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Based on Franckite (the Sword father) method on the kitefoil Forum:

Comparative measurements for mast flex and torsional stiffness

Purpose of the method: investigate what makes a fast ? comfortable ? kitefoil .... using a method as reproductible and scientific as possible ... though cheap.

A very straight forward protocol:
1- a cantilever beam test with 5kg applied at a 85 cm distance from the "wall"
2- torsional stiffness test : 5kg applied at a 30cm distance from the mast

Measurements taken at 85cm and 30 cm in each case with respect to the initial position without weight
Metric to Imperial conversion: 5 kg = 11 pounds at
1 - 85 cm = 2.79 feet (2 feet 9 15⁄32 inches)
2 - 30 cm = 11.81 in - 0.984 foot (11 13⁄16 inches) - 25cm = 9.84 in

The results will be updated with your values and the Kitefoil Forum ones (Spotz, Ketos, Zeeko, Carafino and MHL measurements are missing).
Results expressed in mm would be an advantage.

Click on the table below to expand ...

I think these will be very useful data once the table gets filled. I must make some effort to measure the foils I have. I can add an MHL Lift, and two of my own designs.

I think for foils with a keel of less than 85cm, we should just measure the bending at the end of the keel.

In fact, I think we should maybe measure all the foils at the end of the keel. A longer keel would require a stiffer keel, and I think the data will be more useful to compare how stiff a foil feels for its given design versus how good the construction is irrespective of the design?

I also think we should measure the bending from the wing tips somehow. A low aspect foil might require less torsional stiffness than a high aspect foil?

What do you guys think?

Excellent..
Thanks

@christoff
I understand your reasoning and it makes sense.
One number may not suit everyone.
But, any deflection value should scale easily if both the moment arm and load are provided.
The same procedure will also help standardize comparisons between fuselage and foils.
-Bill

An update, now that the importance of stiffness has become more obvious with the arrival of the chineese foils.

The protocol is far from perfection. It will evolve very soon because it doesn't currently take into account the influence of the mast length: a 95cm mast will seem less stiff than the same mast cut to 70 cm. It shouldn't.
To take this into account, in dec. 2013, I wrote a complementary method based on windsurfing masts stiffness method. Can't find it. I will rewrite it and submit it.

Table to be soon updated with a few 2013 model figures in flex and torsion (mm): Spotz (26/9), Horue 79cm (16/5)...
Looking forward to introducing 2017/2018 model values and Tegirinenashi JNJ hydrofoil stiffness and torsion figures.

Here are exact figures for Sherman tank (oops, LF 2016 Rocket/Happy):
flex = 13 mm
torsion = 3-4 mm
P.S. With this impressive torsion number, I may reevaluate using JNJ mast, and perhaps go for some LF+JNJ hybrid build, gaining some weight, but improving stiffness.

Hmm,

is this method really measuring the mast deflection?

Or bending in almost one singe point marked in red?
The plate/mast connection design would be critical with this test, however, the mast can still be super stiff.

Thoughts?

PS

I do believe in only optimal results. That is how everything is built, including F1 or a fighter jet.

downunder wrote: ↑

Thu Aug 17, 2017 3:55 am

...The plate/mast connection design would be critical with this test, however, the mast can still be super stiff...

This is why, designs like zeeko (which features plate without collar) would likely to exhibit poor flex number. And their marketers came with nothing better than filming head on masts collision with their competitor. Really?

That is the problem for Zeeko. The topic is how to measure, the method that is not that great. Even the torsion is not taking into account the 30cm bending of that metal (?) rod.

It's a good start tho.

I would happily measure my masts, and not that difficult to find a rod or plate that does not bend for the torsinal stifness measurement - major problem is to find a "stiff" plate or wall to attach the mast to

Weight and support and so on is relatively easy, but finding a superstiff fixed reference plate/wall to attach it to, almost impossible for most

The table with comparisons should also have the weight of the mast (and the length), as the weight is almost the most important, combined with the flex/stiffness

PF

Yes, you need to be creative, if you are unwilling to drill holes in garage wall (or worse, your residence ):
Here are the numbers for JNJ:
flex = 11-12 mm
torsion = 22 mm
This is not entirely surprising because bending the mast stretches fibres, while there seems not much resistance along twisting direction.