Tall Ships Stability

Uwe Delfs Jespersen Master Mariner

Copenhagen 2015

Content

Introduction ...... 1 Statical stability ...... 2 Dynamic stability ...... 7 Conflicts between mathematical models and experience ...... 8 A different approach ...... 9 Traditional telltales ...... 16 Additional threats ...... 16 National differences ...... 18 Inclining Experiment ...... 18 Stability in port ...... 20 Overall Guidance for OOWs regarding the Stability of their Vessel ...... 21 Reactions and Comments ...... 23

The author wishes to thank Jesper Amholt Kramp, Master Mariner, for invaluable guidance and assistance, in fact, for leading in the right direction in the first place, and no less Capt. John Etheridge for his indispensable contributions as well as his help, advice and critical review.

Introduction

This pamphlet is written principally to address the Stability syllabus of the International Sail Endorsement Scheme launched by Sail Training International and the Nautical Institute and is not a comprehensive study of sailing ship stability. The aims are to:

• Recap on some relevant theory, • Illustrate the limitations of mathematical models, • Offer some practical advice to compensate for this at sea and in port, including refit.

Discussing ships’ stability is studying the war between righting forces (the good ones) and heeling forces (the bad ones). It is also by common agreement a matter of transverse stability only. Longitudinal aspects are referred to as trim and have hardly any comparable bearing on the safety of the ship.

Make sure the righting forces are stronger than the heeling forces at all times and you’ll be ok.

It sounds easy, but sometimes it isn’t. First of all, you do not want your righting forces to be greater than strictly necessary. An exceedingly stable vessel is a very stiff one, which means that it is loath to react, but not immune, to the sudden impact of outer forces, such as waves and gusts. This makes for very jerky movements, extremely uncomfortable for the people, and very strenuous for everything on board - the cargo, the rig - in other words, potentially dangerous conditions may arise. The obvious question would be, how much stability is necessary? We will see that there is a lot of agreement in principle, but not at all in detail.

Stability can be affected in many ways. Some very obvious ones can not be considered here, such as changing the shape of the hull (a wider beam, for instance) or outriggers; others, like adding or moving ballast or using (counter-) balancing tanks may well be worth considering.

In general terms the officers of a ship have to accept the stability characteristics of their ship as a given set of facts. Regrettably this often means that the topic does not quite receive the attention it rightfully deserves, for a number of reasons. Particularly older (sailing) ships are (by some) regarded as being on the safe side forever. After all, they have been around successfully for so long, haven’t they. Also, all discussion of stability tends to become very complex very rapidly. Different flag states have different requirements, focusing on varying aspects. Best to leave well alone, then, and trust the experts’ judgement. The ship’s stability book is approved by the relevant authorities, so all’s fine, one should think.

And yet, sailing ships have been known to capsize, even in our modern times. Next to fire on board that is the very worst case scenario, one that must be avoided at all cost, and a closer look at the details is asked for.

The truth is that stability can and should be closely watched at all times and even be influenced to some extent by measures taken on board (such as reducing sail, relocate weights etc. ).

1

Statical stability The vessel’s ability to return to the upright position if heeled by an external force

a. the righting moment

The reader is familiar with the concepts of describing the balance of forces. The distribution of weights on board, together with the shape of the hull, are normally such that the ship floats upright, with equal freeboard on both sides, as long as no other factors come into it. What forces there may be at play are in equilibrium. Namely the weight of the ship that presses her down, which is counterbalanced by her buoyancy. The two are thought to act at imaginary points called G for center of gravity and B for center of buoyancy. These are positioned vertically on top of each other. As the ship heels by an external force G remains stationary (relative to the absolute point of reference K on the baseline), but B moves sideways. At this point it is important to note that ‘heel’ is caused by an external force as opposed to listing which is caused by a shift of G. The resulting horizontal distance between G and B is called a righting arm known as GZ (see fig. 1). The length of GZ varies with different heeling angles (Ɵ = “theta”), which can be illustrated by a “righting arm” or “GZ” curve.

The relative positions of: K point of reference on baseline (keel) G Centre of gravity M Metacentre B Centre of buoyancy Z horizontal projection of G at a given heeling angle Ɵ N horizontal projection of K at a given heeling angle Ɵ

GZ = KN – (KG x sin Ɵ)

fig. 1

Righting arm curves for two ships “A” and “B”

fig. 2

2

Fig. 2 shows two such GZ curves. The blue one (ship “A”) shows the maximum length of the righting arm at about 40° of heel to be 0,48 meters, with a positive range of stability of just above 90°, whereas the red one (ship “B”) has a maximum righting arm of 0,29 meters in length at 35° of heel, with a positive range up to about 70°. “A” has the better stability by far.

The shape of the curves in fig. 2 is the standard model for representing the stability of a ship. They may, however, look different, if additional data are taken into consideration, such as the additional buoyancy that a deckhouse (closed watertight) may provide, see fig. 3. Ship “B” has now a positive range of stability of 85°.

similar to fig. 2, but showing the effect of extra buoyancy provided by the superstructure of ship “B”.

fig. 3

righting arm curve for ship “A” (as above). The dotted line shows the effect of large quantities of water on deck when heeling beyond the angle of deck edge immersion (at 20° +).

fig. 4

Fig. 3 & 4 are included here merely to draw attention to the well founded suspicion that the stability information found on board many existing ships may not accurately reflect the real state of affairs, particularly so in the region of higher heeling angles ( > 45°), and in older ships with stability information compiled decades ago with, possibly, many alterations and additions in the interim. In fact, often the stability information available does not even go beyond 60°. 3

Minimum intact stability requirements

The model described here forms the basis of most of the minimum requirements for the (statical) stability of ships as laid down by maritime authorities all over the world. Most of these requirements have the character of describing minimum or maximum angles and areas under or between curves. They may, however, be different in different flag states. In other words, what is considered as sufficiently stable in one country may not pass muster in another.1

b. the heeling moment

While there is no fundamental disagreement on how to describe, calculate or represent the righting moment, the story is very different when it comes to deal with the opponent, the heeling moment.

There is a standard model, albeit with varying parameters, mainly adapted from theories developed for the average cargo ship. It looks more or less like this:

similar to fig. 2 with another curve added. The green line represents the heeling arm of a steady wind force.

fig. 5

In fig. 5, another curve is added, showing the heeling influence of a steady wind blowing with a certain speed. At the intersection of the blue and green line the steady heeling angle of ship “A” can be deduced (about 21°), ship “B” can be seen to be heeling over to 28°.

1 From the International Code on Intact Stability of 2008 (SOLAS) (The entire resolution runs to almost 100 pages. This is the core of the description of minimum requirements by IMO): “Criteria regarding righting lever curve properties The area under the righting lever curve (GZ curve) shall not be less than 0.055 metre-radians up to φ = 30° angle of heel and not less than 0.09 metre-radians up to φ = 40° or the angle of down-flooding φf if this angle is less than 40°. Additionally, the area under the righting lever curve (GZ curve) between the angles of heel of 30° and 40° or between 30° and φf, if this angle is less than 40°, shall not be less than 0.03 metre-radians. The righting lever GZ shall be at least 0.2 m at an angle of heel equal to or greater than 30°.” 4

There are various formulae for the calculation of the magnitude of the heeling force at any given angle. One method is this (often used, but not universally accepted):

Ha heeling arm p wind pressure (kg/m2) Ha = p * As * LSv * (cos Ɵ)2 / (depl * 1000) As sailarea LSv vertical distance between center of effort and center of lateral resistance v2 p = v wind speed (m/s) 16

Probably more significant at this point however is the difference in size of the coloured areas between the green heeling curve and the red or blue righting curve (see fig 6). These areas illustrate the “amount” of righting forces, commonly called reserve stability. Ship “B” (buff) is left with rather little of it, compared to “A” (light blue).

comparison of righting forces for “A” and “B”.

Wha = wind induced heeling arm

fig. 6

This way of showing the ship’s righting force against the effect of heeling forces can of course be repeated for varying wind speeds, see fig 7.

different heeling angles at different wind speeds.

fig. 7 5

Add to this the effect of different sail setting arrangement (such as all set, partly set, storm canvas), and, last but not least, different loading conditions (such as departure and arrival), and yet another different set of curves can be constructed, see fig. 8.

Example of use:

This ship, with all sails set and in departure condition, sailing in a steady wind of 8 m/s, may be expected to heel steadily to about 9°.

fig. 8

6

This set of curves seems to give a pretty clear impression of what steady heeling angles may be expected for different wind speeds, different sail settings, and even differing loading conditions.2

Further down we will see, however, that all this is based on a number of assumption and conventions, that do not stand up to empirical analysis. (Steady heeling angle = a somewhat theoretical steady angle under the impact of a steadily present heeling force, without influence of sea, rolling, gusts etc.)

Dynamic stability The amount of work taken to bring a vessel back to its upright position

Anything may be expected at sea but a steady state of affairs. Neither wind speed nor sea state are steady, ever. And the ship will respond to that in a number of ways, none of them adding much on the positive (righting) side of the “war”.

A number of ways have been devised to calculate and illustrate these effects, and they are, without doubt, extremely important to bear in mind for all ships, particularly for many special purpose ships and of course, sailing vessels. Bearing in mind the multitude of influencing factors the difficulty of this approach becomes immediately apparent, and the instinctive reaction of the responsible sailor is first of all to rely on experience.

One of the most commonly used models is based on an assumed scenario. A ship, resting unrestricted in the water with zero heel, is subjected to a sudden impact of a heeling force (wind) of a given strength. We have seen how the steady heeling angle for this wind force is calculated to a certain degree of reliability. In this described scenario, however, the ship will heel over beyond the steady heel angle. The question is, how far.

steady heeling angle (15°) and derived heeling angle due to the effect of dynamic forces (28°). The two coloured areas are roughly equal in size.

fig. 9

Fig. 9 shows, that a maximum dynamic heel is expected at a point, where the area between righting and heeling arm curves beyond their meeting point at 15° (magenta) is equal to the area up to that point (yellow). No mitigating influence from damping or inertia is taken into account here.

2 In the event of a non-standard loading condition such as loading extra stores or equipment on a deck-house top, a separate stability calculation should be considered in order to confirm that the ship remains within safe limits. 7

Another likely player is the effect of the ship’s rolling movement in the sea. Various ways, some of them with considerable intricacy, have been devised to calculate this rolling angle. It is safe to say that 25° may be considered as a typical “roll motion amplitude” for most traditional sailing ships.3

Fig. 10 serves to illustrate this effect of roll motion.

With a given steady heeling angle (15°) the roll motion may be assumed to be between 10° to windward and 40° to leeward (25° to each side). The coloured areas are not necessarily equal in size.

fig. 10

Conflicts between mathematical models and experience

Most of the above has proven its value for the maritime industry for a long time and continues to do so.

However, for the purposes of enabling the OOW on a sailing ship to make a sound assessment of his ship’s stability in any given situation there are a few shortcomings in the models and the overall approach.

First of all, complexity does not add to the appeal of the models for everyday use. The number and magnitude of parameters used vary considerably between different countries and administrations. In some cases for example, even water salinity or barometric air pressure are entered into the equations.

Secondly therefore, there is a distinct vagueness in many aspects that gives reason not to put too much trust in the values of the derived results, values and limits.

In the words of Barry Deakin4:

“Calculations of statical stability can be carried out to a high degree of accuracy, but their application to the case of a vessel in a seaway involves many assumptions, in particular, that the influence of waves is negligible. Similarly the conventional calculation of wind heeling and its effects on stability incorporate a number of assumptions which should be questioned. Those of most interest here are as follows:

3 More detailed information can be found in SOLAS: MSC 83/28/add2 Annex 13 4 B. Deakin, B.Sc. MRINA, Wolfson Unit, M.T.I.A., University of Southampton, see: http://www.wumtia.soton.ac.uk/sites/default/files/uploads/pages/RINA1990BD.pdf (1990) 8

I. The wind is of uniform velocity at all elevations. II. All sails are aligned along the ship’s centerline. III. All sails have a heeling force coefficient of unity. IV. Overlapped sail areas produce no heeling moment. V. The heeling moment is maximized with the wind on the beam. VI. Heeling moments vary with cos2 (Ɵ). VII. When considering response to a gust, the increase in wind speed is instantaneous. VIII. When struck by a gust the vessel is upright. IX. The vessel’s inertia and damping have no effect on its gust response.”

Up to this point we have only examined the basic theory and the shortcomings of mathematical modelling.

A different approach Following the loss of the barque MARQUES in 1986 the maritime authorities in the UK attempted a closer look at existing stability requirements and instigated closer research into the matter.

The outcome of this research is interesting in two ways. One is that long held beliefs proved to be untenable, the other is the attempt to look for safe stability from a very different angle (no pun intended).

Myths and facts

As mentioned above, a number of assumptions were identified as underlying the traditional concept of stability assessments for sailing ships. Basically they were all found to lack both theoretical and experimental / empirical justification.

i. A considerable range of relevant data were collected, both at sea and by experiment. Not surprisingly it turned out that simple modelling was a vain hope, due - to name but a few factors - to:

• different wind speeds at different elevations, • terrain characteristics • various atmospheric conditions • horizontal angle to the wind • characteristics of rig and sails (shape).

Otherwise comparable data were found to vary up to 40 % due to these factors.

ii. The magnitude of the heeling moment exerted by wind is usually assumed to be a function of the ship’s angle of heel, traditionally considered to be cos2 Ɵ. (Both sail area “weight” and the height of the center of effort “arm” are assumed to decrease as heeling increases.)

9

Analysis of the gathered data showed that the assumption may be valid in principle, but that closer correspondence with reality is found at about cos1,3 Ɵ.

The angle of heel as determined by looking at curves crossing. In this example the angle is 28°, 32° or 34°, depending on what exponent for the cosine is used.

fig. 11

iii. Furthermore, the traditional way of thinking could result in individual stability requirements that were effectively paradox:

“A major criticism of existing regulation is that, in evaluating the stability of a vessel by assessing its ability to withstand certain conditions, with either full sail or a fixed proportion of that sail area set, a vessel with poor stability characteristics can gain approval by removing part of the rig. Thus, there are sailing vessels approved by various authorities which sail without, say, their topmasts. These vessels are severely handicapped when the winds are light, being unable to set sufficient sail to make good progress, but are still able to set the same sails in more dangerous storm conditions, as they could prior to regulation.”5 iv. The obvious scare for the sailing ship officer is a sudden increase in the force of the wind. This, it seems, can be described as being one of two distinctly different phenomena, gusts and squalls. The actual speed of the wind is eternally fluctuating, yet sudden increases in otherwise stable circumstances, called gusts, are found to be limited both in variation and duration. Variation rarely exceeds an increase in the wind speed by 40% of the average speed at any given time - which effectively doubles the wind pressure. Allowing for this one is left with a sufficiently safe margin as far as gusts are concerned.

The situation is vastly different when met by a squall, where more substantial changes in the atmosphere surrounding the ship are experienced. Be this due to a front passage, a thunderstorm or any other possible reason, the increase in the wind speed can be very large indeed, so much so, that any kind of mathematical prediction is futile.

It is up to the OOW to assess the surrounding situation and consider whether the situation is stable (with nothing worse to fear than gusts) or unstable, where changes in the weather, squalls, are likely to bring about considerable changes , always keeping in mind that wind pressure increases as the square of wind speed.

5 ibid. 10

The real threats

As a conclusion to all these observations a radically different approach was suggested (and adopted by UK authorities and others).

Instead of focusing primarily on initial stability (GM) and on establishing the ship’s power to carry sail, attention is now directed at distinct and/or critical points on the path along the righting arm curve. These are:

1. angle of deck immersion Ɵi (fig. 12)

2. downflooding angle Ɵf (fig. 13) 3. capsize angle in a squall (fig. 14) 4. angle of vanishing stability (fig. 14)

fig. 12 fig. 13

Capsize angle and angle of vanishing stability are two very different things.

fig. 14

Reaching the angle of deck immersion, i.e. the point where green water will come on deck (in fig. 12 about 19°) may not be dramatically alarming in itself. It is nonetheless a point that deserves attention, mainly due to the fact that it is quite an undisputable one, easy to recognize.

The downflooding angle, by contrast, is bad news altogether, obviously. That’s where water can flow into the ship in threatening quantities, dramatically increasing free surface effect and reducing freeboard .6

6 Small openings may be disregarded. Critical flooding is deemed to occur when reaching the lower edge of openings that have an aggregate area in m2 greater than (vessel displacement in tons / 1500). 11

Like the angle of deck edge immersion it is dependent on the shape and construction of the ship. Measuring or calculating it may be a bit difficult. But in any case, for purposes of calculating stability data, should Ɵf be larger than 60°, then 60° should actually be used.

The capsize angle is of a slightly theoretical nature, and rarely mentioned, but deserves no less attention. In the presence of a squall sufficiently strong to heel the ship over as the blue curve in fig. 14 shows, all reserve stability (the area under the GZ curve) is completly blanketed, with the two curves touching at an angle of 48°, which may well be less than the downflooding angle. The ship will have no reserves left to rise herself. This, it must be noted, is also taking place long before the angle of vanishing stability (about 69° in fig. 14) woud be reached. In other words, putting faith in the latter being at around 70° or more, may be over optimistic.

1. Staying safe in a gust

It is fairly straightforward to determine what is safe in terms of calculating the maximum steady heel angle in stable atmospheric conditions (i.e. gusty at worst). Safe, by the premises stated above, is not to come anywhere near the downflooding angle. GZf The heeling arm (HA1) in gusts to cause downflooding has been found to be = 1.3 cos Ɵf

where HA1 is the magnitude of the actual wind heeling lever at 0° which would cause the ship to

heel to the down flooding angle Ɵf (or 60°, whichever is least), which is not where one wants to be.

GZf is the value of the ship’s GZ curve at the down

flooding angle Ɵf (or 60°, whichever is least).

The safe angle (HA2) is where the the force of the wind is but half of what it takes to cause downflooding:

HA2 is the mean wind heeling arm at any angle Ɵ 1.3 degrees = 0.5 * HA1 * cos Ɵf . See fig. 15.

fig. 15

Thus, with a downflodding angle (or equivalent) of 60° this ship could heel over to 34° and still be safe in a gust. Technically speaking, that is.

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2. Staying safe in a squall

As mentioned above staying safe in a squall is very much harder to guarantee. Only through a somewhat roundabout way can the OOW determine whether he still has reason to feel safe under squally conditions as desired. A set of curves can be calculated for the ship (and is to be found on all British registered Tall Ships, to be sure) that enable the user to compare the prevailing condition with what may happen in a squall.

These curve are usually called Curves of Maximum Steady Heel Angle to prevent down flooding in Squalls, or just squall curves, see fig. 16.7

Curves of Maximum Steady Heel Angle to prevent down flooding in Squalls

based on the correlation

2 V2 HA2 ──── = ──── 2 V1 HA1

or:

2 2 V2 V1 ──── = ──── HA2 HA1

where: V2 windspeed resulting in ƟX V1 squall speed X (set to 10, 20, 30 m/s resp.) HA1 heeling arm to cause Ɵf HA2 heeling arm resulting in Ɵa Ɵa arbitrary heeling angle

The curves are constructed by a series of calculations for various values of Ɵa.

fig. 16

7 Details on how these curves are constructed may be found here: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/295019/sib_sailing_13_may_10.pdf and here: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/295063/ly3-4.pdf 13

At the root of computing these curves are a few assumptions:

• The squall curves are based on comparing the righting arm at Θf and the righting arm at the (steady) heel angle preceding the squall. • The formulas and procedures then determine the value of the squall wind speed that will heel the

vessel to Ɵf.

If it is presumed that a 10, 20 or 30 m/s squall will heel the vessel to Θf, the corresponding steady heel angle can be found for the vessel (with the same sails set in a lesser wind speed which might precede the squall.)

It must be admitted that all this may sound rather technical. The following example may shed some more light on how to extract useful information out of these curves8:

Consider in fig. 17 a ship “X” sailing with a mean heel of 8° in what averages to a wind speed of 10 m/s, as plotted in fig. 17. This ship will be in danger of heeling to the down flooding angle in squalls of close to 20 m/s. By bringing down the mean angle of heel to 4° ( at X’ ) the same ship would be able to withstand a squall of up to 30 m/s.

One must bear in mind that estimating ‘mean wind speed’ can be very difficult on a rolling ship and further errors can be introduced by a defective or poorly sited anemometer.

These squall curves, useful as they may be, should always be augmented by comprehensive notes on their use and limitations, if they are included in the Stability Book.

fig. 17

8 Occasionally these curves are referred to as cross curves. However, this may lead to confusion. Cross (or KN) curves are commonly prepared for the purpose of enabling statical stability curves being drawn for the ship in any sailing or loading condition. 14

The critical angle(s) revisited

Experts tend to agree today that the positive range of stablity should be in the region of 90° or more rather than 80° or less, with good reason. In fact, the latter would no longer be allowed in a number of countries.

This is, however, a condition nigh impossible to satisfy for a number of older, traditional ships. What does that mean in terms of ensuring the safe conduct of such ships?

Let us go back to ships A and B in fig. 2 and add the curves necessary to establish the “Maximum Steady

Heel Angle to Prevent Down Flooding in Gusts” (as in fig. 15). In both cases here Ɵf is set to be 60°.

In fig. 18 below HA1 and HA2 for “A” are 1,32 m and 0,66 m, thus the maximum heeling angle is about 34°.

For “B” it’s not that simple. Even before any windforce could heel “B” over to her downflooding angle the ship’s reserve stability would be entirely obliterated at a point, where the GZ curve and the wind heeling curve meet, at 50°, the capsizing angle. Ship “A” would still be able to right herself from 50°, ship “B” certainly not.

It is clear that, while finding a maximum heeling angle by halving the critical windforce works fine for ship “A”, it would not save ship “B”. The equivalent derived angle for “B” (about 22°) should be reduced to, say 14° so that some reserve stability remains.

(Incidently, it is, by this method, not even possible to identify a capsizing angle for ship “A”.)

fig. 18

15

Traditional telltales

While staying well clear of Ɵf is good policy for the OOW on “A”, it may not be enough for his colleague on “B”.

No specific recommendations or limiting values have yet been defined for ships like “B”. The easy way out (for the regulator) is to refuse or reduce the right to sail for “B” (as is the case in the UK, for example).

The fact that a number of sisters of “B” have been plying the seas for decades without mishap, may prove nothing other than that they have been lucky so far.

But it can also be argued that these ships can still be sailed perfectly safely if due observence is paid to the limitations of the ship’s stability characteristics. It is possible, and has been done by responsible officers at all times.

The important thing to keep in mind is obviously that it is better to err on the safe side than to be overconfident.

After all, stability is one thing, but the working conditions on deck and aloft are seriously impaired if the mean heeling angle moves significantly beyond 20° or so. One may soon reach a point where the situation becomes seriously threatening not only in terms of reduced stability, but also because the crew is increasingly incapacitated by slanting decks, ropes entangled in the scuppers or trailing through freeing ports, difficulties in furling sails aloft, etc.

As a consequence, reaching the angle of deck edge immersion (as a mean heeling angle) is a wakeup call for most experienced Tall Ship sailors. Going beyond that may spell a lot of trouble, staying below that should keep even ship B reasonably safe. This rule-of-thumb is applicable in any situation, day or night.

Some sailors resort to another time-honoured rule-of-thumb on occasion. They count the seconds of the ship’s rolling movement from hard over to one side to hard over to the other and back again. If the number of seconds this rolling period lasts gets anywhere near the number of meters in the ship’s beam the metacentric height may be critically small. This could possibly be the case in “arrival” conditions for example, with slack tanks and many persons aloft. (Of course this only works with no sails set.)

In any case, whatever may help to improve the ship’s stability, or minimise identified threats, should be taken into consideration at all times. For example, and as a general rule, slack tankage should be minimised, ballast tanks pressed up and cross-connection valves closed at sea – these procedures will normally be covered by the SMS.

Additional threats As hinted above the OOW should not ever consider stability as an isolated phenomenon. A vast array of additonal aspects will influence his decision making, not the least human factors such as the experience and physical readiness of the crew and trainees.

Equally relevant at this point are a few other aspects.

16 i. Virtually nothing has been said so far on the influence of the sea state. Heeling forces are exerted not by the wind alone. Waves cause a lot of movement by themselves, if only of short duration (per wave). The interaction between wind, sea and the ship’s own stability and inertia is far too complex to allow any condensed measurable description. But the sea state must be taken into account as the possible straw on the camels back by further increasing the danger the ship might find herself in due to a violent squall, for instance.

Just one further observation is offered here: The ship is considerably more vulnerable on top of a wave crest than in a trough, not so much due to greater exposure to the wind, but mainly because the ship’s stability may be further reduced if the waterline slopes away downward on one or both sides. ii. When discussing stability as above it is silently assumed that the ship has the wind more or less on the beam, where the exposure of the sails is at its greatest and the heeling forces can do their worst. When running before the wind the effective sail area is much reduced (due to the sails largely blanketing each other), the direction of heeling forces is far from athwardship, so apparently danger is much reduced (not to mention the reduced apparent wind speed.) However, sailing in this condition also means that no relevant observation of mean angle of heel, as required to use the squall curves, is possible. Whatever heeling one experiences is likely to to be generated by the seas rather than the wind. (This is not reflected in fig. 10 above.) iii. The ability to stay on a chosen course relative to the wind is of the greatest importance. However, in adverse conditions steering may become difficult in many ways. Without even considering non-power- assisted steering there are two different critical aspects to bear in mind:

• The steering power of the rudder is gradually reduced with increasing heeling angles. Any attempt to come up into the wind can thus be made virtually impossible without the additonal help of sails aft. • Steering power may periodically be all but lost altogether in high following seas, bringing with it imminent danger of broaching to. iv. Yet another of the silent assumptions is that the wind blows horizontally, parallel to the seas surface. Strictly speaking that is hardly ever the case. But in the vicinity of high shorelines in particular or near large clouds the vertical (downward) component of the wind direction may be considerable.

White squalls, microbursts and similar phenomena can not be discussed here, but one of the things they do, because of their large down-draft element, is completely obliterate the reduction of heeling moment with increasing heeling angles. v. In quartering seas the danger of broaching is increased when the stern is both lifted and slewed downwind while the bow may be buried deeply into the trough – this decreases rudder effectiveness, increases windage aft and decreases the corrective power of the headsails. vi. Free surface effect of water on deck must always be considered, especially during fire-fighting when there will be many other distractions. Fire-fighting systems and equipment must consider water use and drainage – fog systems are very effective and use minimal quantities of water. ‘Dump’ valves can quickly drop water from between decks into the bilge thus reducing free-surface and KG. They should normally be kept closed to avoid up-flooding so fire and maintenance plans should include their use.

17 vii. Ice accretion is rarely a problem in the context of ISES but should be considered when planning programmes, voyages and passages. viii. Normally the stability information of a ship is put together when the ship is built, or possibly occasioned by major alterations. Otherwise it remains perfectly static over the years. That may not be the case for the stability itself, however. Small alterations may have accumulative effects. These small alterations may be minor structural changes, additions or alterations to the spars and rig, new boats, antenna domes, all kinds of additional equipment and stuff. These may effectively move the ship’s center of gravity (G) upward and it is prudent to compare the actual distribution of weights to what shown in the ship’s stability book. Any inconsistencies should be investigated and if in doubt one may want to perform a validating inclining test to ascertain that the all important metacentric height, GM, remains within accepted limits. There have been several losses due to inappropriate modifications affecting stability and/or downflood angles.9

Bearing this in mind, it is important to keep full records of additions and alterations, particularly in respect of the ballast, rig, sail plan and spars.

National differences There is no such thing as a typical Tall Ship. Differences in size, hull characteristics, sail distribution and area - they all vary endlessly within as well as across regions. Maybe it is a fitting mirror of this that national stability requirements are very different as well. It is therefore necessary that one acquaints oneself in sufficient detail with the stability requirements as stipulated be the flag state authority of the ship at hand. There may be underlying implications not necessarily entirely in agreement with what one has learned as being in accordance with good seamanship.

For a Tall Ship today stability is hardly at risk due to any cargo carried. Provided that the stability requirements are met (and the ship’s integrity is maintained) there is no other real enemy than the wind and its heeling force. Keeping the heeling angle as small as possible at all times is good policy. Rare are the occasions where a steady heeling angle not exceeding the angle of deck immersion can seriously threaten the ship. For many ships, particularly large squareriggers this coincides favourably with the fact that a squaresail performs at its best as long as the heeling angle is small.

Inclining Experiment (Heeling test)

Strictly speaking an inclining experiment serves to determine lightship parameters of a ship, from which its stability characteristics can be established for varying conditions of loading. The findings of the experiment, performed for every newbuilding or after major alterations, are a cornerstone of the complete stability information available to the master. Considerable precision is asked for to perform the experiment, and the assistance or supervision of a trained surveyor or inspector is advisable, if not mandatory.

Such an experiment may, however, prove useful even when conducted on a somewhat less ambitious scale.

9 The UK Large Yacht Code (LY3) sets out specific circumstances when a complete stability reassessment is required. 18

As mentioned above, after a couple of years doubts may arise as to whether the information found in the stability booklet is still entirely to be relied upon. Is the actual metacentric height equal to what it is believed to be according to the available data? Changes to the distribution of weights on board may by themselves be insignificant - however numerous - but they may have an accumulative effect impossible to assess. Therefor one may eventually want to either allay or substantiate suspicion. Where is G now?

A few preliminary checks may offer some hints. Is the draught and trim of the ship in total agreement with what it should be, when checked against the available stability information? Both should be found entered in the logbook regularly, and by looking into records a few years back hidden changes may be revealed.

The Basics

In fig. 19 a weight w is shown to be moved over a horizontal distance d. This will result in a horizontal shift of the position of G to G1. Consequently, the ship will heel to a certain (limited) amount. Also, a pendulum with the length pl will be deflected, the deflection being s.

The two resulting triangles are mathematically equal. So:

w * d G G1 = GM * tanΘ and also: G G1 = w * d / depl or: GM = ──────── Δ * tanΘ

Finally, of course: tanΘ = pl * s

w movable weight

d distance of w moved

G G1 distance of center of gravity moved

Pl length of pendulum plumbline

s plumbline deflection

fig. 19

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In other words, all that needs to be done is to find a suitable weight to shift from one position to another athwardships and rig up a pendulum and measure the length of the plumbline and the deflection to determine the metacentric height GM.

Knowing that KG = KM – KG one can now verify whether G has moved up or down or remained where it was initially in accordance with the data in the stability booklet.

Preparations and conditions

While the underlying principle is quite simple, the actual performance is somewhat more demanding. First of all a suitable (number of) weight(s) is to be found. It is to be such that the inclining moment w * d results in Θ being somewhere between 1° and 4°. Such a weight is likely to be in the region of 0.5 to 1 per cent of the ship’s deplacement, depending, a. o., on how stiff the ship is. This weight may be a block of concrete, a box of scrap iron, water in barrels or tanks, or even a number of persons. In any case the total weight must be ascertained precisely, of course.

Altogether the list of necessary preparations as well as surrounding conditions necessary to be found (no wind, calm water, moorings to be slack etc.) is most adequately described here: https://www.gov.uk/government/publications/large-commercial-yachts-inclining-test-guidance-notes

Interpretation

Depending on just how meticulous one went about performing the experiment and assuring that all parameters are correct, conclusions can be drawn. The more effort one had put into ascertaining all the necessary details, the more one may rely on the validity of the outcome. Still, regardless of how unsatisfying it may be as a result, it is safer to find that further action or greater caution is asked for, than becoming over-confident on findings that are not necessarily endorsed by the supervising authorities.

Stability in port Stability should be considered at all times, including dry-dock, since inattention at this critical time could result in a dangerous situation when returning to sea-service. Listed below are some items to keep in mind:

• Permanent ballast: This should be shown on a plan included within the Stability Information Booklet; its condition and securing should be checked during maintenance/dry-dock periods. If the vessel has a separate ballast keel, close attention should be paid to keel bolts and the adjacent hull structure. Any alterations to ballasting should be approved by the flag administration and formally noted in the Stability Booklet. • Freeing Ports: The efficient function of these and other means of drainage eg ‘dump valves’, is vital and should be covered by planned maintenance and specific verification after a refit period. • Suspended loads: If planning to use ship’s gear to handle heavy loads consideration must be given to the effect on stability especially if heavy weights from below have been temporarily removed or shifted and not secured in position, abnormal free surface and down-flooding conditions. Good communication is vital. A risk assessment and ‘tool-box talk’ is recommended; this may show that employing a shore crane is a safer option! 20

• Dry-docking: As with any vessel it is important to minimize KG, list and trim when docking and to ensure identical conditions when undocking. Wind and weather will still affect the vessel; windage and vibration induced by the rig may indicate that additional support and/or lashing is required, especially if the vessel is hauled out on a slipway.

Overall Guidance for OOWs regarding the Stability of their Vessel Maintaining a safe reserve of stability requires observance of good operational and maintenance procedures – not merely plotting heel against wind speed on a graph. There are many sources of information and advice available which may be of particular value to officers involved in the ISES. Some are mentioned above but are also included below for completeness:

Ship/Company specific items:

• The ship’s Stability Information Booklet (including ‘Squall Curves’, if applicable) The Safety Management System – this may (or should) contain instructions regarding passage planning (including operating limits), maintenance of water tight integrity, drainage (scuppers, freeing ports, ‘dump valves’), limitation of free surface effect in bilges and tanks, structural alterations and sail carrying. • Master’s Standing Orders – usually emphasise the SMS and may focus particular attention on specific procedures or items relating to water tight integrity and stability. • Master’s Night Orders – May relate current and expected weather conditions to stability restrictions eg. prohibition on hoisting certain yards without authorisation. • Experience of the Master and Senior Officers – they will (should) always be happy to share knowledge.

General Sources of Information:

• The Large Commercial Yacht Code (LY3) – applies specifically to UK flag vls up to 3000gt. • UK MCA Model Stability Information Booklet – contains much useful, practical guidance. • Transportation Safety Board of Canada – Report on the loss of ‘Concordia’. • Tall Ships America, Safety Under Sail Forum: Sailing vessel Stability, Part 2, MCA Squall Curves Moderation (2012) • Polish Register of Shipping No29/p ‘Guidance for Sailing Vessel Stability Calculation and Evaluation. Text in Polish and English it concentrates on regulations and includes material relating to Icing, the Stability Booklet, Roll Amplitude and Damage Stability.

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Reactions and Comments

This pamphlet serves the basic need very well. Captain Walter Rybka U.S. Brig NIAGARA

The material is inclusive and detailed enough while still remaining manageable for the candidates ... Well done thank you for your efforts. Capt. Tony Anderson SALTS Sail and Life Training Society

I have reviewed the material and find it excellent; great job! Captain Dave Wood, USCG Ret.

… regarding the Stability data that I thoroughly enjoyed! We in NZ as with the rest of you guys have been wrestling with Sailing vessel Stability for some years now and this is a tremendous step forward, well done. Thank you for the simplicity and the straightforward coloured approach. Paul Leppington Lecturer, New Zealand Maritime School, Manukau Institute of Technology

In my opinion the Tall Ships Stability Chapter seems like a very useful tool for those of us who have not recently studied or had the need for stability calculations for many years. Well done, Captain Marcus Seidl S.S. STATSRAAD LEHMKUHL

An excellent job; two thumbs up. Captain Martyn Clark Director Los Angeles Maritime Institute

excellent and to the point. I appreciate the fact that you have avoided the voice of a lecturer, and I think others will too. Capt. Daniel S. Parrott Professor of Marine Transportation Maine Maritime Academy

What a great gift to the sail training world Peter Cardy Independent Governor Southampton Solent University formerly Director of Sail Traing International

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Interesting report for stability criteria improvements. It's an always ongoing subject of discussion. The challenge is to find the adequate and, not less important, workable and understandable criteria for the seafarers involved. • My experience is, that most officers on board have a certain reluctance to study the stability booklet and trust more on their intuition in the judgement of the weather and ship's behavior. I think a lot can be gained to educate them in understanding and recognition of potential dangerous weather systems and the stability book in general. • It is quit common, that the naval architect makes a so-called 'sail-matrix', where the amount of sail is recommended depending on wind direction and force. It is not a 'holy grail' but a tool for the officers to base their decisions on. To present this with pictures of the ship with the sails to be used in mentioned circumstances is very practical and don't need any study to understand. • The in SOLAS mentioned minimum allowable GM value of 0,15m is absurd: For sailing ships with traditional hulls (heavy displacement) a value of 0,50 cm is a much more realistic minimum, although most ship, can not comply to the other SOLAS or Flag administrations criteria with such a GM value. Therefore the GM value should be 0,75 m minimum for most ships. • I can not find myself in the argument, that with high GM values, the ship will be too 'stiff' and sometimes even dangerous to operate. Movements of a sailing ship are almost always slow due the 'damping effect' of the rig and the large moments of inertia, also caused by the rig. As a designer, I am much happier with a GM of 1,50 m than one of 0,75 m and I'm sure the crew shares my opinion here. • The UK approach to take the downflooding angle as a starting point for the calculation of the maximum steady heel angle is in principle a good and intelligent one. The question remains, which opening is a probable downflooding opening. A weathertight door which is always open is certainly a downflooding point, while it is not recognized in the calculations as such. • Non weathertight roundhouses are actually having a positive effect on the dynamic stability at larger heel angles, while this is neglected in the stability booklet, because of the international accepted criteria for stability calculations. • Easy handling of the rig is an attention point in my view. In comparison: on a small yacht you can simply ease the sheet(s) to limit the heel angle. On a tall ship it is not very trustworthy to need 10 or more people to reduce a sail, how romantic it may look and how common practice it often is. Easing the sheets without the danger of taken away by it or leaving the crew with burned hands is not desirable. In general, the amount of man-power and energy needed to control the rig is often too high in my view. This can only be yield for a short period of time and is in-adequate for heavy conditions during a couple of days.

Some other points which in my view is not recognized enough: • Speed: While the rudder can only function with a certain water flow, it is important to have and keep enough speed to control the course of the ship, and therefore navigate safely in especially heavy conditions, with larger heel angles. (where the rudder operates in a plane you don't want; a double rudder configuration angled at approx. 15 degrees with the vertical plane is much better here, but I already hear the 'dinosaur' under the owners and officers protesting ...... ) It is however in contrary with the traditional thoughts of reducing sail to stay save. The only real solution is probably to make the ship 'stiff' enough to carry the amount of sail needed for a speed, to which the ship can be controlled sufficiently. • The dimensions of the rudder is an other aspect in this context: Much older tall ship has ridiculous small rudder blade surface areas with a very low efficiency due the fact that the rudder is made of one steel plate only. A minimum surface area related to the lateral underwater body looks very logical to me.

Klaas Huizenga, KHMB Y&S Design Naval Architect 24

1. It is a good thing that the stability of sailing vessels is addressed by STI. Due to its complexity it is a subject that is often neglected on board. The notion may live that the ship has a never changing stability, because there is not a changing cargo situation. 2. Discussion is, or may be deemed to be, too complex to be discussed by most sailing ship officers although they should be the users of the stability information available to judge the stability and its safety margins at any moment during transit. 3. Improvement of this situation may be reached by designing a comprehensive format of the stability booklet that is more use-full onboard and may be integrated in ISM or standing orders. Development of which may be the working area for designers and scholars with practical input of crew. 4. The stability properties of the ship are computable during the design-stage, or can be determined afterwards. The different models for static and dynamic stability are available and known to designers. The check of these values by design is done with the inclining test. This is a one-off check of stability assumptions. 5. When in transit transverse inclination is a variable that is constantly measurable and could be entered in a pre computed stability form, supplied by design, which could again be part of ISM and/or shipboard routine, but also renders information to designer about the quality of the stability assumptions used. Variables that are recordable and can be entered are: a. Draught forward and aft to determine the actual displacement at departure b. Heeling angle alongside b. Tank condition, including free surface effect. c. Heeling angle at any given moment. d. Weather condition. 6. In reverse , recording of these data on a existing ship may be used in determining or refining the stability properties of an existing ship.

The paragraphs 1 and 2 of the stability pamphlet; “Staying save in a gust” and “Staying safe in a squall” appear very useful to me. The evaluation and determination of quality of this kind of information is for others than me. But I can Imagine that a graph of this type specific to the ship can be a tool for Captain and OOW’s to evaluate the risk of setting the safe amount of sail when squalls or gusts are expected. This combined with useful ISM protocol for sailing in gusty or squally weather conditions may improve safety.

Further important point is on page 12 of the Pamphlet that the size of the sail area is often determined or maximized by the authority or Class via Stability rules. There are ways around this. More important is the notion that it is about time that the judgement of crew in the amount of sail carried by a ship is taken in account. On the other hand this implies that useful tools to evaluate stability for crew should be made available.

Rchard Tefsen Sailmaster BV

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