Part B

International Loadline Convention 1966-69-2005

Under the International Convention on Load Lines, 1966 (as modified by the 1988 Protocol), the tabular freeboard is only the starting value. The final assigned summer freeboard is obtained by applying prescribed corrections and deductions that reflect hull form, depth, reserve buoyancy, strength, weathertight integrity, and the amount of enclosed superstructure. imorules

Q1. Corrections to tabulated freeboard

Starting point

Determine the ship’s type:
- Type A: usually tankers with high inherent weathertight integrity, small deck openings and good subdivision.
- Type B: all ships that do not meet Type A conditions—most dry-cargo ships, container ships, bulk carriers and general-cargo ships.
From the appropriate Type A or Type B table, obtain the tabular freeboard corresponding to the convention length $L$ .
Apply the relevant corrections in the specified order.

A typical calculation sequence for a Type B ship is:

[ \boxed{ F_{\text{assigned}}

F_{\text{tabular}} \pm C_{L<100} \pm C_{C_B} \pm C_D \pm C_{\text{deck line}}

D_{\text{superstructure/trunk}} \pm C_{\text{sheer}} } ]

Then verify:

F_{\text{assigned}} \geq F_{\text{minimum}}

and also check the separate minimum bow-height requirement.

Main corrections and their principles

Correction / deduction	Effect on freeboard	Underlying principle
Correction for ships under 100 m	Usually increases Type B freeboard when the ship has little enclosed superstructure	A short dry-cargo ship with limited enclosed volume has less reserve buoyancy and is more affected by waves; it needs greater protection than the basic table alone provides
Block-coefficient correction	Increase if $C_B > 0.68$ ; reduce if $C_B < 0.68$ , subject to limits	A fuller ship has less buoyancy recovery as it heels or trims, greater exposure to green seas, and generally needs more reserve buoyancy. A finer ship has more favourable form characteristics and may receive a reduction
Depth correction	Increase where actual depth exceeds the tabular/reference depth; may reduce where it is less, within limits	Greater moulded depth gives more hull volume above the loaded waterline. The Convention gives credit for this reserve buoyancy, but only to a controlled extent and subject to minimum-freeboard rules
Correction for deck-line position	Adjusts freeboard if the deck line is not at the normal intersection of freeboard deck and shell	Freeboard is measured from the upper edge of the deck line. If the deck line is shifted from its normal datum, the numerical freeboard must be corrected so that the actual load line still represents the intended reserve buoyancy
Superstructure and trunk deduction	Reduces freeboard	Efficient enclosed superstructures and qualifying trunks create protected enclosed buoyancy above the freeboard deck, improve protection of deck openings, and reduce the likelihood of downflooding
Sheer correction	Excess sheer reduces freeboard; deficiency increases it	Sheer provides extra height, reserve buoyancy and protection at the ends, especially forward where green-sea boarding is severe. Deficient sheer removes this protection
Minimum bow-height check	May require an increase in freeboard or bow height	Adequate bow freeboard is needed to reduce deck wetness, green-water loading, hatch-cover damage and progressive flooding risk
Minimum freeboard check	Prevents excessive reduction	Regardless of deductions, the final assigned freeboard cannot be less than the Convention minimum applicable to the ship

The Convention chapter on freeboards specifically includes corrections for ships under 100 m, block coefficient, depth, deck-line position, superstructures/trunks, sheer, minimum bow height and minimum freeboards. imorules

Block coefficient correction

The block coefficient is:

C_B=\frac{\nabla}{LBT}

where:

$\nabla$ = moulded displacement volume,
$L$ = Convention length,
$B$ = moulded breadth,
$T$ = relevant moulded draught.

For a Type B ship, the standard reference form is approximately:

C_B=0.68

The usual correction principle is:

C_{C_B} \propto (C_B-0.68)

Therefore:

$C_B>0.68$ : add freeboard.
$C_B<0.68$ : deduct freeboard, subject to the permitted maximum deduction.

A full-bodied bulk carrier, for example, normally attracts an increase because it is more vulnerable to deck immersion and may have less favourable reserve-buoyancy characteristics than a finer vessel.

Depth correction

The relevant depth is the freeboard depth, measured amidships from keel to the top of the freeboard deck beam at side, including the thickness of the freeboard deck at side where applicable. assets.publishing.service.gov

A deeper ship possesses more potential enclosed buoyancy above the assigned waterline. Thus, subject to Convention limits:

Extra depth can justify a freeboard deduction.
Inadequate depth requires an addition.

However, additional depth alone must not be used to claim unsafe reductions where the vessel lacks proper structural strength, effective enclosure, closing appliances, or adequate stability. The Convention expressly assumes adequate structural strength, stability and avoidance of excessive structural stress at the assigned draught. assets.publishing.service.gov

Superstructure deduction

An enclosed superstructure is valuable because it creates reserve buoyancy that remains available when the vessel is deeply laden or heeled.

The deduction depends principally on:

\frac{E}{L}

where:

$E$ = total effective length of enclosed superstructures and qualifying trunks,
$L$ = Convention length.

The more of the vessel’s length covered by effective enclosed superstructure, the greater the permitted deduction. If effective superstructure length equals the full ship length, the maximum applicable deduction is available; if it covers only part of the length, the deduction is proportionately smaller. laws-lois.justice.gc

Sheer correction

Sheer is treated separately because a raised deck line at the ends gives real safety benefit, especially in waves. The Convention correction is based on the difference between actual and standard sheer. A shortage of sheer generally increases freeboard; excess sheer may reduce it. assets.publishing.service.gov

Q2. Sheer: definition and calculation

Definition

Sheer is the longitudinal curvature of the freeboard deck measured at the ship’s side. It is the vertical distance between the actual deck line at side and a reference line drawn parallel to the keel through the sheer line amidships.

In simple terms:

The deck is lowest around midships.
It rises toward the bow and usually toward the stern.
The upward rise is called sheer.

The Convention measures sheer from the deck at side to a reference line parallel to the keel through the amidship sheer line. assets.publishing.service.gov

Simple sketch

                  Actual freeboard-deck sheer line

      Forward                                          Aft
         ↑                                               ↑
         |                                               |
         |                                               |
     ____/ \_________________________________________/ \____
         \                                               /
          \_____________________________________________/
                Reference line through midships
                   parallel to the keel

                         Midships

At midships:

\text{Sheer ordinate}=0

At the ends, the vertical ordinate is at its maximum.

Standard sheer

For a normal ship, the standard sheer ordinates are expressed as fractions of $L$ .

Position from nearest perpendicular	Standard sheer ordinate
At FP or AP	$\dfrac{L}{50}$
At $\dfrac{L}{6}$ from FP or AP	$\dfrac{L}{150}$
At $\dfrac{L}{3}$ from FP or AP	$\dfrac{L}{500}$
Amidships	$0$

Thus, standard end sheer is:

S_{\text{end}}=\frac{L}{50}

For example, if:

L=150\ \text{m}

then:

S_{\text{end}}=\frac{150}{50}=3.0\ \text{m}

So, at FP and AP, the standard freeboard-deck line is 3.0 m above the midship reference line.

Standard sheer sketch

Forward end                                              Midships

      Standard end ordinate = L/50
                    ↑
                    │
                    │\
                    │ \
                    │  \
Reference line  ____│___\_______________________________
                  FP  L/6  L/3             0 at midships

Ordinate at FP      = L/50
Ordinate at L/6     = L/150
Ordinate at L/3     = L/500
Ordinate amidships  = 0

The same method is used aft, but the aft standard sheer is generally smaller than forward standard sheer for conventional freeboard calculations. This reflects the greater exposure of the bow to head seas and green-water loads.

Method of calculating actual sheer

For a newly built ship, calculate sheer separately at the forward and aft halves.

Step 1: Establish the reference line

Draw a straight line parallel to the keel through the deck line at side amidships.

             Freeboard deck at side
                  /
                 /
________________/___________________  Reference line
             midships

Step 2: Measure actual ordinates

At each side, measure the vertical distance from the reference line to the freeboard deck at:

Forward perpendicular, FP.
$L/6$ abaft FP.
$L/3$ abaft FP.
Amidships.
$L/3$ forward of AP.
$L/6$ forward of AP.
After perpendicular, AP.

The midship ordinate is normally zero.

FORWARD HALF

FP            L/6            L/3              Midships
|--------------|--------------|------------------|
↑              ↑              ↑                  ↑
S1             S2             S3                 0

Actual ordinates measured vertically from the
midship reference line to the deck at side.

Step 3: Compare with standard ordinates

At every station, find:

[ \text{Difference}= \text{Actual sheer ordinate}

\text{Standard sheer ordinate} ]

Positive result = excess sheer.
Negative result = deficient sheer.

Step 4: Calculate the mean excess or deficiency

The ordinates are not simply averaged; they are combined by the Convention’s prescribed weighting method. The midship region has no sheer ordinate, and the end sections are given appropriate significance because the protection near the ends—especially forward—is most important.

For examination purposes, state:

The actual and standard ordinates at FP/AP, $L/6$ , and $L/3$ positions are compared. The excess or deficiency is integrated or weighted over the forward and aft halves in accordance with Regulation 38 to obtain the mean sheer excess or deficiency.

Step 5: Apply the correction

The Convention sheer correction is based on:

[ \text{Sheer correction}

(\text{mean deficiency or excess of sheer}) \times \text{applicable factor} ]

The factor depends upon the extent of effective enclosed superstructure. In the Convention text this is represented by a relationship involving the proportion of superstructure length; the correction reduces as the ship has more effective enclosed superstructure. assets.publishing.service.gov

Effect of superstructure on sheer credit

An enclosed superstructure itself provides reserve buoyancy and weather protection. Therefore, it may receive sheer credit if its actual height exceeds the standard superstructure height.

If a superstructure is higher than standard by $Z$ :

Add $Z$ to the end sheer ordinate.
Add $0.444Z$ at the $L/6$ station.
Add $0.111Z$ at the $L/3$ station.

These increments are applied at the relevant end. lk.rs-class

Extra superstructure height = Z

At end:       add Z
At L/6:       add 0.444 Z
At L/3:       add 0.111 Z
At midships:  add 0

How sheer improves safety and stability

Strictly speaking, sheer is primarily a reserve-buoyancy, seaworthiness and downflooding-protection feature rather than an initial-stability feature such as $GM$ .

It contributes to safety in the following ways:

Increases hull volume above the loaded waterline at the bow and stern.
Increases reserve buoyancy as the end immerses due to waves, trim or heel.
Improves longitudinal restoring characteristics in pitching and trimming.
Reduces frequency of bow immersion and deck wetness.
Helps protect hatch covers, vents, doors and deck machinery from green-water impacts.
Reduces the likelihood of water accumulating in exposed wells or entering deck openings.
Improves the residual buoyancy margin at the ends after limited flooding.

Low-sheer vessel                     Vessel with adequate sheer

      Green water                          Green water
         ↓                                      ↓
 ________/\________                     _________/\_________
        /  \                                   /    \
_______/____\_______                    ______/______\_______
 Low forward reserve                    Greater forward reserve
 buoyancy; more wet deck                buoyancy; better protection

The key examination statement is:

Sheer does not usually create a large change in initial transverse $GM$ , but it materially improves reserve buoyancy, the range of stability, resistance to deck immersion, longitudinal behaviour in waves, and protection against downflooding.

Limit on total deduction or addition

The assigned freeboard cannot be reduced without limit. The final value remains subject to:

Minimum-freeboard provisions.
Minimum bow-height requirements.
Adequate strength and intact stability.
Full compliance with the conditions of assignment: hatch closures, doors, ventilators, air pipes, freeing ports, guard rails, machinery-space protection, cargo ports and other opening protections.

The Convention allows a limited reduction from tabular freeboard when the vessel earns credit for favourable form, depth, enclosed superstructure, trunks and excess sheer; conversely, a vessel with unfavourable form, inadequate depth, deficient sheer or inadequate superstructure receives additions. The final assigned freeboard must never be below the prescribed minimum. imorules

Q3. Effective length of superstructure

Definition

The effective length of superstructure, denoted by $E$ , is the length of an enclosed superstructure that is accepted for freeboard-deduction purposes.

It may be less than the actual geometrical length $S$ , because the superstructure earns credit only to the extent that it provides meaningful enclosed buoyancy and weather protection.

                Actual length of enclosed superstructure = S

      ┌──────────────────────────────────────────┐
      │              Deckhouse / bridge          │
──────┴──────────────────────────────────────────┴──────
      <--------------- S ----------------------->

      <----------- E ----------->
      Effective length credited for freeboard

For an ordinary enclosed superstructure:

E=S

only where its height is at least the required standard superstructure height.

Standard superstructure height

The standard height is:

H_s=1.80+\frac{L}{300}

in metres, with Convention limits for long ships.

For example, for a 150 m ship:

H_s=1.80+\frac{150}{300}

H_s=1.80+0.50

\boxed{H_s=2.30\ \text{m}}

So an enclosed 80 m bridge/deckhouse with an effective height of at least 2.30 m can usually receive full length credit:

E=S=80\ \text{m}

Why effective length is used

The reason is that not all structures of equal plan length give equal reserve buoyancy.

A tall, fully enclosed, strongly built, weathertight superstructure:

Adds real enclosed buoyancy.
Remains effective when the vessel heels.
Delays downflooding.
Protects openings on the freeboard deck.
Improves survivability in waves and damaged conditions.

A low or poorly enclosed deckhouse may provide less useful buoyancy and may be immersed or downflooded sooner. It should therefore not receive the same freeboard deduction.

High enclosed superstructure             Low superstructure

      ┌───────────────┐                   ┌───────────────┐
      │               │                   │               │
      │    H ≥ Hs     │                   │    H < Hs     │
──────┴───────────────┴──────          ───┴───────────────┴───

Full buoyancy credit                    Partial buoyancy credit
E = S                                  E < S

Dependence on superstructure height

Yes. The effective length depends directly on actual height if that height is below standard.

For an enclosed superstructure other than a raised quarterdeck:

\boxed{ E=S\left(\frac{H}{H_s}\right) \qquad \text{if } H < H_s }

where:

$S$ = actual length,
$H$ = actual least height at side,
$H_s$ = standard superstructure height.

If:

H\geq H_s

then:

\boxed{E=S}

This proportional reduction is specifically reflected in load-line rules: if height is less than $H_s$ , effective length equals actual length multiplied by the ratio of actual height to standard height. laws-lois.justice.gc

Example: ordinary enclosed superstructure

Given:

L=150\,\text{m}

S=90\,\text{m}

H=1.80\,\text{m}

First calculate standard height:

H_s=1.80+\frac{150}{300}=2.30\,\text{m}

Because:

H<H_s

E=90\left(\frac{1.80}{2.30}\right)

E=70.43\,\text{m}

\boxed{E\approx70.4\,\text{m}}

Although the actual deckhouse is 90 m long, only 70.4 m is credited in calculating the freeboard deduction.

Raised quarterdeck

A raised quarterdeck is treated more conservatively because it often has reduced height and a different contribution to reserve buoyancy. Its credit is limited; in typical Convention-form rules, the effective length is restricted to no more than $0.6L$ , and height conditions are more restrictive than for an ordinary enclosed superstructure. laws-lois.justice.gc

Link to freeboard deduction

The total effective length enters the superstructure/trunk deduction through the ratio:

\frac{E}{L}

For example:

Small E/L → little protected buoyancy → small freeboard deduction
Large E/L → much protected buoyancy → larger freeboard deduction
E/L = 1 → maximum superstructure-related deduction

For a vessel where total effective enclosed-superstructure length equals the ship’s full Convention length, the maximum deduction is available. Where $E<L$ , the deduction is calculated from a reduced relation based on $E/L$ . laws-lois.justice.gc

Exam-ready summary

Tabular freeboard is selected from the Type A or Type B table using Convention length.
It is adjusted for: ship length below 100 m, block coefficient, depth, deck-line position, enclosed superstructures/trunks and sheer.
It is then checked against the minimum freeboard and minimum bow-height requirements.
Sheer is the rise of the freeboard deck at side from midships to the ends; it improves reserve buoyancy, reduces deck wetness and gives protection against green seas and downflooding.
Excess sheer earns a deduction; deficient sheer requires an addition.
Effective superstructure length is not necessarily the actual length: a low superstructure earns only proportionate credit because it supplies less protected buoyancy.
For a normal enclosed superstructure below standard height:

\boxed{E=S\frac{H}{H_s}}

The greater the effective length $E$ , the greater the permitted deduction from tabular freeboard—subject always to minimum safety limits.

IMO Fire Safety Regulations-Layers of Protection & SOLAS Chapter II-2 Moment of Inertia