HOW TO CHOOSE THE PROPER ENGINE
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


WHAT ABOUT BUYING HORSEPOWER BY WEIGHT?
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.
