Yacht Auxiliaries - Part One

Before the advent of fin keel yachts, inboard engines were always shaft drives, much like cruisers of the day. The prop ran in either an aperture ahead of the rudder or to one side of the sternpost, as in designs such as the Herreshoff 28 and 36.

But as fibreglass replaced timber and fin keel hulls with separate rudders became popular, yacht designers began looking for ways to maximise accommodation and reduce engine installation costs, particularly as correctly setting up an inboard engine is a fairly labour-intensive task. The result was the saildrive, where like a sterndrive a leg and gearcase protrude below the hull and the propshaft is replaced by three short driveshafts with two 90° angles between them.

THE BEAUTY OF SAILDRIVES
Saildrives are a wonderfully compact way of fitting a powerful diesel engine into a yacht without the engine encroaching much on accommodation. There's no need to have an angled-down propshaft, so the prop's line of thrust is more efficient and finicky propshaft alignment is unnecessary. Propshaft vibration is virtually eliminated as the entire engine, gearbox and saildrive assembly sits flexibly-mounted on a moulded fibreglass bed which can also be trimmed to fit the hull shape of virtually any yacht.

Usually two flexible diaphragms are used to keep out water above where the saildrive leg passes through the hull. Between these diaphragms is a water-ingress alarm to indicate failure of the outer diaphragm and warn the yacht's owner of this before water breaches the inner diaphragm.

As the cooling water intake is located in the saildrive, there's no need to fit a separate water intake with seacock, and in rough water or when heeled while motorsailing, the cooling system always receives plenty of water. Gear lifespan is increased as there's always water flowing around the drive gearcase to cool the oil.

COMPLETE AND COMPACT
Volvo's 50 hp MD22L diesel is a great example of how compact a powerful saildrive auxiliary can be. In shaft drive form, this 2lt OHC four measures 877mm from alternator pulley to propshaft flange, whereas complete with bed and saildrive it's 17mm shorter. Total height of the motor from grommet (in line with the sump) to heat exchanger top is 685mm, only eight more than the shaft drive version, while width is no more upwards of the crankshaft line.

Complete with its 120S saildrive the MD22L weighs 243kg, only nine more than in basic shaft drive form with reduction gearbox. The 120S drive also has a usefully low 2.2:1 gear ratio, reducing prop revs of this high speed automotive-based diesel to 1360 at Wide Open Throttle. And even in its uprated MD22P form, where it develops 59 hp, the prop revs are 1820 and not too fast for pushing light to medium displacement racing yachts.

Because the weight of a saildrive auxiliary is concentrated in a relatively small area, the performance of a racing yacht is enhanced by keeping weight out of the hull ends. Drag from a saildrive is also generally lower than a shaft drive and can be further reduced by fitting a geared folding two-bladed prop.

CRUISING FLAWS
However, for cruising yachts a lot of thrust is lost, and for maximum efficiency under power you still need to fit a three-blader. Indeed, in cruising yachts the saildrive concept has some real flaws. As the drive leg is aluminium, you can only use non-copper antifouling which is nowhere near as effective as 'traditional' slow-leaching copper paints.

Another problem is coral build-up around the cooling water intakes and coral particle intake when operating in tropical waters the main reason why charter operators on the Great Barrier Reef prefer shaft drives. The cooling water intakes of their yachts are usually fitted with an effective filter or screen which can be cleaned from inside the hull by simply closing the seacock at the skin fitting.

If you intend crossing oceans, the possibility of a leak in the diaphragm can be a real problem when several days from the nearest port. A leak in a sterngland is much easier to control than one in a fairly large rubber diaphragm, which can only be fixed by slipping the yacht. This also applies to servicing the leg and gearcase or changing the gear oil in the gearcase.

Servicing a shaft drive engine's gearbox is simpler than a saildrive and may be done from inside the hull, eliminating the need to slip the yacht.

THE CHOICE IS YOURS
To reduce the fore and aft engine angle underway, some diesel manufacturers offer angled-down gearbox output shafts. Volvo's MD22L and MD22P are available with a 7° down angle which should allow the motor to be installed on an even keel and run at no more than 5° underway. To prevent lubricating oil starvation most engine manufacturers prefer their diesels to run at no more than 10 underway.

For heavier cruising yachts shaft drive inboards have a bigger choice of gear ratios to allow coarse-pitch, thrust-efficient props to be swung. Both the Volvo MD22L and MD22P are available with 2.4:1 or 3:1 ratios which reduce the propshaft revs to 1250 and 1000 revs for the MD22L or 1670 and 1330 revs for the uprated motor. As a rule of thumb prop revs for a heavy displacement yacht should not exceed 1500 at WOT.

Saildrives can generally handle no more than 60hp and there are some really compact shaft drive diesels around the 80hp mark. One is Volvo's TMD22, the turbo-charged version of the MD22P, which develops 78hp at 4500 revs and is available with the same gearbox as the naturally aspirated motor. The weight is only 242kg with gearbox and installation dimensions the same as the MD22P.

As with all forms of power, choosing an auxiliary is a compromise that should best suit your lifestyle. If you have a 'round the buoys' harbour racing yacht and are close to slipways, by all means fit a saildrive. But if you cruise long distances and to remote corners of the world, then fit a shaft drive diesel.