5003 Lectures - Faculty of Engineering and Applied Science

E5003 - Ship Structures I 55
© C.G. Daley
This gives us a set of station buoyancy forces due to the wave (net of still water).
These forces should be in equilibrium (no net vertical force). We can calculate the
moment at midships from either the net effect of all forces forward, or all forces aft
(the two moments will balance).
There are other ways to do this kind of calculation. 3D cad programs such as Rhino
can be used to find the still water and wave bending moments. Assuming that we
have a hull modeled in Rhino, we can find the still water buoyancy forces for the
fore and aft halves of the vessel by finding the volume and location of the centroids
of the two submerged volumes.
The procedure would be as follows;
1. Produce solid model of hull
2. Cut the model at both the centerline and waterlines.
3. Find the volumes and centroids of the two halves.
4. Calculate the buoyant moments about midships.
A similar procedure would determine the wave values. The only difference would be
the need to draw the trochoidal wave as a surface.
The example below shows use of Rhino to calculate the Bouyant BM for a large
vessel. The centroids of the two half volumes are shown.
BMB = 109,000 x 1.025 x 53.97 (m3 x t/m3 x m = t-m)
= 6,029,798 t-m
or
BMB = 123,000 x 1.025 x 58.58 (m3 x t/m3 x m = t-m)
= 7,385,473 t-m
average: BMB = 6,707,376 t-m (sag)