How to tie a sling for SHE's prototype HotPot

David Delaney, Ottawa,
May 7, 2003

Solar Household Energy, Inc. (SHE) is developing the HotPot, an excellent greenhouse pot for solar panel cooking.  The sling shown here makes carrying the HotPot easier and safer.

Below, the sling on an upside-down greenhouse.


How to tie the sling.

The knots recommended below are chosen to be extremely secure and permanent once tightened. They can be very hard to undo, so don't pull them really tight until you're happy with the sling.

The sling shown here is of nylon, which softens at 180C (356F) and melts at 219C (426F). Some nylons have higher (better)  softening and melting temperatures.  Under the conditions tested so far,  none of the external surfaces of the HotPot reaches temperatures that pose any threat to the sling. The circumstances tested so far are: 1. An empty HotPot in full sun (but not in the reflector). 2.  A HotPot full of water at 94C. A limp cord, like braided nylon, is much to be preferred to a stiff rope.

Obtain three pieces of braided nylon cord at least 1/8" (3.2 mm) in diameter. Be sure to get pure nylon, or at least be aware of the composition of the cord. The other fibers in mixed fiber ropes tend to have a much lower softening temperature. Experience may show that the higher softening temperature of nylon is not necessary. The cords shown here have the minimum diameter of 1/8". A diameter of 1/4" (6 mm) would not be too large. The measurements given here are for a 1/8" cord.  I don't know if they will work for 1/4" cord. The greenhouse loop cord might have to be a bit longer for 1/4" cord, since the the knots will take up more of the thicker cord.  For 1/8" cord, the two equal length cords (the handle cords) should be 35 inches long (900 mm). The longer piece (the greenhouse loop) should be 62 inches long (1600 mm). The ends should be melted smooth with a flame to keep them tidy.

Start with the longer piece, which will form the greenhouse loop--the loop that encircles the bowl of the greenhouse. First make a water knot joining the two ends into a loop. A water knot could also be called a follow-though overhand knot. Here's how to tie it.

Here's how the loop should look. When stretched into a line, the inside measurement of the resulting loop should be between 27" (685 mm) and 27.5" (700 mm).

Now make four overhand loop knots in the greenhouse loop. First, here's how to tie an overhand loop knot.

Before tying the overhand knots, you must mark the greenhouse loop in four places as indicated in the drawing below.

Now tie four small overhand knots into the greenhouse loop, with each of the ink spots you have just made at the tip of a loop, as shown below. This is the correct size of the loop eye--about 1/4" (6 mm) inside diameter.


The result is shown below.

Now attach each of the four ends of the two handle cords to one of the loop knots using a buntline hitch.  Once upon a time, the buntline hitch fastened halyards and outhauls to the clews of sails--a very demanding application. The buntline hitch is hard to undo--it jams solid-- so don't pull it really tight until you are finally satisfied with the position of the knot.  Once jammed tight it will never come undone by itself.

Here's how to tie a buntline hitch to a loop.  (If you know knots this procedure is easy to remember,  since the buntline hitch is an upside down clove hitch on the standing part.)

Now, form up the knot as shown below before sliding it down to close the loop. This next view shows clearly that an "upside down" clove hitch forms the main part of the buntline hitch.

Slide the knot closed and firm up the knot to finish the hitch, as shown in the last view above.

Tie the buntline hitch four times to attach each of the four ends of the two handles to one of the the loop knots, as shown below.

The sling is now complete. The greenhouse sits in the central rectangle.

The sling is very secure if the short sides of the rectangle are under the handles of the greenhouse. The sling is UNSAFE if the long sides of the rectangle are under the handles of the greenhouse. The short sides are just long enough so that the loop knots nestle right at the ends of the handles of the greenhouse.

Here again is the sling on the upside-down greenhouse.


More solar cooking pages...