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Windage

To determine the appropriate anchor for any particular vessel, many factors must be taken into account. The most important of these, is its windage determined by two factors; (1) the surface area and, (2) the frontal shape of the vessel as presented to the wind, i.e. the windage area. On flat surfaces presented at 90 degrees to the wind, the "Shape Co-efficient" is something like 1.2 (i.e. 1.2 times the surface area). For round objects such as masts and wires, the shape co-efficient is 1. On flat or rounded convex surfaces presented at an angle to the wind, the shape co-efficient is about 0.7 whilst on aircraft it is less than 0.10. The wind pressure for one square metre on shape co-efficients of 1.2, 1, 0.7 for varying wind speeds can be found from the simple graph given below. (Figure 2). Wind pressure (Drag) is determined by the formula:

d=c*p*v2*s/2

where p is the density of air, v is the wind velocity, s is the cross-sectional area at right angles to the direction of the wind and Cd is the coefficient of drag that varies with shape or profile.

Pressure Exerted by Wind
Fig 2

Pressure Exerted By Wind on One square metre of Surface Area.

The main requirement is to know the frontal surface area and shape of the vessel under consideration, not the length of the boat. In conjunction with this, is to know the expected wind velocity against which an anchor may be required to hold - for practical purposes, you can forget about water forces such as tidal current and waves, as these are minimal compared to wind forces.

The frontal surface area is a good guide to the windage of most objects, but with a boat riding at anchor, a certain amount of asymmetrical yawing and heaving on waves takes place. Rarely is a vessel directly head-on to the wind and waves. Horizontal yawing can be up to 30 degrees either side of the wind direction, thus exposing considerable beam to the wind direction. Heaving on waves causes an additional surge factor that can further add to the loads imposed (the weight or displacement of a vessel has very little bearing on its anchor loading in any respect other than this surge factor). Because of the veer and surge effects in very extreme cases, the load can be twice that provided by the head-on surface area - so for safety's sake, you should allow for double the loading calculated on the head-on windage area.

Every boat owner should find out by a calculation or otherwise, the wind loading on his vessel. A general table of co-efficients for different types of vessels is given below in Figure 3.

BoatCross Sectional Area mm Drag Coefficient, Cd
20 ft Etchel30.65
20 ft Bertram40.75
20 ft Houseboat81.20
26 ft Folkboat40.70
26 ft Triton, Tophat, Spacesailer50.70
32 ft Clansman70.70
32 ft Mariner Cruiser80.85
36 ft Grand Banks0.90

Fig 3

The 26ft. Folkboat area is made up roughly of:
Hull3.00 m2
Spars & Rig0.75 m2
Furled Sails0.25 m2

Power boats generally are 25% greater in windage than sailing vessels of similar length. Whilst they may have twice the windage area in beam, topside bulwarks and bridge, they do not have spars and rigging with furled sails lying on deck.

Wind Velocity

As noted, pressure increases as the square of velocity.

On a vessel of 5 square metres, wind pressures for different wind speeds are given as follows:

Wind VelocityWind Pressure
10 knots6 kg
30 knots54 kg
60 knots (120 kph)200 kg
100 knots600 kg

Fig 4

These pressures should be doubled for the reasons given, so that a vessel such as a 26' Triton or Top Hat or a 20' Bertram should have an anchor that will hold 400 kg in a 60 knot (Force 11) wind.

A Nomogram, from which you can determine the wind pressure on any vessel up to 20 square metres for wind velocities up to 100 knots is given in Figure 5. The Force shown is in kilogrammes and includes the surge and veer factor of 2 times.

Nomogram
Fig 5, Nomogram A
Wind pressure on varying surface areas 1m2 - 20m2
Force = Pressure x Area x 2

To use nomogram, follow the example shown:

Given: V = 32 knots; Cd = 0.7; A = 5

then Force = 125 kg


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