Principles of Yacht Design

Stability statistics In general, modern yachts have larger GMs than traditional ones due to their larger beam/draft ratio. The scatter is very large, however. In a survey of American IMS yachts around 1990 the lowest value was 0.67 m and the highest 2.1 m. Contrary to what could be anticipated, there was no definite trend of increasing GMs with length. This is an effect of the reduction in relative beam for larger yachts (to be discussed in Chapter 5). The vast majority of yachts had a GM in the range 0.75 -1.5 m. As appears from Fig 4.9 the YD-40 has aGM of 1.45 m and is thus relatively stiff.

As for the stability range, several yachts in the IMS fleet had a positive righting moment up to 180° while there were other yachts which developed negative stability at 100° of heel. The average was 122°, which, as we have seen above, must be considered a relatively low value from a safety point of view.

A rapid way of judging the stability of the yacht is to compute the so-called Dellenbaugh angle. This is approximately the heel angle the hull will attain when sailing to windward in a 8 m/s breeze. The anelc is computed from a simple formula (given in Fig 4.21). containing the sail area, heeling arm, GM and displacement. The heeling arm is defined as the vertical distance between the centre of effort of the sails and the centre of lateral pressure of the underwater body. (Both will be discussed later, particularly in Chapter 8.) Most modern yachts fall within the band of Fig 4.21, which gives the Dellenbaugh angle versus the waterline length. The difference between stiff and tender yachts is about 6° for all lengths. For a 10 m Lwl yacht the angle is therefore between 13° and 19°, and the value for the YD-40 is 13.6°, which confirms the finding above that the yacht is quite stiff. Note that the Dellenbaugh angle says nothing about the stability at large angles of heel.

Fig 4.21 Dellenbaugh angle

Dellenbaugh Angle

Dellenbaugh angle = 279 •

[13.6 degj

Dellenbaugh angle fdegj

As : Sail area (triangular) [m2] HA : Heeling arm [m]

m : Displacement [kg] GM : Metacentric height [m]

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