DRYDOCKING
OBJECTIVES
1. Understands the stability requirements for drydocking a vessel.
1.1_ Discovers the effect on the vessel=s displacement on taking the blocks
aft when the vessel is trimmed by the stern.
1.2_ Explains the virtual loss of metacentric height due to the vessel taking
the blocks.
1.3_ Illustrates the methods by which the virtual loss of metacentric height
due to drydocking can be calculated.
1.4_ Determines the virtual loss of GM due to drydocking under various trim
conditions.
1.5_ Manipulates calculations to determine whether it is safe to drydock a
vessel in a given condition.
1.6_ Determines the maximum trim at which a ship can enter a drydock to
maintain a minimum stated GM.
1.7_ Determines the depth of water in the dock when positive stability is
exhausted.
1.8_ Calculate the draft when the vessel takes the blocks fore and aft.
EXERCISE
1. A vessel of 3480 tonnes displacement, KG 7,2m, drafts 3,12m forward and
3,68m aft is to be drydocked. Calculate the GM when the vessel takes the
horizontal blocks fore and aft. The following particulars are to be taken as
constant KM = 8,08m; MCTC = 140; CF is 70m foreside of AP.
2. A ship 140 metres in length floating at drafts - f = 5,0m; A = 6,0m is displacing
7100 tonnes. TPC = 11; MCTC = 110 t-m; KG = 7,9m. KM = 8,6m enters
drydock. Calculate the GM and the mean draft at the instant the ship takes
the blocks. (Assume KM constant and CF 4m abaft amidships.)
3. A ships is to be drydocked. Find the GM on touching the blocks fore and aft
if the ship=s draft is 4,11m forward and 5,11m aft. The following information
is to be considered constant: Displacement is 6300 tonnes, KM = 8,6m; KG
= 7,9m. MCTC = 146 t-m; CF is 69m from aft.
4. A ship of 165m in length and 9020 tonnes displacement is entering a graving
dock at a draft of 3,00m F, and 4,15m A. Her CF is 3,7m forward of
amidships and MCTC = 241.
Calculate the force on the after block just before the ship takes the blocks fore
and aft. If the metacentre at this instant is 9,45m above the keel, calculate
the virtual loss of metacentric height.
5. A ship of displacement 11 500 tonnes and length 180m is floating at drafts of
-2-
F 6,7m and A 8,5m with CF 4,0m foreside of amidships is to be drydocked.
At the moment of taking the blocks the draft is 6,9m. Calculate the change in
metacentric height if the KG = 8,0m; TPC = 15.
6. A ship of 122m in length floating at a draft of 3,96m F and 4,56m A is to enter
a graving dock.
From the following hydrostatic particulars of the ship, to be assumed constant,
calculate her draft at the instant when she takes the blocks fore and aft, and
also her effective metacentric height at that instant. Displacement = 5300
tonnes, TPC = 16; KM = 7,65m; KG = 7,00m; MCTC = 100; CF 1,44m abaft
amidships.
7. A ship of 4000 tonnes displacement is to go into a graving dock. Her length
is 140m and CF is 4,2m forward of amidships. MCTC = 100; KM = 7,42m;
KG = 7,12m. Calculate the maximum trim by the stern if the ship is not to
become unstable before touching the horizontal blocks fore and aft during
docking. (Assume hydrostatic values remain constant.)
8. From the following information calculate the maximum trim required before
entering drydock in order that a GM of not less than 0,5m is maintained at the
instant of taking the blocks fore and aft.
Displacement = 5 000 tonnes
KM = 7,0m
KG = 6,3m
MCTC = 100
CF = 60m from AP
9. From the following details of a ship about to drydock calculate the GM at the
instant the vessel takes the blocks fore and aft:
Displacement = 5 000 tonnes
KM = 7,0m
KG = 6,3m
Drafts = Forward 4,25m
Aft 5,05m
MCTC = 100
CF = 60m from AP
-3-
10. From the following details of a ship about to drydock:
Calculate: a) The GM at the instant the vessel takes the blocks fore
and aft.
b) The draft fore and aft at the same instant.
Displacement = 18 750 tonnes
Length BP = 200m
KM = 10,75m
KG = 10,05m
Drafts = Forward 9,5m
Aft 10,55m
MCTC = 350
CF = 105m forward of AP
TPC = 50 (Average over the range of drafts required)
11. From the following information calculate the maximum trim required before
entering drydock in order that a GM of not less than 0,80m is maintained at
the instant of taking the blocks fore and aft.
Displacement = 19 900 tonnes
KM = 10,85m
KG = 9,70m
MCTC = 350
CF = 105m from AP
Allow = 0,1m for the effect of free surface
