From fishing boat skipper to boatyard manager to my longtime position with Boating, I’ve been blessed with a lifetime of jobs that enable me to be on the water all the time. In the course of this aquatic career I’ve learned a thing or two. Even a bit about props, though let me state right here and right now that my understanding of how a prop works is more intuitive than anything else. Despite having propped, run and re-propped hundreds of different boats; despite having formal educational training; despite avidly reading technical books and journals; and despite an ongoing personal dialogue with boatbuilders, boat designers, engine makers and prop shops, the best summary I can give you of why one prop works for a given boat/power package and another, very similar wheel doesn’t cut the bilge water is that prop selection remains a black art.
While the primary physical attributes of a propeller — number of blades, diameter and pitch — are well-known, there are a host of less easily understood characteristics that affect the performance delivered when the thing starts to spin. These include skew, cup, camber, rake, disc-area ratio and the oft-cited but ill-understood slip, cavitation and ventilation. Couple this plethora of variables with the fact that, when a boat’s inclination, aka its running angle, changes — and inclination changes with speed variations, sea state and the captain’s input via trim tabs or trimmable drives — so does the angle of attack of the prop. So if nothing else, it’s easy to see that there’s nothing easy about understanding propeller performance. You’ve got as good a chance of bending your brain around string theory.
Particle physics is a good example, especially if you include yourself in what I call the Bernoullian Group, because propellers are rocket science. Each blade of a prop is foil, like a wing or sail, and delivers lift according to the theories of the physicist Daniel Bernoulli. That the “lift” is horizontal, instead of vertical, is of little consequence: low pressure on one side, high pressure on the other, and the prop moves between the two. Since the boat, via the shafts or engines, is connected to the prop, it goes along for the ride. Simple, right?
What I call the Newtonian camp is divided into two factions, one that believes the prop pulls itself (and the boat it’s connected to) through the water and one that maintains that the prop pushes. Sheesh.
There are also the so-called Archimedians. They are proponents of the screw analogy: A prop can be understood by likening each revolution to a screw moving through a piece of wood. Fair enough.
The Turn-Up Test
Then there’s the most common question posed by boaters who haven’t had the opportunity to make the water their way of life: What is pitch?
Here’s the way I see it and how I explain it to friends: Pitch is the distance the prop moves through the water in one revolution. A 28-inch pitch prop will move — you guessed it — 28 inches for every turn of its shaft.
You can estimate boat speed from pitch by multiplying the distance by revolutions per minute — the shaft rpm, not the engine rpm. An engine turning 3,200 rpm through a 2:1 reduction means the prop is turning 1,600 rpm. Got it?
You also have to consider slip when estimating speed from rpm and prop pitch. Slip, for our purposes, means simply that no prop is perfect and it won’t move the distance of its pitch with every revolution. So take 10 percent off the product you derive from multiplying pitch times rpm. (Why is every “standard” fudge factor 10 percent? Anyone know? The decimal system?)
Probably the most important thing you need to know about pitch is how it affects engine performance and long-term durability. More pitch and the engine works harder and can’t rev up as much. Less pitch and the engine works less hard and can make higher rpm. The primary consideration in prop selection should be that the engine operates at full throttle within the rpm band recommended by the manufacturer. Even though most of us don’t operate at wide-open throttle (WOT) all the time, the fact that an engine can turn up to, or near, its highest rating means that it’s working most efficiently at all engine speeds (rpm).
Here’s a good rule of thumb: For every two inches of pitch added, you lose 400 rpm; for every two inches of pitch subtracted, you gain 400 rpm. And here’s a method for selecting the best prop, assuming you can get your boatyard or prop shop to lend you some demo wheels (or you can borrow from friends):
•Have a selection of propellers to test with.
•Use an accurate tachometer to measure rpm.
•Use an accurate speedometer or some means to measure boat speed.
•Safely test each setup at WOT.
•Make sure every test is with identical settings: boat load, trim angle, engine height, water conditions, etc.
•Power-trim-equipped engines should be tested at their optimum trim angle. Optimum trim angle is the highest trim position that can be run without excessive venting (slippage) either in a straight line or in turns.
It would be nice if that were it. But it’s not. You have those other variables: diameter, number of blades, cup, rake, skew and so on, including the fact that we all use our boats differently. Some may cruise a 23-footer with five or six people most of the time. Others may run the same boat with just a couple aboard. Some always run with a full tank, and some carry just enough for an evening’s jaunt. It’s not easy selecting props if you’re looking for a certain characteristic such as hole shot, better towing, more top end, decreased vibration or better fuel economy, to name just a few, because in changing a variable to achieve a goal, you still have to stay within the rpm bounds stated above. Remember: The closer to the max-rated rpm your prop allows your engine to turn , the better the engine’s long-term reliability and longevity.
So, given that restraint, for most of us prop selection comes down to about two choices: a prop with lower pitch if we carry a heavy load or run in a seaway a lot, or a prop with just one or two more inches of pitch if we tend to run lightly loaded and/or on calm water most of the time. Propped this way, an engine will turn up perfectly on most days, and on those days when you vary from your norm, it still won’t lug or over-rev — or keep the boat from losing either too much hole shot or too much top-end speed.
For those who want the utmost, I suggest you seek out and use a dedicated prop shop. These guys prop thousands of boats for boaters with dozens of different notions of what the term prop performance means. They can build a custom prop, rework a manufacturer’s original prop or recommend an off-the-shelf model that comes close to the parameters you lay down for them.
Let me close by saying that the best layman’s reference for how boat propellers work that I know of is Propeller Handbook, by naval architect Dave Gerr. It’s short, at 140 pages, but a very dense read. You need to psyche yourself up for some intellectual gymnastics to benefit from it. Internalizing the theory will give you the language needed for understanding the practice.
There just isn’t a simple and succinct way to understand propellers any more than there’s a simple and succinct way to understand why subatomic particles, vibrating at one frequency, make one thing an orange while the same particles, vibrating at a different frequency, make another thing an apple. An understanding of propellers doesn’t fit into the sound-byte-laden, Twitterfied, IIRC-LMAO-IMO-B4 place the world has become. But it’s a worthwhile pursuit for any self-respecting boat geek.