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ENGINE SIZE (a rule of thumb guide)


There are many different theories of how to calculate the engine requirements for each vessel. However, these are often so complicated that they are virtually useless to us yachtsmen, requiring a maths degree or a dedicated computer programme to even begin to understand them. This has left the yachtsman with a lot of misunderstanding, regarding the correct amount of power, or size of engine needed to drive their yacht. In fact many yachtsmen seem to believe that the bigger the engine they fit the faster their yacht will move. This belief probably stems from their dealings with motor vehicles where this is largly true.

Listen in nearly any yacht club and you will eventually hear one of the members proclaiming how he needed every one of his yachts enormous number of horsepower to get him home on time. How although the designer recommends an engine of 'x' size, the owner knew better and fitted one twice the size. And wasn't he right to do so! He says knowingly, especially in these waters; he's had to use his engine so much in the last two seasons that it's worn out. Needs a new set of piston rings!

And he doesn’t know what the designer was doing when he recommended that propeller. The 'member' feels sure his yacht needs a bigger one!

Well folks, there are some things that don’t change. The relationship between the load and the effort required to move that load has never changed. In our case the load is comprised of several things – these are the weight of the yacht, the wetted area, how foul the bottom of the yacht is, the windage, the state of the sea and the waterline length, or wave making ability of the hull.

All of these are slightly variable, but this does not make it difficult to calculate the average load for yachts in general.

The size of the engine (i.e. the effort), that is required to move these loads is also slightly varied by the efficiency of transferring the available power into the water. This is altered by the revolutions per minute of the engine, the gearbox ratio and the size and number of blades of the propeller.

One thing that is not variable is the fact that engines are designed to work efficiently and last longest by being used in the correct way. This is especially true of the Diesel engine. If you run a diesel improperly, in this case at very light loads, the bore will glaze and there will be a loss of compression. It will quickly need a new set of rings and a glaze-breaker running down the bore before the new rings are fitted. (A glaze-breaker is a set of stones, which re-hone the surface of the bore).

For yachts, there are certain 'rules of thumb'. If you look in the old textbooks there are 'rules of thumb' for everything from anchor sizes to mast thickness, from the size of a bunk to the sail area needed for each vessel.

In this case the 'rule of thumb' says this: - For an average cruising yacht to make good speed in flat calm conditions ONE horsepower for each ton of vessel is needed. If the yacht is required to go well into the wind and a head sea then THREE horsepower for each ton is needed. This was based on a yacht being expected to average approximately three quarters of its hull speed while under power and having a properly matched propeller. With today’s faster pace of living and the necessity of arriving on time, more is expected.

Also today, there are other things that alter the size of engine that is required. These are the additional loads that you may want to apply which include a high power alternator and refrigeration compressor.

With these additional loads and the fact that many owners now expect their yacht to be capable of travelling at maximum hull speed, some designers are recommending engines of up to 5hp per ton of vessel. Nevertheless, this is very high as it will be rare for the yacht to be motoring flat out into a strong wind and a head-sea; whilst at the same time running the fridge compressor, charging the batteries at maximum and so using the full potential of the engine. A more efficient figure for the modern sailor is probably around FOUR horsepower per ton. This will power the yacht at close to maximum hull speed and allow a margin for fitting a high capacity alternator and fridge compressor. On the other hand there will not be so much excess power that the engine is constantly running at a light load and damaging the engine.

I have noticed that many of the 'white plastic blobs' produced by the production line manufacturers these days have over sized engines. This is so that the weekend or charter boat sailor will be able to motor at hull speed in virtually any conditions to arrive back at the marina on time. Not something that many long term cruisers like to do.

All the big engine and propeller manufacturers have computer programmes to match all the variables and recommend the best combination for each vessel. Some of them will do a free check of your yacht with their programme, obviously in the hope of eventually making a sale.

Whether you are just listening to the 'member' in the club, thinking of purchasing a new yacht, or are going to re-engine your existing yacht, the 'rule of thumb' of around FOUR horsepower per ton gives a remarkably good guide that things are correct.

Our 'member' has made a classic mistake, he has not only wasted cash by buying an engine that is too big, but he will regularly have expensive maintenance bills due to the inefficient use of his engine. All this can be avoided by listening to the experts, safe in the knowledge that their figures should be a close match to the 'rule of thumb'.



© Paul Fay 2001