10. Longitudinal Stability

Question 1

Define MCTC.

Answer 1

Moment to Change Trim by One Centimetre (MCTC)
The moment required to produce a change of trim of one centimetre.

Expressed in cm.

Question 2

What is Trim?

Answer 2

Difference between the draft at the FP and the draft at the AP.

Stated in (cm) and whether vessel is trimmed by the head or stern.

If both drafts are the same then the vessel is said to be “even keel”.

Trim can be adjusted by moving weights in a longitudinal direction. Trim is the longitudinal equivalent of list.

Question 3

Longitudinal Metacentre and a KM (KML).

Answer 3

There is a Longitudinal Metacentre. It is very large in relation to the transverse metacentre (KMT) and is never used for routine stability calculations. Ships might capsize transversely they do not do so longitudinally!

Question 4

Longitudinal Centre of Flotation (LCF)

Answer 4

The point about which the ship pivots when the trim is changed.

Stated as a distance from the AP or distance forward/aft of amidships.

Question 5

What is Longitudinal Centre of Buoyancy (LCB)

Answer 5

The longitudinal point through which all the forces due to buoyancy can be said to act. Stated as a distance from the AP or distance forward/aft of amidships.

This is the longitudinal equivalent of the transverse Centre of Buoyancy (KB).

Question 6

List the Steps to Calculate Change of Trim CoT, find new drafts.

Answer 6

Step 1: Calculate Sinkage, Calculate the sinkage (increase in draught) from loading the weight.

Step 2: Apply Sinkage to both drafts

Step 3: Calculate change of Trim

Step 4: Apply change of Trim, Half of the change will be forward and half will be aft.

Step 5: Calculate weight of water to be pumped.