The Naval Architect September 2012

In-depth | SUPERYACHTS Figure 2

forces which could be prejudicial to the ship, its complement, its equipment and to safe carriage of cargo”. Te second generation intact stability requirements will make criteria on excessive stability mandatory. At this time China and Germany have

proposed Level 1 criteria for excessive stability. A consolidated criterion probably will be established during the 55th SLF Subcommittee meeting.

For ships without a tumblehome hull form,

ΔGM can be estimated with the following approximate formula: ΔGM = { IU IU

- IL and IL

defined as: dU

} / { 2.V(d) } (2) where:

flat water plane at an upper draught (dU a lower draught (dL

), where dU Min(D-d,L.SW

are the moments of inertia of the ) and are

and dL

= d + δdU [δdU is determined as δdU /2)];

Min(0.75d,L.SW

dL = d - δdL [δdL is determined as δdL /2)];

condition; D is the depth to the freeboard deck; [SW

as SW =f2 = = d is the draught of the considered loading

is reference wave steepness determined (L)];

V(d) is the displacement volume of the considered loading condition.

Surf-riding / broaching Broaching is a violent and uncontrollable turn that occurs despite maximum steering efforts to maintain course. It is accompanied with a large heel angle, which has the potential effect of causing partial or total stability failure. Broaching is usually preceded by surf-riding which occurs when a wave, approaching from the stern, “captures” a ship and accelerates the ship to the speed of the wave. Surf-riding is a single wave event in which the wave profile does not vary relative to the ship. During the 53rd

SLF Subcommittee meeting in January 2011, it was agreed that Figure 3 40 The Naval Architect September 2012

Excessive stability In Part B (recommendations) of the 2008 IS Code, IMO recognised the potential hazards caused by excessive stability: “It is advisable to avoid excessive values of metacentric height, as these might lead to acceleration

a ship is possibly vulnerable to surf-riding if its waterline length is less than 200m and the Froude number corresponding to design speed in calm water is more than 0.3. If a ship is found vulnerable to surf-riding according to the above conditions, the operational guidance relating to surf-riding contained in MSC.1/Circ.1228 would be invoked.

If

this is not practical, the Level 2 criterion for surf-riding must be evaluated.

Level 1 criteria applied to superyachts Motoryachts with a length over all between approximately 40m and 100m, a gross tonnage of between 500 and 3,000GT, and carrying 12 or fewer passengers, are oſten referred to as superyachts. Yachts in this size bracket generally comply with the requirements of

the SOLAS-equivalent

legislation issued by the UK Maritime and Coastguard Agency (i.e. The Large Commercial Yacht Code). Superyachts are generally characterised by

fine hull lines, high breadth to draught ratio’s, voluminous topsides with excess freeboard and relatively high design speeds. As such, their hull form differs considerably from most merchant vessels. A total number of thirteen contemporary

superyachts, designed and built by various SYBAss members, was selected for testing the draſt vulnerability Level 1 criteria for

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