Neptune Atlas
Hull form and resistance
How much fuel a ship burns depends less on its engine than on the shape of the hull it pushes through the water. The water's resistance to the hull has two parts, and they behave very differently as speed rises. Which part is larger decides whether a ship is worth slowing down, whether a bulbous bow helps it, and how much a dirty hull costs.
Two resistances, one of them predictable
Frictional resistance is the drag of the water rubbing along the hull. It grows with the area of hull under water and roughly with the square of the speed, and it is the larger part at the speeds most cargo ships sail. It is also the part that gets worse over time, because it depends on how rough the hull surface is.
Residuary resistance is mostly the cost of making waves. The ship creates a wave at the bow and another at the stern, and it must supply the energy in them. This resistance is small at low speed and then rises very steeply, because beyond a certain speed the ship is trying to climb its own bow wave.
The point where one type of resistance becomes larger than the other depends not on speed alone but on speed compared with length. Naval architects use the Froude number for this: the speed divided by the square root of the length times the acceleration due to gravity. In practice it means that a longer ship can go faster before wave making takes over. That is why fast ships are long and slim, and why a short ship sailed fast burns a very large amount of fuel. Most of that fuel goes into making waves.
Block coefficient, and why bulk carriers look like boxes
The block coefficient is the volume of the hull under water divided by the volume of the box that would just contain it (length times breadth times draught). A very full hull comes close to 0.85, while a slim, fast hull is far lower. A full hull is not a sign of poor design. It is the answer to a limit.
Berths, locks and channels limit a ship's length and breadth, and the load line and the port limit its draught. A ship that cannot be longer, wider or deeper can only carry more cargo by filling out the box it is allowed. So a Capesize is close to a rectangular block with shaped ends. It carries a very large deadweight for its size, and it is slow, because a full hull at speed spends nearly all its power making waves. A container ship has a slim hull. It carries less deadweight, but it is faster. Both designs are the best answer to the problem each one is solving.
| Hull | Fullness | What it gives | What it costs |
|---|---|---|---|
| Full and slow | High block coefficient | The most deadweight within a fixed length and beam | Wave making rises early, so driving it fast is not economic |
| Fine and fast | Low block coefficient | High service speed for a reasonable amount of power | Less cargo for the same main dimensions |
The bulbous bow works at one design point
A bulbous bow, the rounded bulge below the waterline at the front, works by making its own waves that are out of step with the bow wave. The two partly cancel each other, and the resistance from wave making falls. That cancelling effect is designed for one particular speed and one particular draught.
Away from that design point, the bulb stops helping. A ship that sails slowly, or spends much of its life empty at a draught the bulb was not shaped for, carries extra hull area under water for no benefit. When the industry slowed its ships down after 2008, this became a real issue: owners reshaped and replaced the bows of existing ships. Owners only did this because the original bulb added resistance at the new, lower speed.
Fouling, and why a performance claim starts with a drydocking date
Frictional resistance depends on how rough the hull is. In warm water, a hull first grows a thin layer of slime, and then weed and shells. This roughness is not only a matter of appearance. From the day the ship leaves drydock, it steadily increases the power needed to keep the same speed.
That is why, in a dispute about a ship's performance, people ask for the dates of the last drydocking and the last hull cleaning before they discuss the engine at all. It is also why a time charter includes terms on hull cleaning and propeller polishing. And it is why the question of who pays when a ship waits for weeks in a warm port is a negotiated sharing of a real cost, not a formality. A fouled propeller does more harm per square metre than a fouled hull, which is why polishing is treated separately.
Trim, squat and shallow water
Three effects change resistance even though nothing has been done to the ship.
Trim. The same weight carried a little differently, with the ship sitting slightly more bow-down or stern-down, changes the shape under water and therefore the resistance. Adjusting trim is one of the few operational measures that costs nothing and can be measured, which is why it appears in almost every efficiency programme.
Squat. In shallow water, the water flows faster under the hull, the pressure drops, and the whole ship sinks lower and changes its trim. At speed the ship sits deeper than its draught at rest. This limits where it can safely navigate, not only its resistance, and it is one reason a draft survey is done with the ship stopped and properly moored.
Shallow water resistance. Separately from squat, limited depth changes the pattern of waves and raises resistance sharply. A ship that reaches its normal speed at sea will not reach it in a shallow channel, and no contract clause can change that.
How the numbers on a description were produced
In model tests, a small model of the hull is towed through a test tank and its resistance is measured. The result is scaled up to the full-size ship by treating the frictional part, which scales with the Reynolds number, separately from the residuary part, which scales with the Froude number. The ITTC publishes the procedures the industry uses for this, including the 1978 performance prediction method. When the finished ship is delivered, its speed and power trials are carried out and corrected under ITTC procedures and ISO 15016. A trial in real weather must be corrected for wind, waves, current and water depth before it can be compared with anything else.
This leads to the main point. The speed in a ship's description is either a corrected trial result or a guaranteed figure, given at a stated draught, in stated weather, with a clean hull. It is a carefully defined reference condition, and the ship almost never sails in exactly those conditions. Treat the number as the starting point of a calculation, not as a fixed property of the ship.
References
- ITTC, Preparation, Conduct and Analysis of Speed/Power Trials, procedure 7.5-04-01-01.1, 2021. ittc.info
- ITTC, 1978 ITTC Performance Prediction Method, procedure 7.5-02-03-01.4, 2017. ittc.info
- ISO, ISO 15016:2015, Ships and marine technology: guidelines for the assessment of speed and power performance by analysis of speed trial data, 2015. iso.org
- IMO, Improving the energy efficiency of ships. imo.org
- IMO, International Convention on Load Lines, 1966. imo.org
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