Why should I get a saildrive for my boat?

The prop ran in either an aperture ahead of the rudder or to one side of the sternpost, as in designs such as the Australian-built Herreshoff 28 and 36.

But as fibreglass replaced timber, and fin keel hulls with separate rudders became popular, yacht designers started 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. Like a sterndrive, a saildrive’s leg and gearcase protrude below the hull. However, the propshaft is replaced with three short driveshafts with two 90-degree angles between them.

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 that can also be trimmed to fit the hull shape of virtually any yacht.

In most applications, 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 that's used to indicate failure of the outer diaphragm and warn the yacht's owner 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 motor sailing, the cooling system always receives plenty of water.

Gear lifespan is increased as there's always water flowing around the drive's gearcase to cool the oil.

Volvo's 50hp D2-50 diesel is a great example of how compact a powerful saildrive auxiliary can be.

In shaft drive form, the 2.2-litre in-line four-cylinder D2-50 engine measures 897mm from alternator pulley to propshaft flange, whereas complete with bed and saildrive it's 717mm.

The height and width of both engines is the same.

Complete with its saildrive, the D2-50 weighs 253kg, only 10kg more than in basic shaft-drive form with a reduction gearbox.

The D2-50 stern drive has a choice of three gearboxes, with counter-rotation versions, that add trolling modes or either an 8.0-degree or straight shaft output.

The D2-50 is available in 50, 60 and 70hp tunes. 

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 yachts using saildrives tend to lose a lot of thrust, so for maximum efficiency under power, you still need to fit a three-bladed prop.

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 that can be cleaned from inside the hull by simply closing the seacock at the skin fitting.

If you intend to cross 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 stern gland 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.

To reduce the fore and aft engine angle underway, some diesel manufacturers offer angled-down gearbox output shafts.

Volvo's D2-50 is available with an 8.0-degree down angle that should allow the engine to install on an even keel and run at no more than 5.0 degrees underway.

To prevent lubricating oil starvation, most engine manufacturers prefer their diesels to run at no more than 10.0 knots underway.

For heavier cruising yachts, shaft drive inboards have a wider choice of gear ratios to allow coarse-pitch and thrust-efficient props to swing.

As a rule of thumb, prop revs for a heavy displacement yacht should not exceed 1500rpm at WOT.

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.