“What’s the Right Propeller for my Boat?”

We hear this question more than any other, and hear it many times a day.  The answer is really pretty simple….The right propeller for your boat is one that has the right diameter and pitch to properly load your engine throughout its’ entire power output curve”.

In a perfect world, this would be accomplished with props that varied in pitch depending upon engine rpm.  While these propellers do exist, they are generally too expensive for pleasure boat application, and we have to make do with fixed pitch propellers.  So, given this restraint, where do we start to find the right prop for your boat?

First, let’s clear up a couple of common misconceptions:

A book that lists the proper propeller for all production boat and engine combinations does not exist.  If you’ve ever been to a boat building plant you know that each boat is pretty much hand made, and even identical models come out quite a bit different in weight and bearing.  When you add the variety of options available, differing engine outputs, and the personal gear that each boat owner brings aboard, it’s easy to see why it would be impossible to compile this kind of data.

Computer simulation programs, while very helpful in getting “in the ballpark” for sizing props, are estimates that are based on averages compiled across thousands of boat samplings.  They can be right on the money, but can also be off by a mile, especially as speed increases. 

But don’t despair, the best place to start in your search for the right prop is on your own boat with engine manual in hand.  You see, all engines are rated at “X” horsepower at “X” rpm’s (Specs section of your manual), and simply stated, the more rpm’s you turn, the more horsepower your engine makes.  A fixed pitch prop should be sized to allow the engine to achieve its’ rated rpm’s at wide open throttle (WOT).  Every engine and propeller manufacturer on the planet endorses this principle, so if you don’t agree with it, you’re on your own.  If the engine turns more than rated rpm’s at WOT it’s not being properly loaded at cruise rpm’s where you’re losing hp, speed, and performance.  If the engine turns less than rated, it’s not developing hp, is being overloaded, and will suffer internal component damage.

A WOT test is the first step in determining the right prop for your boat, and is fairly straightforward: 

Take your boat out with its’ normal load of fuel & people on a calm day. 

After the engine(s) warms up, accelerate to WOT.  Stay at WOT only long enough to record rpm’s achieved.  (Don’t trust your installed tachs here, most accurate readings come from an electronic tach or phototach used right at the engine).

Late Spring/Early Summer is probably the best time of year to perform this test because your bottom is still nice and clean and the weather is not too hot (it’s not uncommon to lose 10 to 20% of engine horsepower in the hot and muggy “dog days” of July & August according to engine manufacturers).

Once you know your WOT rpm’s, we have formulas to work with that are very accurate in evaluating adjustments that may be made to pitch, cup or diameter if you’re over or under rated rpm’s.  We also have a precision measuring system to determine the true pitch and diameter of the propellers you ran when making the test.  This is important because you won’t get good results when making adjustments unless you know exactly where you’re starting, and the markings on the prop are usually off substantially even on brand new propellers.

So take the time to do a WOT test.  You’ll know in short order if you’re propped right or if changes can be made to improve performance.

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