1. Fuel Economy of Running On One Engine Vs. Two
I have never seen an article or even a web document on the question of running on one engine instead of two for fuel economy.
Anecdotal evidence and word of mouth indicates a 25%-33% increase in fuel economy, running at trawler speed (6-9 knots). A 25% savings on a Great Loop trip would be significant. I have a 51' Bluewater with twin Crusader 350s (gasoline) and plan to do the Loop over the next few years.
Feathering props are becoming a reality in the sailboat world. Some people also discuss whether the shaft should be locked to keep the non-active engine shaft from heating the transmission without cooling systems being turned on. This could be done with either putting the engine in gear or using a shaft-lock system like those found on sailboats.
What say the experts?
Robert
The technique of, “trailing” an engine isn’t new, aircraft such as the Navy’s long range anti-submarine warfare and surveillance P-3 Orion do this regularly when loitering over a location or while on patrol. The primary reason is to save fuel, thereby increasing the aircraft’s range and time aloft. The dormant engine is feathered to prevent the propeller from turning the engine. There’s no reason the same approach could not be used for trawlers and other recreational vessels while they operate at displacement speeds in order to achieve greater fuel economy. Precautions must, however, be taken before operating under this scenario. Some marine transmissions are designed to trail or windmill without incurring damage or undue wear. Other, however, are not designed for this type of use and must therefore be immobilized or other procedures outlined by the transmission manufacturer must be followed. Some require that the transmission be intentionally overfilled with lubricant and then drained back to the normal operating level before trailing begins. This submerges all of the bears and bearings in lubricant and the “dunking” is good only for a limited amount of running time, at the end of which the procedure must be repeated. If your transmission manufacturer advises against trailing, then the shaft must be immobilized to prevent it from spinning. A few smaller mechanical transmissions can be placed in gear, which prevents them from turning. Most transmissions, however, are hydraulically actuated and thus will turn when trailed. In this case, these transmissions/shafts must be manually locked using a locking device. Manual devices may be manufactured and installed by competent boat yards (the lock should incorporate a mechanism that prevents the engine from being started while it’s engaged). Shaft brakes designed for sailing vessels may work, however, they are often too small for the pressure that is exerted by larger, powerboat propellers (and I haven’t seen a new one installed in several years). Additionally, if the trailing engine is equipped with a scoop intake hull strainer, the seacock must be closed to prevent water from being forced into the engine. Likewise, if the vessel is equipped with dripless stuffing boxes, which often include cross-over water supplies, the plumbing leading to the dormant engine should be turned off if the shaft is immobilized. If, on the other hand, the dormant engine’s shaft is allowed to turn, it should have water supplied to it for lubrication and cooling purposes. If these procedures are followed, you may enjoy the improved fuel economy afforded by operating on one engine and it’s likely the operational engine will be operating under greater load, which is beneficial, especially for diesels.
Steve D'Antonio, Technical Editor