HR SOLUTIONS-POLYSAIL

Material Strength Testing

We wanted to find out whether our white polytarp material could hold up to the wind pressures that most sails are subject to. We used a scale developed in 1806 by Admiral Beauford of the British Navy that has been refined and is in common use to determine wind strength and pressure on sails on the open ocean. On inland lakes, wind pressures increase by about three Forces. The numbers in blue show the Force equivalent and pressures on sails on inland lakes and waters. We wanted our sail materials to stand up to at least 10 lbs. of pressure per square foot.

For most small sailboats and dinghies that might use white polytarp sails, the maximum pressure exerted on sails would be unlikely to be over 10 lbs. per square foot because the boat itself would probably not survive a Force 9 gale on the Beauford Scale. In fact, few inland water dinghy sailors relish being out in winds over 30 knots because even the most experienced sailors can end up in the water with those winds. Our guess is that most sailors who would use polytarp sails would rarely experience more than about 5 lbs. per sq. ft. of wind pressure on their sails unless they were caught by an unexpected gale.

BEAUFORD SCALE

To see whether our white polytarp sails could withstand a pressure of at least 10 lbs. per square foot, we placed a 42 lb. lead acid battery on a 4' x 3' piece of suspended white polytarp. The "footprint" of the battery is 6 1/2" x 10 1/4" or 66.625 sq. inches or .46 sq. ft. This means that the pressure of our 42 lb. battery was exerted on about .46 sq. ft. of the material. This is roughly equivalent to 90 lbs. per square foot of pressure exerted by the battery on the material.

 

The pictured tarp was about three years old and was originally used to cover the bow of Hot Tub II when I transported the boat under the bed cover of my pickup. This piece of polytarp is not reinforced at the edges as we normally would reinforce a sail. Usually we double the material back over double faced tape and rope, then apply vinyl tape over the edges before installing grommets. Instead, this material is covered only with vinyl tape at the edges and then grommeted. The age of the tape and material along with the lack of reinforcement might explain why the tape lifted at some of the points of stress. Other than that problem, there was little evidence of damage from the battery pressure.

The final picture in this series shows the white polytarp material after the battery was removed. I could find little evidence of stretch or indentations from the battery.

This test suggests that this inexpensive material should hold up well when reinforced properly and used for sails on smaller, homebuilt wood boats under 20 feet LOA.

This page last updated 2/22/01