Knowledge Base
Sail & Rigging Calculation
You've been merrily sailing along for years, never really knowing the actual numbers of wind force that your sails and rigging are exposed to.
Common sense tells you to reef the sails to accommodate windy conditions and that works for you!
You might even enjoy testing the limits on windy days just to see what "she will do"; heeling over until the toe rail gets wet.
Whatever floats your boat but beware...
A catastrophic sail or rigging failure can be life threatening, not to mention expensive to the point of stimulating the Marine Trade's Economy!
As responsible sailors it is important to know our vessel's limitations and the effect WindForce may have.
Let's start with a simple calculation comparing 15 & 30 knots of wind.
Convert Knots to Mph
15 knots x 1.15078=17.26 mph
30 knots ×1.15078=34.52 mph
Wind Pressure Formula (psf)
P=0.00256×V2
Formula Applied
P=0.00256 × (17.262) =.76 psf
P=0.00256 × (34.522) =3.05 psf
(1191.6)
Calculate Sq, Ft of Sail
Luff x Leech x Foot
40H x 32 x14B
.5 x (B)(H) = 280 Sq, Feet
WindForce at 15 Knots
= 280 x .76
213 Lbs.
WindForce at 30 Knots
= 280 x 3.05
854 Lbs.
Double the Wind Speed
is FOUR times the WindForce.
On days when the forecast calls for gust factors more than 10% of the winds it's a good idea to take caution.
If you observe your sails to be strained during 10-15 knot winds, this is an indicator that you should definitely REEF the sail to prevent damage when in higher winds and gusts.
But what about the rigging?
Forestay Strength Calculation
Since the forestay carries 50 to 70 % of the horizontal load let's use 60% of the pressure load to estimate pressure per foot.
Force on forestay=0.60×854≈512.4 lbf
Force per foot=40 ft512.4 lbf≈12.81 lbf/ft
Calculate the Working Load Limit
1/4 1x19 wire breaking strength is 8200 lbs.
Use a 5:1 Safety Factor
WLL=8200/5 =1,640 lbs.
Calculate WLL for a 44 Foot Stay
1640/44 = 37.27 Lbs. Per Foot
The forestay is safely able to handle nearly 3 times the load at 30 Knots.
At aladofurler.com every roller furler is designed for reefing on any stay.
We recommend that you always know your sail and rigging specifications.
"Reef Often and Wisely"
The AladoFurler.com Team