June 2004 Issue
PS Advisor: 06/04
Saildrive Pros and Cons
Do you have any information on the care and maintenance of sail-drives? Do you consider saildrives in general to be a viable alternative to shafts, or are they a convenience for boat and engine manufacturers?
We've received several letters asking about the pros and cons of sail-drives. There's more to be discussed than can fit here, but here are the essentials, from Mike Muessel at Oldport Marine in Newport, RI:
"Saildrives are neither fish nor fowl in the sailboat propulsion field, and are a close relative of the popular powerboat outdrive units.
"For the sailboat manufacturer they represent a quicker, more versatile and non-technical engine installation route as compared to a conventional inboard installation. Gone is the propeller shaft, along with its attendant coupling, stuffing box, cutless bearing, stern tube, and strut. Also missing are the alignment procedures between the engine and shaft that must be done on the production line.
"The saildrive can easily be mounted facing forward or aft, and because it isn't constrained by shaft angle or aperture location, it offers the builder more versatility in engine placement.
"There's also a certain propulsion efficiency in the saildrive, because the thrust is parallel to the boat's waterline, whereas the downward angle on a standard shaft means that some of the propeller's thrust is wasted.
"On the other side of the coin, the saildrive is a constantly immersed aluminum object in a hostile marine environment. If placed too far forward of the rudder, it offers little prop wash on the rudder for low-speed steering, and if placed well aft of the keel, it can increase drag. And don't forget the large hole that must be cut in the hull to accommodate the unit.
"We sold and helped to install a Yanmar saildrive unit on J.P. Mouligne's class-winning 50-footer in the Around Alone Race six years ago, and I must tell you, I didn't want to be the one to cut a huge hole in his beautiful Finot-designed carbon fiber hull to slide the leg through.
"Large hull holes or not, saildrives were still the required emergency power packages of choice in that race. We strapped a huge 200-amp alternator and a large water ballast pump to that little 18-hp engine, and it never failed him during that race.
"Some competitors thought the units needed more fairing, and proceeded to sand through the factory- applied barrier coating in an attempt to reduce drag. I was quick to point out that messing with the barrier coat is a recipe for electrolysis, and it must not be disturbed. Small scratches can be repaired with outdrive barrier coat.
"Because of the short shaft length in the saildrive legs, one should be careful to pick a prop that is rubber-bushed to reduce shifting shock on the drive system. Some folding props also feature shock-absorbers on the blades to further reduce clunking.
"A good boot seal to keep the ocean out of your boat is essential with these units, and I recommend you look for a double seal with a water detecting alarm between them, like Yanmar uses.
"Monitor the drive zinc closely and change it long before it disappears. The new zincs are split, so the prop doesn't need to be removed to replace them.
"Follow the maintenance instructions carefully and be sure to use the recommended oil. Different units use different oils, so be careful."
So the answer is yes, we definitely consider saildrives to be viable alternatives to the standard shaft/strut/prop system, but as always, it's necessary to balance the trade-offs.