How A Saildrive Works

A saildrive takes the engine’s output straight down through the bottom of the boat and turns it aft into a propeller, in one sealed leg. There is no shaft running at an angle through the hull, no stern gland, no shaft log.

It is the standard arrangement on most production cruising yachts built in the last thirty years, and it behaves quite differently from a shaft installation.

The layout

The engine sits above the leg, usually further aft and lower than a shaft installation allows. Drive goes down through a vertical shaft inside the leg, through a bevel gear at the bottom, and out to a propeller that sits horizontally, close under the hull.

Because the propeller is horizontal rather than angled, thrust pushes the boat forward rather than partly downward. That is the main engineering argument for the arrangement.

The diaphragm is the thing to understand

The leg passes through a hole in the hull. What keeps the sea out is a rubber diaphragm — a moulded rubber seal clamped between the leg housing and the hull.

It is a rubber component, below the waterline, holding back the ocean.

Manufacturers publish a replacement interval for it, typically expressed in years rather than hours, and it is a haul-out job. Some installations have a double diaphragm with a telltale between the two, so a failure of the outer one can be detected before the inner one matters.

If you own a saildrive boat and do not know when the diaphragm was last changed, that is the single most important thing to find out about the installation.

Anodes on a saildrive

A saildrive leg is aluminium sitting in seawater, with a bronze or composite propeller on the end of it. It is an aggressive galvanic environment and the leg is the expensive part.

Saildrive anodes are typically a ring or collar around the leg plus one on the propeller nut. They are consumed quickly in warm marina water and they are the only thing standing between the electrolyte and an aluminium casting that costs more than a full engine service.

Some manufacturers specify a particular alloy and warn against others. This is worth checking against your own manual rather than fitting whatever the chandlery has.

Oil in the leg

The leg has its own gear oil, separate from the engine. Its condition is diagnostic:

• Milky or creamy — water is getting in, usually past the lower seal behind the propeller. Fishing line wrapped behind the prop is a common cause.
• Metallic glitter — gear wear.
• Low level — a leak, either out into the sea or in through a seal.

Checking leg oil is a quick job and it catches expensive problems early.

What owners get wrong

• Treating leg servicing as part of the engine service. It is a separate item with its own schedule.
• Painting over the anodes at antifouling.
• Antifouling the leg with the wrong paint. Copper-based antifouling on a bare aluminium leg causes exactly the corrosion the anodes are fighting. Aluminium-safe products and correct priming matter here.
• Fishing line behind the prop. Left in place it cuts the seal.

Frequently asked questions

How often does a saildrive diaphragm need replacing? The manufacturer sets the interval for your model, and it is time-based rather than hours-based. Check your own manual, and treat an unknown history as overdue.

Can a saildrive diaphragm be changed in the water? No. It is a haul-out job.

Why is my leg oil milky? Water is entering, most often past the propeller shaft seal. Fishing line wrapped behind the propeller is a very common cause. Investigate before running further.

Is a saildrive better than a shaft? Different, not better. Saildrives are efficient and quiet, and leave the hull clean. Shafts are simpler, more forgiving of neglect, and have no rubber component holding out the sea. See our comparison of drive types.

Do saildrives need different antifouling? Yes. The leg is aluminium and needs an aluminium-safe system. Copper-based antifouling applied directly to the leg will cause corrosion.