JANUARY 2013STABILITY

STABILITYJANUARY 2013

The ship’s beam on waterline is a variable determining stability. The hull, when it is equal to the ship’s beam, increases stability. This is pos-sible because the plates of the hull go up vertically keeping the same dis-tance from the deck to the waterline over a significant length, in order to guarantee this distance in different loading situations, that is to say dif-ferent immersion situations. A roun-ded hull would come down to the water with a constant angle reducing drastically the beam on waterline and especially in medium-low loading situations.

Stability is a key feature to a ship. Sta-bility tests have to be carried out for any new construction in all shipyards in the world. It is one of the most important factors when it comes to safety and appointments on board. A good stability (like on every Ocean King) increases the possibility of ad-ding transportable goods by placing them in upper zones that are more accessible than on any other ship. We are available to compare our pro-ducts’ stability data with any other shipyard in the world.

Questions frequently asked to nume-rous more or less expert sellers about how much some ship or other rolls make us smile. And answers make us smile even more. “Not much or har-dly at all” are the answers given, as if these were units of measurement, objective data, to compare with other products! Nowadays, there are units of measurement for everything. Who would buy a diamond if instead of a “River” the seller would say it is only a pretty gem? If instead of a “carat” they were only sold a good weight of gold but which, in the end, is not so much. Nonsense! Maybe this sounds crazy because a diamond is worth thousands of euros? Why then are these answers accepted for a yacht which is worth millions as if they were the rule?

Would it not be better to know it all about a million-euro yacht before buying her? Testing the ship’s perfor-mance at sea is possible and should be carried out. We do it and provide the results and specific calculations that we manage to obtain in collabo-ration with the best universities.

Stability, as we will demonstrate later on, is measured in metres and is re-lated to the metacentric height. On an Ocean King yacht, this number is included between 1.4m (no load) and 1.8m (maximum load) which is a number usually found on genuine tug boats.

In order to understand the meaning of these figures, imagine it would necessitate exerting a force of about 4 tons on one side of the upper deck to generate a yaw of barely 1.5 degree on an Ocean King 88’. This calcula-tion is made on a boat empty of fuel (having 30 tons of fuel below the hull would lower the barycentre and split the yaw in two).

In practice, it is like placing 40 people on one side of the deck and the yacht would only heel over from 10cm. Feel free to compare the figures with any yacht on the market. But whether or not other shipyards will manage to provide you with these is another matter! Tests in naval docks supply all possible details on our yachts and their performance in rough seas.

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STABILITYOCEAN KING AT SEA

A ship’s stability represents a key element to her safety since it is her ability to fight against capsizing (horizontal stability). In other words, stability is the aptitude of the hull to regain its equilibrium after rocking (rolling and pitching)* which is caused by the wind and swell. There is form stability (plain, rounded hulls), and weight stability which is obtained on medium-large sail boats by means of appropriate weights (ballast) that are placed on the keel to balance yaws and stabilize the hull. In any case, stability depends on the position of two specific points characteristic of the ship which are:

The Centre of Gravity (or barycentre) G: it is the point on which all weight of the ship and load is exerted

The Centre of buoyancy (named C hereafter): it is the point on which all forces of the water are exerted on the hull (Archimedes’ principle). On calm waters, the ship is balanced by two equal and opposite forces exer-ting on the same vertical line: the ship’s weight P and the pressure S which keeps her floating (fig.1). The weight always has an effect on the centre of gravity G whereas the pressure always goes through the centre of buoyancy C.

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DATA ON THE SHIPSBuoYAnCY & STABILITY

* The boat rolls when the hull rocks around her longitudinal axis and pitches when the hull rocks around her vertical axis.

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Archimedes’ Principle (SA): a body immersed in water is subject to a pressure (SA) from bottom to top (exer-ted on the centre of buoyancy C) equivalent to the weight P (exerted on the centre of gravity G) of the volume of water displaced.

Reserves buoyancy: it is the difference between the pressure relative to the total volume of the hull and the one relative to the hull at maximum load. The position of C varies in relation to the ship’s yaws (because they modify the shape of the immersed part of the hull) whereas G remains at the same spot even if the ship yaws (G will change only if a weight is added or moved from bottom to top).

Steering torque: when the ship yaws, the immersed part of the hull changes and the centre of buoyancy C is displaced to C’ which creates a new vertical pressure S greater than C’. The torque X arises from the pressure of the weight P (always pushing down) and the pressure of the water S (always pushing up). It is a transverse stability torque which tends to straighten up the hull by exerting a pressure until G and C are back on the same vertical line.

Metacentre (M): it is the point of intersection of the vertical pressure S, greater than C’, and the longitudinal plan of symmetry. M represents the ship’s stability limit and should always be greater than G in order not to reverse the torque’s trend, which would provoke a turn in the opposite direction that is to say the capsizing of the boat.

Metacentric height: it is the distance, measured in metres, between the points M and G. The greater the meta-centric height, the greater the steering torque and the more stable the boat.

Some examples of metacentric height on military and non-military boats:

Aircraft carriers: 2.5 … 3.0m (most stable boat in the world)Ocean tugs: 1.5 … 2.5mOcean King: 1.4 … 1.8mCruisers: 1.0 … 1.6mFrigates: 0.7 … 1.5mYachts: 0.3 … 0.8m

If the metacentric height is too high as on aircraft carriers, the boat becomes “rigid” in rough waters, making it compulsory to keep her parallel to the waves’ profile (in marine jargon “ride the waves”). If the metacentric height is low, the boat will be less stable and will roll more in rough seas (in marine jargon “cut the waves”) and the boat would be called “shifting boat”.

For the sake of the crew on board, a metacentric height between 0.8m and 1.8m as for frigates or cruisers would be desirable for all leisure yachts (but it is hardly feasible). Our Ocean King offers a number in this range!

Weight stability: when a hull which is ballasted at the keel tends to yaw, C is only slightly displaced, G falls significantly and the distance X between the two vertical lines G and C rises. This allows a distinct improve-ment at the moment of righting and great stability.

Form stability: at the contrary, when a large hull tends to yaw, C will is displaced dramatically which also allows a great improvement when righting due to the increase of X.

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Factors influencing stability: so that the barycentre and the original stability characteristics remain unchan-ged, it is necessary to place heavy loads symmetrically in relation to longitudinal and transverse floorings of the hull and to place these at the lowest maximum level. There must be no stagnant water which, in great quantity, acts as a heavy metal ball on the hull’s movements causing dangerous and unusual yaws.

For instance, let’s take two boats having the same length. One is 7m wide and the other 8m. Stability, as it was de-monstrated, highly depends on the beam and is proportionate to its square. When comparing the two boats, even if there is just one meter beam difference, the stability for the 8m wide boat will be 25-30% higher than the second one! This is why the beam of our Ocean King is much wider than any other boat on the market.

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FOR MORE INFORMATIONConTACT uS

M o n A C o o F F I C ECHRISTIAN PATRON

7 Avenue des PapalinsMC 98000 - MONACO

Tel.: +336 89 19 20 38

christian@mysealtd.com

L o n D o n o F F I C EALINE SERRET

Imperial House - 15/19 KingswayLondon WC2B 6UN

Tel.: +44 20 7836 6498Fax: +44 20 7504 8640

aline@mysealtd.com