Even for long-distance cruising, twin-engine propulsion packages often make more sense than single-screw installations...
Historically, single-engine installations in yachts have been thought to be more economical than twin-engines set-ups for cruising — both in terms of initial cost and ongoing operation. But that is mostly a myth.
True, a single engine of a given HP sometimes costs less initially than two engines whose combined HP are equal that of the single engine. However, because most cruising wander far from immediate help (even if only operating alongshore and island-hopping), most cruising owners and skippers want a way to “get home” in the event a main propulsion engine fails. Consequently, the common approach to a single main-engine installation is to also rig some form of alternative “get back to port” propulsion power - perhaps using a jackshaft or hydraulic PTO or even an electric motor driven by a genset. The downside of such solutions is that they are themselves expensive to assemble and install and far less reliable than simply splitting the marine propulsion system into twin engines and screws.
Admittedly, that is a sweeping generalization. But it’s one that stands up whether you’re looking at Caterpillar, Cummins, John Deere, Kubota, MAN, MTU, Volvo Penta, Yanmar, or other engines built for the marine market. But don’t take my or anyone else’s word for it… work the numbers for yourself.
Just remember, while you’re at it, a single-engine main propulsion installation does not necessarily use less fuel because, single engine or twin, a given yacht requires a given amount of horsepower (and fuel burn) to achieve a given level of speed. Whether you supply all the required horsepower from a single engine or half of the required horsepower from each of two smaller engines, the rate of fuel burn at a given boat speed will be virtually the same. (https://www.seattleyachts.com/news/powering-up-your-yacht)
To be sure, it doesn’t always work out that way since a 200-HP engine may weigh more than ½ of a 400-HP engine. But then again, it might work just fine — as it happens to in the following comparison. Consider, for example, the following two John Deere Marine Diesel engines:
Suppose we need 500 total HP to drive a yacht at the desired speed under specified load and sea conditions. And we can find that power in either a John Deere 6135SAFM85-M3 marine diesel or in a pair of 4045SFM85-M4 marine diesels. Are there serious disadvantages to using the twin-screw set-up? Not really. The pair of 4.5-liter 4045s weighs less than the single 13.5-liter 6135 engine; their weight-to-HP ratio is better; and their rated RPM at the target HP, although somewhat higher, is far from pushing the envelope. Moreover, in this case, any added cost involved in having two sets of shafting and props is more than offset by the lower overall price of the pair of smaller engines.
Since the marine sector of the market is relatively so small compared to the overall engine market, for many years engine choices were drastically restricted by the very limited selection of available engines and horsepower ratings. This meant, for a given HP requirement, it was often difficult to find two smaller engines to replace a single engine to drive a given load. These days, however, the selection has grown, and in general, twin-engine rigs don’t suffer weight-wise or cost-wise in comparison to single engine set-ups — as many people. even some pros in the industry, continue to believe. And the situation has been improved even more by increases in the array of available marine gears, as well.
Always keep in mind, the advantage of a twin-screw installation does not have anything to do with performance. Driving a given yacht at a given speed under the same load and sea conditions depends on the amount of thrust delivered by the propeller or propellers. Which is related to the total HP developed by the engine or engines.
The advantage(s) of twin-screw installations have to do with engineering redundancy — and the operational margin of safety that introduces. For running home (or to a safe port) on a single engine is much less risky and generally more reliable and no more costlyy than limping home on an expensive “emergency” wing engine driving your vessel’s single screw via a connection that would have Rube Goldberg grinning from his grave. — Phil Friedman
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