Greg from Sheb. wrote:
Thank you both for the help. Seems like the gap between 6.0, 5.2 and 4.5 is more consistent in percentage of additional sail cloth if I go with this progression:
4.5 > 5.2 : +15.5%
5.2 > 6.0 : +15.3%
6.0 > 7.0 : +16.6%
But knowing that wind force is not linear, it seems to make sense to reduce the gap between sails the stronger the wind becomes. Here's what I found online:
"When wind speeds increase from 10 to 15 knots, the force doubles. When wind speed doubles, the force exerted increases by a factor of 4. If the force at 30 kts is 3.6 lbs/square foot the force at 60 knots calculates out to 14.4 lbs/ft(3.6 X 4 = 14.4)". "The wind force at 48 kts (Storm Force winds) is double what it is at 34 kts (Gale force winds)".
No wonder why sailing in 30kn with gusts at 40kn is so hard!
For more math see here:
forum.gcaptain.com/t/wind-speed-to-force...er-square-foot/15221
Greg from Sheb. wrote:
Per the website above, here's how to calculate Wind Force (in lb/sft). Source: U.S. Navy Sailors Handbook
Force per square foot experienced when wind is blowing perpendicular to a surface is calculated using this formula:
F = 0.004V(squared)
F = wind force measured in pounds per square foot
V = wind velocity in knots
Wind Speed (kts)______Force (lbs/square foot)
10__________________0.4
15__________________0.9
20__________________1.6
25__________________2.5
30__________________3.6
34__________________4.6
Wind speeds for my Ezzy Tigers in MPH
15+ mph: 6.9
20+ mph: 5.8
25+ mph: 4.7
30+ mph: 4.2
To add to the other factors and formulas
Weight of the Wind
weather.mailasail.com/Franks-Weather/Weight-Of-Wind
"Boyle’s law... simply says that, for a given pressure, the density of a gas varies inversely with the temperature where temperature is in °K (0°C = 273°K)."
In dry air
"...cold air is denser than warm air and cold air will create a greater force than warm air at the same speed."
The effect of water vapour
"...moist air is less dense than dry air. This is because water vapour is a relatively light gas compared to oxygen and nitrogen - the main constituents of the air. The chemical formula for water is H2O ie two molecules of hydrogen to one of oxygen. When water vapour content increases, the amount of oxygen and nitrogen in a given volume of air decreases."
Extreme example
"...in the tropics with a temperature of 32°C (89.6 F) and 90% humidity, the air density is 1.14 kg/m3. In a northerly airstream around the UK, we might have temperatures of 10°C (50 F) and 10% humidity giving an air density of 1.25 kg/m3. So with
cold dry air, the
force of the wind on a sail would be about
10% higher than with warm humid air
at the same wind speed."
Over the years we've always felt this but were never able to put numbers to it. On really hot humid days 90 F + the wind felt "soft".
Last year we had a day 93 F and the 6.9 was required in wind that looked like 5.8. Then I sailed a day that was 50 F and the 5.8 worked fine in wind that looked 6.9. Meter readings confirmed both cases.
So on cold dry days you really are feeling those gusts.