Trim by the Head vs Trim by the Stern: Ship Trim Explained

Ship trim is the difference between a vessel’s forward and aft drafts. A ship is trimmed by the head when the forward draft is greater, and trimmed by the stern when the aft draft is greater.

This guide is written for deck officers and cadets who plan loading and ballast conditions. It covers how trim is produced and calculated, and why a small stern trim is the normal seagoing condition. It also explains why trim by the head is avoided, and the few cases where a slight head trim is accepted.

What Is the Trim of a Ship?

Trim is the aft draft minus the forward draft, read at the perpendiculars and expressed in metres or centimetres. It describes the ship’s longitudinal inclination, the fore-and-aft counterpart of list. As a verb, to trim a ship means adjusting cargo or ballast to reach a planned trim.

Officers read the forward, midships and aft draft marks before departure, or take them from remote draft gauges. British usage spells the word “draught”; the meaning is identical. Trim is a key figure in every loading and ballast plan, because it governs propeller immersion, steering and fuel consumption.

Trim by the Head

A ship is trimmed by the head (also “down by the head” or “trimmed by the bow”) when the forward draft exceeds the aft draft. The bow sits deeper and the stern rises.

Trim by the Stern

A ship is trimmed by the stern when the aft draft exceeds the forward draft. A moderate stern trim is the normal seagoing condition for most merchant ships.

Even Keel

A ship is on an even keel when the forward and aft drafts are equal and trim is zero. Even keel gives the smallest maximum draft for a given displacement.

For that reason, ships are often brought to even keel to cross a draft-restricted bar, channel or berth. Ballast is transferred from the deeper end to the shallower end until the two drafts match. Draft surveys are also more accurate near even keel, because the trim corrections are smaller.

What Causes a Ship to Trim?

A ship trims when its longitudinal centre of gravity (LCG) and longitudinal centre of buoyancy (LCB) are not on the same vertical line. Weight and buoyancy then form a trimming moment that rotates the hull fore and aft.

The ship rotates about the longitudinal centre of flotation (LCF), the centroid of the waterplane. As it rotates, the underwater shape changes and the LCB shifts. Rotation stops when the new LCB lies vertically below the LCG.

• LCG forward of LCB: the bow goes down and the ship trims by the head.
• LCG aft of LCB: the stern goes down and the ship trims by the stern.
• LCG directly above LCB: the ship floats on an even keel.

In practice, the LCG moves whenever cargo, ballast, fuel or stores are loaded, discharged or shifted. Fuel burned from aft bunker tanks during a passage gradually moves the LCG forward. The relationship between these points is covered in more detail in LCB and LCF and their relation to ship stability.

How Is Trim Different From List and Heel?

Trim is the fore-and-aft inclination of a ship. List and heel are both transverse inclinations, to port or starboard. List is caused by weight inside the ship, while heel is caused by external forces.

• Trim: longitudinal, measured as a draft difference in metres between bow and stern.
• List: transverse and persistent, caused by an off-centre centre of gravity, such as uneven cargo or ballast. Measured in degrees.
• Heel: transverse and temporary, caused by wind pressure, waves or the forces of a turn. It disappears when the force is removed.

How Do You Calculate Trim?

Trim equals the aft draft minus the forward draft. A change of trim, in centimetres, equals the trimming moment divided by the moment to change trim one centimetre (MCTC), taken from the ship’s hydrostatic tables.

• Trim = aft draft − forward draft
• Trimming moment (t·m) = weight loaded or shifted (t) × longitudinal distance from the LCF (m)
• Change of trim (cm) = trimming moment ÷ MCTC
• Change of draft aft = change of trim × (distance from aft perpendicular to LCF ÷ LBP)
• Change of draft forward = change of trim − change of draft aft

Worked example: a ship of 150 m LBP floats at 6.80 m forward and 7.20 m aft, a trim of 0.40 m by the stern. MCTC is 200 t·m/cm and the LCF is 72 m forward of the aft perpendicular.

The crew transfers 200 t of ballast 120 m aft, from the forepeak to the aft peak. The trimming moment is 24,000 t·m, giving a change of trim of 120 cm by the stern.

The aft draft increases by 120 × 72 ÷ 150 = 57.6 cm, and the forward draft decreases by the remaining 62.4 cm. The final drafts are about 6.18 m forward and 7.78 m aft, a trim of 1.60 m by the stern.

Most stability booklets also contain trim tables, which list the draft changes for loading 100 t at set positions along the hull. On board, the loading computer runs the same calculation for every stage of a cargo plan. Each condition must also meet the IS Code stability criteria.

Is Trim by the Stern Positive or Negative?

There is no single sign convention. Most textbooks and many loading computers show trim by the stern as positive, but some software displays it as negative. Always check the sign legend in the stability booklet or loading program before entering or reading trim values.

Why Is a Small Trim by the Stern Preferred?

A small stern trim keeps the propeller and rudder deeply immersed, improves course-keeping and lets tanks drain towards their aft suctions. Most ships also behave better in a seaway in this condition.

• Propulsion: deeper propeller immersion reduces racing and air drawing in a seaway.
• Steering: a fully immersed rudder keeps its effectiveness, and the ship holds a straighter course.
• Draining: cargo, ballast and fuel tanks drain towards suctions sited at their aft end.
• Seakeeping: the bow has more freeboard and ships less water forward.

Trim is set by loading the ship and by shifting water between the forepeak, aft peak and double-bottom ballast tanks. On tankers, a stern trim is kept during discharge so that cargo tanks can be stripped completely.

Why Is Trim by the Head Avoided?

Trim by the head lifts the stern, which reduces propeller and rudder immersion and lowers the bow into the sea. The ship becomes harder to steer, more likely to ship green water forward, and slower for the same power.

Propeller Immersion and Racing

With the stern lifted, the propeller may not be fully immersed, especially in ballast. A partly immersed propeller draws air and loses thrust, so the ship makes less speed for the same fuel.

When a light ship trimmed by the head pitches, the propeller comes out of the water intermittently and races. The sudden load changes and vibration can damage the propeller shaft, thrust block and shaft bearings, and the main engine governor has to cut fuel to prevent overspeed.

Steering and Course Stability

Trim by the head reduces directional (course) stability. More of the hull’s lateral resistance sits forward, so the ship turns readily but is hard to steady and tends to yaw or sheer.

This means a head-trimmed ship may actually show a tighter turning circle, yet its steering performance is poorer. The helmsman needs more and larger rudder corrections to hold a course, and checking a swing takes longer. A trim by the stern has the opposite effect: better course-keeping, with a larger turning circle.

In ballast, a head trim can also leave the upper part of the rudder clear of the water, cutting its effectiveness further. In following or quartering seas, poor steering control raises the risk of broaching-to, where the ship is slewed broadside to the waves and cannot be brought back on course.

Green Water and Bow Damage

A head trim reduces freeboard at the bow. In heavy weather the ship takes green seas over the forecastle, which can damage mooring fittings, ventilators, air pipes and forward hatch covers.

Tank Draining and Loss of Suction

Tank suctions are normally fitted at the aft end of each tank. With a head trim, liquid runs forward, away from the suction, so ballast and cargo tanks cannot be emptied completely.

The same applies to fuel and lubricating oil tanks at low levels. In heavy weather, a tank can lose suction at a critical moment and interrupt supply to the main engine or generators, with a blackout as a possible result.

Squat in Shallow Water

Full-form ships with a block coefficient above about 0.7, such as tankers and bulk carriers, tend to squat by the head in shallow water. A ship already trimmed by the head loses even more under-keel clearance forward, increasing the risk of grounding at speed.

When Is Trim by the Stern Excessive?

Stern trim becomes excessive when the forward draft is too small for the sea state, or the bow blocks the bridge view. The ship then slams, is pushed off course by wind, and loses forward visibility.

• Windage: a raised bow presents a large wind area forward, while the deep stern resists turning. The ship becomes hard to manoeuvre at low speed.
• Slamming and pounding: with a shallow forward draft, the bow bottom comes out of the water when pitching and slams back in. Repeated slamming can damage forward bottom plating and framing.
• Bridge visibility: the raised bow creates a large blind sector ahead, especially on ships with the bridge aft. Lookout and pilotage both become harder.

SOLAS Chapter V Regulation 22 limits the blind sector ahead of the bow. Under all conditions of draft, trim and deck cargo, it must not exceed two ship lengths or 500 m, whichever is less.

For oil tankers in segregated ballast, MARPOL Annex I Regulation 18 sets a minimum ballast condition. The midships draft must be at least 2.0 + 0.02L metres, trim by the stern must not exceed 0.015L, and the propeller must be fully immersed.

How Does Trim Affect Fuel Consumption?

Trim changes the hull’s underwater shape, which changes its resistance through the water. For every draft and speed there is one trim with the lowest resistance, and sailing at that trim reduces fuel consumption and emissions.

Trim affects both frictional resistance, through the wetted surface area, and wave-making resistance. The second effect is usually larger, because trim changes how deeply the bulbous bow is immersed. A bulb that is too high or too deep no longer cancels the bow wave efficiently.

Most ships are designed for a specific draft, speed and trim. Away from that design point, the optimum trim varies, and published trim-optimisation studies commonly report fuel savings of 2% to 4%. Results depend on ship type, size and operating profile.

How Is Optimum Trim Found?

Optimum trim is established through model tank testing, operational data or Computational Fluid Dynamics (CFD). CFD is now widely used to produce trim tables or software that recommend the best trim for each draft and speed.

The IMO’s 2022 SEEMP guidelines, Resolution MEPC.346(78), list optimum trim among recognised energy efficiency measures. A ship’s Ship Energy Efficiency Management Plan (SEEMP) is mandatory under MARPOL Annex VI and supports its CII rating.

What Is the Difference Between Static and Dynamic Trim?

Static trim is the trim of the ship at rest, read from the draft marks before departure. It is set through the loading plan and ballast distribution.

Dynamic (running) trim is the trim while under way. It differs from static trim because the pressure field around a moving hull changes sinkage at the bow and stern. Trim optimisation systems measure it in real time with draft sensors and inclinometers.

Is Trim by the Head Ever Acceptable?

Yes, within limits. A slight trim by the head is sometimes accepted in calm water, at certain drafts and speeds, when trim-optimisation data shows lower resistance. It is never planned for heavy weather or light ballast passages.

Some container ships and other fine-form ships with bulbous bows show their lowest resistance near even keel or slightly by the head at part-load drafts. A head trim may also appear during intermediate loading stages, as long as the loading computer confirms stress and stability limits are met.

Why Do Ships Enter Drydock Trimmed by the Stern?

Ships enter drydock with a small trim by the stern so that the stern lands first on the aft keel blocks. The ship then settles progressively along the block line as the dock is pumped out.

A head trim would land the forefoot first, concentrating load on lightly built bow structure and risking an uncontrolled settle. The trim is kept small, because the upthrust at the aft blocks reduces effective stability until the ship sits fully on the blocks. The dock master’s docking plan sets the permitted trim and requires the ship to be upright.

Does Trim Mean the Same Thing on a Small Boat?

The principle is the same: trim is the boat’s bow-up or bow-down attitude. On planing powerboats, though, “trim” usually means adjusting the running angle while under way.

Skippers do this by changing the outboard or sterndrive angle, or by using trim tabs on the transom. Bow-down trim helps the boat get on the plane and handle a chop, while bow-up trim reduces wetted area for speed in flat water.

Frequently Asked Questions