General arrangement and planning
28
Single and twin installations
The most effective method of propulsion is generally
achieved with a single installation. If more power is re-
quired two engines, each with a separate propeller
shaft can be installed.
Improved manoeuvering is gained with twin installa-
tions and separate propellers as the power output can
be controlled separately and independently for each
engine. One engine can be run reverse and the other
ahead when for example manoeuvring at low speed.
Choice of reduction ratio
The propeller shaft usually has lower speed than the
engine. This is normally achieved with the reduction in
the reverse gear.
As a rule the largest possible ratio should be chosen
for slow-going displacement boats. It then follows that
the propeller diameter can also be relatively large with
high thrust within the applicable speed range. Depend-
ing on the hull type and speed range, a smaller ratio
can be chosen for higher speed, if required. See the
table. This is to obtain highest thrust within the re-
spective speed range. If the ratio is chosen outside of
the recommendations the thrust can be lower than the
optimum calculated power. The boat’s top speed is
not necessarily affected.
A check must always be done that the hull has suffi-
cient space for the propeller according to information
on page 27.
Ratio, Ratio Main type of Speed
approx. rpm approx. rpm operation range
2500–3250 3000–4000
Work boats,
2.5:1–3.5:1 3.0:1–4.0:1 Displacem. boats, 4–10 kn
High pulling power,
Towing, Trawling
Work boats,
2.0:1–3.0:1 2.5:1–3.5:1 Displacement boats, 7–15 kn
Low speed planing boats,
mainly free run
Semi-planing to planing
1.5:1–2.0:1 2.0:1–2.5:1 boats, Patrol boats, 16–15 kn .
Sport fishing
and Pleasure boats
— 1.5:1–2.0:1 Planing boats,
Patrol boats, 25–40kn
Sport fishing,
and Pleasure boats
Engine revolution range 2700–3900 rpm with con-
versional shaft/propeller system
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