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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.

peter_pan
2005-04-27, 10:36
Verlager,

You pose an interesting question....Your tie down material assumptions and area statements are dangerous and incorrect. Your discussion of the issue is also clouded by mixed examples.

First. A Bungie cord should never be used to tie down a tarp...Before any tarp failures, it is more likely the bungie will pull out a stake...Especially true if ti ultra light stakes are used, which have minimal surface area....Such a stake will become a missle and could lead to injury or death...Please do not use a bungie in this manner or suggest that others do so.

Second. Good site selection out of the wind should virtually eliminate wind...not just reduce it by 15 % or so...The point here is that we should encourage good site selection, not just minimal wind protection...this point is even more true if a storm is approaching or raging.

Third. When an 8x10 tarp is folded in half to create an "A" the surface area of a side is 40 square feet not 20 as cited in your post...

Fourth. If your concern is for an 8x8 hung on the diagonal, you should use an appropriate example to model that problem, not a different geometric shape with differing number of tie outs. A tarp , actually 7.5x7.5, has an area of 28.1 square feet on one side of an "A" made on the diagonal ridge. This is a much smaller "sail area"...

Fifth. Storm conditions require extra diligence in ones preparations... If this problem is for a "field" test, any shelter should be pitched to its design optimum and use all available tie out points. The 8x8 tarp has three tie outs per side just as most larger 8x10 (some have 5 on the 10' side)...Use of all tie outs significantly changes the area per tie out point...for the 8x8 tarp this become less than 10 sq feet per point...( i don't know how this should affect the modelling, but it is obviously a major issue)...

Sixth. If breaking strengths are the issue, they are best approached initially in lab or developmental testing... Here the focus can be on each element.......there are several elements to be considered even here.......the actual silnyl itself... the size and nature of the corner tab/grommet (Width, nylon, polester, polypropelene)... the method of attachment ( triple Z, Double Z, simple Z, bar tack 1/2", bar tack 1", double bar tack 1/2", double bar tack1")...The corner design ( simple rolled hems with tab tacked on or grommet applied, triangle gusset added ((how large and what material)), back folded 2x-inserted and sewn tab- then tab refolded out and sewn((pick from preveous))and rolled hems where the seams begin and end in the 6 layers of tarp material and tab material with at least 1/2 " back stitching in this area)... diameter of the tie out cord- its material and strength are also factors...

Seventh. as you note, most folks should prefer to have a cord fail first. There is a definate trend to lighter cords because of this point and the obvious weight reduction...

FTR... The Jacks R Better 8x8 Tarp is the smallest and lightest manufactured tarp designed for hammock use ( Stock HH fly and Silnyl extended length poncho excepted)...Its surface area per side is 28.1 sq ft...It has 3 tie outs per side...They are 3/4" nylon gross grain... It has the triple Z for tab sewing... And it uses the strong 2x folded corner etc .....There have been no reported tab or corner failures for this tarp...There are known cases of stake pull out...

Pan

Verlager
2005-04-27, 15:11
First. A Bungie cord should never be used to tie down a tarp...Before any tarp failures, it is more likely the bungie will pull out a stake.. I knew this 20 years ago, but I forgot it. I accept all of your points, as it is apparent that you have an engineering background, which I lack.

Thank you, Jacks 'R' Better, for the corrections to the errors in my previous post, the geometry lesson, and for the insights into tarp design and testing considerations. I went back and edited the original post. I want to take it further and see what I can develop.

-Verlager

No harm in considering ways of reducing this stress factor. Yes, sensible hikers should select calmer areas for their campsites; taking advantage of shelter afforded by natural terrain. Some might seek the nearest trail shelter or try to walk to out of a potential storm area. Others, daunted by the possibility of sleeping in a hammock with 2" of water, simply opt to stay home if the weather looks bad.

But being aware of these potential forces against the tarp, could the design of those big advertising banners that have slits cut in them to allow the wind to pass through be applied to a tarp? They are largely unaffected by wind because the slits dissipate the wind force.

Cut reinforced slits in the tarp 1/2 up from the ground of a length of 3'. Add a loose fitting panel in the back to deflect or duct the wind and rain. The reduced stresses would allow the tie-out points to withstand a more violent wind. It's just nylon, relatively cheap, and it can be sewn in any design imaginable.

Lanthar
2005-04-28, 10:47
The one thing that makes sense in the original post, that I think many people forget, is that you want the fail point to be something OTHER than the tarp. Thre is probably no need to use 3/8" spectra lines for tie-outs, for example.

peter_pan
2005-04-29, 15:51
Verlager,

Calculating wind force and its consequences to tarps of various designs probably has some merit. Applying the laboratory to the practical can clearly be a second step. It seem this issue is relevant to all tarps and as such would probably be best addressed as general wind force problem to which you could put the issue of various tarp area, shapes, pitches and manufacture designs.

I’m not sure that your sailing model is appropriate for this analysis. As a minimum I recommend that you cite the source of the formula so that it and any necessary modeling assumptions can be reviewed by those interested in pursuing this issue. It would seem that getting an agreed model to use would be the first step.

As an example, an "A" pitched tarp would have a side slope, the from the ridge line towards the ground that could vary from 30-60 degrees from the perpendicular between the ground and ridge line. This will cause deflection that significantly varies any force models. This does not seem to be provided for in the stated model. The wind angle you state as assumed, "90 degrees to the long angle of tarp" means the wind is 90 degrees to port or starboard, or from the side…this sailing model, is based on a mast that is straight up, 90 degrees from the plane of the water at resting position of the craft. This assumption is wrong given that no one pitches the tarp straight down.

Recommend that you post subsequent reflections, analysis, modeling, data etc. as new posts on the tread. When you significantly revise the tread starter, as you have done, or any post it makes it very difficult to follow the discussion and the contribution of others.

If there are any experts in this area, please feel free to join in.

Pan

Verlager
2005-04-29, 19:34
Calculating wind force and its consequences to tarps of various designs probably has some merit. Applying the laboratory to the practical can clearly be a second step. It seem this issue is relevant to all tarps and as such would probably be best addressed as general wind force problem to which you could put the issue of various tarp area, shapes, pitches and manufacture designs. Yes, I will remove all named references to your tarp at once. Sorry about that!


I’m not sure that your sailing model is appropriate for this analysis. As a minimum I "recommend that you cite the source of the formula " so that it and any necessary modeling assumptions can be reviewed by those interested in pursuing this issue. It would seem that getting an agreed model to use would be the first step.

I did. Just click on the link below. Use the "mouse."


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


As an example, an "A" pitched tarp would have a side slope, the from the ridge line towards the ground that could vary from 30-60 degrees from the perpendicular between the ground and ridge line. This will cause deflection that significantly varies any force models. This does not seem to be provided for in the stated model. The wind angle you state as assumed, "90 degrees to the long angle of tarp" means the wind is 90 degrees to port or starboard, or from the side…this sailing model, is based on a mast that is straight up, 90 degrees from the plane of the water at resting position of the craft. This assumption is wrong given that no one pitches the tarp straight down.Yes, you are partially correct: a gentle breeze will be deflected by the angle the tarp was pitched at. But a severe wind creates a temporary crater in the side of the tarp, and it is this semi-flattened area that the modelling seeks to address. The original pitch of the tarp lessens somewhat in a high wind. As the tarp distorts under the force of the wind, it flattens closer to perpendicular, doesn't it?


Recommend that you post subsequent reflections, analysis, modeling, data etc. as new posts on the tread. When you significantly revise the tread starter, as you have done, or any post it makes it very difficult to follow the discussion and the contribution of others.The original post was terrible .. it had more bad than good, and I decided that it was simply unlimited fuel for detractors and backbiters, therefore it had to be streamlined.

youngblood
2005-04-29, 20:52
Wasn't the original quest of this thread to select guyline that would break before the tarp in an effort to 'protect' the tarp? Anyway you do this you are going to have to make assumptions that aren't necessarily going to hold true in every situation, but that is probably about the best you can hope for without getting complicated. To do that couldn't you just compare the burst strength of silnylon integrated over the surface area and compare that to the breaking strength of the guyline multiplied by the number of guylines?

Youngblood

peter_pan
2005-04-29, 22:07
Verlager,

I read your reference. The context of the article’s discussion is about capsizing sailing vessels and how much sail a vessel can handle under varying conditions. An understanding of the dynamics of sailing is necessary to understand the appropriateness of assumptions and the parallels to tarp applications. The context for this entire article is in reference to counter balancing fin forces and it is on a moving vessel. I doubt taking one formula out of context, especially when it contains a controversial undefined "coefficient" will yield any useful information. But let us look further at it.

First, the entire article is a sea level computation. The factor(s) associated with elevations of 1000, 2000, 3000, 4000 and 5000 feet need to be addressed. How is a good question.

Second, the "coefficient" is not defined and the author states it is highly controversial. I believe it is referring to the coefficient of the efficiency of the sail both design and materiel. It is important to note that from 0 to 20 degrees of wind attack angle it is equal for all three displayed coefficients . This is also true at 90 degrees, which is your stated assumption for review. At these point the operable coefficient is 1.0-1.2. Taking the coefficient to be 1.5 as, "worst case" when assuming a wind attack of 90 degrees as you have is a flawed assumption. Since we are not searching for the wing effect for propulsion, and since we wish to apply any results to a static model ( staked down tarp) at best the coefficient should be assumed at 1.0 or neutral.

Third the author states that shape is important. "Tall skinny sails can reach their maximum at much lower angles than short fat sails. They can also be more efficient at low angles of attack because they have less drag and thus can provide better performance sailing into the wind (but they are harder to control than short sails so the effect may not be always real)". Again this model is about aerodynamics, not a fixed model, that is what the coefficient addresses.

Yes, I can sail and received a captains license for 13 foot vessels from the recreation services sailing school at the Naval War College, Newport, RI in 1989.

Since you agree that the angle of pitch causes deflection, it seem ludicrous to assume it away at higher wind speed, and a then add a table that starts displaying data at 5 knots.

All experienced tarpers know that the more severe the weather the greater the requirement to flatten the profile by widening the angle of pitch. This is especially true if a sheltered area is unavailable.

I can not see how this model is useful, it in no way approximates the use of a tarp by a hammocker or ground camper,

Youngblood make a good point. The breaking of a guy line is preferential to tarp failure....seems the data search should be reduced to that level.

It is also preferential for a stake to pull out as long as it is not propelled by a bungie cord. Using a few inches of sling shot rubber or shock cord as a slack absorber in concert with the guy line is a totally different issue from a fully loaded bungie. I repeat my caution...UNDER NO CIRCUMSTANCES SHOULD BUNGIE CORDS BE USED TO STAKE TARPS.

Pan

Verlager
2005-04-30, 04:16
I can not see how this model is useful, it in no way approximates the use of a tarp by a hammocker or ground camper,

Youngblood make a good point. The breaking of a guy line is preferential to tarp failure....seems the data search should be reduced to that level. I can see how a tarp user (or manufacturer) would prefer a tarp guyline to fail first. Railing that "tarps aren't sails", "sails move and tarps are stationary", "do the calculations 5000' above sea level!" accurately suggests that I lack appreciation for these critical criteria. Perhaps you shouldn't release any data on the breaking strength of various points of the JRB tarp until more accurate modeling is present, and then only to trained QA/QC people.

Of course I'd like to see a more accurate method of calculating wind forces. I think we agree that the formula cited is, at best, "fuzzy math." But the formula and table illustrate that force increases exponentially as wind speed increases. That fact, with a basic table, is all I wanted. Anything over and above that, is gravy.

I read your bio at http://216.83.168.206/About%20Us.htm "NewPage0", and I thank you for your many years of military service to country. I like your resume.


Since you agree that the angle of pitch causes deflection, it seem ludicrous to assume it away at higher wind speed, and a then add a table that starts displaying data at 5 knots.

But a severe wind creates a temporary crater in the side of the tarp, and it is this semi-flattened area that the modelling seeks to address. The original pitch of the tarp lessens somewhat in a high wind. As the tarp distorts under the force of the wind, it flattens closer to perpendicular, doesn't it?