Here’s one way to think about adding a controllable-pitch propeller (CPP) to a marine drive train: It’s like adding a few more gears to the transmission of a loaded truck climbing steep hills.
Perhaps this analogy falls a little short in flat seas when a fixed prop of optimum diameter and pitch works just fine. When it comes to head winds, heavy seas or a vessel carrying a highly varying payload, however, CPPs show their worth. The energy needed to make headway can increase drastically in these conditions, and the match between a fixed-pitch prop, its reduction gear and a specific engine can be anything but optimal.
In addition, the argument for shouldering the extra cost and complexity of a CPP increases with each increase in the cost of a barrel of fuel. Today, we are already at the point where controllable-pitch technology is the answer for many mariners.
Commercial and recreational boat owners in Europe have been battling high fuel costs for decades, and one of their tried and true solutions to the problem has come from the development of CPPs.
Companies like Saab and Hundested have led the way, and the reliability of their drive train products are well known.
The classic example of this technological development is a Hundested CPP and a venerable diesel like the slow-turning Gardner, a combo that has moved from Scandinavian workboats to the recreational boater interested in covering more miles at sea while consuming less fuel.
A cost-benefit analysis of this European experience validates the technology, but before a boater jumps on the CPP bandwagon, it’s worth taking a close look at the pluses and minuses of propeller pitch control.
Still no free lunch
First of all, the no-free-lunch rule is in full effect, and along with the upside of some increases in performance and fuel economy comes a downside associated with increases in cost and complexity. For example, a control rod traverses the hollow propeller shaft of the Hundested system, functioning as the link that controls the angle of attack of the propeller blades. It is a rugged and reliable design, but it increases the complexity of the drive train.
Some feel that with a CPP system you can do without a reverse gear, thanks to the unit’s ability to rotate the propeller blades either side of complete feathering and therefore create either forward or reverse thrust via blade angle alone.
Unfortunately, the speed with which the blades can be switched from forward thrust to thrust astern is slow and can make docking and tight maneuvering tenuous at best.
Consequently, more and more boat owners with reversible-pitch props also opt for a reverse gear and use the CPP solely to fine tune the relationship between engine rpm, prop pitch and the load affecting the vessel.
The issue of load needs some explanation and goes well beyond a reference to payload or the sum of the weight associated with fuel, water, provisions, etc. In many cases the weight of fuel and water may amount to 10 or 20 percent of the displacement of a vessel, and as the fluid is consumed, what may have been a perfect full-trim match between a fixed-pitch prop and a vessel’s propulsion system now becomes hindered by an underpitched prop. Heading downwind with following seas can be another means of creating a underpitched-prop scenario.
Heading into a seaway with a building breeze, full tanks and a liveaboard’s array of necessities can cause just the opposite reaction. The loads on the vessel increase due to aerodynamic as well as hydrodynamic drag, and what was the right prop choice for calm seas and flat water instantly turns into an overpitched-prop problem.
Strong points of fixed props
Before we dismiss the fixed prop, however, let’s not lose sight of a few of its strong points. First of all its hub is small and contains no moving parts, a feature that drives down its cost and at the same time ups its efficiency, albeit only when it’s well matched to the conditions in which it is being used.
Another big plus is the cup shape and blade skew that can be added to improve performance, features that can’t be used in a feathering context.
The big problem is the one-size-fits-all nature of the prop and the need to pick the right compromise. The boat, engine, drive train and propeller relationship is a bit like the resonance of a musical tone or picking the right size plow for a specific horse to pull. Too large and the beast will be overwhelmed by the load; too small and its energy won’t be harnessed efficiently.
Picking the right prop is part art and part science, and the more a pro practices the alchemy, the better he gets at conjuring the right pitch and diameter to suit the needs associated with how a specific vessel is most often used. If identical boats are cruised in differing conditions, their props also should differ.
For example, if one vessel tends to be used for fast passagemaking in calm conditions and a sister ship is often called on to carry full loads in lengthy windward slogs across rough bodies of water, there is good reason for differing pitches to be selected for each propeller.
The first vessel experiences less drag-induced load and may spin a prop with an inch or more pitch. The rough-water passagemaker needs less pitch in its prop so that the engine is not overloaded by wind, sea and payload factors.
When it comes to inputs on propeller selection, manufacturers rank right up at the head of the pack. They usually won’t specify a brand name or a specific pitch, diameter or even blade count, but they certainly will offer very specific details about the rpm that must be reached by whatever prop is installed, and this is the catch-22 of CPPs.
With a CPP, it’s easy to dial in a pitch that meets the manufacturer’s recommendations. Simply increase the throttle setting to full speed (after an adequate warm-up period), and if the recommended rpm is reached or exceeded, increase the prop pitch until the rpm drops 50 to 100 rpm below the manufacturer’s recommended number. Slowly decrease the pitch until the number is correct again.
The opposite approach can be used to reduce pitch to cope with a setting that did not initially reach the manufacturer’s rpm. The real temptation comes into play at cruising rpm, when increases or decreases in drag-induced load cause adjustments in pitch to be desirable. Deciding just how much to dial in or dial out becomes the big question.
Before tackling the problems associated with under- and overpitching propellers let’s look at a bit of internal combustion theory – a technology steeped in metallurgy and heat transfer – and see what happens to a diesel engine as pitch increases.
A propeller behaves just like a wood screw, and as pitch increases, more stringent twisting is required. Whether you accomplish this with your wrist and a screwdriver or with a turbocharged Cat diesel, torque demands remain directly proportional to changes in pitch.
Marine diesels convert reciprocating piston motion into rotary energy, and depending on design, each will reach an optimum torque at a very specific rpm range. Ideally, the diameter and pitch of a prop and its other physical characteristics complement the reduction gear and allow an engine to reach the manufacturer’s recommended rpm at full throttle, yet still afford good thrust when the power plant is throttled back to a cruising rpm. That’s where a temptation to dial in too much pitch can cause problems – a classic case of too much of a good thing.
Dialing in more pitch when loads are lighter makes good sense, just as reducing pitch in a tough slog can be helpful. However, if too much pitch is cranked into the equation, the engine can be overloaded, head and exhaust temperatures rise, fuel is not completely combusted, and fuel economy suffers.
This also can lead to serious internal problems and potential failure of the power plant. Black smoke and an unusual increase in exhaust gas temperature are telltale signs of overpitching.
Underpitching is not quite so detrimental, but performance suffers, and the engine actually sounds as if it is undertaxed by the propeller blades at the end of the drive train. Prop gurus calculate the impact of factors such as cavitation, cup shape, blade skew and other esoteric engineering issues that help or hinder efficiency.
Take caution to avoid overpitching and placing excessive loads on the engine when using CPPs. This being said, the upside of being able to pick the right blade load and thrust for a given condition is significant.
There are also secondary advantages, such as those derived aboard a twin-screw long-range motor vessel, which can shut down one engine and feather its prop for fuel efficiency, and alter pitch to limit stern walk in close-quarters maneuvering. All in all, the technology works well, and as more and more concern is given to the miles per gallon, products by Hundested and Saab will become more familiar in U.S. boatbuilding.