POWERING UP

Often Using Higher HP-Rated Engines Makes More Sense...

So, you’re having a yacht or other vessel built — or perhaps undertaking a major refit and repowering project. And you’re naturally considering not only which engine make and model to choose, but grappling with selection of power rating. Well, would you be surprised to learn that a significant percentage, if not all of the advice you will commonly receive may be misinformed or misguided.

Contrary to a common old wives’ tale, accepted by many in the marine industry,

Engines with higher HP ratings don't necessarily consume more fuel than those with lower ratings.

Because fuel consumption is directly related to power production. And an internal combustion engine only produces power in response to a load. All other factors held constant, an engine with a higher HP rating will not produce any more power than one with a lower HP rating, when both engines are faced with identical loads — provided only that each engine is sufficiently powerful to adequately respond to the load in question.

For a given load, and holding all other factors constant, the actual fuel consumption of two engines with different HP ratings, will effectively be the same.

Horsepower is a measure of work accomplished. As such, it is the product of torque (force) and engine rpm (movement). Torque is produced by combustive force in an engine’s cylinders acting through its chain of pistons, connecting rods and crankshaft. Assuming similarly efficient fuel burn between different engines (a reasonable assumption in most cases), combustive force is determined by the quantity of fuel burned. The upshot is that for a given load — or we may say, for a given application in a specific vessel — the actual fuel consumption of two engines with different HP ratings under equal loading conditions will be effectively the same.

Horsepower Rating Versus Cost

For this reason, in terms of fuel consumption and range, choosing an engine with a lower HP rating may not really be advantageous. Granted, it is likely that, of two engines able to handle a given anticipated load, the one with the lower HP rating will have a lower first cost. That is to say, if your anticipated load dictates a 500 HP engine, selecting one of similar weight, but with a maximum 700 HP rating will probably cost more initially — although not ultimately in terms of fuel consumption, maintenance, or longevity.

A proper analysis requires consideration of the effect of maximum HP rating on longevity or MTBF (mean time between failure). For example, if you run an engine consistently for long periods of time at 95% to 100% of its rated horsepower output, you are almost certainly going to seriously reduce its operating life. Consequently, the idea is to select an engine or engines with power ratings that assure the bulk of your operating hours will be at 80% to 85%, or less of maximum rated output. By decreasing the percentage of operating hours at engine loadings above 85% or maximum rated output, you will increase MTBF, and will as a result reduce the ultimate cost per hour of running time of your engine(s).

Yachts Never Diet

In the case of pleasure craft, you should also consider that no yacht ever loses weight as it ages. What we refer to in the industry as “weight gain” is a never-ending, never-reversing process that begins when the vessel moves from design and engineering to actual construction, then to outfitting and commissioning, and finally to, and during real-world use. This life cycle is always — yes always — accompanied by a growth in overall, loaded weight. The result is an increasing load for propulsion engines to face over the life of the vessel. All due to the fact that we always put things onto and into a yacht — tenders, jet skis, dive tanks, air compressors, and all other manner of gear and equipment. But we rarely, if ever take anything off.

If the engines you select start out life in an application initially projected to see them loaded to 95% or more of their rated power, you can almost certainly expect that somewhere during their life in the craft in question, they will end up running overloaded, that is, producing power at levels above their rated maximum. This will mean accelerated stress, wear and tear; decreased MTBF, and ultimately higher cost per hour of running time than if you had specified and initially paid for engines with a higher power output rating.

Real World Yacht Construction

However, that’s not the only problem with engines whose power production ratings are only marginal for the application at hand. My experience in large-yacht construction is that a significant number of vessels specified with engines whose rated maximum power output is very close to the design loads initially projected, end up underpowered for their performance targets, by the time their construction is completed and they are launched.

As construction and final engineering progress during a new-build, previously unanticipated weight growth may force you to accept lower vessel performance than originally targeted — unless you take remedial action. In such cases, there may be steps you can take while the build is still in process. For example, one might be able to boost projected thrust by upsizing the vessel’s propellers. However, because of the higher loading produced by such larger propellers, such a move will only be successful if the engines selected at the early stages of project development and specification have a sufficient margin of reserve power available to accommodate the higher propeller loading.

In my experience as a yacht builder, it's prudent to select engines whose power production ratings are 10% to 25% higher than the designing naval architects and engineers originally anticipated would be necessary. Of course, you need to ensure that the overall weight of the higher-powered engines is not significantly greater than the weight of those originally specified. (In many cases, it won’t be.) And you have to ensure that the higher-powered engines fit within the maximum physical envelope delineated in the design when the original engines were chose. In this way, you can provide for a margin of reserve power, without incurring and penalties in terms of fuel consumption and range. In other words, if it turns out that the additional power is not required, the vessel will not be carrying around any additional weight, nor will the higher-powered engines be using any more fuel than the lower-powered ones originally specified. A bonus will be that the engines with the higher power ratings, when run at a lower percentage of rated maximum power, will generally last longer and be more reliable.

— Phil Friedman

Author's Note: This article is not intended to be a technical paper, although it is technically correct. For more technically detailed discussion, you're invited to visit the author's yacht building blogs at:

YachtBuildAdvisor.com and BoatbuildConsult.com

About the author: With 30 some years background in the marine industry, I've worn numerous hats — as a yacht designer, boatbuilder, marine operations and business manager, marine industry consultant, marine marketing and communications specialist, boating magazine writer and editor, yacht surveyor, and marine industry educator. I am also trained and experienced in interest-based negotiation and mediation. And in a previous life, I taught logic and philosophy at university.

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Text and photos Copyright ? 2014 by Phil Friedman — All Rights Reserved