by Nick Nicholson
When we installed a Max-Prop VP feathering prop on Calypso back in 1996, we did it with two purposes in mind: to reduce drag under sail, and to increase maneuverability in tight quarters, particularly in reverse gear. During a survey prior to turning her over to her new owners, we were reminded of yet a third advantage of this versatile propeller, an advantage that, to us, more than compensates for the price difference between the VP and standard versions of the Max-Prop, and almost any other prop, for that matter: the ability to change propeller pitch without hauling the boat.
Neither the Max-Prop nor the Max-Prop VP is cheap. A 17″ three-blade standard Max-Prop retails for about $2,700. The same size VP version of the prop—the size and model on Calypso—retails for a whopping $3,200. But read on.
Like most sailors, we have never paid as much attention to the details of prop pitch and diameter as we should. Diameter is a bit of a no-brainer. Any boat that does a significan't amount of motoring should have as big a prop as you can install, subject to the normal requirement that the minimum vertical clearance between the prop tip and the hull or aperture be at least 15% of the prop diameter. For example, there should be at least 2.7″ of tip clearance for an 18″ prop.
Determining correct propeller pitch, as any designer will tell you, is a movable feast. Yes, there are well-established formulas—both simple and complex—for determining proper pitch, but these generally give you only a starting point. It’s not unusual for a new powerboat to go through several iterations of props early in life before arriving at the magic combination that gives the best performance under a variety of conditions.
We sailors generally take what the boat comes with, and don’t give it a second thought. If the boat moves ahead at about the speed we expect, that’s good enough for us.
It really shouldn’t be.
Our intention here is not to give you a magic formula for making sure you have the right prop, but to give you a cautionary tale about what to look for, and in our case, how we were able to correct our long-standing mistake.
Sailors generally do not use their engines in a particularly effective way, as we consider them auxiliary propulsion, to be used only when the wind lets us down. As a result, we use the engine primarily in calm conditions, when relatively little power is required to move the boat. If the wind is blowing hard from where we want to go, we can usually generate more propulsive force from our white vertical awnings than from the chunk of iron nestled in the bilge. If the wind is blowing us in the right direction, we’re happy, and the engine can rest in peace.
Such was the case with Calypso.
What our recent pre-sale survey showed was that we had grossly over-propped the boat, to the point that we were unable to pull much horsepower out of the Perkins 4-108 without overloading it. We simply couldn’t get the engine far enough along on its power curve to use the engine effectively if it became necessary to power into strong winds or a head sea. We had more or less known that all along, but had never gone through the exercise of putting it right.
This was never really a problem for us until we were faced with the necessity of motorsailing upwind while heading up the Red Sea two years ago.
The Red Sea has notoriously lousy sailing conditions. On our trip, for example, we had 40-knot winds from almost dead aft from the southern entrance to the Red Sea all the way to Eritrea. From there, we had flat calms for several hundred miles, almost to the Sudanese-Egyptian border. These were followed by the classic northern Red Sea sailing menu: 35 knots of wind and steep seas, from right where you want to go.
Any good cruising boat should be able to sail upwind in 35 knots of breeze. Add six-foot seas with crests that seem to be about a boatlength apart, however, and you have a recipe for misery. In such conditions most sailors, ourselves included, simply look for a reef to hide behind, licking our wounds and waiting for better conditions.
The extremes—flat calms or strong headwinds—were easy to deal with. It was the transition phases that gave us problems. During our mad dash up the Gulf of Suez, flat calms gave way to rising headwinds—not a problem normally, but what seemed to happen was that the bigger seas associated with the stronger winds would arrive before the winds themselves. We found ourselves struggling into rising seas while still lacking enough wind to sail efficiently, despite the fact that Calypso is a good light-air boat, either upwind or downwind.
At the engine RPM we normally used for motoring in calms, we couldn’t punch through the rising seas and make more than a couple of knots of headway. Running the engine up higher on the power curve led to rising temperatures—the classic signs of an overloaded, over-propped engine.
We faced a similar problem in our transit of the Suez Canal, when our pilot—among the most obnoxious humans I have ever dealt with—exhorted us to drive the boat faster and faster into moderate headwinds and sloppy chop.
While hauled out in Turkey a few months later, I backed an inch of pitch out of the prop—a 10-second job with the Max-Prop VP and the boat on the hard. This turned out to be overly conservative. We knew what the problem was—and the solution—but it was hard to make ourselves aggressive enough at making the change.
Like most serious cruising sailors, we are obsessed with getting the most mileage out of a tank of diesel fuel. With our over-propped engine we could easily squeeze 1,000 miles out of our 120 gallons of fuel at 5.75 knots—as long as the seas were calm. Changing the prop pitch would enable us to pull more power out of the engine for reserve use, but would mean an increase in fuel consumption in normal use. It would, however, also be much better for the engine.
We motored across much of the Mediterranean. We motored most of the way from Gibraltar to the Canaries, all at our normal pace, although we discovered we had to increase engine RPM slightly to achieve the same speed. We sailed the next 4,000 miles to the Caribbean and back to the US in good, strong winds.
Then came the survey. The very thorough surveyor had us run the engine up to full no-load RPM, which was terra incognita for me. On our trial runs under power he discovered that we could barely turn more than 2,200 RPM at full throttle.
Chastened, we knew it was time to reduce pitch aggressively.
This is when the advantage of the Max-Prop VP came into play. With a fixed prop, changing pitch means hauling the boat, taking the prop to the shop for a re-pitch (or simply buying a new prop, which might be easier), and reinstalling. With the conventional Max-Prop, changing the pitch still requires hauling the boat, adjusting the internal pitch mechanism, and putting it all back together.
With the Max-Prop VP, all that was required was a thorough external cleaning of the external pitch adjusting ring, as any fouling on the prop hub would make it difficult to adjust the pitch. I then removed all the lubrication plugs from the prop hub, screwed in my grease gun, and gave the hub a fresh shot of grease. Because the pitch ring hadn’t been moved in a year and a half, it required some gentle prying to free it up.
All of this was done underwater, with no prop disassembly. It was also done by free-diving, rather than using scuba tanks, which had already been removed from the boat. If the prop had already been lubricated and cleaned, the whole job would have taken about a minute.
The cost was zero, and the price differential of about $500 between the standard Max-Prop and the VP was almost paid for by avoiding a single haulout.
As it was, the hardest part—and this is no joke—was reading the pitch numbers on the prop hub in the murky waters of a canal in Ft. Lauderdale. Without my underwater light, and with my faltering middle-aged eyes, I puzzled over the numbers—which are hard enough to read with the boat on dry land—for a good 10 minutes gathering the confidence to adjust the pitch.
After re-pitching, another set of runs under power showed that we had increased engine speed under load by almost 800 RPM, and the engine was running significan'tly cooler when pushed hard. Calypso is still slightly over-propped, but she is very close to having a good all-purpose pitch setting. With the Max-Prop VP, you can afford to do these corrections in small steps until you get it almost perfect.
The new owners now cruise under power at about 2,200 to 2,300 RPM, something we could never achieve. This is very close to the optimum cruising RPM for this engine, according to several 4-108 experts we consulted, although it is still below the 70% of rated RPM that most engine manufacturers quote as an ideal speed for diesel engine efficiency and long life.
Calypso’s designed full-load displacement is about 30,000 pounds. Empty of fuel, water, and stores, she weighs only about 28,000 pounds. But in full cruising mode, with her big ground tackle, extra tankage, heavy dinghy and outboard, six months’ supply of stores, enough spares to go around the world, and all the other stuff that goes into a boat bound across oceans, she probably goes closer to 34,000 pounds.
This range of displacement is not unusual for a serious cruising boat. Because of Calypso’s large dry displacement, the percentage of displacement change between empty boat and full boat is less than it would be in a lighter boat. Loading the same amount of gear on a lighter boat could be catastrophic to performance under both sail and power. The extreme example of this would be an overloaded cruising catamaran—and we’ve seen plenty of those.
Unfortunately, there’s only a single condition of loading that is ideal for a particular combination of propeller pitch and diameter. Unless you have a true variable pitch prop—and these are in a class by themselves in both complexity and cost—your prop will almost never be exactly right for your boat. The best you can do is find a compromise that meets somewhere in the middle.
Before taking off on a long cruise, try to find a prop pitch that will give reasonable all-around performance. If you have an adjustable-pitch prop, such as a Max-Prop or Max-Prop VP, this will be relatively easy. With a fixed prop, you’ll spend more money and effort getting it right.
There are two ways to approach this. The simplest is to load the boat to an “average” condition. For a long-range cruiser, I would define this as full fuel and water, all the ground tackle, all the bits and pieces—everything but the consumable stores and personal gear.
You can then do your engine trials in this condition, and in flat water, recognizing that it is only an average which will not be valid at the extremes of loading and sea state.
The other method—more complex and time-consuming—is to conduct trials with the boat completely full and completely empty, in flat seas and rough seas, and average the results. Most of us simply don’t have enough time for such an extensive series of tests.
Let’s assume you’ve chosen the simpler “average loading” method. Make multiple runs on reciprocal courses, each run of about a mile duration— enough time for the engine to show signs of overheating if it’s going to happen. You should be able to achieve something close to full rated RPM without excessive exhaust smoking or engine overheating. There should be no struggling whatsoever at 70% of rated RPM. The engine should run at normal operating temperature.
In flat water, at average loading, an engine speed of about 70% of rated RPM should yield a boatspeed in knots of between 1.1 and 1.15 times the square root of the waterline length in feet if the engine power, prop diameter, and prop pitch are reasonably matched to the boat.
There’s a good chance that engine, boat, and prop will not be particularly well-matched. Engines come in discrete intervals in size. We have seen the Perkins 4-108, for example, in everything from a heavy 30-footer to a moderately heavy 54-footer, with the same gearbox. In the 54-footer, the engine is used almost exclusively for no-air situations, while the smaller boat is almost a true motorsailor.
You’re stuck with the engine you have, and the maximum prop diameter is determined by the space where the prop fits. The only readily adjustable variables are number of blades, and prop pitch.
Figure that every inch of pitch removed from your prop is worth about an additional 200 RPM of engine speed. If you come up 200 RPM short of max on your tests, or the engine shows signs of overheating even if you can achieve max RPM, you’re at least an inch over ideal pitch.
Multiple runs will give you a feel of just how much pitch might need to be removed. With the Max-Prop VP, a quick dive over the side is all that’s required to make adjustments. Fine-tune until you get the pitch that gives the best all-around performance.
To put it bluntly, all the sophisticated prop calculations in the world are not as good as a day of field tests. If you have any reservations at all about this process, however, you should talk to your engine manufacturer, boatbuilder, or a mechanic you really trust before changing your existing prop. Different engines have different sweet spots—the point at which efficiency and longevity coincide—and this point may be somewhat different from that suggested by pure rule of thumb.
Chances are, your boat is over-propped, particularly if you’ve added lots of goodies to it over the years. Boats get heavier over time, not lighter. The prop that your boat came with may well not be the correct prop when you take off to go world cruising. Whether you spend the money to upgrade to a fully adjustable prop like the Max-Prop VP, re-pitch your existing prop, or spend a few hundred dollars on a new fixed prop, take the time to figure out if what’s on your boat is what should be there. Your engine will thank you in the long run.