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Why do I need a much bigger board AND bigger kite for a heavier rider?

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sergei Scotland
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Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby sergei Scotland » Mon Aug 06, 2018 9:54 pm

Stupid question?
I am only asking because lift/water resistance is proportional to square of speed and only linear with surface area, i.e. lifting force is:
F = k *v*v*S
where V is speed and S is surface.
General rule of thumb seems to be 135x40 for 75 kg rider 150*45 for 95 kg rider, correct?
Plus 9-10 m kite for 75 kg and 14m for 95 kg for the same conditions.
So kite roughly 40-50% more powerful required plus+ board surface area 1.25 size of smaller board.
So for only 26% increase in weight (95/75=1.26) people recommend 40-50% increase of pull PLUS 25% increase in board surface area? 1.5*1.25=1.875 - i.e. 87% combined kite + board increase...
Does not make sense to me to have 87% increase combined for only 26% increase in weight.

I would say for 26% increase in weight I might want say 13% increase in surface area and 13% increase in kite size?
13% increase in surface can be something like 135*45 cm (compared to 135*40)
13% increase in kite area is going from 10m to 11.3m .

My understanding is this somehow above increase values are actually not enough?
Correct? Why? Where did I go wrong?

Why most people suggest 20-25% increase in board area plus 40% increase in kite surface
????????? :nono: LOL

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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby _Jakobie » Mon Aug 06, 2018 10:18 pm

Ehm..

I use the same kite size, and same board size as my buddys... Riding a 140x43 Brunotti Youri 2017 with my 16m in 5-7m/s, 12m in 7-9/10m/s, and 9m from +10m/s.
They are around 70-80kg, im around 100kg and have no problem going out on a 9m or 12m when they are out on the same, i never take a bigger kite that them.
I have never heard of that rule og thumb you mention. :?

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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby hayes » Mon Aug 06, 2018 10:23 pm

I’d say your estimations on sizes are a bit off, most riders will got for a slightly bigger board if they’re bigger, but I wouldn’t imagine a 150x45, that’s pretty big, also kite sizes are a bit too much of a gap from my experience

Guys at my local all vary in weight but if it’s 12m weather then usually everyone’s out on 12s, just varying degrees of power, maybe the lightest on a 10 or 9m, all similar size boards, weights varying from 55-95kg

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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby FLandOBX » Mon Aug 06, 2018 11:33 pm

The size of kite and board depends a lot on (1) level of experience and (2) riding style. A beginner needs a larger board than a proficient rider. An airstyle rider needs a larger kite than a wakestyle rider. I think these variables are more important than the weight of the rider.

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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby grigorib » Tue Aug 07, 2018 2:55 am

Well, befinners are inefficient and they start with larger sizes.
Then they can ride well
Then there comes that phase of “I can ride smaller than anyone else” that day
Then they need more and more power to boost
Then they need slightly smaller size again to loop and turn fast

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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby fluidity » Tue Aug 07, 2018 8:46 am

You also have more drag on a heavier board and displace more water with more weight.
However, the biggest jumpers usually go way overpowered. They are usually skinny with a good muscle to bone ratio and little fat.

sergei Scotland
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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby sergei Scotland » Tue Aug 07, 2018 4:53 pm

Thanks guys for straightening me out,

it looks like my understanding on how big board I need and how big kite I need was actually wrong.

Looks like 13% board surface area increase and 13% kite increase for 26% increase in weight would be my new rule of thumb...

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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby Matteo V » Tue Aug 07, 2018 5:16 pm

grigorib wrote:
Tue Aug 07, 2018 2:55 am
Well, befinners are inefficient and they start with larger sizes.
Then they can ride well
Then there comes that phase of “I can ride smaller than anyone else” that day
Then they need more and more power to boost
Then they need slightly smaller size again to loop and turn fast
Funny! Not many riders make it to the 3rd part of riding smaller kites than anyone else. But if you do, then the next phase of "going a bit up in size" is pretty much inevitable. Yo-yo is pretty common and I hope to get off that one day. - Maybe becoming an advanced rider is about nailing a kite size you like best for a given wind speed when you can ride 3 different ones. But it is nice to be able to be able to have the skill to (happily) ride smaller kites or larger ones when you need to pick based on a guess of what is going on out on the water.

One of the many variables that the OP is missing is wind consistency. The amount of turbulence, and the type of turbulence is 30-40% of that equation. If it is blowing 20kts for 2 seconds, then 12knts for 4 seconds, then a 15° degree wind direction shift loading the kite with 24kts for 1 second, then a 10° wind direction shift unloading the kite for 3 seconds, what size board do you need? - Or rather, how do you come up with an equivalent "steady wind speed" for that turbulent wind condition?

Even if you think you could find all of the variables and solve for them, I could throw another one at you like "what was the actual caffeine content of your morning coffee" and "how much sugar was in your breakfast meal". - Rider competency, capability, and rider condition affect a few percentage points too.

I post here regarding the academics of kiting. But I kite by feel. Academics are important. Feel is crucial.
Last edited by Matteo V on Tue Aug 07, 2018 5:28 pm, edited 2 times in total.

grigorib
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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby grigorib » Tue Aug 07, 2018 5:22 pm

sergei Scotland wrote:
Tue Aug 07, 2018 4:53 pm
Thanks guys for straightening me out,

it looks like my understanding on how big board I need and how big kite I need was actually wrong.

Looks like 13% board surface area increase and 13% kite increase for 26% increase in weight would be my new rule of thumb...
Look. board doesn't use whole surface to push water out. Depending on width/length/shape/rocker/channels that will also differ.
Kitewise - larger size would pull more, but would be slower. And at certain moment you start entering a LW kites domain.

So overall - I would want to know the formula but I wouldn't dare to name even variables leaving multipliers for later.

What rider(s) weight you consider kite quiver for?

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Re: Why do I need a much bigger board AND bigger kite for a heavier rider?

Postby Matteo V » Tue Aug 07, 2018 5:37 pm

grigorib wrote:
Tue Aug 07, 2018 5:22 pm
What rider(s) weight you consider kite quiver for?
What is a 170lb guy riding? - 12m

So I weigh 225lb, and need to upsize at least 1 size to 14m or go to a 15m.


But what if you are skilled at light wind or just use smaller kites than almost all the other kiters?

Then try the 12m.


It's not "rocket surgery" or "brain science". And you all know how in depth and detailed I get on the academics. Even I just guess on this question and go for it. The "academic reasoning" behind me just guessing is that you can use a range of kites in a given wind speed which will yield different performance curves for specific applications. If you can only do 1 trick, or focus only on 1 application for the kite/rider/board system, then figuring out some equation may help you. But the rest of the kiting world boosts when overpowered (picked too big of a kite), or carves hard when underpowered. There is lots to play with when things are not "just right".


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