Beware the Unprotected Saildrive
Paying close attention to anodes and your sailboat’s bonding system will help prevent corrosion that can quickly cripple these power units.
So, how’s your drive? I’m not talking about your daily commute or personal energy level here. I’m talking about your saildrive, if you have one. Increasingly, boatbuilders are adopting saildrives over the traditional shaft and propeller, and with good reason: They’re much easier to install, and they offer a considerably quieter ride when the wind dies and you simply must fire up the diesel. During CW’s Boat of the Year competition in 2009, for example, a third of the new boats we looked at were equipped with saildrive propulsion, so they’re here to stay. And though early adopters of the drives experienced some well-publicized problems, the technology has evolved considerably since then.
I can remember some of the conversations with my colleagues when saildrives first appeared. Thoughts of failure of the rubber bladder that seals them to the hull were at the top of the list of why they were a bad idea. Some of us had lingering memories of Outboard Marine Corporation’s early attempt at offering saildrive functionality, basically by using an outboard-engine power head with a lower unit mounted inboard. They were very problematic and only lasted a few years before they disappeared into the not-such-a-great-idea department.
Well, we’re past all of that. Today, both Volvo Penta and Yanmar offer complete saildrive solutions. The initial fears of rubber-bladder failure are simply no longer an issue. I haven’t heard of one case in which this has occurred. Certainly this is a maintenance item that needs addressing at some point, but premature failure has not been a factor.
What has been a factor is rampant corrosion of the aluminum drive units, with total replacement—at great expense—sometimes required. In some cases, the rapid decay has been so severe that drives appear to have dissolved off the bottom of the boats to which they were fitted. Many online forums and discussions have addressed this problem, and much misinformation about the causes for this corrosion has been distributed widely. I hope to end much of the confusion right now.
The essence of the problem centers around one basic corrosion-mitigation premise: that there exists a proper relationship between the surface area of the anode, typically a zinc or aluminum-alloy fitting, and the surface area of the cathode, which includes the saildrive, the propeller, and any other metal below-the-water fittings. What this means simply is that the amount of metal exposed to the electrolyte (the water in which the boat floats) must be proportional to the amount of sacrificial anode installed on the boat.
It’s important to remember here that drive manufacturers install anodes on their drive units at the factory that are intended to protect the drive units only as shipped with their standard propeller. Let me repeat that: The anodes supplied are engineered to protect the drive units and standard props only. Many sailors will add a bronze feathering propeller to the drive, which means that more anode area is going to be needed, though too often this is overlooked. This simple alteration is going to decrease the anode service life because more cathode area has been added. Additional bronze through-hull fittings may also be added, again requiring additional anode surface area.