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Propeller apparent slip: engine distance against the log

Why it matters A slip that climbs week after week can mean a fouled hull or a damaged propeller; that means more fuel for the same voyage.

Apparent slip: the engine distance against the log distance through the waterBy engine (pitch × revolutions) 389.2 nm; by log, through the water, 368.2 nm; the difference, 21.0 nm, is the apparent slip. Over the ground 372.1 nm, which carries the current and is not used. Apparent slip = 21.0 ÷ 389.2 × 100 = 5.4 %.by engine 389.2 nmpitch × revolutions — the theoretical runby log, through the water 368.2 nmslip 21.0 nmover the ground 372.1 nmcurrent +3.9 nm in it — not used350360370380390400apparent slip = 21.0 ÷ 389.2 × 100 = 5.4 %One scale · axis broken below 350 nm

Words on this page

Propeller
the bladed wheel at the back of the ship; it turns, pushes water backwards and drives her forwards.
Pitch
how far the propeller would move in one full turn if it turned in something solid.
Engine distance
pitch × the number of turns on the counter: the theoretical run if the propeller never slipped.
Log distance
the distance the log, the ship's water-speed meter, says she has gone through the water.
Distance over the ground
the distance made over the sea bed, as on the chart; it includes the push of any current.
Apparent slip
the engine distance minus the log distance, as a percentage of the engine distance.
nm
a nautical mile, 1852 metres.
Hull
the part of the ship's body under the water; weed and shell on it slow her down.

An everyday picture

Picture riding a bicycle on firm tarmac. Each turn of the wheel carries you one full wheel round.

On soft sand the wheel slips a little. The same number of turns carries you less far.

A propeller in water works the same way. Its pitch is the distance a turn on tarmac would give. The water gives way a little, like the sand.

Some slip is normal, because the propeller drives the ship by pushing water backwards.

The rule

  • Engine distance = pitch × revolutions. It is the theoretical run: how far she would go if the propeller never slipped.
  • Through the water she makes a little less. The shortfall against the log distance is the apparent slip.
  • Divide the shortfall by the ENGINE distance, not by the log distance. Then multiply by 100.
  • Leave the distance over the ground out; it has the current in it. A slip that slowly climbs over weeks points at a fouling hull or a damaged propeller.

Formula

  • Apparent slip (%) = (engine distance − log distance) ÷ engine distance × 100
  • Engine distance (nm) = pitch (m) × revolutions ÷ 1852

Worked example

Distance by engine, from the pitch and the counter389.2 nm
Distance by log, through the water368.2 nm
Distance made good over the ground372.1 nm
  1. Shortfall through the water = 389.2 − 368.2 = 21.0 nm (the 372.1 nm over the ground is not used)We compare the run the propeller 'promised' with the run she really made through the water.
  2. Apparent slip = 21.0 ÷ 389.2 × 100 = 5.4 %We write the gap as a share of the theoretical run, so days can be compared.

Answer5.4 %

Note: slip over the ground

Worked from the distance over the ground, 372.1 nm, the same sum gives 4.4 %. Many of the reports ships send every noon give this slip over the ground.

It is a different quantity. The ground distance has the current in it: here 3.9 nm with her. This question asks for the apparent slip through the water. That is the figure that tells you about the propeller and the hull.

Note for mariners This is the apparent slip, worked from the log; the real slip uses the speed of the water reaching the propeller (the wake); it is usually larger.