Feb. 22, 2005--FIXED OR VARIABLE PITCH PROPELLER?
by Michel & Jane DeRidder

While we were building MAGIC DRAGON in 1964 in Vancouver, the local Gardner engine dealer had a Hundested VP-1 variable pitch propeller on the shelf priced at $2000. At that time, that was the price of a new Volkswagen. So we opted for a so-called three-bladed sailing prop (its long narrow blades were supposed to minimize drag) and a direct drive to keep the prop small and cut the drag under sail. Although, with a perfectly clean bottom in very calm water, our 35hp engine would push us at close to 7 knots wide open, it took very little to slow us to a crawl. We soon discovered that we often had to motor in calm winds but not necessarily in calm waters. The small prop just did not have sufficient push. A more efficient three-bladed propeller did not improve things much.

So in 1969 we re-engined with a 75 hp engine and again, to keep the prop small, we stayed with the direct drive. I was hoping that going from a 13-inch to an 18-inch prop and having more power at slower rpm would solve the problem. It helped some. But the prop drag was greater too, so we installed a Federal self-feathering propeller. This became another problem. The self-feathering prop has the advantage of rotating its blades 180 degrees between forward and reverse lock so the shape of the blades is a true pitch in both directions. This makes an efficient propeller in reverse from a stand-still start. However, we discovered that if we had way on, the propeller would start turning before the blade reached the reverse lock, which meant we had reverse shaft rotation but opposite propeller hand--still pushing us forward! Not good when trying to slow down or stop. It was so bad that we did not dare keep that prop on when coming back up the west coast of the US where we had to maneuver in ports full of expensive pleasure boats.

When back in BC in 1970, a friend was installing a Hundested VP-3 variable pitch propeller on his new motorboat. He priced the VP-1 for us at the factory in Denmark. We could get one shipped with his for a mere $600, shaft and all. We had already invested more than $1000 in all our various props and spares. (The Federal alone had been $500.) So we took the plunge.

This time we added a 1.75:1 reduction and we tried the 24" three-bladed VP-1. Finally we had powering efficiency for the first time. We can cruise at an economical 6.5 knots on a gallon of fuel per hour at 1000 rpm, at 1400 rpm we get 7.5 knots on a gallon and a half, and in calm water we could push along at over 8 knots, but at a price--nearly 5 gallons of fuel per hour--so we don't go that fast. In a seaway we can pitch the prop to suit the power we need from the engine.

THE CASE FOR VARIABLE PITCH PROPS ON SAILING BOATS

A variable pitch system is an additional weight in a yacht. But a light engine with all the paraphernalia of a variable pitch installation is no heavier than the old style marine engine, and is much more versatile. Although cost and weight are factors to be taken into consideration, probably the main reason for the lack of wider utilization of adjustable pitch propellers in yachts is lack of awareness of their advantages.

In sailing boats there can be totally different power requirements: cruising in calm water where hull speed is achieved with moderate power, motoring into head seas as in going in or out of harbour where tacking is impossible or difficult, bucking currents in narrow passages where considerably more power might be required, and motor sailing where it may take only a slight boost of power to get up to hull speed. Each of these different power requirements can be matched with suitable propeller pitch by the intelligent use of a variable pitch propeller. An adjustable pitch prop that can be feathered while sailing by turning the blades so they align themselves parallel to the motion of the boat is an enormous advantage, particularly in light air. Dragging a large propeller is akin to dragging a garbage can. Being able to get rid of that immense drag by feathering the propeller could double the speed in light winds.

WHAT IS A VARIABLE PITCH PROPELLER?

A variable pitch propeller is designed with individually rotating blades, the rotation taking place in their own axis at 90 degrees to the axis of the shaft. Depending upon design, the hub of the prop contains either a gear or a cam, activated through the main shaft by a secondary shaft that either rotates or moves fore and aft to change the angle of attack of the blades, that is, to change the pitch of the propeller. The secondary shaft mechanism, usually located inboard at the forward end of the shaft is mechanically or hydraulically activated by controls in the wheelhouse or cockpit.

ADVANTAGES OF VARIABLE PITCH PROPS

A variable pitch propeller offers the possibility of varying engine load at different rpm, as do multiple gear ratios in automobile transmissions. But unlike automotive three or five independent gear ratios, propeller pitch can be varied smoothly and progressively over a large range of ratios. With a matched propeller of an adjustable pitch type, an engine can be loaded as desired at various shaft rpm.

This means that a variable pitch propeller coupled to a marine engine is efficient not just at normal cruising speeds, but also at both maximum power, and at best economy power. This versatility is not possible with a fixed pitch propeller in most boats. By varying the pitch of the propeller to suit varying conditions, you can enable an engine to run more efficiently, resulting in lower fuel consumption and less maintenance. Not only can you make use of the full power of the engine when you need it, but you can also use less power and obtain good efficiency at low engine rpm. Whatever power is needed can be delivered at the best rpm of the engine for that particular output.

Truck engines which run at low loads carbon up terribly. Boat engines turning too small a propeller have the same problem. Engines run with insufficient loading don't get proper combustion. Carbon deposits are the result. It was interesting to see some years ago that even after 6000 hours of operation there was no carbon in the cylinder heads of our four-cylinder 75-horsepower Isuzu diesel engine. They were absolutely clean, having been run at proper combustion temperature at all times, thanks to the VP-1 Hundested propeller. Seventy-five horsepower is much more power than MAGIC DRAGON needs for motoring economically. But with the adjustable pitch prop we have been able to get the horsepower needed, efficiently at 1000 - 1200 rpm, with little wear on the engine.

UNEXPECTED SIDE ADVANTAGES OF VPPs

As well as the two main advantages of adjustable variable pitch propellers, that is of being able to obtain the most efficient use of the engine, and being able to feather the prop when sailing, there are other unexpected advantages.

1. You can set the adjustable pitch prop on low pitch for running slowly through hazardous passages without having to throw the engine in and out of gear to slow down.

2. You can set the prop to minimum pitch as a dragging brake to minimize sailing speed if approaching a landfall too early.

3. A variable pitch prop can drive a sailing generator with the drive shaft more effectively than can a fixed prop.

4. You can even manipulate variable pitch propellers so as to start the engine when the battery is flat. At six knots under sail, we can adjust the pitch of the prop to turn the engine with the decompression lever out, then by shoving the decompression lever in, the engine will start even if the starting battery is dead. The same maneuver could be performed under tow. Unfortunately few engines today have decompression capabilities. Super-light engines do not have this feature because of the additional weight and expense.

REVERSE GEAR BOX ADVANTAGES

There are various sorts of adjustable variable pitch propellers and several different makes. Some companies offer reversing pitch propellers, a feature that saves on having to have a gearbox with its share of maintenance and problems. However, we favor the variable pitch prop with a reverse gear box separate for various reasons. For example, with a reversing pitch prop, you may not get weeds off without diving, for the prop always turns the same way even when reversing, so you cannot change the rotation of the prop to shake off weeds. Motoring at night through the Sargasso Sea, we often had to go back and forth, reversing and going forward alternately, to get rid of weed entangled in the propeller. We've had the same problem with Southern California kelp, and weed in British Columbia, as well as plastic in all too many places.

MAINTENANCE OF VPPs

Maintenance of variable pitch systems is a matter of occasional greasing of the pitch changing mechanism. We've had ours for 30-plus years now. We keep the mechanism greased and our fingers crossed.

PYROMETER USE

Ideally a variable pitch propeller installation is used to best advantage with a pyrometer to sense exhaust gas temperature, since exhaust gas temperature is a direct factor of the load applied to the engine. The greater the load, the hotter the exhaust. Under-loading and overloading the engine is immediately sensed by the pyrometer so that you can adjust the load by changing the propeller pitch for best engine loading. Engine speed and propeller pitch can be regulated for best results at various boat speeds and for any given ideal cruising speed in various conditions.

USING A VPP WITHOUT A PYROMETER

Without a pyrometer to monitor engine loading, the variable pitch propeller can be employed effectively by using the engine rpm counter to 'feel' the loading of the engine. This is the system that we use. To test engine loading at any given prop pitch, we throttle up to find the maximum revs the engine will run with that particular pitch of the propeller. We then throttle back to reduce rpm 10%, to bring the engine to within a reasonable working load. If we should decide for any reason that we want to run on a heavier engine load for a short time and wanted to burn out the carbon for an hour or so, then we throttle back just below the top rpm with the set pitch--something we might choose to do if we have been idling a great deal to charge batteries, for instance.

Another way to determine optimum engine loading is to decide first what rpm you wish to run the engine at. Run it 10% faster and increase the prop pitch until the engine revolutions slow down. Then throttle back to run it at the intended rpm. For example, if you want to run your engine at 1200 rpm, set up the pitch so the engine won't go above 1350 rpm, then throttle back to 1200 rpm.

ABOUT FIXED PITCH PROPELLERS

Matching a fixed pitch propeller to an engine on a given boat depends on various calculation intricacies involving propeller diameter, pitch and rpm, as well as slip, efficiency, cavitation, noise, and cost. It is not just a simple matter of matching the prop to the power of the engine. Boat speed is a major factor too. In different sea conditions, the same power at the propeller will produce different speeds, just as the same speed can require varying power outputs. A fixed prop has to have enough pitch to accommodate speed and slip, but not so much pitch as to overload the engine. It is a rare boat that can be matched with a combination of engine and gear ratios as well as propeller size and pitch that will be efficient in most conditions. The boat speeds and propeller slip have to be just right in a great variety of conditions to achieve this.

The decision as to the size and pitch of a fixed pitch propeller ends up being a compromise, for with a fixed pitch prop, maximum efficiency can only be obtained around one particular set of conditions. If the propeller pitch is set for maximum efficiency at full engine power as it usually is, then when the engine is running at cruising speed or best economy output, the engine will be running too freely and not getting proper combustion. Yet if the pitch is set for economy power, then you cannot make use of the full power potential of the engine, for the engine will not get up to full revs with that particular propeller. Choosing a propeller is a matter of selecting a particular compromise.

The reasons for the difficulty in selecting propeller diameter and pitch originates in the idiosyncrasies of internal combustion engines. Gasoline and diesel engines are capable of limited flexibility of power output. Each engine has a curve of optimum power versus rpm. Outside this curve, efficiency drops rapidly, causing carboning of engines, extra fuel consumption, reduced engine life, and increased maintenance cost, besides the fact that the power of the engine quickly becomes unusable. This peculiarity is the reason why in the automobile, where speed and power requirements vary to the extreme, gearboxes with numerous gear ratios, manual or automatic, are universally used.

On displacement hull applications, where a large propeller is needed to provide the necessary thrust, internal combustion engines develop too much shaft speed for the power. A reduction gear is used, varying from 2:1 to 5:1 or more, depending on propeller size and types of boats. On high-speed planing boats, a closer match exists between the engine (power to rpm ratio) and the boat (power requirements for various speeds). Here, low reduction ratios of less that 2:1 or even direct drives are needed, while hydroplanes often require overdrives. Perfect matches are rare indeed outside of a few modern runabouts and the occasional old-time cruiser equipped with a slow-turning one-lunger, where the power to rpm of the engine resembles the power and speed requirements of the boat itself.

FOLDING AND SELF-FEATHERING PROPS HAVE THEIR LIMITATIONS

There are other ways to get rid of the drag of a propeller while under sail--self-feathering props, folding props, and two bladed props can all do so to greater or lesser degrees. But each of these systems has decided disadvantages that adjustable pitch props do not have. A self-feathering prop can be dangerously unreliable if it does not always reverse when it is supposed to. Though it may reverse from a standstill, it may not reverse dependably when the boat has way on, so that it can not be relied upon for braking and maneuvering.

A folding prop will reverse, but not necessarily efficiently, for any amount of thrust applied in reversing action will tend to fold the propeller. It will only sustain as much thrust as the centrifugal force of the blade will hold, which is minimal. Folding props work satisfactorily on smaller engines where there is more inertia in the blades than power in the engine, but any boat that has an engine capable of more power than a folding prop can handle may be better off with a variable pitch.