GM goes negative while loading heavy lift. The heavy lift is kept back on the jetty. Cannot adjust fuel, ballast, FW or cargo. How to increase GM to ensure smooth loading of heavy lift?
When lifting a heavt lift, Why does GM reduces ? GM reduces because of shift of center of gravity of the ship. In fact this is not only the case of lifting heavy lift alone. Center of gravity of the ship would change each time
- a weight is added (loaded) to the ship
- a weight is reduced (Discharged) from the ship
- A weight is shifted from the ship
The shift of center of gravity is in the direction of weight added, removed or shifted. And the distnace the original center of gravity would move in that direction is calculated by formula
GG1 = w x d / W , Where
w is the weight added, removed or shifted.
d is the distance between center of gravity of the weight and vessel
W is the final weight (Displacement) of the ship
Now when ship's crane lifts the heavy lift, the weight of the heavy lift is condered to be acting at the top most point of the boom, directly above the heavy lift. This reduces the GM of the vessel as center of gravity of the ship move upwards.
If this caused the negative GM and to not to allow the GM to become negative, we need to increase the GM of the vessel before lifting of the heavy lift. This will be done by shifting the center of gravity downwards by placing heavy weights in lower part of the vessel (such as by ballasting the DB tanks).
But if we cannot do that as the question asks. We are left with two options
First is to use the shore crane for loading this heavy lift.
Second is to use the ship's crane with as flat boom as possible. See the below picture to understand how the height of the boom can have the effect on shift of center of gravity (& hence GM ) of the vessel.
The only limitation being that lifting a heavy weight with flat boom is not recommended as the SWL of the crane's component may reduce with flat boom. But if the SWL of the crane allows it, we can see if this can ensure the positive GM during the lift.