9.2 Single Engine or Twin? - Some Comparisons.
The following is part of an article I posted to rec.boats.cruising in June of 1996 as part of a thread on Trawler FAQs. A question had been asked regarding the relative advantages of single engines vs. twin -- in a trawler in particular.
"> My reading, personal experience, and discussions with other trawler owners have indicated the following regarding twin engines vs. single engines in these boats. Twin engines have twice the purchase cost, twice the maintenance cost, higher fuel cost, and usually, though not always, somewhat higher speed. They also provide greater maneuverability in very tight quarters around docks without using spring lines -- a skill that's not hard to learn but is probably essential in brisk crosswind and/or cross-current situations with single-engine trawlers, even those with bow thrusters. Another vital skill in handling single-engine trawlers in tight spots is "backing and filling," a technique that will allow you to turn the boat 90 or 180 degrees within slightly more than her own length. There are other single-engine techniques as well, but this is a start. It's amazing what you can do with some practice! Still, twin engines should give you better control in these situations.
Twin engines may also provide an increase in reliability, though not as much as one might think. This is because the most likely cause of engine stoppage (I am told by engine "experts") is blockage of primary fuel filters which is usually a single-thread failure point -- i.e., both engines usually use a single, common primary filter, so when one engine stops the other does also. The most common method of avoiding blockage (whether the boat has one or two engines) is to add a vacuum gauge after the primary filter to indicate the degree of blockage long before it becomes completely blocked so the primary filter can be changed before the engine(s) stop. Even more elegant is to use a day tank into which the day's anticipated fuel is pumped through a separate filter so you are certain the fuel is clean even BEFORE it gets to the primary filter. Finally, dual, parallel primary filters on a manifold permit rapid switch over to a clean filter without stopping the engine(s), if the filter became blocked before it can be changed. On the other hand, twin engines do provide redundancy for failures of fuel pumps, water pumps, cooling water leakage or blockage, lubrication oil leakage or blockage etc.
One of the most serious causes of engine and/or drive train damage and failure is accidental grounding or striking large semi-submerged objects with the screw. Here, the single engine designs usually fare much better than the twins. The reason is that the single screw is usually protected by the keel which is directly in line with the screw and projects below it. In the case of the full-keel, full displacement trawlers (like the Nordhavns and Krogens), the screw is almost totally protected by the keel. On the other hand, most of the twin engine boats, especially the semi-displacement designs, have relatively less protection of this kind. I have seen at least one boat with extended skegs/keels in front of both twin screws to minimize this kind of failure."
After reading an earlier version of this FAQ, Andy La Varre suggested I add that the possibility of a broken alternator belt would put a plus sign in the twin-engine column. Certainly here's a case where you could keep her moving with the second engine, if you had it. (The immediate problem isn't the loss of the alternator, of course, but the fact that this same belt often drives the fresh-water coolant pump, as well.) He further suggested that if you have a single engine, be sure to keep the spare fan belt right by the engine (perhaps along with the wrenches for the alternator bolts and the bar for putting tension on the belt), rather than hiding these things in separate spares or tool boxes. Great suggestion!
For more thoughts on the relative advantages of single and twin engines, see, "Single or Twin Engines -- Which Is Best?" by Bill Parlatore in the Fall 1996 issue of Passagemaker. Lots of input from a wide variety of engine experts.
For additional information on using spring lines for maneuvering in tight spaces -- whether with single engines or twin, see "Boat Handling Under Power - A Motorboat and Yacht Owners' Guide" by John Mellor.