This is one of those discussions that comes up from time to time, borders on politics and religion. :roll: I have a few experiences to share.
I have twin Yamaha 80's on my CD25 which I've owned 8 yrs since new. Never had twins before, but had this yearning desire to have them. Prior boats I had a get-home-kicker, usually an older, cheap vintage model of sorts (not the best idea). Had to use a 15hp kicker on a 20ft Crestliner (aluminum, modified V), took me nearly 4 hours to go 12 nm crabbing into the wind/waves, got back at dark, but was very thankful none-the-less.
First off, I use the twins' ability to turn the boat on a dime quite regularly, especially when approaching a mooring ball with the wind on the nose or just off. These boats, with their shallow draft, tend to fall off the wind rapidly at a moments notice, so I love the extra turning torque that the twins provide (1 eng fwd, 1 reverse) -- not an immense amount of torque, but sufficient in most conditions. It's saved my azz from hitting another boat close by several times.
It seems I usually hit some seaweed on every 3-5 trips, such that it wraps around and clogs the water intake, the temp alarm sounds, I shut down that engine while still running at speed, raise it up to clear the clog, lower it, restart and resume. I've gotten very good at this. I've had my passenger friends ask what was that alarm -- oh, just seaweed, already cleared it and resumed before they knew enough to get shook. ha. This is more of a convenience feature of twins, not a necessity.
Last year coming home at night from Catalina (22 nm), had my daughter with me, mid-channel one engine quit while at speed, its voltage regulator fried and took out the main fuse that powers the ignition system. I have a very simple battery system, just 1 battery per motor. I continued at 6-7 knots in moderate seas, no problem. The replacement regulator's heatsink was nearly double in size, must have been a design problem. This incident bordered on a safety issue, I was in the 4-mile wide shipping lanes when it died.
And just on my last trip, the starboard engine's temp alarm sounded soon after I dropped it to idle to approach the dock. I shut it down, continued on with one engine, docked, no problem. There were boats all around me and a moderate breeze. This would have been dicey to anchor, start the kicker, pull up the anchor then dock among all these boats. I looked at my maintenance logs, 4 yrs since I replaced the water impellers, that was pushing it, so they're getting new ones this weekend.
Another time, one of my batteries went dead (my fault, too much TV the night before on an old battery). So I started the engine with the good battery, let it run for a few minutes, then paralleled the dead battery with the battery switch, let it charge up for awhile, then switched back to separate batteries (very important), and started the other engine. The battery worked fine for the trip (Delta Extravaganza 5 days), replaced them both when I got home. Again, convenience, not safety.
Sure, the initial cost of two smaller motors is higher, and the maintenance is double (but not very costly because I do most myself), and the time to maintain a little more. But then there's just the sheer satisfaction and enjoyment of throttling up and syncing twins! A single just doesn't have that satisfaction. And having twins purring along while you're doing 8-12 knots in nasty conditions, waves breaking over the boat and engines, is well, just very reassuring. What can I say.
Would I get twins again? Depends on whether I'm going to be offshore a lot, what size boat, water/weather conditions I'm likely to encounter, etc. So far, I'd say yes, love my twins!
My 2c.
