you would cover than compared to a larger pitch. You might be saying to yourself, "Well why doesn't everyone run large pitch propellers if it means going faster?". Good question… and here's why.

With larger pitch propellers you sacrifice your hole shot. Remember, outboards don't have a selection of gears like a car, they only have one ratio in the gearbox, so choosing the right propeller is a bit like choosing the correct ‘gear’ for your outboard. A ‘gear’ that will allow you to take off and plane quickly from idle, gives you reasonable top end speed and also ensures the engine meets the manufacturers RPM requirements.

Diameter
If you were to draw a circle around the propeller using the propeller tips as the outer spectrum of the circle, then the propeller diameter would be the diameter of this circle. As a rule, boats traveling at slower speeds that may require good thrust in reverse as well as forward would use larger diameter propellers, whereas boats traveling at faster speeds would require a smaller diameter propeller. Effectively, the faster the boat, the smaller the diameter.

Choosing the right propeller
If you don't have a propeller, or you believe your current propeller is dramatically underperforming, then the best place to start is to find out what size propeller was standard from the outboard manufacturer and what maximum RPM the outboard is recommended to. This will give you a base to work from.

Changing up or down a size in pitch will typically increase or decrease your RPM between 200 and 400 RPM. As a basic guide, you need to see what RPM your engine is doing at WOT (wide open throttle). If you're not sure what this is, your aim should be for around 5500 RPM with the throttle down as far as it will go. Then, depending on your hole shot and speed, you can change the propeller up or down in pitch and diameter to find the perfect size for your outboard and boat combination.

Hopefully, this has proved informative or helpful in your quest to better your boats performance. There are a whole array of factors, like boat weight (laden and unladen), tidal influence and engine height that can be brought into the equation when trying to find the perfect propeller but, I've tried to keep it as simple as possible.

If you are still confused, don't worry you're not alone! There are many in the marine industry that struggle with correct propping and in my experience, it can be a bit of an art form to calculate and choose the right propeller for each and every boat and outboard combination. If you work through the information above in a methodical sequence then I'm sure you'll come out on top.

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