Verlager

2005-04-26, 21:53

Understanding the strength of wind forces is important to tarp rigging.

Included here are sample calculations and a formula for

calculating maximum wind force at various speeds against a tarp.

Important note: These figures are based on winds at 90 degrees in relation to the long axis of the tarp, in an open area, with no trees or rocks to obstruct the wind. Pitching the tarp obliquely angled to the wind, setting up in dense woods or on the leeward side of hills are ways to reduce the effective wind force.

Though the use of bungee cords with tarps is risky, I use them because:

A. I have heavy, long solid steel tent stakes which I place nearly flush into the ground at a 35 degree angle. They are unlikely to come out of the ground with an oblique pull.

B. Without the bungees, the tarp fabric could be damaged if I accidently stepped on it, or pushed against it while entering on leaving the hammock.

C. There is nothing wrong with using bungee cords in pitching a tarp, except with tent stakes. They can be used fine at the top tie-in places.

Addendum A. found at http://homepages.apci.net/~michalak/1apr04.htm

I. Essentially the force of the wind on an object at sea level is of the equation: F = .0034 x A x C x V x V. "A" is full sail area in square feet. The value of C varies with "angle of attack" which is the angle the wind acts upon the sail as shown below. But it tends to peak at about 1.5 for most good sails. Really super sails might peak at something like C=2.0 and a clunker might be down around 1.0, but when running downwind all sails will have a C of about 1.2. C = 1.5 is actually a fairly high value and few aircraft wings can operate at such a high value in real life without resorting to articulated flaps or leading edge slots.

[Since we're figuring for worst case scenario, we will use C = 1.5 ]

II. The "V" in the equation is wind speed in knots. Since this value is multiplied twice, "squared", its effect can quickly overwhelm other factors. For example a 15 knot wind will produce 2.25 times as much sail force as a 10 knot wind. A 20 knot wind will produce 4 times as much force as a 10 knot wind.[Simple enough] 1 knot = 1.15077945 miles

Tarp is 7.5 x 7.5 = 56.25 sq. ft. /2 = 28.12 sq. ft. ; Number of tie outs (not for use with bungies!) = 5 ;

Plugging in these numbers to the formula: F = .0034 x A x C x V x V

A = 28.12 "sail" area in square feet C = 1.5 V = 40 MPH x 1.15 (knots = mph x 1.150) = 46

F = .0034 x 28.125 x 1.5 x (46˛) = estimated total load of 303.513 lbs. ; / 5 = 60.70 lbs. load per tie-out

Area = 28.1 sq. ft, table:

Wind speed in MPH: Total Force in lbs.:

______________ 05 _______________4.74

______________ 10 ______________18.95

______________ 15 ______________42.64

______________ 20 ______________75.87

______________ 25 _____________118.45

______________ 30 _____________170.57

______________ 35 _____________232.17

______________ 40 _____________303.24

______________ 45 _____________383.79

______________ 50 _____________473.82

______________ 55 _____________573.32

Important note: These figures are based on winds at 90 degrees in relation to the long axis of the tarp, in an open area, with no trees or rocks to obstruct the wind. Naturally, if you camp in the forest, these possible maximum loads are significantly reduced.

Maybe someone with a hiking web site and programming skills could write a javascript that would allow a user to input tarp's area and wind speed and then calculate the total force.

Included here are sample calculations and a formula for

calculating maximum wind force at various speeds against a tarp.

Important note: These figures are based on winds at 90 degrees in relation to the long axis of the tarp, in an open area, with no trees or rocks to obstruct the wind. Pitching the tarp obliquely angled to the wind, setting up in dense woods or on the leeward side of hills are ways to reduce the effective wind force.

Though the use of bungee cords with tarps is risky, I use them because:

A. I have heavy, long solid steel tent stakes which I place nearly flush into the ground at a 35 degree angle. They are unlikely to come out of the ground with an oblique pull.

B. Without the bungees, the tarp fabric could be damaged if I accidently stepped on it, or pushed against it while entering on leaving the hammock.

C. There is nothing wrong with using bungee cords in pitching a tarp, except with tent stakes. They can be used fine at the top tie-in places.

Addendum A. found at http://homepages.apci.net/~michalak/1apr04.htm

I. Essentially the force of the wind on an object at sea level is of the equation: F = .0034 x A x C x V x V. "A" is full sail area in square feet. The value of C varies with "angle of attack" which is the angle the wind acts upon the sail as shown below. But it tends to peak at about 1.5 for most good sails. Really super sails might peak at something like C=2.0 and a clunker might be down around 1.0, but when running downwind all sails will have a C of about 1.2. C = 1.5 is actually a fairly high value and few aircraft wings can operate at such a high value in real life without resorting to articulated flaps or leading edge slots.

[Since we're figuring for worst case scenario, we will use C = 1.5 ]

II. The "V" in the equation is wind speed in knots. Since this value is multiplied twice, "squared", its effect can quickly overwhelm other factors. For example a 15 knot wind will produce 2.25 times as much sail force as a 10 knot wind. A 20 knot wind will produce 4 times as much force as a 10 knot wind.[Simple enough] 1 knot = 1.15077945 miles

Tarp is 7.5 x 7.5 = 56.25 sq. ft. /2 = 28.12 sq. ft. ; Number of tie outs (not for use with bungies!) = 5 ;

Plugging in these numbers to the formula: F = .0034 x A x C x V x V

A = 28.12 "sail" area in square feet C = 1.5 V = 40 MPH x 1.15 (knots = mph x 1.150) = 46

F = .0034 x 28.125 x 1.5 x (46˛) = estimated total load of 303.513 lbs. ; / 5 = 60.70 lbs. load per tie-out

Area = 28.1 sq. ft, table:

Wind speed in MPH: Total Force in lbs.:

______________ 05 _______________4.74

______________ 10 ______________18.95

______________ 15 ______________42.64

______________ 20 ______________75.87

______________ 25 _____________118.45

______________ 30 _____________170.57

______________ 35 _____________232.17

______________ 40 _____________303.24

______________ 45 _____________383.79

______________ 50 _____________473.82

______________ 55 _____________573.32

Important note: These figures are based on winds at 90 degrees in relation to the long axis of the tarp, in an open area, with no trees or rocks to obstruct the wind. Naturally, if you camp in the forest, these possible maximum loads are significantly reduced.

Maybe someone with a hiking web site and programming skills could write a javascript that would allow a user to input tarp's area and wind speed and then calculate the total force.