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Code of Practice
11 Intact Stability
11.1 New Monohull Vessels
The standard of stability to be achieved by a new vessel will be dependant upon its length.
11.1.1 Vessels of 15 metres in length and over or carrying 15 or more persons
11.1.1.1 The centre of gravity (KG) of a vessel should be established by an inclining experiment and a curve of statistical stability (GZ curve) for the loaded departure with 100% consumables should be produced.
Notes:-
1. The above condition may include a margin for growth not
exceeding 5% of the lightweight with the VCG positioned at the upper deck amidships.
2. Buoyant structures intended to increase the range of positive stability should not be provided by fixtures to either a mast or rigging.
11.1.1.2 The GZ curve required by 11.1.1.1 should have a positive range of not less than the angle determined by the formula in the table in 11.1.2.6
11.1.1.3 In addition to the requirements of 11.1.1.2, the angle of steady heel obtained from the intersection of a "derived wind heeling lever" curve with the GZ curve referred to in 11.1.1.1 above should be greater than 15 degrees (see Figure 1).
In Figure 1
'DWHL'=the "derived wind heeling lever" at any angle ( degrees = 0.5 x WLO x Cos 1.3(where WLO=GZfCos1.3(f
FIGURE 1
(Figure Missing)
Noting that:-
WLO - is the magnitude of the actual wind heeling lever at 0 degrees which would cause the vessel to heel to the 'down flooding angle' ((f) or 60 degrees whichever is least.
GZf is the lever of the vessel's GZ at the 'down flooding angle' ((f) or 60 degrees whichever is least.
(d - is the angle at which the 'derived wind heeling' curve intersects the GZ curve. (If (d is less than 15 degrees the vessel will be considered as having insufficient stability for the purpose of the Code).
(f - the 'down flooding angle' is deemed to occur when openings having an aggregate area, in square metres, greater than:-
Vessel's displacement in tonnes,1500 are immersed,
Moreover it is the angle at which the lower edge of the actual opening which results in critical flooding becomes immersed. All openings regularly used for crew access and for ventilation should be considered when determining the downflooding angle. No opening regardless of size which may lead to progressive flooding should be immersed at an angle of heel of less than 40 degrees. Air pipes to tanks can, however, be disregarded.
If as a result of immersion of openings in a deckhouse a vessel cannot meet the required standard those deckhouse openings may be ignored and the openings in the weather deck used instead to determine (f. In such cases the GZ curve should be derived without the benefit of the buoyancy of the deckhouse.
It might be noted that provided the vessel complies with the requirements of 11.1.1.1, 11.1.1.2 and 11.1.1.3 and it is sailed with an angle of heel which is no greater than the 'derived angel of heel', it should be capable of withstanding a wind gust equal to 1.4 times the actual wind velocity (i.e. twice the actual wind pressure) without immersing the 'down flooding openings', or heeling to an angle greater than 60 degrees.
11.1.1.4 A 'Stability Information' booklet, based on the Department of Transport's model booklet, should be submitted to and approved by the Department and placed on board the vessel. The booklet should include details of the maximum steady angle of heel for the worst sailing condition. The steady angle of heel is to be calculated in accordance with 11.1.3. The prevention of down flooding in the event of squall conditions. Details of the development of such curves are given in the Model Stability Information Booklet for Sail Training Ships.
Vessels of less than 15 metres in length and carrying 14 or less persons
11.1.2.1 General
The stability of a vessel should be determined by the methods discussed below and its area of operation should be dependent upon the standard which it is shown to achieve.
11.1.2.2 Vessels without external ballast keels
.1 Stability assessment
The centre of gravity (KG) of a vessel should be established by an inclining experiment and, in addition, a curve of statical stability (GZ curve) for the loaded departure, 100% consumables should be produced.
Notes:- 1. The above condition may include a margin for growth not
exceeding 5% of the lightweight with the VCG positioned
at the upper deck amidships.
2. Buoyant structures intended to increase the range of positive stability should not be provided by fixtures to either a mast or rigging.
.2 Permitted are of operation
The permitted are of operation is dependant upon a vessel's range of stability as indicated in the table in 11.1.2.6.
11.1.2.3 Vessels fitted with external ballast keels
.1 The stability assessment of a vessel may be made by any one of the following methods:-
Method 1 - as for vessels without external ballast keels, see 11.1.2.2.1 above;
Method 2 - by the formula shown in 11.1.2.4;
Method 3 - by the 'STOPS' Numeral developed by the Royal Yachting Association (RYA) and discussed in 11.1.2.5.
Notes:- Method 1 should be used for a vessel fitted with more than one of the following:-
1. roller furling headsail;
2. in-mast or behind-mast roller furling mansail;
3. a radar mounted higher than 30% of the length of the vessel above the waterline.
.2 Permitted area of operation
The permitted area of operation is dependant upon a vessel's range of stability or its STOPS Numeral as indicated in the table in 11.1.2.6.
11.1.2.4 Formulae for estimating range of stability
The range of positive stability for a vessel fitted with an external ballast keel may be estimated from the following formulae:-
Estimated range = 110 + 400 degrees
(SV - 10.0)
SV = Beam² BR X DCB X (DISPLACED VOL) 1/3
Noting that:-
Beam = greatest beam measured, excluding rubbing strips, in metres.
Ballast Ration (BR) = weight of ballast in tonnes contained in the keel divided by the full displacement in tonnes.
Displaced Volume = the volume of a vessel's displacement, in m3, at the operational draught.
Draught of canoe body (DCB) in metres is taken by measuring the maximum draught at the 1/8 of the full beam from the centreline in way of the transverse section at greatest beam as follows:-
FIGURE 2
Figure Missing
Once the estimated range of stability has been determined it is necessary to study the table in 12.1.2.6 to ascertain the area of operation which the range permits.
1.1.2.5 Assessment using RYA 'STOPS' numeral or use of SSS numeral calculated by the Royal Ocean Racing Club
.1 A vessel can have its area of operation based upon the RYA 'STOPS' Numeral'.
Information on the derivation of the STOPS numeral may be obtained from the Certifying Authority.
Once the STOPS Numeral has been determined it is necessary to study the table in 11.1.2.6 to ascertain the permitted area of operation.
.2 A SSS numeral calculated by the RORC will be accepted in place of STOPS numeral, provided that it includes a self righting factor based on an inclining experiment and shown on a valid IOR or IMS rating certificate.
11.1.2.6 Table showing permitted areas of operation and STOPS numeral for a vessel of less than 15 metres in length
| Permitted area of operation | Code Category | Minimum required standards |
| Range of Stability (degrees) | STOPS numeral |
| Unrestricted | 0 | 90 = 60 x ((24 - LOA)/17) | 50 |
| Up to 150 miles from a safe haven | 1 | 90 = 60 x ( (24 - LOA)/17) | 40 |
| Up to 60 miles from a safe haven | 2 | 90 = 60 x ( (24 - LOA)/17) | 30 |
| Up to 20 miles from a safe haven | 3 & 4 | | 20 |
Stability information will not be required in booklet form. The owner/managing agent should, however, present documentary evidence to show that the required range of stability of STOPS Numeral is in accordance with the table in 11.1.2.6 for the intended and permitted area of operation.
11.1.2.8 Guidance on stability assessment
It should be noted that the Certifying Authority may require a full stability analysis for a vessel which has been modified from the original design, particularly if the freeboard has been significantly reduced or the modification has involved the addition of a mast-furled main sail, a roller-reefing headsail, a radar antenna or any other item of equipment which may have caused the position of the vertical centre of gravity to be situated at a higher level than that intended by the designer.
A flow diagram showing the procedure for assessing stability is shown in Figure 3.
11.2 Existing Monohull Vessels
11.2.1 When stability information has been approved by the Department under existing criteria, published in October 1987, this will continue to be acceptable subject to the following:-
.1 a vessel does not undergo a major conversion; or
.2 the owner/managing agent elects to re-submit a vessel for stability approval based on the new criteria.
11.2.2 An existing vessel which does not comply with 11.2.1 should comply with 11.1.
11.3 Multihull Vessels - New and Existing
11.3.1 A multihull vessel should be provided with a 'Stability Information' booklet, giving details of the maximum advised mean apparent windspeeds for each expected combination of sails that may be set, for each of two displacement conditions.
The displacement conditions used in the stability booklet should comprise the maximum displacement condition with full stores, fluids and spares, and the minimum displacement condition with 10% fluids and no stores or spares. The hull and outfit weight used for calculating these conditions should be based on a weighing of the actual completed vessel. Spars, standing and running rigging may be weighed separately.
The 'Stability Information' booklet should be based on the Department of Transport's Model Stability Information Booklet for Mutlihull Sailing Vessels published by HMSO. The publication includes notes for consultants which explain methods and assumptions for calculation of the maximum hull righting moment and Maximum Advised Mean Apparent Windspeed (MAMAW). The value of MAMAW for a vessel determines the permitted area of operation, which is given in 11.3.5 below.
11.3.2 For each combination of sailplan and displacement condition (maximum and minimum), the windspeed (in knots) should be calculated at the point when the maximum wind heeling moment equals the maximum hull righting moment. The maximum advised mean apparent windspeed (MAMAW) = 2/3 calculated windspeed.
11.3.3 The wind heeling force developed on the sails and hull should be taken as:-
Force (Newtons) = 0.20 x A x (square of windspeed in knots)
(Force (kg) - 0.02 x A x (square of windspeed in knots))
where: A =lateral profile area of sails, masts and above-water hull (square metres)
The effective lever of the wind heeling force should be taken as the vertical separation of the geometric centres of area of the above-water and below-water profiles of the vessel, including sails.
11.3.4 The maximum hull righting moment for each combination of sailplan and displacement condition may be calculated by either of the following two methods:-
.1 Conventional method
Determination of righting moments by traditional naval architecture methods.
A full righting moment analysis should be used for a multihull of unusual form and for a trimaran with floats when each float is incapable of easily supporting the displacement of the vessel.
.2 Simplified method
This method may be used for:-
(a) a catamaran of normal form; and
(b) a trimaran with floats when each float is capable of easily supporting the displacement of the vessel.
The approximate maximum hull righting moment (kg.metres) is given by:-
Displacement (kg) x (b - (KG x sin(HM))(
where:- b = the spacing of the centreline of the float to the centreline of the vessel.
KG = estimated vertical centre of gravity of the vessel, with spars and sails (hoisted), above the bottom of the canoe body, conservatively taken as 75% of the depth from the bottom of the (main hull) canoe body to the top of the main coach roof.
Hm = estimated angle of heel of maximum righting moment.
It might be noted that provide the vessel complies with the above requirements and is sailed in conditions where the maximum advised mean apparent windspeed (MAMAW) is not exceeded for the actual combination of sailplan and displacement condition, it should be able to withstand a wind gust of 1.5 times the actual windspeed without capsizing.
11.3.5 The permitted area of operation for a vessel should be determined by reference to the minimum acceptable value for maximum advised mean apparent windspeed calculated for the largest working sailplan of the vessel in the minimum displacement conditions, as given in the following table. The working sailplan comprises sails that may be set when proceeding with the true wind less than 60 degrees off the bow, and includes any sail of a weight that is capable of withstanding winds of more than 10 knots. The working sailplan should be detailed in the 'Stability Information' booklet.
| Permitted are Of operation | Code Category | Minimum acceptable value for Maximum Advised Mean Apparent Windspeed (MAMAW) (knots) for minimum displacement condition |
| Unrestricted | 0 | 18 |
| Up to 150 miles From a safe haven | 1 | 16 |
| Up to 60 miles From a safe haven | 2 | 14 |
| Up to 20 miles From a safe haven | 3 & 4 | 10 & 12 |
11.3.6 A 'Stability Information' booklet, based on the Department of Transport's model booklet, should be submitted to and approved by the Department and placed on board the vessel.
11.3.7 The 'Stability Data' page from the 'Stability Information' booklet should be copied and mounted in a suitable position for the ready reference of the crew when at sea.
