Lot of good points above. There are plusses and minusses, but the short answer is you generally see outboards only on smaller boats. There are good reasons for this. I have a 15 HP o/b on a 30' racer
, and it is fine in benign conditions. Then you have to have a boat and a crew that can sail its way out of trouble.
1. Weight is not a particularly big issue. Horsepower for horsepower, smallish outboards on small yachts are light vs. diesels, paricularly once you factor in all the weight hanging off a plumbed in diesel. If weight in the stern was the real issue, relative to diesels, you would not see so many lightweight racers using them.
2. But smaller outboards generally have small props that cannot use their nominal HP to drive the boat into wind
and waves. I generally have to motor sail under a small jib
to get much progress against winds over 25 knots. Going through the engine
calculations, I need a 10" diameter prop and lower revs to get good use of my 15 HP.
3. Above 25-30 knots in a seaway I get two problems. First, stern lift
with power loss and over-revving. My o/b is in a well; this would be worse with stern hung o/b. Second, prop lift
when heeling (note need to use sail to get upwind above ~25 kts w/s). My o/b is central. A side monted o/b would lift more on a particular tack.
4. Fuel: safety
. Petrol is volatile, highly explosive (in tanks
and as vapour), and goes stale reasonably quickly. ~60% of my lifeboats rescues in spring relate to o/b fuel problems, typically stale fuel or oiling (2 strokes). O/b fuel is also commonly stored in platic tanks in locations where accidents can happen more easily (vs. bilge
5. Fuel: range. O/b driven boats simply don't go as far, HP for HP, on a litre of fuel. So on long trips you carry a lot of it, largely nullifying the lower engine
weight of an o/b. And tote tanks are generally small, increasing the likelihood that you have to refuel at sea. My 20 litre tote tank will get me 20, maybe 30 nm in flat seas. The 10hp diesel on a sister ship will take her well over 100 nm on 20L of diesel.
6. I have not found reliability
is a major issue. I know some diesel-powered boats that have had outrageously expensive problems that I could fix at a fraction of the price
by getting a new engine. I know diesel powered boats that have so many silly little (engine killing) problems they are always slowing a group of yachts down. And I know some people who have lots of problems with dingy o/b. Good outboards are pretty reliable if you keep the fuel fresh, maintain the engine well, and keep the plugs clean. And as for any engine, carry essential spares.
IMHO outboards are a good option on small boats (well below 30'), particularly those that sail in sheltered waters, or sail conservatively in coastal areas. But they are not designed for slow speed yachts, and will always be out-performed by a good diesel. They are an uncomfortable compromise on any cruising yacht, and potentially hazardous on boats that cannot sail upwind in a big sea.