I am surprised that several of you are ambivalent about singles vs duals. I thought that there would be definite opinions when it came to this question. Maybe it's not the deal breaker that I thought it was. I see a fair amount of my time on the boat singlehanded. This brings up the question of whether or not one person can handle (docking) a 40' boat with a single engine or whether duals would improve the handling of the boat. Opinions?
KDA, an advantage of being new is you are unaware of the many TF pages sacrificed to twins vs single debate. I have twins, had a single, prefer twins. Yes you can use thrusters to good effect, but the 40ft trawler with single which just arrived on my marina, in some breeze, lost its bowthruster towards the end of the 600 mile delivery, presenting its new single handing owner with challenges.But, some can do miracles with single no thruster. There is little ambivalence, and plenty of definite opinions, on single/twins.
Heh... as Bruce mentions, there are bazillions of bytes on here about singles vs. twins... Wading through those will turn up several nuggets you can consider, and it'll also show "ambivalence" probably isn't the right word.
To counterpoint Bruce's note, I'd say for a new-to-me trawler, I'd probably prefer a single, with bow thruster. For the boat we have now, twins are more appropriate. IOW, the debate has some nuances that include boat, boating style, cruising area, etc.
OTOH, I'd be a bit picky about which exact single diesel that is... and there are thrusters and then there are thrusters. His example, for instance, could have been an undersized 12V thruster that's been mis-used over time, with a puny battery bank located too far away... yaddy yaddy yadda. I don't worry too much about reports like that, other than to make a mental note about being sure my set-up would be up to snuff if I were to go that way.
It happens we ran a single-screw boat for several years, too, and that helps. There are some things I couldn't have done then, single-handing... but I could do more of those thing single-handed now than I could then... simply from more practice with boats over the years.
There would still be some instances where one crew person would be excellent, mostly to handle spring lines in a situation where the thruster couldn't overcome wind or current. In both of those instances, anchoring out for a while is a semi-easy single-handed option.
Think I'd rank single/thruster and twins about equal in handling around the dock. In various situations, one or the other might be slightly better, but in general, approx the same. Single with both bow and stern thrusters, maybe a slight step up. Twins with bow thruster maybe the same step up. Twins with both bow and stern thrusters, getting even better. All that, very subjective, just off the top of my head...
-Chris