Can you explain the difference between a "saildrive" and a regular drive shaft and stuffing-box arrangement? Also, what are the advantages and disadvantages of each?
Mark Matthews responds:
Thanks for the question. The saildrive is another way of approaching the problem of having the engine inside the boat and the drive aparatus—the prop—outside without allowing water in. Saildrives, which are essentially an amalgam between a standard diesel engine and the lower-end unit of an outboard, operate the same way as a standard propellor shaft in that they pierce the hull through a sealing gasket.
With a standard propellor shaft arrangement, stuffing boxes are used to seal out water where the propeller shaft exits the hull. The shaft passes through a threaded sleeve and a hollow nut. In conventional stuffing boxes, this nut is filled with packing material that is screwed onto the sleeve. The idea is to tighten the nut to compress the packing material and form a watertight seal. With saildrives, instead you have a set of resilient diaphrams that surround the lower end unit and are bonded to the hull to keep the water out. If you're not a fiberglass guru, installation of saildrive units is best left to those who work day in and day out with the stuff.
The stuffing boxes on conventional shafts have packing material that needs to be changed periodically. These units allow a minimal drip of water into the interior of the boat when the shaft is in motion, but they should not drip when the shaft is still. Dripless mechanical seals are an option if you're worried about water entering. These require minimal maintenance, although they should be inspected regularly; if the flanges slip, a large volume of water can enter the boat rapidly.
One advantage of the saildrive is that it's able to maintain engine alignment more easily for a smoother ride, which also presents a more hydrodynamic surface for less drag and more thrust. Many serious racing sailboats use saildrives for propulsion, principally due to their reduced drag. While I don't have the benefit of first-hand experience with a saildrive unit, I understand that they are lighter in weight and offer performance advantages over conventional prop-shaft arrangements.
As far as the disadvantages go, these units depend on the type of boat you have. No matter how strong the hull, when you cut through the skin of a boat with a large hole, you will need to reinforce the edge of the new opening. Cutting a hole means that the loads around the opening that would have been distributed across the area are now focused on the edges. Also, saildrive units have gear lube that should be drained and replaced annually. And a watchful eye should be kept on corrosion as well as many of these units have different kinds of metal working together. Check the zinc and check the seal between the unit and the hull on a regular basis.