same as decreased stability) or through increasing the radius of inertia (putting
weights farther from the axis of rotation). Here are examples how these factors can
be used to effectively dampen a boat's movements in a seaway:
1.
O n sailing yachts you can hoist a heavy anchor up the rigging (decreasing meta-
centric height and increasing radius of inertia without increasing weight).
2.
O n an Atlantic passenger ship the motion in a seaway was unsuitable for pas-
sengers. The cure was 200 tons of stone on top of the upper deck.
The same effect has been measured on the Albin 25. The period of roll was measured
with an empty boat, with one man (85 kilograms) positioned in different places. The
following times were measured:
low position
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.05 seconds
at the helmsmans place
. . . . . . . . . . . . . . . . . . . . .
2.1 5 seconds
on top of the cabin
. . . . . . . . . . . . . . . . . . . . . . . . .
2.40 seconds (an increase of 25%)
Ballast and movements in a seaway
The above technical discussion on the period of roll has been given to give a back-
ground to the following advice regarding ballast or no ballast. Several owners of the
Albin 25 think that the boat will have a calmer motion if ballast is used and many more
ask questions regarding ballast. One publication has advised that a ballast of 500
kilos be used down in the bilges. With conflicting advice i t seems advisable to discuss
this question more thoroughtly. The reasons usually given as to why'ballast should be
used is the desire to obtain better stability and calmer motion in a seaway. Here we
have two conditions contrary to each other and where i t will be necessary to choose
a suitable compromise.
Increased stability
is not necessary. This is clearly shown in the stability curve fig. 5.
When the weight of the boat is increased with equipment, provisions etc., the stability
is increased and much more so if everything is stowed deep down in the boat.
Calmer movements in a seaway:
The main part of the discomfort experienced in a
motorboat in a seaway is caused by the athwarthship movements
-
the rolling.
There is the unhappy case where the period of the wave train is the same as the
boats natural period of roll and the motion of the boat therefore is increased. An
alteration of the course may change this. Each boat and each size of boat has a type
of sea which is especially uncomfortable.
Albin 25 has been designed for comfortable motion in normal use, loaded with a full
crew and all necessary equipment. The hull form gives a very comfortable movement
in a headsea. In certain types of seaways and also in open anchorages the boat can
develop relatively quick, uncomfortable athwarthship movements because of its form
stability. In open waters with a short and uncomfortable sea, calmer athwarthship
movements can be had through using:
1.
Sails.
A mast in itself is enough to increase the roll period and give a calmer
motion. With sails and a good wind the movement is considerably dampened.
2.
Mechanical stabilization.
There are several different systems, as a rule they are
too expensive.
3.
Lengthening the period
of roll.
Through his own efforts the owner can increase
the period of roll b y stowing all equipment and cargo (provisions, drink, etc.) as
high up and as far out to the sides as possible. A dinghy and anchor stored on
the deck helps for instance to give a better motion. A n increase in the total weight
of the boat increases the radius of inertia if the increase of weight does not ride
on the axis of rotation.
To
use ballast is not advised if i t is possible through other efforts to improve the
movement of the boat to satisfy your personal taste. If ballast is used to dampen the
movements i t should be stowed high up and as far out to the sides as possible. Such
ballast must be fastened thoroughly so as not to come adrift in a seaway. If ballast is
placed deep down, the meta-centric height is increased (which shortens the period
of roll). The radius of inertia is increased considerably if ballast is placed high up and
far out to the sides. It is a completely faulty use of ballast to place i t far down hoping
for better motion, and the effect may be only 25% of what the right position will give.
Don't forget that every increase in weight also decreases the speed, increasing fuel
consumption. For instance
500 kilograms of ballast results in a reduction of maximum
speed b y about 3/4 of a knot (10%) and a reduction of cruising speed by
about 1/3 of a knot (5%).
Fuel consumption and speed
Fig. 6 shows how much power must be delivered to the propeller to give the Albin 25
a certain speed. The power shown is about equal to the
DIN power minus 5% to 10%
(losses in alternator, gearbox, shafts, bearings, etc). The efficiency of the propeller at
around
58%
is good. It follows that the thrust is a little more than half of the available
6
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