Nautica Digitale
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There are precise relationships between engine's power, weight and displacement; complex calculations are not required to understand those values, which could be, on the other hand, very useful to the serious yachtsman


When buying something, we always refer to a unit which is able to quantify the precise amount of good we ask for. For example, there are things we buy on number, like screws or medicines, although the most used units are weight and volume: two liters of wine, twenty liters of fuel, one liter of paint or a 200 milliliters bottle of alcohol are all measurements of volume. 1000 cubic centimeters make a liter thus speaking of milliliters or cubic centimeters is actually the same. One kilo of fruit, one hectogram (the tenth part of one kilo) of ham and half a kilo of bred are all measurements of mass commonly called "weight".

At this point it is necessary to clarify that this is not an academic discussion on measurements: in fact technicians and engineers may find my introduction approximate, but I believe this is the proper way to begin engine selection understanding, especially for those who do not have a strong technical background. Nowadays energy is supplied by modern mechanic or electric means, but, not many years ago, horses were used to produce it: a good horse was identified by its weight and its volume (muscular mass), and this is why the term "horsepower" identified, till the recent introduction of kilowatts, the amount of labor produced by a machinery. Just like horses, kilowatts are related to weight and volume: let's see how.

One kilowatt is one thousand watts: a one kilowatt outboard marine engine (corresponding to about 1.5 hp) is a very small engine but is also able to light ten bulbs of one hundred watts each at the same time. A 6 kilowatts engine is still a small engine at sea, but has more power than an average house electrical plant and could be enough to give power to boilers, washing machines, ovens and bulbs. Let's step back in time and assume we are going to buy 8 horses (corresponding to about 6 kW) at the local market: our choice should be based on weight and volume, examining the horses' size (weight) and their muscles (volume) which will supply us of the energy we need. The example perfectly fits the out board marine engine selection. Here enclosed are 3 diagrams: the first two show the relationship between power and, respectively, weight and volume. Power, weight and displacement (volume) of all the out board engines currently available on the Italian market have been plotted for both two and four stroke engines.

On the first diagram, the following criteria has been adopted for engines' plotted weight selection: high power engines show the weight of the most complete model with long transmission, while the smaller engines available have been considered in their minimal configuration and with short transmission; middle range engines, use an average of the two previously described methods. Now, and looking at the first diagram, lets go to the "outboard engines market" to buy power versus weight: by entering the diagram at 6 kW (4 lines below 10 kW) its clear that there is an offer of models ranging from 20 to 40 kilos. This means that the average 6 kW engine has a 30 kilos weigh, which is 5 kilos for each kilowatt: obviously one should expect a 40 kW engine to weight 200 kilos, while, thanks to the same diagram, such engine weights about 80 kilos.

What is wrong? Actually nothing, because if one buy a horse which weight half than another he is not buying a horse half stronger, but much more weaker and on the other hand the larger horse will deliver more than the double of the power. All this means that engine's power and weight have a logarithmic relationship and the diagram is a logarithmic one. It can be read with the same simplicity of a linear graph but every line tell us the value by which the closest number in the scale must be multiplied. If, for instance, the fist line is "ten", the second will be "twenty", the third "thirty" and so on till "one hundred"; the first line after "one hundred" will be "two hundred" and so on. Now lets assume we want to buy power versus volume (just like a liter of gasoline or milk). If we enter the 6 kW line we will find we need 200 cubic centimeter of displacement, which are 30 cubic centimeters per each kilowatt. So, just like before, one should expect 1300 cubic centimeters to achieve 40 kW, while, according to the second diagram, all is needed are 800 c.c., which are 20 c.c. per kilowatt.

The two diagrams follow the same criteria (which is logarithmic) and the same trend with the same apparent direction: they, probably, can be put one on the other, scaring to death many mathematics experts, but making sense for our purposes. In fact if the "power to weight" and the "power to displacement" diagrams follow the same trend, it means that both displacement and weight are equivalent measures of out board marine engines.

This is why the third diagram has been produced, where the relationship between weight and displacement has been plotted (again for all the out board engines available on the Italian market). Here two main characteristics must be noticed: -first: there has been no need for a logarithmic diagram to plot input data on a line; -second: with obvious approximations and necessary reserves it has been possible to determine a relation between weight and displacement that we have called "outboard density" in harmony with the density of a solid which is the weight to volume ratio expressed in g/c.c. . From the diagram the "outboard density" is about 0.1 kg/c.c.: this indicative value make sense only for the type of engines we have considered so far, and do not apply to inboard engines. Special attention has to be paid to 4 stroke out board engines.

According to the first and second diagram they require more weight and displacement to achieve the same power, but the "density" diagram do not show relevant differences with two stroke engines. Considerations on power and displacements relationships has been already done on the February 1996 issue of Nautica (#406), and practically are equal to those one can make on weight. However the weight to power ratio can be dramatically effected by engine's accessories (such as trim or built in fuel tank), transmission length and starting system (manual or electric). Weight is, anyway, a very important factor to be considered when buying an out board engine: on smaller ones it means a comfortable object to transport, and for larger models the maximum weight carrying capacity of the boat have to be considered. Actually one should not buy a horse bigger than the stable or, even worst, larger than the coach it has to drawn.

It could sound strange but, using the two first logarithmic diagrams, one could buy out board engines by weight or by volume: choosing kilos or liters (which means one diagram or the other) is about the same thing if one consider that, according to the third diagram (the "density" one) to obtain the same result a liter of displacement is more or less 100 kilos of engine.