Monday, October 8, 2012


So, you’re having a yacht built…or perhaps, undertaking a major refit and repowering of an older vessel. And you’re naturally considering not only which engine make and model to choose, but grappling with the decision as to selection of power rating.
Well, let me tell you that a significant percentage, if not all of the advice that you will commonly receive may be misinformed or misguided. One of the most common “old wives’ tale” you will be exposed to is:
 “Higher HP engines always consume more fuel than those with lower ratings.”

Fuel consumption is directly related to power production. 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 the identical load, provided that each engine is sufficiently powerful to adequately respond to the load in question.
Horsepower is a measure of work accomplished. As such, it is the product of torque and engine rpm. 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 yacht — the fuel consumption of two engines with different HP ratings will be effectively the same. 
For this reason, in terms of fuel consumption and range, choosing an engine with a lower HP rating may not be advantageous. If anything, it is likely that, of two engines able to handle the load that will be placed upon them, the one with the lower HP rating will have a lower first cost. Thus, if you need, say, a 500 HP engine, selecting one with a similar weight, but a maximum 700 HP rating will probably cost more — although not ultimately in terms of fuel consumption.
Against that, however, you need to consider the effect of maximum HP rating on longevity or MTBF (mean time between failure). 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. So the idea is to select an engine or engines with power ratings which assure that the bulk of your operating hours will be at 80% to 85% of maximum rated output. For by increasing MTBF, you will actually be reducing the ultimate cost per hour of running time of your engines.

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 the overall, loaded weight of a vessel, and consequently with an increasing load for her propulsion engines to face. 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 that is 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 yacht 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.
But that’s not the only problem with engines whose power production ratings are only marginal for the application at hand. My experience in 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.

As construction and final engineering progress in a yacht build project, previously unanticipated weight growth may force you to accept lower vessel performance than originally targeted. In such cases, there may be remedial steps that can be taken 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, this kind of 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.

In my experience as a yacht builder,  it is 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. Provided you watch to see that the overall weight of the higher powered engines is not significantly higher than those originally specified (and in many cases they won’t be), you can provide for a margin of reserve power, without incurring and penalties in terms of fuel consumption and range, if it turns out that the additional power is not required. A bonus will be that the engines with the higher power ratings, when run at a lower percentage of rated maximum power, will last longer and be more reliable.
My next post will deal with why total fuel consumed provides a better basis than engine hours for scheduling maintenance.

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