One of the never ending questions whenever power boaters get together is the thorny question of whether a boat should have one engine or two. There are arguments for both of course. Owners of twin screw boats will tell you that with two engines you have a spare if one should break down and that maneuvering in close quarters is easier. Owners of single engined craft will argue that only having one engine is simpler, you don't have to buy two of everything, there is more room in the engine room and if you are a skilled boat handler you should be able to dock a boat just fine with only one engine.
For the last forty years I have only ever owned a boat with only one engine, true most of those boats have also had sails as auxiliary power if the engine were to fail but I have never as yet had a an engine that would not start and left me stranded. There have been times when I have sailed off the mooring and gone considerable distances without ever turning on the iron topsail and even come along side all under sail mostly for the smugness factor but I was fairly confident that the engine was there and would start if I ever needed it. The proponents of the two engine approach will often drive considerable distances in their cars with only one engine with nary a thought of what might happen if things go awry but in truth motors whether they be in cars or boats are pretty reliable. There will always be times when unexpected things happen and the engine might break down but these are generally pretty rare and it seems that a straw poll of some Boat US and Seatow guys that I know often say that they get calls out not because the engine has actually broken down but because the owner has run out of gas or has left the stereo on to long and flattened the battery.
I have operated my fair share of twin engined boats and I will agree that these can be easier to operate around the docks but having an engine costing 30 grand just to make things a bit easier around the docks is going a bit overboard if you will pardon the pun.
A 40 foot Nordhavn has completed a 24000 mile journey around the globe on one engine without it appears any sort of major mechanical mishap. Testament if ever there was one to the suitability of long distance power voyaging on one engine.
Most engine failures can almost always be attributed to poor maintenance, providing that a diesel engine gets a clean supply of fuel, regular oil changes, cooling water and a goodly supply of clean air it will run for years.
I used to work for the BBC many years ago and the transport department had what they called a twenty two and half mile an hour agreement. This meant that the average speed of any of the trucks was calculated to be twenty two and half miles per hour over the life of the truck. This figure was calculated allowing for time that the truck would be stuck in traffic, running on congested streets and the open road. It worked out pretty accurately. Therefore if we interpolate this to the average cruising boat with say 1500 hours on the engine that works out to a measly 33,750 road miles. For a diesel engine this is nothing and is just about run in.
Most boat engines be they diesel or gas rarely wear out most rust away from lack of use.
So although I have not settled the debate as to one or two engines I am firmly down on the side of the single engine camp. For my money it is favorite for one thing there is more room in the engine room and thus maintenance is easier and more likely to be completed correctly and as Nordhavn have proved a single engine is perfectly capable of safe long trips and I don't expect they worried to much about not having that second engine as they pulled into the docks at Dana Point marina!