Saildrive: vibration, noise or milky oil

A saildrive is a drive leg that goes straight down beneath the engine and through the hull: the gears that turn the propeller run in their own oil, and the seal between the hull and the leg relies on a rubber diaphragm (also called the membrane or hull seal). This arrangement is compact and discreet, very common on recent sailboats and catamarans, but it has two weak points: water getting into the leg, and the diaphragm ageing. This guide helps you read the signs and know when the boat has to come out of the water. Your drive manufacturer’s manual always has the last word.

1. What you notice

• Milky oil on the dipstick or when draining the leg, the colour of milky coffee: water has mixed with the oil.
• An oil level that rises without any being added (water is getting in), or that drops (oil is escaping, often leaving a sheen around the boat).
• New vibration, at certain revs, or only in reverse.
• Noises: a rumble that rises with the revs, a knock at idle, a noisy jolt when shifting gear, rubbing under the hull.
• Water or salt traces around the base of the leg, inside the boat.
• An alarm: some legs have a moisture sensor in the diaphragm; if it goes off, take it seriously.

2. Safety first

A leaking diaphragm is a leak in the hull. If seawater is coming in around the base of the leg, inside the boat, treat it as an emergency: check that the bilge pumps work, watch the level, head for a harbour and arrange a haul-out — see water in the bilge. Milky oil with no water in the boat is not a leak in the hull, but it is not a fault to leave running: seawater rusts the gears and bearings from the inside.

For vibration: if it appears suddenly, after an impact or when crossing an area of pots, reduce speed, shift to neutral and stop the engine — there may be a line in the propeller. Never dive under the boat with the engine running: stop it, remove the key, and only dive in calm water, never alone, if you know how.

3. Likely causes, from most to least common

Milky oil or water in the boat

• The propeller shaft lip seals — at the bottom of the leg, behind the propeller. They wear, and fishing line or a rope wound around the shaft cuts into them. This is the most common cause of milky oil.
• The leg’s other seals — a drain plug not tightened properly or its washer not replaced, O-rings between the parts of the leg.
• The diaphragm — cracked, hardened, swollen by diesel or oil left in the bilge, or come unstuck. Water then enters the boat, around the leg. Rarer, but the most serious case.

Vibration

• A fouled propeller — shells and weed, especially after a long stay in harbour or at anchor.
• Something caught in the propeller — a line, a net, a plastic bag.
• A damaged propeller — a blade bent or chipped after an impact.
• A folding or feathering propeller — a blade that no longer opens, a hinge that is seized or worn and has play, a lost blade.
• The anode — loose or badly refitted, it can vibrate; worn right down, it no longer protects the aluminium leg, which starts to corrode (paint blisters, pitting).
• Engine mounts — sagging, split or oil-soaked: the engine moves too much, vibrates, and strains the diaphragm.
• How the engine sits — a saildrive has no shaft line to align like a conventional inboard, since engine and leg form a single block; but the assembly must sit at the right height on its mounts, with no strain on the diaphragm. Badly adjusted mounts cause vibration.

Noises

• A knock at idle — often the worn damper plate between the engine and the leg.
• A jolt or noise when shifting gear — a badly adjusted control cable, idle speed set too high, or a tired clutch in the leg.
• A rumble that rises with the revs — bearings or gears in the leg, often after a period of milky oil.

4. What you can safely check yourself

• The leg oil, engine cold, boat level: level and colour on the dipstick. Write them down, with the date and engine hours, to compare next time.
• The base of the leg, inside, with a torch: water, salt, oil, cracked or swollen rubber. Keep this area clean and dry, free of diesel and oil.
• The engine mounts: rubber split, sagging, come unstuck or soaked.
• Shifting gear, at idle, in harbour, boat moored: forward and reverse should engage cleanly, without a knock.
• When the vibration happens: at what revs, forward or reverse, since what event (impact, long stay in harbour, antifouling). These notes save the mechanic time.
• The propeller and anode, boat out of the water (or by diving, engine stopped and key removed, in the conditions given above): clean and intact blades, folding blades that open and close freely by hand, anode tight, unpainted, not too worn.
• Around the shaft, behind the propeller: wound fishing line, traces of oil.

5. When to stop and call a professional

Water coming in around the leg: call immediately, haul out as soon as possible. Milky oil: haul out soon, to find where the water gets in and replace the seals — on most legs the shaft seals can only be changed with the boat out of the water — then drain and flush the leg. Replacing the diaphragm is a big job (the engine is lifted or moved) for a mechanic. Also call if the vibration persists after cleaning the propeller, if the leg rumbles, if a folding blade has play, or if the leg’s aluminium shows corrosion.

6. Keeping it from coming back

• Check the leg oil at every engine oil change, and replace it every year or as the manual says — see inboard engine maintenance.
• At every haul-out and antifouling: clean the propeller, grease a folding propeller according to its manual, replace the anodes, and apply to the leg only an antifouling compatible with aluminium (many manufacturers prohibit copper-based ones).
• Inspect the diaphragm every year, and replace it at the interval set by the manufacturer (often seven years), even if it looks sound.
• After sailing through an area of pots or nets, take a look at the propeller at the next safe opportunity.

On a used boat, the date of the last diaphragm is one of the first questions to ask — twice on a catamaran with two legs, see buying a used catamaran. Record every job in the Hullteam log: the part, the date, the engine hours, the invoice. That is what will give the boat its value at resale — see which proof really counts.