The following comment by Bill is at the same the reason why the strut is the heavy weight solution.
>>This is a stability equation and has to do with 'buckling failures'.
A trick to get around the buckling problem is to induce the buckling failure mode yourself. = PREBEND spinnaker poles. Due to the prebend it can not buckle downwards anymore.
By bending the pole upward you force the failure mode to be directed upward which can be stabilized again by the little wires from the midpole to the bridles. See F18 pole setups. This is lighter (spectra dyneema HT line), cheaper and easier to repair. This has the advantage that also sideways buckling is stabilized which may not be the case with just the strut.
This means that the reason to opt for a strut is only one. Lengthening the luff of the jib. Supportwires to the sides are then still needed to stabilize the pole to the sides or the pole needs to be made stiffer to have sufficient "I" when you want to do it without support wires.
Another trick is to make the angle of the tip support wires relative to the pole as big as possible. The I-20 does this I believe. Here the tip support wires are not lead back to the bridlewires but have their own attachement point forward of the bridle wires.
For cats up and including 20 foot a tube of 40 mm by 2 mm is stiff and strong enough for the all used spinnaker sizes (not hooters). For 18ft and all 16 footers 40 mm by 1.5 mm is by far enough.
I've used 3.55 mtr. by 35 mm by 2 mm for several years and this was on the borderline of buckling WITHOUT midpole support wires. With them I had no problem what so ever not even in windforce 6 (25 knots) although I wished I hadn't pulled the kite then.
40 mm = about 6/10 inch 2 mm = about 1/12 inch
Weight of a 3.75 mtr. by 40 mm by 2 mm pole incl support lines (dyneema) = 2.5 kg = 5/5 lbs
Weight savings on a 3.75 mtr 35 mm by 2 mm pole EXCL strut = 2.5 - 2.2 kg = 0.3 kg = 3/4 lbs. Any guesses what a strut of say 2 ft weights ? Bills 3/4 inch SS strut weights 400 grams = 1 lbs. And than it is not stabilized in horizontal plane.
Two lines of dyneema and prebend in the pole add only grams to whole setup.
The tornado guys are only using the strut to keep the jib area the same to what they had before. It is not a stability device as they still have the lines - wires for that.
Wouter
