Prop selection is a serious endeavor. Too much diameter or pitch can destroy engines in a very short order. Before we get into the mechanics, let me get my qualifications out of the way — because most errors come from boat owners listening to unqualified people who love to sound like experts. I traveled all over the world as Cummins Engine Company's trouble shooter, and also worked for Detroit Diesel and Caterpillar doing troubleshooting and forensic failure analysis. A qualified marine professional isn't hard to spot. The tech at the prop shop knows a lot. The guy in the slip next to you is like taking medical advice from someone at the gym.
The Three Dimensions That Matter
There are three dimensions you care about: Diameter, Pitch, and Bore Size.
- Diameter — simply the diameter of the propeller.
- Pitch — how far the propeller travels with one complete revolution. Imagine a big piece of Styrofoam: you spin the prop one full revolution driving it into the foam — how far did it travel? That's pitch.
- Bore Size — the size of the hole at the center of the prop. It must match your shaft size.
Props are stamped — even after being altered. Look closely and you'll see something like 24 x 22: diameter is always first, pitch is second. A prop shop can alter pitch to some degree and reduce diameter, but they cannot increase diameter — so think carefully before requesting a reduction.
Alloys: Inboards vs. Outboards
Inboards use brass or bronze. Bronze is a harder composite of both metals — more expensive, but definitively better. Any flexing is to be avoided; the harder the propeller, the less it flexes.
Outboards use aluminum or stainless steel. Aluminum props are designed so that if you strike something, one of the ears will pop off rather than transferring the shock load to the lower unit. With stainless props, you stand a much better chance of destroying a lower unit — or worse. I recently witnessed the shock load of a stainless prop hitting a substantial metal object snap a crankshaft. The hub system (a rubber grommet press-fitted into the prop center with female splines mating to the outboard's male shaft) is designed to break away on impact. A new hub press-in beats a new lower unit every time.
The Danger of an Oversized Prop — Engine "Lug"
If your engine is governed to spin 2,350 RPM and you have too much prop — either too much pitch or too much diameter — and it only spins to 2,100 RPM, you will destroy the engine in short order.
Without getting overly technical, the engine enters a condition called Lug: overloading the crankshaft bearings, as if you were driving a truck uphill in too high a gear. The tops of the main crankshaft bearings and the bottoms of the connecting rod bearings get destroyed.
Interesting side note: an overspeed condition does the exact same thing in reverse — over-revving causes the bottoms of the mains and tops of the rod bearings to fail.
The Challenge of an Undersized Prop
The entire prop sizing challenge exists because the rev limiter will stop the engine with bare shafts fitted — so there's a real possibility you have too small a prop and wouldn't know it. The engine spins to its governed RPM and everything looks normal. The only tell-tale is poor performance.
It's almost rare to see a boat with the wrong props. Unless you know of several boats of the same model and power that are dramatically faster than yours, don't start playing games with props. There can be many other causes of poor performance: fuel delivery, turbo issues, shaft problems. If you do know of two comparable boats that are significantly faster — after ruling out engine performance issues — then start comparing prop sizes. One or two knots difference? Leave it alone.
Using a Prop Calculator
Most prop companies, including Michigan Wheel, offer a free online tool where you enter your parameters to get a starting point. You'll need: engine HP and governed RPM, transmission ratio, boat length, beam, displacement, and speed both actual and desired.
It's not exact. If you get it right on the first pass, go buy some lottery tickets. And be forewarned: you are not going to make a legitimate 28-knot cruise boat into a 32-knot boat by changing props. You will destroy healthy engines. You won't make her faster or burn less fuel — all props are designed for maximum efficiency at full RPM, where nobody runs.
Understanding Cup
The solution to improving midrange efficiency is something called a cup. Cups used to be rated 1 to 11 (11 being most severe); today you'll see the terms mild, medium, and severe. Old-school professionals still use the numbers. The higher or more severe the cup, the more efficiently the prop runs at midrange and cruise RPMs. You may gain a knot or two and see a fuel consumption benefit — it won't be dramatic, but why not take what you can. Note: the full range from mild to severe cup equals roughly 1 inch of pitch or 1 inch of diameter, or approximately 125 RPM.
Reading Exhaust Smoke
Exhaust smoke is one of the clearest diagnostic tools available:
- Brief puff of black smoke at throttle-up — normal. The fuel injection senses increased load and dumps fuel as the turbo spools up from increased exhaust gas temperature. It must clear quickly.
- Sustained black smoke — too much fuel to air ratio. Either the prop is overloading the engine, or on older boats, the turbocharger is failing. Note: newer engines with ECMs often won't show black smoke even when the turbo isn't making boost — the computer prevents the fuel dump for emissions reasons.
- Lingering white smoke — unburnt fuel, usually an indicator of water in the fuel.
Knowing these basic dynamics should keep you out of trouble and help you make informed decisions — or at minimum, ask the right questions of the right people.
