What it is depends on who you are...
A saildrive seems to be all of the following:For the designer, it's a way to gain space for other uses while giving the engine space the minimal amount of room possible in which to shoehorn the engine. No prop shaft, so no access to the stern gland either. (Designers don't work on engines, I've come to learn).For the boat builder, s/he not only gets to offer a boat with 'more space' (uh-huh...) but can do a powerplant install easily with less skilled workers at less cost. They just cut a big hole to drop the leg thru, install the rubber gasket that they say will last a long, long time (uh-huh...) and they can even promise minimal vibration to the buyer.For the engine maufacturer, it helps create a bigger sale since an entire power train is purchased from them as a unit vs. perhaps just the engine which will be married up with someone else's transmission, perhaps with a V-drive from a 3rd vendor, plus hardware (shaft, strut, etc.) from yet more vendors. This way, all the 'buying' is concentrated in the engine manufacturer's direction.And for the owner...well, s/he gets to figure out how to lube the leg annually or every 100/200 hrs of run time if s/he doesn't haul annually (some of us get 1.5, 2 or even 3 years on a bottom job with routine maintenance), how to get sufficient room to reach all the pieces needing service, and how to change out anodes (zincs) when underwater.In fact, there are some good attributes of sail drives but I don't think this idea was ever conceived with the customer/end user as the primary beneficiary.Jack