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Line strength for hydrofoiling ?

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dave1986
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Re: Line strength for hydrofoiling ?

Postby dave1986 » Thu Oct 03, 2019 10:52 pm

Peter_Frank wrote:
Thu Oct 03, 2019 9:14 pm
I think it is simply thinner Dyneema lines, but well made.

Dont care about where to get them (but would simply ask Liros if I had to), I am lazy so get "Famous Frank" (Frank Rosin in here) to splice them on ordered length :thumb:

Only thing is, they tend to sing more, these lines.

8) Peter
What performance benefits do you see from thinner lines in light winds?

I can see that there would be less drag and therefore better upwind angles... but do you expect it'll make a detectable difference to the low end when using an Ultralight foil kite?

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Re: Line strength for hydrofoiling ?

Postby foilholio » Thu Oct 03, 2019 11:47 pm

Thinner lines weigh less, so affect the kite similarly like a lower weight kite and they have less frontal area and so less drag. In light wind less drag is far more important than high wind. Drag makes the difference to how well you go upwind or how long and far your can fly aka "float" in jumps. Lower drag is the difference in light wind between going upwind or not, which I think is the difference between enjoying kiting and not. A lower weight kite also reduces induced drag, because the kite needs to produce less lift and with it drag to support it's weight. It is not much, but many little things add to help.
TomW wrote:
Sun May 13, 2018 10:13 am
I'm not a math person, but know that F= MA.
F is force in Newtons, M is mass in kg, and A is acceleration in m/s.

You can then look at it 2 ways.
1. The kite pulling your body up out of water or for a jump.
F= 80kg x 2ms = 160N.
You might say your body is moving faster.
F= 80kg x4ms = 320N
Or you are pulling up water too:
F = 100kg x 2ms = 200N

2. Your body might be fixed point, and kite is exerting power against it. How much force can a kite produce in the scenario of kite luffing and regaining power suddenly?
This seems to me the most likely failure. It could easily be 400N or more.

Now remember you have 2 lines, but might not be equally stressed at some moment.

Finally, lines are rated in kg. Which is confusing because it must really mean Newtons.
So seems to me that 280kg rated lines x 2 lines is sufficient for 80kg foiler.

You might say you need 2.5x safety margin, then the 500kg lines make sense.
You forget to account for Gravity in your calculations. 80kg is 785N. 2 front lines is 560kg or 5492N. No real point switching between N and kg. With a rider of 80kg and lines rated to 560kg you can accelerate at 560/80 or 7G, -1G if accelerating against gravity so 6G. Most lines with age will fail at half that so 3G. I would say most kiting forces are within 0.3-2G, with very rare amounts at or above 3G. Aswell you need to factor with these forces that some are very sort duration and the elasticity of the line can adsorb much greater forces for shorter periods. Dyneema is not very elastic though, but with that the length of the line is important.

I would say if you didn't jump you could use lines for 2G new or 4G old. So for 80kg rider that is is 80x2G/2lines or 80kg lines or x4G so 160kg lines. I would say for rear lines you could use half or less strength lines.

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Re: Line strength for hydrofoiling ?

Postby Regis-de-giens » Fri Oct 04, 2019 9:33 am

I am afraid that this estimation method is not suitable "alone" ; indeed one easy way to assess this "permanent regime " force during a ride (exclusing jumps and kite sudden opening) is the angle your body has against gravity ; that is directly the counter-reaction accounting for all weight, drag, water pulling, ect ...
And it can go beyond your own weigh as soon as you incline more than 45 degrees . For exemple at 60 degree vs vertical (with kite very close to the water, say well charged ), the total tension of your 4 lines is your weight * 1.73 = 140 kg for a 80 kg rider ; to be spread into approx. 40 kg in each front line and 14 kg in each rear line ... (by the way at this angle your hydrofoil shall support twice your own weight on its single wing... )

Well ... this is obviously not the only force you need to take into account to rate your lines ... as such sewing line will break very fast ... so other loads will count , like all the dynamic (kite driving vs rider inertia ) + gusts + wear (i.e. riging on ground , salt inside the line) + fatigue by cyckling + concentration loads factors for fatigue (like splices) ;

So despite I love use the theory when "available" , I think that in this particular case the test and feed back from testers is more than recommended to choose one's line diameters . In my case for my current light wind steup:

- SK99 in 1mm (front) and 0.8 mm (rear) => no rupture for 2 years and about 50 hours hydrofoil use in 5-11 knots + about 20 small jumps (3 meters high 60 kg);

- Now I have ordered 0.7 mm line (not found in SK99 unfortunately) for rear and will go for 0.8 front and 0.7 rear ;

- I had previsously 110 kg front and 70 kg rear during 8 month without breaking ( no jump) (poor quality dyneema lines)

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Re: Line strength for hydrofoiling ?

Postby Foil » Fri Oct 04, 2019 12:15 pm

interesting subject with some crazy hard to understand facts with confusing maths.

All I can add from my experience is that to me its the feeling you get if you use thinner lines,
the feeling of being unsure your lines will hold,
to me that the most important element to enjoying my ride out onto the water,
if like me you have experienced a snapped line at the peak of a jump, then you never forget it
I recently bought a set of 4 lines 16mtr sk99,
all 4 lines are the thin 1.5mm all 8 ends came finished with a simple splice and stitched.
a few days after loading them on to my bar I was learning how to jump,
then the worrying started as the lines do look so thin, this feeling had me checking the specs for the line, which looked fine,330 kg looked good,
but the feeling I was risking a failure was always there as I was boosting, the stronger the wind, the worse the fear of failure got.

eventually It was this feeling that made me revert to the standard Ozone sk99 lines,
now I smile as i load up the line for a boost, instead of worrying about a ping event.
whatever the maths say, it was not enough to stop the fear of repeating a snapped line event whilst boosting.

anyone want to buy a set of hardly used 16mtr sk99 1.5 mm lines before i put them on Ebay?

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Re: Line strength for hydrofoiling ?

Postby Mossy 757 » Fri Oct 04, 2019 4:17 pm

I race foil kites on 1.3mm Dyneema SK99 I got from Greenhat. I try not to boost with it, but sometimes I do and it seems to work fine. I would never throw a kiteloop on this stuff, but it's held up for a little over a season now without any issues or indication of wearing thin prematurely.

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Re: Line strength for hydrofoiling ?

Postby Ramkiter » Sat Oct 05, 2019 5:29 pm

anyone want to buy a set of hardly used 16mtr sk99 1.5 mm lines before i put them on Ebay?
Might be interested sent you a pm


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