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What going round a traffic separation scheme costs in time

Why it matters The master signs the plan on this figure; too big a number pushes her into a crowded lane, too small a number hides a late arrival.

Two whole passages on one time scale: round the outside at sea speed, or through the scheme with a slow laneTop bar: round the outside, 252 nm ÷ 13.6 kn = 18.53 h, all at sea speed. Bottom bar: through the scheme, 188 nm of open water at 13.6 kn = 13.82 h, then 26 nm of lane at 9.4 kn = 2.77 h, 16.59 h in all. Going round adds 18.53 − 16.59 = 1.94 h. The extra miles alone, 38 nm at 13.6 kn = 2.79 h, compare with a direct passage all at sea speed (15.74 h) and overstate the cost.PASSAGE PLAN · TWO PASSAGES, ONE TIME SCALEround the outside252 nm ÷ 13.6 kn = 18.53 hthrough the scheme16.59 h in allopen water 214 − 26 = 188 nm at 13.6 kn · 13.82 hlane 26 nm at 9.4 kn · 2.77 hgoing round adds18.53 − 16.59 = 1.94 h38 extra nm alone: 2.79 hcompares with a lane at sea speed024681012141618h

Words on this page

Traffic separation scheme (TSS)
a marked area at sea where ships going opposite ways keep to separate lanes, like a dual carriageway.
Lane
the one-way strip of the scheme a ship follows through it.
Direct route
the shorter way, straight through the scheme.
Planned track
the way the passage plan really takes, here round the outside.
Open leg
a stretch of the route in open water, steamed at full sea speed.
Passage
the whole voyage from here to there, every leg counted.
Passage plan
the written plan of the route, its legs and speeds, which the master signs.
nm and kn
a nautical mile (nm) is 1852 metres; one knot (kn) is one nautical mile an hour.

An everyday picture

You can drive through a town or take the ring road round it. The ring road is 10 km longer. But the road through the town has a 5 km stretch at 60 km an hour.

Someone says the ring road costs 10 km at 100 km an hour, six minutes. That forgets the slow stretch on the other route.

Time both routes door to door and compare. The ring road costs four minutes, not six, because the town road was never fast all the way.

The rule

  • Work the passage that is planned, round the outside. Every mile of it is at sea speed, because she never enters the scheme.
  • Work the passage it is compared with, through the scheme. It is shorter, but it is not all at sea speed. Take the lane out of the direct distance; that part is at sea speed. The lane itself is at the lane speed.
  • Add the two parts of the through passage. That is its whole time.
  • The cost of going round = time round the outside − time through. Two whole passages, compared.

Formula

  • Time round the outside = planned distance ÷ sea speed
  • Time through = (direct distance − lane) ÷ sea speed + lane ÷ lane speed
  • Time added = time round the outside − time through

Worked example

Direct distance, straight through214 nm
Distance by the planned track, round the outside252 nm
Length of the lane if the scheme were transited26 nm
Speed on the open leg13.6 kn
Speed the lane would be transited at9.4 kn
  1. Round the outside = 252 ÷ 13.6 = 18.53 hThe planned route: all of it at sea speed, because she stays outside.
  2. Open water on the direct route = 214 − 26 = 188 nm; 188 ÷ 13.6 = 13.82 hThe other route's fast part: the direct distance with the lane taken out.
  3. The lane = 26 ÷ 9.4 = 2.77 hThe other route's slow part: the lane, at the lower speed.
  4. Through the scheme = 13.82 + 2.77 = 16.59 hFast part plus slow part is the whole time through the scheme.
  5. Time added by going round = 18.53 − 16.59 = 1.94 hThe difference between the two whole passages is what going round costs.

Answer1.94 h

Common mistake

Timing only the extra miles: 252 − 214 = 38 nm at 13.6 kn.

That gives 2.79 h, and it is too much. The route it is compared with is slower in the lane, by 0.85 h. Take that off and the true cost is 1.94 h.

Note for mariners The hours are only half the decision; crossing traffic in the lane and Rule 10's duties weigh on the other side; they are not measured in hours.