Stability After a Deck Load Is Added
A 9 m fibreglass commercial fishing vessel has a lightship displacement of 6.0 tonnes with KG 1.35 m. Its loaded operating condition is 8.5 tonnes with KG 1.55 m and a maximum permissible KG (from the stability booklet) of 1.70 m for that displacement. The skipper takes on a deck load of 400 kg of bait and ice in insulated boxes whose centre of gravity is 1.9 m above the keel, plus 150 kg of pot rope stowed at 2.2 m. No other weights change. The forecast is for 25 knots on the beam on the return leg, and the boat has a low freeboard aft.
1. Calculate the new KG after the deck load is added and determine whether the vessel remains within its permitted limit.
Read the worked answer
Use the combined weight and moment method. Existing condition: W = 8.5 t, KG = 1.55 m, so vertical moment = 8.5 × 1.55 = 13.175 t·m.
Added weights and moments.
- Bait and ice: 0.400 t × 1.9 m = 0.760 t·m.
- Rope: 0.150 t × 2.2 m = 0.330 t·m.
- Added weight = 0.550 t; added moment = 1.090 t·m.
New condition.
- W = 8.5 + 0.55 = 9.05 t.
- Total moment = 13.175 + 1.090 = 14.265 t·m.
- KG = 14.265 ÷ 9.05 = 1.576 m.
Check against the limit. The booklet limit is 1.70 m at 8.5 t. At 9.05 t the permitted KG will be slightly higher or lower depending on the curve, but the increase of about 0.026 m is small and the vessel is very likely still inside the limit - roughly 0.12 m of margin. So the arithmetic says compliant.
Caveat. The limit is displacement-dependent. I would read the actual permitted KG at 9.05 t from the booklet rather than assume the 8.5 t figure applies. If the curve falls with displacement, the margin could be less than it looks.
2. Given the beam wind and low aft freeboard, assess whether the calculated result is sufficient evidence of safety, and describe what else you would check or change.
Read the worked answer
The calculation answers only one question. It shows the loaded KG is under the tabulated limit. It does not show the vessel is safe in a beam wind with low aft freeboard.
What KG does not capture.
- Free surface effect. Bait and ice in boxes, and any liquid in partially filled tanks, act as a virtual rise in G. Several small boxes are less severe than one large slack tank, but the effect is not zero and should be estimated.
- Wind heeling lever. A beam wind of 25 knots produces a steady heeling moment proportional to the lateral area above the waterline. A low aft freeboard means the immersed lateral area grows quickly as the boat heels, so the righting arm curve flattens early.
- Deck load and watertight integrity. Deck-stowed gear raises the centre of the exposed area and can trap water. If the aft deck ships water, added weight high up compounds the problem.
- Dynamic effects. Beam seas cause rolling; the vessel may be caught at the angle of maximum heeling lever, not in static equilibrium.
Actions. Lower and secure the load, move the rope below deck or into a locker near the centreline, lash the boxes, and drain any free water. Check the righting lever curve against the wind heeling lever. If the margin is thin, reduce the deck load, delay the return leg until the wind eases, or take the sheltered route. A compliant KG number is a starting point, not a clearance to sail.