Saildrive versus shaftdrive

Over 60% of production yachts and sail and power cats powered by Yanmar are now fitted with saildrives. And with new production builds, the figure is higher than 70%. Those are the statistics from Michael Blair of Power Equipment, the Australian distributor for Yanmar diesels.

Shaftdrives are now mostly limited to the repowering of older full keel yachts or steel hulls, where the installation of a saildrive is impossible or impractical.

SAILDRIVES
It's not hard to see why fibreglass boatbuilders prefer saildrives. Saildrives eliminate the need for two separate holes in the hull bottom - one for the propshaft and the other for the cooling-water intake. So once the hole for the drive leg has been cut in the hull, it's just a matter of glassing-in the fibreglass engine bed, which forms a well around the hole.

The leg is then lowered through the well and bolted to the bed, followed by the engine. The flywheel drive splines are aligned with those on the head of the leg, which houses the ahead/astern gearbox. This eliminates the need for careful alignment of the gearbox output shaft to the propshaft - which if slightly out can create annoying vibrations through the hull - because the engine and drive unit are isolated from the engine bed via flexible mounts.

Cooling-water intakes are located just above the lower unit gearcase torpedo. The water is led up the leg to an outlet above the diaphragms, where a hose carries water to the engine's seawater cooling pump. There's no need for a separate cooling-water intake with a seacock mounted to one side of the engine, which may suffer water starvation when motorsailing with the hull heeled on a tack upwind.

If accommodation space aboard is limited, the engine may be installed aft of the drive leg, and for cats the ahead/astern linkages may be reversed to provide counter-rotation.

A fairing is needed around the drive leg to reduce the drag of having a square well in the hull bottom, as the leg does not touch the sides of the well and passes through a rubber diaphragm into the bilge. Yanmar uses two separate diaphragms: one that keeps out seawater and another atop this as a backup. As the lower diaphragm is constantly immersed, Yanmar fits an automatic sensor to alert the operator to any water between the diaphragms.

Saildrives are ahead of shaftdrives for thrust efficiency. Because the lower unit gearcase is parallel to the water surface, there is none of the thrust loss that occurs with shaftdrives, which may have a down-angle of well over 7°. And as there's no need for a long propshaft, the saildrive leg can be positioned closer to the fin keel, resulting in less thrust loss when the yacht is pitching heavily.

But because there are two sets of right angles from the flywheel to the prop, frictional losses reduce actual propshaft power to less than a shaftdrive.

SHAFTDRIVES
If you have a full keel transom-sterned yacht such as a Folkboat/Stella and H28 or full keel counter-sterned yacht such as a Tophat 25 or Nicholson 32, a shaftdrive is your only choice. The deep bilges allow the engine to be mounted below the static waterline and only a short shaft is needed to reach the prop, which usually spins in an aperture ahead of the rudder, or in the case of a Stella and H28, to one side of the sternpost.

Back in the seventies many small marine-specific diesels (such as Bukhs, Saabs and Volvos) had flywheels ahead of the engine blocks to enable an engine to be shoehorned well aft in the bilge. Unfortunately, most diesels are now industrial-based and have flywheels aft of the engine blocks, forcing the engine further forward into the accommodation space, so these engine design changes have to be allowed for when repowering your yacht.

Though it's time-consuming to correctly align the gearbox and propshaft, shaftdrives have the distinct advantage that the stainless steel propshaft and bronze prop allow the use of slow-leaching copper antifoulings. These paints last much longer than the non-copper paints needed for aluminium saildrive legs, and if your boat is steel, stainless steel and bronze are less susceptible to electrolysis than an aluminium leg.

Although to my knowledge very few (if any) saildrive diaphragms leak, if your boat is left afloat unused for long periods, there's more security in having a small stern gland than relying on rubber diaphragms.

When I interviewed bareboat charter operators in the Whitsundays about their drive system preferences, their main gripe was the fine coral particles from reefs in the area. They said that these particles blocked saildrive cooling-water intakes with monotonous regularity. Once the unscreened intakes of a saildrive leg had clogged, the hull had to be slipped and the drive leg removed to clear the blockage, unlike the separate cooling-water intakes of shaftdrive motors, which could be cleaned by simply diving over the side and removing the grit with a screwdriver.

Saildrives generally have only one gear ratio choice and the ratio is the same ahead and astern to facilitate counter-rotation. But even with small shaftdrive Yanmar diesels such as the 1GM10, three different ratios are offered and the two taller ratios have a higher ratio ahead than astern. This allows the engine to reach its torque band faster and reduces overloading when going astern to stop the yacht or when using the engine to back off a sandbar.

To reduce overall size and weight, most saildrive gearboxes have dog clutches whereas shaftdrive boxes have cone clutches that allow for a more progressive take-up of ahead or astern gear and have a longer lifespan. Shaftdrive maintenance is also lower than saildrives because only the shaft and prop are exposed to seawater and the gearbox may be serviced from within the hull.

CHOICE DEPENDS ON APPLICATION
As most new fibreglass yachts come standard with saildrives your drive system choice is severely limited.

If you're fitting-out from scratch a fibreglass yacht with a separate keel and rudder for harbour racing, the ability to concentrate engine and drive system weight amidships makes a saildrive very attractive.

But if you're fitting-out a cruising yacht, especially a heavy displacement hull designed for passagemaking that won't be frequently slipped, then I'd recommend staying with a shaftdrive.