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EnginePage 7 of 7

Engine power with one unit out: the limit goes on what is left

Why it matters Running the remaining units too hard can wreck the engine at sea; the master's promise to the charterer must rest on a safe figure.

One unit out: the limit fraction goes on the power that is left, not on the MCRMCR 11100 kW from 6 units of 1850 kW. With one unit out 11100 − 1850 = 9250 kW is left, and the engine may be loaded to 0.85 of that: 9250 × 0.85 = 7863 kW. She is making 7800 kW now, inside the limit.unit out6 units × 1850 kW = MCR 11100 kWMCR 11100 kWunit out: 11100 − 1850 = 9250 kWallowed: 9250 × 0.85 = 7863 kWnow 7800 kW — inside the limitBars to one power scale

Words on this page

Engine
the big engine that turns the propeller.
Unit
one cylinder of the engine, with its own piston and fuel injection; a big engine has several.
Unit out
a unit stopped and not firing, for example after damage.
MCR
maximum continuous rating: the most power the engine is built to give, hour after hour.
kW
kilowatt, the unit of power.
Limit fraction
the share of the remaining power the engine may safely run at.
Turbocharger
a fan driven by the exhaust gas that pushes extra air into the engine; out of tune, it can surge (the air flow kicks back with a bang).
Torsional vibration
the shaft twisting back and forth a little as it turns.
Barred speed range
engine speeds to pass through quickly, because the vibration is strong there.
Charterer
the company that hires the ship and pays for her time.

An everyday picture

Six horses pull a heavy cart. One goes lame and is taken out of the harness.

First, the cart has only five horses' pull left. It cannot have the pull of six.

Second, you do not drive the five flat out for the whole journey. The team is now uneven, so you ask a bit less of each.

In the ship, the horses are the engine's units. The bit less is the limit fraction, applied to what is left.

The rule

  • Take the unit out first: power left = MCR − the output of one unit.
  • Then keep the remaining power under its own limit. With one unit out, the engine no longer runs as designed. The turbocharger is no longer matched to the gas flow (surge margin, exhaust temperatures), the firing becomes uneven. The crankshaft's torsional vibration has changed too, so barred speed ranges can move.
  • Apply the limit to the power that is LEFT, never to the full MCR.
  • This is the figure the master can promise the charterer for the whole passage. Before giving it, compare it with the power she is making now.

Formula

  • Allowed power = (MCR − output of one unit) × limit fraction

Worked example

Maximum continuous rating of the engine11100 kW
Output of one unit at that rating1850 kW
Fraction of the remaining output the engine may be loaded to with a unit out0.85
Power the engine is developing at present7800 kW
  1. Power left = 11100 − 1850 = 9250 kWWe take away the power of the unit that is out.
  2. Allowed = 9250 × 0.85 = 7863 kWWe run what is left only up to the limit.
  3. Now 7800 kW: inside the 7863 kW limit, 63 kW to spareWe compare that with the power the engine is giving now.

Answer7863 kW

Common mistake

Taking the unit out and then running what is left, 9250 kW, at full load.

With a unit out, the turbocharger, the firing order and the torsional vibration are no longer as designed. Full load on what is left invites trouble on passage. The limit (× 0.85) brings it to 7863 kW.

Note for mariners With a unit out, the limit and any barred speed range come from the engine maker's instructions; no general rule gives them.