By Ed Sherman
All About Saildrives
Saildrive systems are commonplace on modern sailboats, taking the place of standard shaft-and-strut running gear. If you're evaluating a boat propelled by a saildrive auxiliary, get familiar with the pros and cons.
If you’re in the market for a new or recently built sailboat, odds are good that it is equipped with a saildrive system vs. a conventional shaft drive set-up.
Basically a sail drive set-up is much like an outboard engine’s mid and lower section projecting through the boat’s hull. It’s a bit different in the sense that unlike an outboard engine or inboard/outboard (IO) set-up, the engine exhaust is not exiting the boat via the drive leg, but rather through the hull just like with a conventional drive system. However, raw-water intake for engine cooling is drawn up through the saildrive leg, just as with an outboard.
To put the popularity of the saildrive in perspective, last year I sailed 23 new cruising sailboats, and of those 19 were equipped with either a Volvo Penta or Yanmar saildrive system. One thing is clear: boatbuilders are embracing saildrives in a big way. The flexibility of the saildrive configuration on a new hull design is compelling. The fact that the engine and drive system form essentially a single unit eases installation, and by design can keep weight “out of the ends,” which is always desirable with sailboats.
With the saildrive, the weight of a shaft, stuffing box, and propeller are moved forward, almost under the engine. Further, the propeller’s angle of attack is more parallel to the surface of the water, which will enhance efficiency and help to minimize traditional prop-walk characteristics common to conventional drive configurations. There’s also a wide variety of folding or feathering propellers available for saildrives — also desirable for the sailing crowd aboard modern designs with relatively flat underwater surfaces aft of the keel that can accommodate saildrive systems. (One of many examples is the the Hanse 545.)
But for many sailors, questions regarding saildrives abound. Are they a better choice over conventional drive systems? What about the rampant tales of excessive corrosion? How about a rubber boot in the bottom of the boat to keep the water out? What are the advantages and disadvantages of using a saildrive system? Can you easily retrofit a saildrive to your older boat as part of a repower? Let’s work through answering these questions.
Lower Initial Cost, Quieter
Boat builders can certainly save money on installation by using a saildrive system. With no shaft or strut required, engine installation is as simple as dropping a drive leg through a hole in the bottom of the boat and sealing it up with a rubber bladder.
My experience during on-the-water test comparisons has shown that saildrives run significantly quieter and with less vibration than conventional shaft systems. Less noise and vibration are things that make life on board a much more pleasant experience when motoring. Engine and shaft alignment procedures are no longer a part of the maintenance regimen, also a plus. But what about some of the other maintenance concerns?
Three Major Maintenance Areas of Concern
There are really three major issues that a boat owner needs to think about when it comes to saildrives: the integrity of the watertight seal around the drive leg, ensuring that the seals that keep the oil in and the water out of the gear mechanisms are doing their job, and combating corrosion.
Keeping the Water Out
The matter of the gaiter, or boot, that seals the perimeter of the drive leg to the boat’s hull is one that, if neglected, could cause a boat to sink. In fairness, both Volvo and Yanmar drives employ a double seal with a water alarm system that will notify the boat owner if the outer seal begins leaking. Recommended replacement intervals for this rubber seal range from five to seven years between Yanmar and Volvo Penta, and both manufacturers consider this service a dealer-only procedure. Many boat owners have gone much longer than this before replacing the seals, but one has to wonder if an insurance company would pay a claim for a sunken boat if the service interval had been ignored by the boat owner. Certainly this is a good question to ask an insurance agent before something like this happens. Both Yanmar and Volvo provide an “aqua sensor” that will sense water leakage if this bladder begins to leak, but periodic inspection of this critical rubber seal at least every two years is of the utmost importance. Without question this is one of the most important not-to-be-ignored service items related to the saildrive system. Depending upon how difficult engine access is on a given boat, replacement of the bladder and associated service parts can cost in the neighborhood of $1,000-$1,500.
As for the drive leg seals that keep the seawater out and the gear oil in, periodic checks of the transmission oil level and looking for any discoloration (milky white color) will indicate if problems exist. (See Saildrives: How to Check/Change the Oil.) It is recommended that the gear oil level be checked before each use, so any problems should show up rather quickly. Unfortunately, repairs needed here are going to require hauling the boat for seal replacement. Again, it is recommended that trained service personnel take on this task. Basically the task of resealing a drive leg is not much different than seal replacement on an outboard engine leg, and should not have a repair frequency any worse than with an outboard engine or IO drive, meaning service intervals measured in the 5- to 10-year range depending upon how many hours of use you put on in a given year.
Changing the transmission oil is recommended at 100- to 250-hour intervals depending on the specific drive, and both Yanmar and Volvo provide methods for accomplishing this task without hauling the boat out of the water.
Corrosion Concerns and Solutions
Corrosion is perhaps the most frequent issue associated with modern saildrive units. There are a few different causes for premature or excessive corrosion on these units, and if certain guidelines are followed, problems can be eliminated.
The number-one thing to remember about these drives is that the anodes on the units are engineered to provide corrosion protection for the drive only! Manufacturers recommend anode replacement every 100 hours, but it is of absolute importance to remember that this recommendation is based on some important assumptions that only the boat owner can maintain control over.
The first step in ensuring that your drive’s anode(s) provide good service is to make sure that any of the factory original paint on the drive leg that gets scraped off gets touched up. Also, you need to be certain that the anode material is appropriate for the water your boat stays in most of the time. The standard zinc anodes won’t do the job in fresh water and may even fail in brackish water. For brackish water use aluminum alloy anodes, and in fresh water magnesium alloy anodes are the best choice. Anodes should be replaced when they are 50-percent depleted.
Volvo electrically isolates their drives from their engines to minimize the chance of any electrolytic, more commonly known as “stray current” corrosion, occurring. Yanmar does not, and so one of the potential issues here occurs whenever your boat is plugged into shore power. Without electrical isolation, any boat plugged into shore power is connected electrically to all of its dock mates via the green grounding wire in their shore power system. This connection creates a galvanic cell and means that your boat’s anodes may actually end up protecting one or more of your dock buddies. This is a sure-fire way to deplete your saildrive’s anodes too quickly. The next bit of metal in the galvanic food chain once your drive anodes are depleted is the drive case itself. A galvanic isolator installed in the shore power system will prevent this from happening.
Also, keep in mind that accessory bronze folding propellers, which are popular especially with racing sailors, can add a rather substantial mass of extra metal to your drive and make for extra hard work for the anode(s). This very addition will require that anode consumption be monitored carefully until you establish a known service interval with your drive’s anodes.
Bottom Line on Saildrives
So, which is better, the saildrive or a more conventional set-up with a shaft, stuffing box, and propeller? This is a really tough question to answer. From a user’s perspective, better weight placement, improved motoring performance, and reduced noise and vibration make the saildrive the clear winner. From a maintenance perspective, I think the conventional set-up will always win out. Neglecting prescribed maintenance procedures and intervals with saildrives is going to cost you big. The conventional drive configurations are much more forgiving in this regard.
- Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.