Dry Dock

Question 1

How do you calculate loss of GM when touching the blocks in dry dock

Answer 1

Virtual Loss of GM

There are two methods for ascertaining the virtual loss of GM. In each of the two methods the force P must be known. Force P represents the upthrust at the stern at the moment the vessel touches the keel blocks. The time the keel first touches the blocks until the vessel has taken the blocks overall is considered to be the critical period (Figure 14.10).

Question 2

How do you calculate loss of GM?

Answer 2

The first method considers the movement of the metacentre (M):

Virtual Loss of GM = P x KG / W

The second method considers the movement of the centre of gravity (G):

Virtual Loss of GM = P x KG / W - P

W represents displacement of the vessel.
KM represents the distance between the keel and the metacentre,
KG represents the distance between the keel and the centre of gravity of the vessel.
P is upthrust

• Note: Both methods should be calculated and the worse of the two results used to ensure the safety of the vessel during the critical period.

Question 3

How do you calculate upthrust (P)

Answer 3

P = MCTC x Trim / L

L is distance from Longitudinal Centre of Floatation to P.

This should be calculated for each block but as we enter with a stern trim, P will be Max when touching down occurs.