Stability & CargoPage 15 of 15
Loading to the marks when she will cross into the Winter zone
Why it matters Arrive at the zone boundary a few centimetres deep and she is over her marks, an offence; load from the wrong mark and the charterer loses cargo she could have carried.
Words on this page
- Load line marks
- lines marked on both sides of the ship; they show how deep she may load in each season and zone.
- Summer mark and Winter mark
- the Summer line is the deepest loading in a Summer zone; the Winter line sits a little below it, for the rougher Winter zones.
- Load line zone
- a sea area on the load line chart marked Summer, Winter or Tropical for the time of year.
- Zone boundary
- the line on that chart where one zone ends and the next begins.
- Seasonal allowance
- the distance from the Summer mark to the Winter mark, one forty-eighth of the Summer draught.
- Draught
- how deep the ship sits in the water.
- Load line certificate
- the ship's paper that gives the draught for each mark.
- TPC
- tonnes per centimetre: how many tonnes change the draught by one centimetre.
- Consumption
- the fuel and fresh water she burns and drinks on the way; she gets lighter and rises.
An everyday picture
You load a lorry here, where the road allows 40 tonnes. Further on there is a bridge that allows 38 tonnes, and you must cross it.
You do not load to 40. You load for the bridge, 38 tonnes, because that is where the rule bites.
But you will burn 200 kg of fuel before the bridge. So you may leave at 38.2 tonnes and arrive at the bridge at exactly 38.
The rule
- Start from the mark the voyage must meet, not the mark of the port she loads in. She will be in the Winter zone when she reaches the boundary, so the Winter mark rules.
- The certificate gives both draughts. The gap between them is the seasonal allowance, one forty-eighth of the Summer draught; it is already on the certificate, so do not take it off again.
- She burns fuel and water on the way to the boundary, so she rises. Rise (cm) = consumption ÷ TPC.
- She may sail that much deeper than the Winter mark and still be on it at the boundary. Deepest sailing draught = Winter draught + rise.
Formula
- Rise before the boundary (cm) = consumption ÷ TPC
- Deepest sailing draught = Winter draught + rise
- Seasonal allowance = Summer draught ÷ 48 (check: Summer − Winter)
Worked example
| Summer load draught from the load line certificate | 16.32 m |
| Winter load draught from the load line certificate | 15.98 m |
| Tonnes per centimetre immersion at that draught | 128 t/cm |
| Fuel and fresh water to be consumed before the zone boundary | 480 t |
- The mark to meet is the Winter mark, 15.98 m; the allowance on the certificate is 16.32 − 15.98 = 0.34 m (= 16.32 ÷ 48)The zone she arrives in decides the mark; the certificate already holds the allowance.
- Rise before the boundary = 480 ÷ 128 = 3.75 cm = 0.04 mBurning fuel makes her lighter, so she comes up by this much.
- Deepest sailing draught = 15.98 + 0.04 = 16.02 m (exactly 15.98 + 0.0375 = 16.0175 m)She leaves deeper by that rise and arrives on her Winter mark at the line; a maximum is rounded down in practice, and the exam asks for two decimals.
Answer16.02 m
Common mistake
Starting from the Summer mark, 16.32 m, because the berth is in a Summer zone.
That gives 16.36 m. At the boundary she would sit 0.34 m deeper than the Winter mark allows. That is over her marks in a Winter zone. The mark is set by the zone she will be in, not the zone she loads in. The certificate carries both figures so that nobody has to remember which. 16.36 m is already over her Summer mark at the berth.
Note for mariners The sum gives the maximum, not the figure to sign; the consumption is an estimate and the weather can move the crossing, so sail inside it.