Except to a yacht’s chief engineer, talk of main propulsion engines is neither glamorous, nor exciting. But if you’re in the process of selecting propulsion power for a new yacht intended for charter service, or if you are repowering an older vessel already operating regularly on charter, you might first want to catch up on some of the latest developments in engine technology. In particular, you may want to look seriously at what Caterpillar is calling their “wide operating speed range” (WOSR) option, available now on several of their engine models.
The WOSR option enables operation with full power over a wide speed range, with significantly improved matching of engine output to load conditions. The capability is effective enough in many cases to eliminate any need for multi-ratio gearboxes or controllable-pitch propellers.

This is how it works. Under normal circumstances with a fixed-pitch propeller and single-ratio reduction gear, when loading is significantly reduced (say running light between charter gigs), propulsion engines will run up to maximum governed engine rpm, but will not be producing full available power. Despite having sufficient additional power output available, the yacht cannot be pushed any faster, because engine rpm (and consequently propeller rpm) are limited by the engines’ electronic rpm governor system. In contrast, with WOSR capability, the Caterpillar engine’s governor system senses the low-load condition and allows the engine to turn up another few hundred rpm over normal maximum governed rpm. In turn, this allows the props to turn faster, and consequently, enables the yacht to move faster in the light-load condition.

Variable matching of engine output and rpm to load conditions has heretofore been accomplished by the incorporation of multi-ratio (shifting) gearboxes or controllable-pitch propellers. However, both of these options are relatively expensive and generally add considerable weight. WOSR, on the other hand, is primarily a matter of electronic programming, and so offers serious advantages over these historical methods, for yachts which can benefit from being able, on occasion, to run at higher than normal speeds when lightly loaded.

Utilizing WOSR also offers a way to readily retrofit such capability when completing a re-powering, as opposed to sustaining the higher costs and increased complication of retrofitting either multi-ration gearboxes or controllable-pitch propellers.

The advantages for yachts in charter service are pretty clear, for such vessels are often called upon to run light and fast to a charter pick-up rendezvous. What may not be as obvious is that WOSR can prove advantageous in non-chartered yachts, such as the 152′ Northern Marine “Sorcha,” which has successfully circumnavigated the Pacific Rim. Such yachts often make long passages heavily loaded with fuel, but when alongshore, desire to operate with much lighter loads at significantly higher speeds. In all cases, the tally on the positive side for WOSR is sufficiently significant that I, for one, anticipate other engine manufacturers will follow suit with similar technology very quickly, if they haven’t done so already.

Phil Friedman is the Director of the New Build and Refit Program at Dwight Tracy & Friends Yacht Sales, Fort Lauderdale, Florida, USA. He has a long background in yacht design and construction, including several years as President and CEO of Palmer Johnson Yachts.