Well heeled (part 1 of 2)

Andrew Blyth and Tom Nighy review the stability and buoyancy requirements of sailing boats.

In previous issues we introduced the main concepts of stability and buoyancy assessment using the Stability Standard (ISO 12217) and explained that this standard is divided into three parts, as follows:

• Part 1 - non-sailing boats of hull length greater than or equal to 6m(20ft)
  
• Part 2 - sailing boats of hull length greater than or equal to 6m(20ft)
  
• Part 3 - boats of hull length less than 6m(20ft)

This time we look at the requirements for various types of sailing boat — everything from superyachts to dinghies, monohulls to multihulls. Readers will probably find it helpful to refer to previous articles for explanations of some of the terminology that has already been introduced, as we do not explain again here.

What is a sailing boat?
A sailing boat is defined as any boat that is intended to use sails as the main means of propulsion. However, to avoid differences in interpretation, it was decided that sailing boats must have a sail area (in square metres) more than 0.07 times the fully loaded mass (displacement) in kilograms raised to the power of two-thirds. The table above illustrates what this means in practice.

Intermediate values can be obtained by interpolation, but such figures are only approximations. If in doubt, the formula should be used. Most motor-sailers exceed the sail area described above.

Hazards for sailing boats
Sailing boats can be exposed to the following:

• filling with water when upright, due to insufficient freeboard or watertightness of decks or closures.
  
• filling with water when heeled, either over the gunwale or through openings in the hull.
  
• capsize or excessive heel due to the wind.
  
• capsize due to the effects of waves — (principally in Design Categories A and B)

The first of these is affected by the same considerations as for non-sailing boats and is addressed by very similar requirements.

Because of their different stability characteristics, sailing multihulls are treated very differently to monohulls.

As different styles of boat counter the various hazards in different ways, the standard allows for this by providing a table of options (Table 2 in Part 2, Table 6 in Part 3). A designer or builder can choose whichever option is permitted for the particular style of boat, provided that all the requirements of the selected option are satisfied.

Righting and heeling moments for both Monohulls and Catamarans showing the Angles of Vanishing Stability.

Some more terms explained
In addition to the terms Downflooding Height, Downflooding Angle, Quick-Draining, Watertightness, Flotation Elements and Fully Decked described in the last issue, a few more terms need to be introduced:

• Knockdown is when a boat reaches a heel angle sufficient to immerse the masthead, from which it may or may not recover.
  
• Capsize is when a boat reaches any heel angle from which it cannot recover without intervention.
  
• Inversion is when a boat is turned completely upside down.
  
• Angle of Vanishing Stability is the angle of heel, at which the righting moment becomes zero. See Figure 1.

Monohulls compared to multihulls
As a direct result of their very different hull shapes, these two groups of boats have very different stability characteristics, which affects the way they are assessed.

Monohulls usually achieve maximum righting moment when heeled between 40 degrees and 70 degrees. As they heel, the wind is progressively spilt from the sails until at 90 degrees of heel there is very little heeling effect. Unless downflooding occurs, many monohulls will return to the upright after being knocked flat by a gust of wind in relatively smooth water. If the righting moment remains positive up to about 120 degrees (which usually means that the boat must have permanent ballast in the keel), it will normally recover even if totally inverted by breaking waves.

Note: The term righting moment was explained in European Boatbuilder Issue 8, July.

Multihulls, in contrast, have very high initial stability with a maximum righting moment between 10 degrees and 20 degrees of heel, beyond which the righting moment decreases and ceases to be positive at about 70 degrees to 90 degrees. Even though wind is spilt from the sails as the boat heels, the windage of the hull means that there will still be a considerable wind-heeling moment at 90 degrees of heel. Unless handled with exceptional skill such boats will capsize if heeled beyond the maximum righting moment and, once inverted, they are virtually impossible to recover — except in the case of small beach cats that may right like conventional sailing dinghies. See Figure 2.

Some ‘form stable’ monohulls, such as barge yachts, have stability characteristics like multihulls and so must be treated similarly.

Monohulls
The manner in which these are assessed varies according to the type of boat. Any one of the options in Table 2 of Part 2 or Table 6 of Part 3 may be used. All types of boat can achieve Design Categories C and D.

Fully decked boats
As defined in the last issue, these are the only type considered acceptable by this Standard for Design Categories A or B, as well as Categories C and D. Generally they must satisfy:

• downflooding height and downflooding angle requirements.
  
• minimum requirements for the boat weight and Angle of Vanishing Stability (the latter varies with the weight of the boat).
  
• a minimum value of the Stability Index (STIX) for the relevant Design Category.

For Part 2 click here

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