stock. The wheel turns two full revolutions from side to side giving a rudder angle of
35" each side. If the flexible cable is loosened from the rudder,arm a spare tiller can
b e mounted directly into the rudder arm.
Fuel
tank is made of Argon welded stainless steel plate with a capacity of 75 litres
(16.5 Imp. gal., 20
Us.
gal.).
Water tank is made of Polithene with a capacity of 65 litres (14 Imp. gal.,
17
US. gal.).
Performance and characteristics
Stability
If the power and length
of
a motorboat are increased the speed is increased, but if
the weight (displacement) is increased, there is a
loss of speed. The Albin 25 has
been designed around its engine t o produce the most efficient boat, obtaining the
best speed with available power. This means that the boat must b e light. Therefore
the necessary stability is achieved b y means of a suitable hull shape
-
giving form
stability. Fig. 4 shows just how the force of gravity
(P)
works from the centre of
gravity and how in a heeled position a similar force works vertically up from the
centre of displacement. If we multiply displacement
(P) (in this case 1680 kilograms)
times the length
of
the moment arm (distance 0.25 metre between the two forces) we
get the stability moment which is the "power" which will bring the boat back to an
upright position. Heeling an Albin 25 up to 50
degrees (empty boat with mast and boom and
one person on the cabin top) the accompa-
Fig. 4 .
Section showing stability moment.
nying stability curve was measured a (fig. 5).
For heeling angles larger than 50 degrees the
stability
moment has been calculated
as
shown. As a comparison the sailboat Vega
has about twice as large a stability moment
up to heeling angles of 50 degrees. A normally
loaded Albin 25 weighs more than the test
boat and also has a lower center of gravity.
Because of this the loaded boat gains even
better stability than the curve shows.
It
shows
that the Albin 25 is as good as uncapsizable
and that you can use rig and sail without any
risk of capsizing. It also shows that ballast is
not necessary from a stability point of view.
Ballast decreases speed through increased
weight and if the ballast is put deep into the
boat
it
can also give a less comfortable move-
ment to the boat.
Period of
roll
The results of such good stability give the
Albin 25 a comparatively quick natural period
of roll (time of swing from one side t o the
other and back).
A
longer period of roll gives
a more comfortable athwarthship movement
in a seaway. The formula for calculating the
period of roll
(t)
is:
t =
1.108xradius of inertia
metacentric height
Radius of inertia means the distance from the
axis of rotation t o a theoretical point where
the mass of the boat i s concentrated when
rolling around this axis. The radius of inertia
can b e considered about the same as the
length of a pendulum. The meaning of the
meta-centric height is shown in fig. 4 which
also shows that this height is a measure of
stability. A n increase
or
a
decrease of the
meta-centric height gives an increase or de-
crease of stability. Seemingly
it
is possible t o
lengthen the period o f roll through decreasing
the meta-centric height (the same as raising
the center o f gravity of the boat which is the
5
www.jonesboatyard.co.uk
