Thank you for yet another really good article (“The Fuel Burn,” PassageMaker, March 2018). May I be so bold as to add some factors to think about when running a twin-engine boat on just one engine?
A twin-screw boat has one engine on either side of its centreline and is therefore driven in a straight line. Stopping one engine will introduce asymmetric thrust which in turn will want to turn the boat. To counter this, opposite wheel needs to be applied. So straightaway, we have additional drag caused by rudder deflection as well as the boat being forced to “crab.” Both increase the drag. And more drag = more fuel burn at the same speed through the water.
Beyond this, you need to consider the additional force on the rudder and whether the rudder stock is built to withstand this force for sustained periods of time.
For the dead engine, I absolutely agree that keeping the shaft and prop spinning freely is correct and yes, you need to know if the shaft seal (stuffing box) requires lubrication. Furthermore, you need to know the gearbox manufacturer’s details about freewheeling and whether any limitations apply.
So, additional consideration needs to be given to the asymmetric thrust, the rudder stocks, shaft seals, and gearbox limitations. For me, the only benefit of doing this on my boat is to keep engine hours down. Is it worth it, given the above? Not in my mind. Piers du Pré Fleming 55, Play d’eau Guernsey, British Isles
Thank you for your well-informed letter. I agree with all of your comments. I am not advocating that boat owners run their twin engine boats on one engine. My point is that doing so rarely results in anything other than minimal differences in fuel economy and introduces a host of other problems (as you also pointed out).
If the dead engine prop cannot freewheel (due to transmission concerns), then you are effectively dragging an anchor with the fixed prop and fuel economy will suffer substantially. Additionally, running on one engine can impart stresses to the drive train not normally experienced, raising the risks of creating a new problem. —SZ