3.1 Objectives and definitions
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC.1/Circular.1200 – Interim Guidelines for Alternative Assessment of the Weather Criterion – (24 May 2006) - Annex - Interim Guidelines for Alternative Assessment of the Weather Criterion - 3 Guidelines for Experimental Determination of the Wind Heeling Lever Lw1 - 3.1 Objectives and definitions

3.1 Objectives and definitions

  3.1.1 The purpose of the tests addressed in this section is to ensure uniform applicability of model tests for the determination of the steady wind heeling lever, l w1 (paragraph 3.2.2.2 of the Code). It is allowed by this procedure to consider the steady wind heeling lever as dependent on the heeling angle (see figure 3.1.1).

Figure 3.1.1 Weather criterion when the wind heeling lever is dependent on the heeling angle

  3.1.2 The standard model test procedure consists of two parts. The first part is a procedure for estimating the heeling moment Mwind due to steady wind in a wind tunnel. A blower may be used as an alternative as long as the uniformity of wind speed is comparable. The second part addresses the estimation of the heeling moment Mwater due to steady drifting in a towing tank.

  3.1.3 The steady wind heeling lever, l w1 , is obtained by means of the following equation:

where:
= Mw (Φ) is the total heeling moment (N*m) when the ship is drifting laterally due to beam steady wind (90° heading angle) with an angle of heel Φ ;
= Δ is the displacement (N) of the ship; and
= The drifting force is assumed to be equal to the horizontal force Fwind due to steady wind.

The equation 3.1.3 assumes that the wind force and the drifting force work as a couple. In that case the heeling moment Mw is independent on the point of reduction of the system of forces. However, due to the unavoidable unbalancing of vertical forces arising from direct measurements, the total heeling moment Mw may depend on the point of reduction. For practical purposes, it is considered sufficient to calculate all moments with respect to the point O given by the intersection of the ship centreplane and the waterplane.

  3.1.4 Fwind is related to the wind drag coefficient CD by means of the following equation:

where:
= ρair is the air density (1.222 kg/m3 for full scale prediction);
= U is the wind speed (m/s); and
= AL (m2) is the lateral projected area of the ship exposed to wind in upright position.

  3.1.5 Mwind is obtained at full scale by appropriate scaling of results from wind tests carried out as indicated in paragraph 3.3. Mwater is obtained at full scale by appropriate scaling of results from drifting tests carried out as indicated in paragraph 3.4.

Parent topic: 3 Guidelines for Experimental Determination of the Wind Heeling Lever Lw1
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