Personal observations, as I have had an engine down for most of the last month, all of which time has been spent cruising.
We have traveled from Saltspring Island to Desolation sound and back, mostly on one engine. There have been no difficulties with docking or anchoring, as the "down" engine has a problem only if allowed to run for more than 20 minutes, so I have been able to use it for manouvering. I have a good, new autopilot, so once up to speed the steering is not affected. Friends were with us for the first two weeks, who reported that there was black smoke when up to speed, but which went away as soon as I slowed a little. That taught me that the single engine was being overworked at anything over 6.2 knots. Our normal cruise is 8.2, with no smoke (on 2 engines). I expect the slower speed will result in lower fuel consumption, but I would prefer to cruise faster and when the second engine is back in service, I won't be going any slower than I used to. Fuel consumption is related only to the amount of work being done in the engine room. That, in turn is highly related to the height of the stern wave being created by your passage through the water. With one engine dictating a slower speed, a shorter stern wave will result in fuel economy. I am sure that if I wanted to save fuel, and I slowed to 6.2 knots all of the time, I wouldn't be able to tell how many cylinders were going up and down.
I tried putting this boat on the dock with one engine a few times and found that it can be done, but with only one direction of prop walk it isn't always easy. Any kind of a crosswind or current and I was regretting the loss of control.
When I checked the shaft on the idle engine, sometimes it wasn't turning, sometimes it was, but never fast enough to create any warmth in the transmission, so FF's warnings simply don't apply to my setup.
Like Idleweiss, I won't be going back to singles. I like the security of the second engine. I too have towed many broken down boats over the years, and almost all of them have been singles. I haven't yet had both engines down at once, and I doubt it will happen. I have had an engine down for various different reasons over the years, and every time, I was able to schedule the repair so that it never impacted on scarce vacation time. With a single, there is always a priority on getting the engine running again, as without it, you just don't go. If you are dependent upon a pro to do your repairs, you will find them busiest during that scarce vacation time, so your vacation may be over before you get slotted into their busy schedule. With twins, you can still go, and get the repairs done when you get home.
