Saildrive Care: Leg Oil, Hull Seal, Anodes and Prop

Yachting Monthly calls checking the saildrive oil before every engine start, along with the engine oil, best practice. A saildrive holds a small amount of oil, so even a small leak matters. The dipstick is usually on top of the leg inside the boat, near the back of the engine. Read it the way your manual describes (some are read screwed in, some resting on the threads).

Wipe the dipstick on a white rag:

• Clear, normal color: good.
• Milky, creamy or "like mayonnaise," or the level has risen: water is getting in. The usual cause is worn lip seals on the propeller shaft (often damaged by fishing line) or a bad drain plug seal.
• Dark with a burnt smell or metal glitter: internal wear. Get it checked.

Milky oil does not mean the drive is ruined. Practical Boat Owner's engine expert notes the gears and shafts usually survive, but the bearings may be pitted and the seals need replacing. Do not keep motoring on milky oil; plan a haul-out soon.

Most makers call for an annual oil change, or by hours if you motor a lot; your manual gives the interval and the exact oil. Some drives use gear oil and some use engine oil, so never guess. Yanmar's FAQ says plainly that oils vary by engine model and by drive or gear model, and points owners to the Operation Manual or a Yanmar dealer for the right one.

• Afloat (some models): the oil can be pumped out through the dipstick tube with an extraction pump and refilled from the top.
• Ashore: remove the dipstick first so air can get in, then remove the drain plug at the bottom of the leg and let it drain into a pan.

Fit a new O-ring or washer on the drain plug every time; a tiny crack lets water in. Refill slowly to the mark and recheck after running the engine. Save a sample in a clean jar if you want to compare color next year, or send it for an oil analysis.

Inside the boat, look around the base of the saildrive where it meets the hull. You can usually see only a small part of the diaphragm and its clamp ring. Check for:

• Water or a damp ring around the base (any water here is serious; see the safety note)
• Cracks, splits or swelling in the visible rubber
• Corrosion or loose bolts on the clamp ring

Some saildrives have a water sensor built into the seal area that triggers an alarm at the helm. If yours does, find out from the manual how to test it and test it each season.

The hull diaphragm is a rubber part with a fixed life. Practical Boat Owner reports Volvo Penta's recommendation as an annual inspection and replacement every seven years, sooner if an inspection shows deterioration. Yanmar's interval depends on the model; published figures vary, so read the maintenance table in your own manual. Yachting Monthly gives 5 to 10 years depending on care.

Many owners say seals look fine much longer than that, but PBO also published a case where a seal that had looked new was found split nearly through at the next replacement. Long periods ashore in heat may have contributed. Replacing it means hauling out, disconnecting the engine and lifting or sliding it so the leg can come out, which is why it is a dealer job. Keep the date of the last replacement in your log; buyers, surveyors and insurers often ask for it.

When the boat is lifted, make sure the travel-lift slings sit well away from the saildrive leg and the prop. A sling pressing on the leg can strain the diaphragm. Most boats have sling marks on the hull; point them out to the lift crew.

The leg is aluminum, which corrodes fast in salt water wherever bare metal shows. Look for blistered paint, white powdery corrosion and scratches down to metal. Small spots can be cleaned back and touched up with aluminum-safe primer and epoxy paint. Bottom paint on the leg must be one formulated for aluminum drives. Copper-based antifouling on an aluminum leg speeds up corrosion. Deep pits or corrosion around the seal area need a pro's opinion.

A saildrive can have anodes in up to three places: on the leg, around the propeller (a ring or a cone) and, on some models, inside the boat on the upper gear housing. Yachting Monthly's rule: replace each one when it is about half eaten away, or will be before the next haul-out.

• Use the right metal: magnesium only in fresh water, zinc in salt or brackish water, aluminum in either.
• Use the exact shape made for your drive so it makes good electrical contact. Never paint anodes.
• If you fitted a bronze folding or feathering prop, expect the anodes to wear faster; the original anodes were sized for the original prop.

Anodes that look untouched after a season are a warning too: they may not be making contact.

Anodes that disappear much faster than last year point the other way: stray current or a neighbor's boat. Cruising World's saildrive article, drawing on a Yanmar service bulletin, makes three points worth knowing:

• The anodes on the drive protect the drive only. A bronze prop, through-hulls or other underwater metal may need their own protection.
• If the boat is plugged into shore power most of the time, have a galvanic isolator installed, or your anodes may end up protecting the boats around you.
• Yanmar's bulletin says to avoid copper-based antifouling not only on the leg but on the whole bottom. Check your own drive maker's paint advice before you repaint.

A marine electrician can measure the drive's protection level (hull potential) with a reference electrode; some saildrive owners fit a permanent meter.

With the prop removed, or by looking closely at the gap between the prop hub and the leg, check for fishing line, net or rope wrapped around the shaft. Line works its way into the shaft lip seals and lets water into the oil. Picking out line at every haul-out is cheap insurance.

• Fixed prop: check blades for dings and the rubber drive hub for slipping (the engine revs but the boat is slow).
• Folding or feathering prop: clean out barnacles and old grease, lubricate the hinges or gears with the grease the prop maker specifies, and check that blades open and close freely and evenly.
• Reinstall the prop, washers and cone in the right order with the locking hardware your manual specifies, and fit new anodes while it is off.

Around the outside of the hull opening there is usually a separate rubber fairing (sometimes called the outer seal or gaiter). It smooths water flow and protects the diaphragm, but it is not the watertight seal. If it is loose, torn or lifting, reglue or replace it at haul-out. Water between the fairing and hull is normal; it has drain holes.

• The engine's raw water usually comes in through a grille on the leg and up to a valve on top of the drive. Clear any growth from the grille at haul-out, and make sure the valve opens and closes.
• Look at the shift cable where it connects to the drive. Check that the linkage is secure, the neutral position is correct, and the parts move freely. Grease them if the manual calls for it.

Before winter storage, change the oil so the leg sits with clean oil, and follow your winterizing routine for the engine and raw water side. In spring, check the oil level and color again after launch, look around the base of the drive inside the boat for any water in the first hours afloat, and check again after the first run.