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.
A new trick that some of us who are involved with marine-corrosion issues are now employing involves using a galvanic isolator installed in the green-wire link from a saildrive and engine to a keel bolt, and hence the keel, since these metal components are in most cases connected via the bonding system. This installation effectively isolates the keel, from a galvanic perspective, from the drive in the event that the keel does lose any of its protective barrier coating. (More on coatings in a minute.)
Choose the Correct Anode
Although most people refer to their anodes as “zincs,” they may not be made of zinc, and using the correct anode material for given water chemistry is, in fact, of paramount importance. Anodes come in three basic materials: zinc, aluminum, and magnesium. All three are actually very specialized alloys of each of these metals, and they’re designed to achieve appropriate voltage potentials in salt water, fresh water, and brackish seawater.
The proper selection of anode material can’t be overstressed. One fellow who lost several saildrives finally found out that the zinc anodes installed at the factory on his boat weren’t really doing anything to protect against corrosion in the brackish water in which he kept his boat. In fact, aluminum anodes would’ve been the preferred choice.
The reason for this difference in material selection centers around the relative conductivity of the water in which your boat sits. Salt water is quite conductive, so zinc anodes, which have an electrical potential of around -1,050 millivolts, work just fine. Brackish water is quite subjective depending upon its salinity, so aluminum anodes, with a voltage potential of -1,100 millivolts, perform better due to their slightly higher voltage potential; these actually can also be the best choice in salt water. In fact, most outboard-engine makers now supply these as standard equipment, not knowing where the engines will be used. Magnesium, which should only be used in fresh water, can have a negative voltage potential up to around -1,630 millivolts; it can be quite damaging to aluminum if used in salt water.
The Importance of Coatings
One of the most important components in the mitigation of corrosion is the proper type and application of protective metal coatings. Proper coating effectively insulates metal from the electrolyte surrounding it, again eliminating one of the components necessary to complete the galvanic cell. On a drive unit on which this coating is beginning to be compromised, this puts a higher demand on the anodes, and once they’re depleted, the drive begins to corrode rapidly.
The best base coatings we have available to us today are any of the epoxy barrier coats available, such as Interlux Interprotect. Apply this to all underwater metals, including bronze through-hulls, before any anti-fouling coating goes on. On drives that have already been submerged in salt water, another hidden enemy lurks: soluble salts, which are invisible and tasteless. These microscopic particles cause premature paint-system failure and must be cleaned away before any epoxy or paint is applied. The trouble is, these particles are difficult to clean off. In fact, a typical boatyard pressure washer won’t remove them. Yanmar, for example, recommends both a test kit and a biodegradable neutralizing wash made by Chlor-rid Corporation (www.chlor-rid.com). If you’re recoating a saildrive, this step is super important to ensure that the coating stays on the aluminum.
The primer and anti-fouling are important too. Don’t use any anti-fouling paint containing copper on a saildrive. Yanmar says that the Interlux Trilux 33 system or its equivalent should be used.
The Final Check
You must perform a final check to be sure that you’ve achieved the proper level of cathodic protection. You can do this yourself by using a standard multimeter/silver-chloride reference-cell combination instrument. To do this, make sure your boat is unplugged from shore power. Set your multi-meter to the DC-volts scale. Connect the positive lead from your meter to the boat’s grounding buss (the one to which all the green insulated wires are attached). Submerge the reference cell in water to the same approximated depth as the installed anodes on your boat. If all is well, you should see a negative voltage reading of between -950 to -1,100 millivolts. If the reading is too low—that is, less than -950 millivolts—then you need more anode surface area.
My recommendation is to employ the services of an ABYC-certified corrosion technician to perform what’s known as a Hull Potential Test. Do the test only after the boat has been floating in its usual location for several days. The polarization process doesn’t occur immediately; it usually takes 24 to 48 hours. The correct level of cathodic protection will vary depending on hull material and how your boat is configured. To get an idea of the range of voltages the tester should get, see “Recommended Range of Cathodic Protection for Boats with Saildrives” (left).
Remember that the initial reading when all anodes are new should be toward the more negative, say, perhaps, -1,100 millivolts. This is because as the anodes deplete, the voltage range will shift to a more positive number, eventually indicating under-protection. It is also quite important that the number not be any greater than -1,100 millivolts in order to protect your saildrive. Aluminum is somewhat unique among marine metals in that it can be easily overprotected or underprotected cathodically, and in either case, damage to the casting will occur. Sometimes this happens quite rapidly.
To sum up: If you follow the advice here, your drive should give you many years of good service. But I must emphasize: Diligence is of paramount importance. Don’t let anyone tell you that six to eight weeks of anode life is acceptable; this is a common depletion rate only when something is wrong.
Ed Sherman, the curriculum director for the American Boat & Yacht Council, was a 2011 Boat of the Year judge.