Stability

Of all the characteristics of ship or boat design, or for that matter any floating body, intact stability is the least understood by owners and even builders and promoters.

The standard stability criteria for all the worlds’ commercial shipping are a set of Statuary Rules published by the International Maritime Organisation (IMO).

These statuary regulations cover all types of ships from 6 metre (20ft) water taxi to 500,000 DWT bulk carriers and 100,000 DWT cruise liners. All these ships are commercial vessels.

In most countries pleasure craft are exempt from any stability rules, or for that matter any sea keeping standards.

For this reason it is unusual for pleasure craft owners to be provided with comprehensive stability information.
The little information that is sometimes supplied unfortunately needs to be treated with a degree of skepticism.

The development of a stability standard for shipping has a long history. It has its origin in Samuel Plimsolls Merchant Shipping Act of 1876. This statute was aimed at preventing the overloading of merchant ships. It required that every ship had to have a certain minimum reserve of buoyancy, freeboard and minimum bow and stern heights. The significance of a bow height requirement is interesting as it was determined not only needed for reserve bouyancy, but also recognized as essential in limiting the amount of "green" destabilizing water taken onboard ship.

Since that time further stability rules have been developed. Finally in 1966, IMO, after an extensive study of a large body of casualty reports, settled on a recommendation for oceangoing ships of up to 100 metres (333ft).

This is now the standard criteria for the statutory regulations.

The criteria are based on the work done to heel the vessel to three specified angles of heel, a minimum GZ at greater than 30 degree heel and a minimum metacentre height (GM).

The actual regulation is worded as follows:

(a) The area under the righting level curve should not be less than 0.055 m-radians up to φ=30deg, where φ is the heeling or inclining angle (degrees).

(b) The area under the righting lever curve should be not less than 0.09 m-radians up to φ =40 deg or up to an angle where the non-weather tight openings come under water (whichever is less).

(c) The area under the righting lever curve should be not less than 0.03 m-radians between the angles of heel φ-30 to φ-40 deg or such lesser angle mentioned under Standard (b).

(d) The righting lever should be at least 0.2 m at an angle of heel equal to or greater than 30deg.

(e) The maximum righting lever should occur at an angle of heel exceeding 30deg.

(f) The initial metacentric height GM should be not less than 0.15 m (0.85 for fishing ships.)

Additionally, for passenger ships: Angle of heel <10 deg due to movement of passengers. Angle of heel in turning <10 deg with moment, M = 0.02 V2/L ? (KG – T/2), Where V is service speed and T is draft.

The work done to heel the ship is the area under the GZ curve up to the specified angle of heel, the units being Meter-Radians.

A complete data sheet is shown in the following example for our Watson 48 in the 50% arrival condition.

Arrival Condition

All vessels save river craft should be provided with a properly documented Stability Data book. For new built vessels, particularly those intended for ocean crossing, the owners should demand that a data book be included with all the other ship papers.

The data book should show, among other things, all the hydrostatic properties of the vessel, its tank capacity tables, at least four sailing conditions and any critical condition.

The most important section of the data book is the Inclining Experiment report. This experiment is conducted afloat to establish the location of the vessels vertical and longitudinal centre of gravity. This point is impossible to calculate arithmetically with sufficient accuracy. With commercial vessels it is customary to have pendulum deflections witnessed by a third party; often a Classification Society surveyor, which demonstrates the importance of the experiment.

The accompanying facsimile is from our Watson 48 data book. Although perhaps not self explanatory to a layman the Section 8 Trim & Stability condition sheet shows a summary of the statuary IMO criteria previously mentioned. These are shown in the box below the GZ curve.

heel graph

IMO Stability Criteria

Criteria

Actual

Minimum

1. Area under curve 0-30°

0.0834 M-Rad

0.055 M-Rad

2. Area under curve 0-40°

0.131 M-Rad

0.090 M-Rad

3. Area under curve 30°-40°

0.048 M-Rad

0.030 M-Rad

4. GZ at 30° heel

0.270 M

0.20 M

5. Maximum GZ occurs

34°

>25°

6. Range of stability (downflood)

62.45°

-

7. Initial GM

0.689 M

0.150 M

 

Displacement, Trim And Drafts

 

Statical Stability

Displacement

40.340 Tonnes

KMT

3.336 M

VCB

1.392 M

KG

2.625 M

LCB

6.505 M

GMs

0.717 M

LCF

5.928 M

FSC

0.022 M

MCT

11.283 T-M/Degree

GMf

0.689 M

TPC

0.523 T/cm

KGf

2.647 M

 

Draft LCF

1.873 M

Trim Angle

0.057° By Head

Draft AP

1.867 M

Trim BP

0.014 M

Draft Aft Marks

1.867 M

Draft Fwd Marks

1.474 M

SELF RIGHTING ABILITY

Self righting ability is not a requirement of the IMO stability rules. The reason for this is the practical impossibility of closing off the numerous openings in the superstructure such as the engine room air intakes.

Also to be considered are the windows and skylights. Glass or "composites" are not a building material; certainly this is the case as far as Classification Societies are concerned. Wood, FRP (in certain forms), steel and aluminum are "building Materials"; glass is always considered an "opening". In the event of immersion of the superstructure it is impractical to have a glass (or composites) thickness that could withstand the pressures it would be subjected to. These require watertight metallic covers to be considered in anyway safe. In the case of the Watson 48 GZ curve shown above, will be noted the "flood" Point at 62.45 degrees of heel. This is the point at which the deckhouse windows become immersed, all other openings below this angle including skylights, vents and doors either are "watertight" or have watertight closures.

The only exceptions to the self-righting rule are certain coastal lifeboats which have special buoyancy arrangements to ensure self righting ability. Prototypes of these boats are always proof tested after construction. It is difficult to picture proof testing a 50ft passage maker let along a 10,000 DWT freighter!

Instead the IMO rules are framed to give the hull the hydrostatic power to resist very large angles of heel. The statistics alone show that the IMO criteria are more than adequate to resist capsizing.

Despite this we are often asked if our vessels are self righting. The test for this is: “if all openings in the hull and superstructure are considered watertight will the vessel be hydrostatically unstable in the inverted condition at 180 degree angle of heel.” The Watson range meets this test but due to the reasons mentioned we do not claim self righting ability for our vessels.
 

Last Updated (Wednesday, 26 March 2014 12:29)