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