Most boaters would like to go a little faster and burn a little less fuel, but some take this desire to a whole new level. They shell out good money for the latest hull coating, fuel additive, or other gizmo in hopes of eking out another knot or two. Do they succeed? It’s hard to tell. Product advertising argues convincingly that they do, but there’s a dearth of true head-to-head comparison testing, and anyway, even in the best of cases, the gains are usually relatively meager: For the average boater, a ten-percent improvement in top speed works out to maybe three knots (less at cruising speed); the same improvement in miles per gallon barely registers.
The key is return on investment: Does the modification produce an improvement that justifies its cost? That is precisely the question you need to ask before you start messing with your propellers. Specifically, how can you tell if your boat is running the right props, and is it even worth the time and effort required to find out?
To answer that, start with this premise: The right prop is the one that allows an engine to turn at the speed at which it produces maximum horsepower, and no more. Look at a horsepower-output graph and you’ll see that maximum horsepower occurs at a pretty specific data point in the rpm range. That’s the sweet spot. Below it the engine isn’t making full power; above it output declines, often dramatically.
If a gasoline engine is mispropped, it’s obvious: just look at the tachometer. If the engine is rated at 4800 rpm and the tach reads 5200 rpm, you’re losing speed and efficiency, and you probably need more prop. Diesels are different; they’re governed, which means their maximum rpm is limited, either mechanically or electronically. So the tach could indicate a diesel engine is at its rated rpm when it’s not making full horsepower because it’s not fully loaded.
To tell if you have the right prop on your diesel-powered boat you need to determine engine load. Before the advent of electronic engines, this was a matter of trial and error: Run boat, note rpm and speed, change to props with more pitch or cup, run boat again, and if you get more speed, repeat until speed drops off. Obviously, all those props, haulage, runs, and wrench turning could get costly, so absent evidence of a substantial performance issue (like your buddy’s identical convertible blows past you at every Bimini start) most owners usually contented themselves with the props that came on the boat.
Electronic engines changed everything because somewhere in the blizzard of data that they spit out probably lies engine load. (I say “probably” because not all electronic diesels offer load data.) Call that up, take your boat out and run her up to WOT, and see how close this number comes to 100 percent. An engine may not reach that threshold, but anywhere above 95 percent is usually acceptable to anyone short of the fanatic described earlier.
Whether your engines burn gasoline or diesel, they should be propped so that they reach the desired threshold under real-world conditions—that means with all your stuff aboard, plus a couple of people and tanks that are at least half full. Boatbuilders, of course, can’t do that, so they usually underprop their prototypes just a bit so that when the production versions are loaded up, the engines hit that sweet spot.
Absent an errant encounter with something immovable, props don’t change much over time, so if your boat isn’t as fast as she used to be, the first place to look is not pitch or diameter but all that stuff you keep bringing aboard or the green beard that’s growing on your hull.
But let’s say that for some reason you suspect your boat has the wrong props. How do you determine the right ones? The answer is again trial and error, which as we’ve determined, gets expensive. If you’re lucky and the deficit in load and/or rpm is small, a prop shop can probably correct it by adding cup to the blades. (Cup is the convexity at the end of each blade.) If the deficit is big, you’ll need to find a prop shop with an inventory of loaner propellers. This could get expensive too, but a good prop guy can intuit what a boat needs in a very short time.
But make no mistake: We’re usually talking small changes. Do not expect your 30-knot cruiser to become a 35-knot cruiser after changing props. A 31.5-knot cruiser is more like it. On the other hand, if your engines aren’t hitting their sweet spot, you’re not only missing the performance you should be getting, you could be damaging your engines.
Three things determine the speed at which an engine turns a propeller: the marine gear’s reduction ratio and the propeller’s diameter and pitch (including the amount of cup). All three interact, and figuring out the right combination is a job properly left to boatbuilders and engineers. The only parameter you need to worry about is pitch (and cup), and most of you probably won’t ever need to do that.