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Hull

Hull form and resistance

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What a ship burns is decided less by her engine than by the shape of the wetted surface she drags through the water. Resistance splits into two parts that behave completely differently as speed rises, and which of them dominates decides whether a hull is worth slowing down, whether a bulbous bow helps her, and how much a dirty bottom costs.

Two resistances, one of them well behaved

Frictional resistance is the water shearing along the hull. It scales with wetted surface area and roughly with the square of speed, and it dominates at the speeds most cargo ships run at. It is also the part that gets worse with time, because it depends on how rough the surface is.

Residuary resistance is mostly wave making: the ship builds a bow wave and a stern wave and pays for the energy in them. It is modest at low speed and then rises very steeply, because past a certain point the ship is trying to climb her own bow wave.

The boundary between the two regimes is set not by speed alone but by speed against length. The Froude number, speed divided by the square root of length times gravitational acceleration, is the number naval architects use, and its practical meaning is simple: a longer ship can go faster before wave making takes over. This is why fast ships are long and fine, and why a short ship pushed hard burns fuel in a way that looks absurd until you see the wake.

Block coefficient, and why bulk carriers look like boxes

The block coefficient is the volume of the underwater hull divided by the box that would contain it, length by breadth by draught. A very full hull approaches 0.85 and a fine, fast hull sits far lower. Fullness is not laziness. It is the answer to a constraint.

Berths, locks and channels cap length and breadth. A ship that may not get longer or wider, and whose draught is set by the load line and the port, can only carry more cargo by filling out the box she is allowed to occupy. So a Capesize is close to a rectangular block with ends on it, carries an enormous deadweight for her dimensions, and is slow because a full hull at speed is all wave making. A container ship trades deadweight for a fine hull and buys speed with it. Both are optimal; they are optimising different things.

HullFullnessWhat it buysWhat it costs
Full and slowHigh block coefficientMaximum deadweight inside a fixed length and beamWave making rises early, so she is uneconomic to push hard
Fine and fastLow block coefficientHigh service speed at tolerable powerLess cargo for the same principal dimensions

The bulbous bow is tuned, not free

A bulb works by making its own wave system out of phase with the bow wave, so the two partially cancel and the wave making resistance falls. That cancellation is designed for a particular speed and a particular draught.

Move away from that design point and the bulb stops helping. A ship steaming slowly, or trading much of her life in ballast at a draught the bulb was never shaped for, can be carrying extra wetted area for no benefit. When the industry slowed down after 2008 this stopped being theoretical: bows were reshaped and retrofitted on existing ships, which only makes sense if the original bulb had become a liability at the new operating speed.

Fouling, and why a performance claim starts with a drydocking date

Frictional resistance depends on roughness, and a hull in warm water grows a biofilm and then weed and shell. The added roughness is not cosmetic: it raises the power needed for the same speed, continuously, from the day she leaves the dock.

This is why the last drydocking and last hull cleaning are asked for during a performance dispute before anything about the engine is discussed. It is why a time charter carries hull cleaning and propeller polishing terms, and why the question of who pays when a ship sits idle in a warm port for weeks is a negotiated allocation of a real cost rather than a formality. A fouled propeller is worse per unit of surface than a fouled hull, which is why polishing is treated separately.

Trim, squat and shallow water

Three effects change resistance without anything being done to the ship.

Trim. The same displacement carried slightly differently changes the underwater shape and therefore the resistance. Trim optimisation is one of the few operational measures that costs nothing and is measurable, which is why it appears in almost every efficiency programme.

Squat. In shallow water the flow accelerates under the hull, pressure drops, and the ship sinks bodily and changes trim. She draws more than her static draught at speed, which is a navigational limit as well as a resistance one, and it is one reason a draft survey is taken with the ship stopped and properly moored.

Shallow water resistance. Independently of squat, restricted depth changes the wave pattern and raises resistance sharply. A ship that makes her speed at sea will not make it in a shallow channel, and no clause makes water deeper.

How the numbers on a description were produced

Model tests in a towing tank measure resistance at model scale and extrapolate to full scale by separating the frictional part, which scales with Reynolds number, from the residuary part, which scales with Froude number. The ITTC publishes the procedures the industry uses for this, including the 1978 performance prediction method. Speed and power trials on the delivered ship are run and corrected under ITTC procedure and ISO 15016, because a trial in real weather has to be corrected for wind, waves, current and depth before it can be compared with anything.

Which brings the point home. A description speed is a corrected trial result or a warranted figure, given at a stated draught in stated weather with a clean hull. It is a reference condition, carefully defined, and the ship almost never operates in it. Treat the number as the start of a calculation rather than as a property of the steel.

References

Marine propulsion and enginesOne slow two-stroke turning one propeller with no gearbox. Why that won, and why a speed warranty is a promise rather than a fact. How a ship is measuredTonnage is a volume, deadweight is a weight, and the canals keep a third set of books. Where each number on a ship comes from. Draft survey: weighing cargo by displacementA draft survey does not weigh the cargo. It weighs the ship twice and subtracts, and the errors in between become shortage claims. Time charter equivalentFreight per tonne means nothing next to hire per day until you divide it by the voyage. This is the arithmetic that does it.

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