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Sail Drives

Saildrives are often used on catamarans as they result in a very compact drive system.  Multihulls have narrow hulls, so there's not enough room to walk around an engine, as there is on most monohulls.  So a catamaran's engines are usually under the aft bunks, which usually doesn't leave much room for a gearbox, propeller shaft, external "A" bracket, and still have room for the rudder.  Saildrives attach to the back of the engine and immediately go down through the bottom of the boat, sort of like the leg of an outboard motor.  A pair of flexible seals (the main seal and a backup) keep water from flooding the engine room.  There are a handful of manufacturers who offer saildrives, but Volvo and Yanmar are probably the biggest, and the vast majority of saildrives we see are produced by those two manufacturers.  Since we have Yanmar 4JH2CE engines and SD31 saildrives, the comments below are aimed primarily at Yanmar saildrives, but Volvo saildrives are similar enough that most of the comments apply to both.

Subjects in this article include:

Oil Header Tanks:  How to make sure water doesn't get into your oil.
Secondary Seals:  A cheap and easy way to help preserve your secondary seal.
Copper-based Antifouling Paints:  This is not difficult to do, and will allow you to use the
 same paint on the whole of the bottom of your boat.
Saildrive Boots:  Make your own instead of buying the flimsy official parts.
Gear cluster removed so we can replace the seals
Saildrive gear-cluster removed
so we can replace the seals

Oil Header Tanks
Like virtually all manual gearboxes, saildrives are filled with 90W hypoid oil (ideally, salt-resistant oil if you can find it).  This oil is held in by a pair of oil seals around the propeller shaft.  Those oil seals can wear out so they need to be replaced periodically.  They're usually fairly standard seals, but replacing them means dismantling the bottom end of your saildrive and then disassembling the gear-cluster to get at the seals - a messy and fiddly business requiring some familiarity with the parts and procedures involved (and sometimes requiring special tools).

But it's very important to service those seals before they leak.  Since the saildrive is wholly below water level, the seawater on the outside is at a slightly higher pressure than the oil on the inside.  This means that if the seals start to leak, seawater will enter the bottom of the saildrive and start turning your oil into mayonnaise.  If this happens, you need to haul the boat out of the water and service the saildrive as soon as possible, before the seawater starts to corrode the gears and bearings.

Oil tank & bracket in our port engine room
Oil tank & bracket in our port engine room

In the Seychelles we ran into an excellent father/son machine shop, and I got to talking with the father.  He'd seen a lot of marine equipment - in fact, he was refurbishing a turbocharger for a 5,000 horsepower diesel engine when I met him.  When I explained this problem with the oil seals, he had an excellent idea:  Put an oil header-tank as high up in the engine room as practicable, and run a hose from it to the saildrive.  That way, the oil will be at a slightly higher pressure than the seawater.  If the seals start to leak before their normal maintenance interval, all that will happen is that a bit of oil will leak out.  You'll be able to see the oil level dropping slightly in the header tank and that will tell you that you need to replace the oil seals the next time you haul the boat.  But it's no longer an emergency!

We did this conversion in 2009 when we were in South Africa, and it seems to work well.  The oil-fill / dipstick hole is a standard size, so we just removed the dipstick and replaced it with some pipe fittings to take the hose to the header tank.  We put a small valve in the hose so we can shut the header-tank oil off when we want to change the saildrive oil.

There are a couple of caveats here.  First, there's another oil seal on the shaft that comes in from the engine.  This seal is not used to taking any pressure, and it will now be taking a small amount of pressure.  We haven't noticed any problems with this, but I heard that one boat did.  I don't have any details on that, but I'd think that if that seal can't take 3‑4 feet (1m) of head, it's got other problems and should probably be replaced anyway.

Oil piping into the saildrive - plastic & stainless steel
Oil piping into the saildrive - plastic & stainless steel

Another issue is that the prop‑shaft oil seals should be turned around.  Oil seals are direction sensitive.  The flat side should go on the low-pressure side, while the side with the spring (to hold the seal against the shaft) needs to go on the high-pressure side.  That way, the pressure actually helps to keep the seal against the shaft.  Swapping the seals around is easy to do and not a big deal - just put them in the other way around when you next change them.  In fact, some people I've talked to have put the 2 seals back to back, with the inner seal set to keep the oil in, and the outer seal set to keep the water out.  This seems like a good arrangement, but if you do this, remember to remove the spring from the outer seal and replace it with an O‑ring that won't rust.

Finally, it will probably take a little while before all the air works its way out of the saildrive and up the hose, through the thick oil to the header tank.  While this is happening, it may look like you're losing some oil, but it's just air coming out.

Secondary Seals
Saildrives have 2 flexible seals between the saildrive leg and the hull, to keep the water out.  The primary seal is a monster of thick rubber while the secondary seal, above the primary seal, is much thinner.  The secondary seal is what you actually see from above, and it's really only there for emergencies, in case something happens to the primary seal.  But for the secondary seal to be effective, it has to be able to survive whatever caused the primary seal to fail.

Hose clamp around our secondary saildrive seal
Hose clamp around our secondary saildrive seal

In the Ha'apai Group of Tonga we had a big problem when we caught a rope in our starboard propeller and ripped that engine and saildrive clean off its mounts.  This ripped the main seal that joins the saildrive to the hull, so we had water coming into the engine room.  We were very lucky that the secondary seal had just popped off, and could be reset fairly easily.  In fact, once we reset the secondary seal, it didn't even affect our cruising much, as we just continued cruising with our port engine (although anchoring was rather ... interesting).

It turns out that SunSail (who had managed Ocelot before we bought her) had replaced the $70 clip that Yanmar sells to hold that secondary seal with a large $5 stainless-steel hose-clamp.  The Yanmar part holds the seal so securely that, had it been used, we might well have shredded the secondary seal as well as the primary!  This would have been a REAL problem, especially since we were out in the boonies - a good 90 miles from what passes for civilization in Tonga.

As it was, a hose-clamp is a bit too tall, so it can't be tightened up too much or it slips off.  It certainly holds the water out, but if you yank hard on the hose-clamp or the seal, it will just pop off, instead of ripping the seal.  This allowed Jon to find the hose-clamp, loosen it a bit, make sure the seal was down correctly, put the hose-clamp over the seal, and tighten the hose-clamp gently (even with his heart going a million miles an hour as water was coming in ).  Then the bilge pump could pump the water out.  Since the water was completely opaque, Jon had to do this without being able to see anything, but it was still pretty simple.  It also helped that Chris jumped overboard and stuffed a towel up around the saildrive from the outside, which limited the flow of water coming in.

In our opinion, Yanmar (or any end user with a saildrive) should replace the fancy clip holding the top of that secondary seal with something like a hose-clamp that holds the seal securely enough, but not too securely.  It's an easy and inexpensive modification which could potentially save lots of grief if you ever need that secondary seal.

Draining oil from saildrive
Draining oil.  Note poor condition of
saildrive paint compared to the hull

How to use Copper-based Antifouling Paints
Most saildrives have an aluminum outer casing.  If you paint this casing with normal copper‑based antifouling paints, you run the risk of bimetallic action eating up your saildrive.  They sell special (non‑copper based) antifouling paints to go on aluminum, but we've never found one that lasted more than about 9 months before starting to allow growth.  Since our normal antifouling paints often last 2‑3 years, it's frustrating to have foul saildrives on an otherwise clean hull.

In early 2010 we decided to clean our saildrives back to bare aluminum and then coat them in several layers of epoxy, to act as an insulating layer.  Then we could use normal (long lasting) antifouling paints on the whole bottom, including the saildrives and props.  As I write this (about a week after epoxy-coating and painting our saildrives) it all seemed to go quite well.  Our West System epoxy went on like fine varnish, with no running or any signs of beading up or surface tension.  We put 4 layers of epoxy on (in about 4 hours) before painting on the antifouling.  But only time will tell how effective the whole operation will be.  This is something we'll be watching closely and we'll report what we find here on this page.  We'll especially watch our zinc anodes, to see if they're dissolving faster than normal (the anodes we took off had been on for 3 years).  If anyone else has experience with this, please let us know.

Epoxied & sanded saildrive & prop
Epoxied & sanded saildrive & prop

If you decide to do this yourself, make sure you read our article about how to make epoxy stick to aluminum.  It's fairly straightforward to do, but it's not what the paint retailers will recommend.  It involves stripping all old paint off your saildrives (a small angle grinder with a cup-shaped wire wheel will help tremendously here), cleaning the saildrives well with acetone or alcohol, painting on a good epoxy, and then sanding the aluminum through the wet epoxy.  This will remove the oxide layer that forms on aluminum.  Without any free oxygen near the aluminum, it will also prevent the oxide layer from forming again, allowing the epoxy a good chemical bond as well as a good mechanical key.  The epoxy will turn aluminum colored from the sanded off aluminum, but that's OK.  Only the first layer of epoxy needs to be sanded.  If you're using a good epoxy that doesn't contain solvents (like West System epoxy) then the next layer of epoxy can be applied in about an hour, as soon as the under-layer has started to kick off.  It should be tack-free, but still soft enough to dig a fingernail into.  This will allow the epoxy to bond well to itself.  If the epoxy is solvent free, then the first layer of antifouling can also be applied before the epoxy has fully hardened, but if you allow the epoxy to harden it should be lightly sanded and cleaned with acetone before applying the next coat.

If the epoxy you use has solvents or, even worse, oils or waxes, you'll need to wait until it hardens completely, then give each coat of epoxy a light sanding and wipe it down with acetone or alcohol to clean off any oils before applying the next coat.  This will require 12‑24 hours between coats.  Better to use a high-quality epoxy if possible.

Finished saildrive and propeller
Finished saildrive and propeller

Other things to look out for:  Make sure you mask off your anodes well.  Try hard to epoxy coat everywhere you might want to paint, as you don't want straight paint on bare aluminum if you can help it.  This can be difficult around fiddly bits, like up under the rubber boot, or inside the water-inlet holes.  We sanded our water-inlet holes by folding a bit of sandpaper into a thin strip and poking it through the holes, pulling it back and forth to sand as much as we could.  You also need to be careful around the oil drain-plug.  The plug itself is steel and doesn't need epoxy, and you certainly don't want epoxy to get into the crack between the drain-plug and the casing or into the screwdriver slot, so we'd recommend masking off the whole area for the epoxy phase.  You might even consider squirting a bit of lanolin grease up into the crack with a plastic syringe (we keep a plastic syringe full of lanolin grease handy, as we're constantly using it).

Amanda carving a new saildrive boot
Amanda carving a new saildrive boot

The object of this whole exercise is to provide an insulating layer between the copper in the antifouling paint and the aluminum of the saildrive.  Although the epoxy is only a few thousandths of an inch thick, it should provide good electrical insulation.  Since seawater is so conductive, there may still be a small bimetallic effect, but it should be easily absorbed by the zincs.

Saildrive Boots
Saildrives typically have a bit of rubber that goes between the saildrive and the outside of the hull, to make for smooth water-flow around the saildrive.  Both the Yanmar and the Volvo parts are rather thin and flimsy, so they rarely last more than a year or 2.  Being official parts, they're also, of course, quite expensive.

Both of the official parts actually contact the saildrive leg.  This permits a good seal and good water-flow around the leg, but it also allows engine (and saildrive) vibration to go through the boots, which will eventually tear up both the rubber boot itself as well as whatever adhesive you use to stick the boot to the hull.

Amanda applying 5200 to both surfaces
Amanda applying 5200 to both surfaces

In Tonga, our saildrive boots needed replacing.  But the closest Yanmar dealer was in New Zealand, and getting parts out of them was like pulling teeth.  They ignored our emails, even when we put EMERGENCY in big letters on the subject line.  The only way we got their attention was to call them on a satellite phone ("Oh, THAT emergency email...").  Yeesh!

So instead of going through Yanmar, we went up to the local auto-parts store and bought some truck mud-flaps.  These are thick (1/4" or 6mm) rubber, much sturdier than the standard Yanmar parts.  Amanda carved these into the correct shape but left a small gap (again, about 1/4" or 6mm) between the boot and the saildrive.  This still permits good water-flow around the saildrives, but the boot would no longer be subjected to engine or saildrive vibration, so it should last much longer.

The finished (home-made) saildrive boot in place
The finished (home-made) saildrive boot in place

To hold these boots in place, we first tapered the edges, to provide less of an edge for things to grab onto.  Then we forced them over the saildrives (the props have to be removed) and cleaned all surfaces thoroughly.  Instead of using contact adhesive, we stuck the boots to the hull with fast-cure 5200.  Sika-flex would probably have worked just as well since they're both polysulfide adhesives.  Finally, we made stainless steel strips to go on the edges of the boots, and screwed through the strips and the boots into the hull.  As we write this, it's been well over 5 years and those saildrive boots are still hanging in there.  (But while she was at it, Amanda carved a second pair of saildrive boots, just in case...)

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