Maintenance Mania: Saildrive Lessons

Saildrives by Matt

Oh boy, fun stuff on the blog today! Saildrives may just be the bane of catamaran owners everywhere. Even though many new monohullers have them too, they seem to complain about them half as often (ha ha!). I’ve learned a lot about these undersea monsters in the last two years, and very little of the actual knowledge came from online forums. Those forums provided me with only confusion and angst. 

Of course what follows is only a list of my opinions and feelings. For actual help with your actual saildrive, I recommend you talk to a qualified mechanic. Hahahahahahahahahahahaha!!!!! Good luck with that! 
Yamnar Saildrive
Yanmar technical diagram 69.86fu: The inner workings of your saildrive
A saildrive is basically the lower unit of an outboard mounted through a huge hole in the bottom of the boat and attached to the diesel engine. If this sounds like a horrible idea to you, have a beer because you deserve it! The gap in the hole in your boat is filled by a simple, thick rubber diaphragm. If that rubber bit fails, you’ve got big problems. Conventional wisdom seems to be that these rarely fail except in cases of the saildrive impacting something, but the saildrive manufacturers want you to replace it somewhere between every 5 and 8 years. 

The saildrive is fitted with a water sensor to alert you if any water begins to enter through the diaphragm. Ideally, a failure would occur slowly, setting off the sensor and allowing you time to do an emergency haul out before water began entering the boat. Of course, if you don’t follow the manufacturer’s maintenance schedule and something bad enough happens to result in an insurance claim, we are left hoping that you have very good, very trusting insurance. 

To replace the diaphragm you must remove the entire saildrive, both the upper bit from the hull and engine and the lower bit, so it’s a major job (that must be done on the hard) to say the least. And if you’re doing all that, you might as well do x, y and z too. (For those who don’t know, x is move to Colorado, y is never see a boat again and z is get a divorce)

Those issues are just the beginning of the dramas, because as already stated there seem to be very little evidence of actual diaphragm failures. What happens all the time, as evidenced online, personal experience and talking with knowledgeable techs (I met them both recently), is failure of the propellor shaft oil seals. These two seals squeeze against the prop shaft, one keeping sea water out and one keeping oil in. These are simple $5 rubber parts that see a lot of action, so preemptive replacement is definitely recommended. The process of replacing them is well documented online and is not that complex. And don’t let the Yanmar shop manual worry you, I watched the pros do it and they use the same two hands and large hammer that you or I would.

When replacing the oil seals, especially if you have had any water ingress, it’s vitally important to figure out if there is any obvious reason it should have failed. The oil seals slowly wear down the prop shafts, leaving a gap. By shimming the location where the seal touches the shaft, the depth of the groove created can be kept to a minimum. If the groove gets big enough, water gets in. A badly grooved shaft may need to be replaced, though some online have apparently somehow had the shaft repaired. A tech recently told me that if you are paying for the amount of work that takes, you might as well buy a whole new lower leg and get a warranty to boot.

The saildrive has an oil dip stick that should be checked during every morning pre-departure check. Pay attention to the color and consistency of the oil…a sudden change could mean water ingress. Water that has been emulsified in oil makes a milky consistency. Make absolutely sure to not overfill the oil level. The oil does get warm, which causes it to expand. If the oil level is too high, it puts too much pressure on the seals. Too much pressure might mean a busted seal, which might just cost you an extra haul out.

The question of the header tank: many online “folks” swear by the installation of a header tank onto the saildrive unit. The idea for this is to allow extra pressure that builds up during operation to escape into an overflow tank. That’s a pretty good idea, but my guess (and I’m guessing Yanmar’s guess) is that if you simply don’t overfill the thing you don’t need this. Some also say that if the oil header tank is mounted above the waterline, it maintains pressure on the oil seals around the prop shaft, there by using this oil pressure to keep seawater out. Could be, I honestly cannot say. At this point I’m betting that if that were truly necessary then that’s how new saildrives would be sold.

And before we move on from oil seals, it's worth mentioning that there is another even harder to repair oil seal between the saildrive and the engine. If this seal fails, you'll likely know by puddles of oil on and around the top of the saildrive in the engine room. These failures are more in the "nuisance" category, but if you don't know where it's coming from the leak is hard to track down.

The type of oil in the saildrive is also worth some thought. For each saildrive and engine combination, the manufacturer recommends a specific grade of oil. In some cases, they even specify the brand of oil. My 3YM30 engines and SD20 saildrives simply state "80w90 or SAE90 gear oil". I have found several references to only using Quicksilver or Mercruiser approved marine oil, as this gear oil has better water retaining properties (to keep the water you hopefully never get in there in suspension) and better corrosion prevention properties. It could be poppycock, but why take the risk? The marine stuff is only slightly more expensive. 

So now you’ve hauled out the boat because the oil seal failed, and you say, hey, you know what, I need new bottom paint too. Well guess what? There’s a whole new set of saildrive problems for you to think about. Standard bottom paints have copper biocide in them, and painting that copper onto the aluminium saildrive will destroy it in a few months. You need to use an aluminum safe bottom paint on those (read: more expensive paint that doesn’t work). Yanmar recommends Interlux Trilux for the saildrive, after carefully cleaning and priming the bare aluminum with an epoxy barrier coat. Extra care must be taken to not allow any salts that are on the bare metal to get painted in, so keep it clean with solvent and paint the barrier on immediately. Remember, near the sea even the air has salt, and aluminum and salt don’t like each other too much.

While you are hauled out pay special attention to the saildrive’s anode. Remember, the saildrive is made of aluminum and you need to provide a softer anode, lest galvanic corrosion go after the saildrive like Hastings goes after rotisserie chicken. Also remember, the anode is sized for the saildrive and your prop may require its own anode. My saildrives also have a second, “pencil" type anode located at the top of the unit (in the boat, but not replaceable in the water).

So now your saildrive is in the water and you’re good to go. Now you go to clean the bottom and the saildrive is a forest of growth, while the hull is clean as a whistle! So you carefully clean the drive leg, being extra careful to not use any metal scrapers too forcefully which might nick the paint and expose bare aluminum to salt water. Now you realize, the raw water intake for the saildrive is on the leg, and is pretty plugged up with growth! Yeesh! 

Saildrives pull the cooling water up through a channel and have a built in shutoff valve. These valves are pretty low quality gate type valves and I’m going to be looking for a suitable replacement during my next haul out. They are completely inadequate and dangerous in my opinion. Cleaning the cooling channels of growth is tricky and requires some thought. Luckily I haven’t had that problem yet!

After reading all of this, you’re probably thinking: why does anyone buy these things?? Well, no one does. People buy boats, and boat designers have figured out some definite advantages to building with saildrives. On our Lagoon 380, the saildrives allow the engines to be completely aft of the cabin, which we really like. No grease or diesel smell and much less engine noise and vibration inside keeps us very happy. Most shaft drive catamarans would have the engines under the aft bunks, making service and troubleshooting more fun. 

Saildrives have been around now for decades and they are very reliable. I’m not worried about my boat sinking because I have them, but I accept that it means my list of “must haul out” emergency/urgency items is much longer on my boat than on a shaft drive boat.

What's the biggest home or boat maintenance drama you've faced? 

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  1. This makes me very glad we don't have saildrives. I had no idea you had to use special bottom paint. Do you think either of us will ever get to the Bahamas this year?

    1. The Bahamas: always just out of reach! It's looking bad: we've already eaten through all our "Bahamas provisioning" honey roasted nuts!

  2. Biggest boat drama? We've had our fair share over the years but the biggest one aboard Cambria would have to be when one of the through-hull fittings gave out and water started to pour into the boat. We ended up hauling out and replacing all 17 of them (17? who the heck has 17?). Fun times!

    Stephanie @ SV CAMBRIA

    1. Wow! That does sound very dramatic. I agree, 17 is a crazy number. There are so many things waiting to go wrong on a boat, it's a miracle anyone stays afloat!

  3. You sure are learning a lot about sail drives! What they also don't say in the manuals is that big enough lifts for catamarans are far and few between in the more remote areas of the world! As with cruising, the good and the bad days are also represented in the ownership of a catamaran. You curse them sometimes and, when all the monohulls are rocking in an anchorage, you put your cocktail on the table, make dinner without cursing, smile and go to sleep comfortably. :-)

    1. The lifts are a problem... and in the US, getting a DIY yard and a big enough lift can be a challenge! Like you say, there are pluses, and she is great in a tropical anchorage!

  4. Haha, love your diagram up top! Boat maintenance drama? Probably all 3 of our shaft coupling bolts shearing off underway in the Bahamas before getting to a safe anchorage. Had to drill them out and only managed to replace one while we were underway, so sailed most of the way in to anchor. That night we had a 45 knot wind squall blow through for about two hours and was thinking if the anchor starts dragging I've only got one bolt holding my prop shaft to the transmission. Luckily it didn't come to that and tightening the newly fitted bolts became a routine maintenance item. Fortunately it was a v-drive configuration and the coupling was right there.

    1. Oh golly, that doesn't sound fun! At least we're not the only ones having incidents and events. The real question is why do we go boating at all!

  5. Dealing with painting SD20 saildrives on my 380 right now. Total pain in the butt!

    Yanmar recommends trilux. Trilux recommends Primocon primer. Primocon specifies interlux 353 primer (primer for primer?) which was of course discontinued by interlux in 2010.

    I spoke with the yard manager where I'm at and he recommended sanding with 80 grit, filling any corrosion pits with thickened epoxy, sanding off blush, then painting with interprotect 2000E, then trilux on top.

    Interlux forums are saying Pacifica is better than Trilux.

    I'm half tempted to try the Hacking family west system epoxy approach... but i'm a little reluctant to experiment with expensive sail drives.

    So yeah, saildrive maintenance is kind of a pain... but I feel good, and more secure about everything I've learned while working on them this spring!

    1. Saildrives are definitely a pain....and then there are two of them!


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