Multiple engines provide the ability to power on even if one or more of the other engines quits. Hit a rock, throw a rod, overheat … and you can just run home on the still-working mill.
Is it really that simple, though? Here are some things to consider should you find yourself needing to run on one. There are too many variables in boat systems to make a comprehensive must-do list, so be sure to check with your engine’s maker and boatbuilder.
It’s not practical to lock the shaft of the dead engine. For one thing, a freewheeling prop creates way less drag. For another, with most transmissions being hydraulic, you won’t be able to put them in reverse to lock them without the engine running anyway. So let the dead engine’s wheel spin.
Also, without the engine running, inboard engine transmissions and shaft seals may overheat. If lip seals or face seals are used to seal the prop shafts, make sure they are plumbed with a cooling water crossover from the other engine(s). This way, the running engine cools the shafts of all engines. Older boats with stuffing boxes need not worry about seals overheating.
As for the transmissions, anecdotal experience shows that you can run many makes of marine gears without the engine working without damage (but again, check with the maker). Still, I advise keeping a weather eye on the transmission sump temperature. You can purchase overheating alarms for most makes.
As to handling, it is really going to depend upon your boat. Some gas-powered boats have very small props and tiny rudders. At the least, the boat will steer heavily to one side. At the worst, it may, in fact, be completely unresponsive to turns in the direction of the engine that is running. I advise trying out your boat with one engine for practice so you know what to expect should the actual need ever arise. In all cases — emergencies excepted — do not try to plane. By going as slow as practical, you are putting less strain on the drivetrain as well as mitigating any of the cooling issues mentioned above.
Tilt a dead outboard motor out of the water when running on one (or the remaining) engine(s). Doing so protects the dead engine from seal or other damage and will reduce the drag and may even allow your boat to achieve plane. Still, I advise you to reserve planing for running a breaking inlet, getting crew to emergency medical treatment or dealing with another urgent scenario in which speed and dynamic stability are required. Otherwise, take it slow to reduce the strain on the engine. No sense risking tearing apart the gears in your one remaining engine!
Also, take care — not all steering gear is rigged and installed to deal with one engine tilted up. You could bend or break a tie rod and, in fact, take the maneuverability from the still-running engine. Again, check with the engine maker and the boatbuilder. Once you have discovered the procedure, run a drill to practice and see what the deal is, should you ever really need to run on one engine.
Tilting up one drive may reduce drag, but be forewarned that, if you turn the wheel in order to maneuver, you may overstress the seals and allow seawater to enter. Also, the boat may be rigged such that just one engine of two runs the power-steering pump. (If this were my boat, I know which engine would break offshore!) I advise you to find out in advance and, to repeat myself, to practice on one engine before you are in a position to need to use just one engine to run.
Anticipation and preparation are as integral to seamanship as salt is to seawater. Use these processes and develop your own personal “engine-down” scenarios. Stay safe.
Quick Tip: If steering is impinged by a down engine, you can tow a bucket, or otherwise create drag on one side, to effect coarse turning in open water.