Now, don't take this wrong, but I really don't think you'd ever see any noticeable or measurable reduction in drag from removing the prop from a Westsail 32. Westsail 32s are all about drag and wetted surface--and I say this without any intention of bad-mouthing the boat. It's just more of a fact. There's so much inherent drag, and the boat will never be a stripped-down lean machine, that the drag induced from the current setup can be considered virtually nil. It takes a lot to affect the performance, positively or negatively, of a boat with the D/L ratio of a Westsail 32.
I'm sure exhaustive calculations could prove me wrong in every instance, but I generally consider the amount of drag produced from fixed, three blade, aperture-mounted props on full keel (or modified full keel) boats to be so minimal as to be ignored. There are so many other inefficiencies in the underwater designs of these sorts of boats that the extra drag from an aperture or propeller, to the extent it exists, has no practical effect on the boat, and certainly no more or less harmful to performance than the inevitable less-than-perfect sail trim, or mast tuning, or algae on the bottom, or 12 jerry cans of fuel, or wheels of Brie and fine wine in the bilge, or what have you.
In a perfect, theoretical, scientific setup, I am sure one could prove that, all things being equal, a full keel boat with no prop or aperture would be marginally faster than an identical, traditionally-propped setup. But in the real world, these differences, to the extent they exist, just don't play a big roll in the actual sailability of the boat. (Please note that I'm not talking about fin keel boats with exposed shafts here...I believe prop drag to be a much more important factor in those designs.)
Outboards work to power a lot of boats, but they just aren't for every situation. In theory, you can push any boat with an outboard, but in practice outboards deliver power very differently from something like a diesel engine, with its high torque and ability to deliver that torque cleanly and efficiently to a large propeller mounted well beneath the surface (and therefore in more "solid" water). One important factor in comparing the power from a diesel with that from an outboard is to look at the power and torque produced at lower RPMS. Outboards tend to produce their power only at very high engine and prop speeds, compared to diesels which excel at producing high amounts of power in the lower ranges. This is why diesels are so well suited to heavy boats.
From a purely practical standpoint, the only way I could conceive of an outboard even physically fitting--and therefore being able to operate--on your Westsail, with the stern configuration and the gear you have, would be some sort of side mount, perhaps along the lines of the one used on the Alberg 30
Jean de Sud, owned, of course, by the inventor of your Cape Horn self-steerer.
You could come up with calculations as to how much power your 22,000 lb. Westsail truly needs in order to achieve hull speed under power. I certainly wouldn't go any smaller than this minimum, since calculated and theoretical minimums don't take into account propeller slip, inefficiencies at transferring the calculated power to the propeller itself, and other factors; therefore, you need a margin of additional power over and above that minimum, in any real-world situation. I don't think the outboard would be very efficient at powering your boat, particularly set off to the side in this sort of manner, so you'd have to have something a bit larger to account for this loss in efficiency.
In my opinion, you'd have to own stock in an outboard company to want one badly enough in this situation to make these efforts worthwhile. The money would be far better spent either overhauling your diesel or replacing it. Diesels are wonderful things, and take so little to keep happy. Instead of hating the engine, learn to love it. A loved engine is a well cared-for (rather than hidden and neglected) engine; a well cared-for engine is a happy engine; a happy engine leads directly to a happy boat.
The high topsides of the Westsail would make operating the engine challenging in any mounting location, but all the more so on some sort of side bracket. You'd almost certainly need remote controls, which leads to wires and cables hanging off the boat as well, increasing clutter and complexity.
What you're suggesting isn't impossible by any means. I don't think it's a good idea, nor would it be very fulfilling in the end, but don't let that stop you! Outboards have their place, and can be used as emergency or temporary means in just about any situation, but as a permanent or long-term "solution", they fall short for many reasons, particularly when significant engineering might be required to even allow one to hang off the boat somewhere.
