4.5.1 After a model of a lifeboat has been constructed,
it is very often necessary to adjust the magnitude of the second moment
of mass so that it is in proper proportion with the full-scale lifeboat.
This is easily accomplished by placing two moveable weights in the
model as shown in figure 4.2. The mass of the weights W1 and
W2 are equal to m1 and m2 respectively.
The total scaled mass of the lifeboat model, then, is equal to:
where m0 is the mass of the model (including
equipment placed in the model) without the two weights on board. The
weights are placed in the model at a distance S1 and S2 longitudinally
from the CG and vertically at the CG. The distances S1 and
S2 are selected so that the location of the CG does not
change.
As the weights W1 and W2 are
moved farther apart, the second moment of mass of the system increases.
Conversely, as the weights are moved closer together, the second moment
of mass of the system decreases. The distances at which the weights
should be placed are:
In the above equations I is the required second moment
of mass and I0 is the second moment of mass of the model
without the weights on board. If the two weights have the same mass,
and that mass is equal to ma, the equations can be reduced
to:
Figure 4.2 Geometry for Adjusting Second Moment of Mass
4.5.2 For this procedure to be applicable, I0 must
be smaller than that required. The second moment of mass can be increased
by moving the weights around but it cannot be decreased below that
which would occur if the masses were placed at the CG (S1 and
S2 are both equal to zero). As such, the model should be
built so that its mass and second moment of mass are smaller than
that required. Two weights then can be properly placed in the model
so that the resulting mass and second moment of mass are of the proper
magnitude.