The Why's and How's of Shade Sail Size Calculations
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Since tension is very important to longevity, the goal is to have a sail that looks good and is tensioned well. We have tried to make this as uncomplicated as we can and hope this will help you understand why we are requiring some information, and how it is used to make very good shade sails.
We do not make shade sails with straight edges!
Understanding why the sails have a parabolic curve on each side.
It is important to understand that all our shade sails have parabolic curved edges. Without a curve it would not be possible to obtain good tension on the shade sail. The curve is concave (inward) and at the midpoint would be about 5% of the length of the side. As the shade sail is tensioned, this reduces to between 2% and 4% - depending on the tension.
For this purpose we will use a square as an example, but the principle is the same for all shapes:
- The distance between any two mounting points has to be reduced (in the direction of tension) by sufficient to fit a turnbuckle and D shackle at each end.
- If the point to point was 1000 cm it would be reduced by 32 cm at each end meaning that side of the shade sail would be 936 cm.
- If the 936 cm was straight, then when tensioned is applied the effect is to stretch the webbing - nothing else!
- When a 5% radius is added, the center point of the curve would be 46 cm from the hypothetical line between the points.
- Although the point-to-point dimension remains the same, the length of the webbing increases because it is now the length of the curve.
- Now when tension is applied the effect is to pull this side into a straight line.
- This means the distance between the opposite sides is growing thus tensioning the shade sail.
All a bit confusing? Click on the animation video to see the tightening of the turnbuckles tensioning the shade sail.
Tensioning Animation
It is good to do what we say - it works very well!
This architect customer told us, in no uncertain terms, that he knew more than we did about measuring shade sails.
This customer, who knew nothing at all about shade sails, followed our instructions!
The Three Measuring Steps
STEP 1
When you have your first idea and rough plan, use the online quote system to get an approximate idea of the cost of the shade sail.
STEP 2
Once you have created your final plan, use these dimensions to get another more accurate cost estimate.
STEP 3
This is the most important quote. After all your mountings are in position, ready for the shade sail, take measurements for the final quote.
Critical Tension Accuracy
It is VERY IMPORTANT that the measurements are from the inside of each mounting point as shown above. Avoid using string (it can stretch) or a tape measure (it will sag)—instead use builders twine or a laser. Tension is critical, so a bit too short is OK, but too long is a disaster.
Understanding Our Shapes for 3 to 8 Sided Sails
The six shade sail shapes are guides to the points we need measurements from. They are NOT indicative of the final shape of the shade sail, that is determined by your input.
We require the measurements to be plan view (looking down from above). Always have the point A at top left and then B to the right and continue clockwise.
When dealing with 5 or more sides it is essential that point B can connect directly with all other points (e.g., point B connecting to D through C is not correct). If you get a nil value, a dimension was wrong. The system uses a series of triangles to build the shape.
Understanding Allowances
Click the image to see the points that we need the measurements for (the area in green). When you select "allowance" we will make the sail as per the yellow coloured area.
Direction of Tension
This is how the allowance is calculated. In a rectangle, the direction of tension is obvious. Anything else needs our computer software to calculate it, so we strongly advise that you leave it to us.
Extra 3%
At the point you place the order, you will be offered the opportunity for us to reduce the overall dimensions by 3%. All our professional installers opt for this and about 75% of retail customers. This provides an extra element to tensioning and allows for re-tensioning over time.
Actual Example of an Order
From Input to Final Product
On the left (or above) are three images demonstrating the process.
- Top Image: The customer input on the quotation system for a large 8-sided shade sail.
- Middle Image: The CAD that our system produced for the customer to verify before we manufactured the sail.
- Bottom Image: The finished, installed shade sail.
Look carefully on the CAD for the lines with '32' by them. These are where the turnbuckles will go. Observe the position of the line which is in the direction of tension. You will see that this specific direction of tension can only be calculated accurately by the computer geometry solver.
Click on the images to view them full size.