Critical period

The period since the keel first touches the block until the vessel takes blocks overall.

An upthrust is caused by the blocks, denoted by "P".

P at any instant can be calculated by the following formula:

P = TPC X Change in mean draft in cm.

P is maximum at the instant before vessel takes blocks overall. It can be calculated as:

P = MCTC X t / l        { t = trim in cm, l = dist of CF from AP}

Due to the upthrust, the vessel reduces its GM.

The G moves UP, thereby GM is reduced.

 

M moves down to M', thereby GM is reduced.

Shift of G (Center of gravity) or M (Transverse metacenter) may be calculated as:

The danger is, due to subsequent loss of GM, the vessel may lose positive stability and may capsize.

Maximum loss of GM to be calculated beforehand.

 

It is dangerous if negative GM occurs in dry dock.    (Reasons).