power. The curves for displacements of 1750
kilograms and 2100 kilograms give an idea of
what an increase in weight means. That an in-
crease of power is needed with increased
speed is still more clearly shown - a speed
of 8 knots requires 12 to 15 times as much
power as for 4 knots! At low speed the resist-
ance of the boat is mainly friction (about 95%
of the total resistance) at 8 knots the frictional
resistance is about 55% of the total resist-
ance - the rest is form resistance - wave
resistance plus a few percent air resistance.
It is important to have a clean bottom with the
least possible frictional resistance.
Fuel consumption
at different revs and speeds
also appear in fig.
6.
These values are the re-
sults of many measurements and can be con-
by the tachometer of the boat can at full revs
sidered correct to within
5%.The rpm shown
show up to 150 rpm too little and sometimes
up to 50 rpm too much. At cruising revs the
fault is considerably smaller - about 60% of
the error at full rpm. At idling rpm the fault is
very small. The speeds shown are for calm
weather without any sea. Also in the diagram
there are 3 curves showing the largest pos-
sible distance covered with 50 litres of fuel.
These show how economical it is to use a low
rpm if it is desired to go as far as possible
with a certain quantity of fuel. The curves
shown for one or two knots head current can
also be used if the speed is diminished by
headwinds and headseas. They show how the
most economical rpm increases as head cur-
rent increases.
3
KNOTS
4
5
b
7
a
Do not trust a speed log which has not been
tested and calibrated. It is not unusual that
the log can show more than one knot error.
.
FOR
DIFFERENT SPEEDS
A
ND DISPLACEMENTS
I00
Fig.
6. Fuel consumption and resistance.
Influence
of the
propeller on
manoeuvring
The direction the propeller rotates has a certain influence on a motorboat. Figs. 7 to
13 show this. It can be added that the rudder effect when going astern is not as good
on boats with the propeller sheltered behind a deep keel as on boats where the
propeller and rudder are completely unsheltered. It would be wise to practice all
types of manoeuvering on open water with different wind directions. From such test
manoeuvers it will be possible to judge and make successful manoeuvers in difficult
situations later on. The effect of the wind on the boat is mostly felt at low speeds and
can be rather large. A good knowledge of this effect, the best speed for manoeuvering,
radius of turns in different situations, influence of the propeller, etc. is necessary if
you are to manoeuver without risk of damage to your own or other boats. With the
propeller thrusting
-
propeller wash against the rudder
-
the turning radius is about
a
boat lenght. There is no great difference in turning radius between low and high
speeds. From full speed ahead you can stop the Albin 25 in one boat lenght if the
gearbox is used in the right way.
7
www.jonesboatyard.co.uk
