CP props

My first experience with CP props was in 1974 aboard the Australian National Line's Allunga - a roll-on, roll-off ship powered by three medium-speed diesels coupled to a single CP prop.

The idea behind using three engines instead of one was that downtime through engine maintenance could be reduced by using only two engines underway and servicing the third while at sea. The engines were connected via clutches so that one, two or all three engines could be used at any one time.

A fixed-pitch prop would have overloaded the engines had fewer than the three been used, and the CP prop system enabled blade pitch to be reduced in headseas to reduce engine loading, or increased downwind when engine loading was reduced.

The CP prop improved manoeuvrability too, because the engines didn't have to be idled to change gear or stopped and restarted in reverse as was the case with the ubiquitous direct-drive two-stroke diesels of the period.

A similar system could be used in passagemaking motoryachts. Twin diesels could be coupled via twin reduction gears and a combining gearbox and one CP prop, avoiding the inefficiencies of twin props and enabling one engine to develop full power should the other need repairs.

EVEN BETTER FOR YACHTS
A couple of years later I crewed on a Contessa 25 during my club's winter pointscore and the yacht had been fitted with a 5hp Albin Cadet petrol inboard driving through a two-blade CP and feathering prop.

No gearbox or reduction gear was necessary as the maximum revs were only 1600, and a clutch wasn't needed as the engine was started with the blades in neutral pitch. As the throttle was opened in ahead or astern the blade pitch was increased to covert engine torque to thrust.

When motoring into headseas there was provision for reducing blade pitch, or when motorsailing increasing the pitch to improve fuel efficiency - just like driving a car such as the Honda Jazz with a constantly variable automatic transmission. When under sail only the blades could be angled fore and aft in line with the prop aperture to reduce drag.

The disadvantages of early CP and feathering yacht props supplied with diesels such as the Norwegian Sabb units was that most were two bladed, and compared to fixed-pitch three-bladed props they created more vibration and lacked thrust efficiency, wasting about two thirds of an engine's output. And being designed to perform well over a relatively large range of pitches the blade shape was nowhere near as efficient.

Other problems have been the underwater complexity due to the gearing in the prop hub and the slightly larger prop-shaft diameter to accommodate the pitch-control rod running through the hollow shaft.

As yacht designs moved away from full keel (such as the Folkboat) or keel and rudder skeg (such as the Contessa and S&S 34) to keel and balanced rudder hulls with exposed prop shafts, most fixed-pitch props have been replaced by geared folding props - particularly when used with saildrives where blade-pitch control systems cannot be fitted. Geared folding props are now available in three or four blades (Volvo Penta) and some, such as the Gori, have provision for increasing the blade pitch when motorsailing.

But with these props, engine revs must still be reduced to idle when shifting from ahead to astern, and rapid acceleration in gear can overload small-yacht auxiliaries in blustery conditions.

Most small diesels have fixed fuel-injection timing, which is over-advanced below the maximum torque band, resulting in black exhaust smoke from fuel oversupply when quickly stopping the yacht by rapidly increasing revs in astern gear. Small auxiliaries such as Yanmar's 2GM20 develop maximum torque at 3000 revs with little below 1500.

CP prop systems score over fixed-pitch props by eliminating the need for an ahead/astern gearbox (enabling the engine to be mounted further aft, away from accommodation space), though with most small diesels that rev out to about 3600 a reduction gear is needed to reduce prop-shaft revs to an efficient prop thrust level, and a clutch would allow heat-exchanger engines to be run on the slipway.

The engine revs can be kept at a fast idle and the pitch angle altered from ahead to astern without slowing the engine because the prop-shaft constantly rotates in the same direction. When using the engine to reduce ground-tackle loading while riding out a storm the engine revs can be kept at fast idle to reduce engine-cylinder bore glazing and just a small amount to pitch used to prevent the anchor from dragging.

Other advantages are that, should a eutectic refrigeration system and a large-capacity alternator be fitted, the blade pitch can be reduced when using the full output of these. And as the output of all mechanically injected diesels diminishes in tropical weather - where the air has less density than in cold climates - engine load can be also be reduced by decreasing blade pitch.

Finally, as hull drag increases due to antifouling effectiveness, the blade pitch can be reduced to maintain the manufacturer's recommended engine revs at WOT.

Hulls with props running in an aperture either in the rudder skeg or the trailing edge of a full keel simply cannot use folding props as the blades fold aft when under sail. In comparison feathering prop blades simply turn 90° from neutral pitch to the fully feathered position, requiring very little more space than a fixed-pitch prop.

The prop-shaft diameter need not be much larger than a conventional solid prop-shaft used with a fixed-pitch prop. For example, 10-14in props need a 1.0-1.25in shaft diameter, while 16-20in props require 1.75in. Props from 22-26in need a two-inch shaft.

IMPROVING ALL THE TIME
CP and feathering-prop engineering and design have come a long way since the days of the narrow and inefficient two-bladers. More recent three-bladed designs such as the Kiwi-built Trident have 316 stainless-steel prop shafts and control rods with aluminium bronze props and use ball and roller bearings throughout for a long service life.

The prop hubs are totally enclosed and packed with grease to prevent internal marine growth, while there is inboard provision to grease the control tube and hub.

Many yachties have told me the biggest problem with diesel inboard auxiliaries is that they rarely achieve their recommended WOT rev range even when the yacht is new, mainly due to the boatbuilder fitting too coarse a pitch prop. Yachties who build yachts rarely understand correct propping for performance, fuel efficiency and a long engine lifespan.

Fitting a CP prop will overcome this problem while dramatically reducing drag under sail and providing the ability to handle widely varying engine loads - features just not available with fixed-pitch props!