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Will a light weight rider be faster than a heavy weight?

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BayAreaKite
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Re: Will a light weight rider be faster than a heavy weight?

Postby BayAreaKite » Sat Apr 10, 2021 3:40 pm

Bigger boats are generally faster because they have a longer waterline and can therefore generate more forward thrust under sail. However sailboats operate under displacement theories and foils are lifting bodies and should therefore be compared to airplanes. Heavier planes require more lift which results in more drag. There are two components of drag, lift-induced and parasitic. All things being equal, two riders on the same foils, kite, skill level, tack... the lighter will always be faster due to having less lift-induced drag.

Floating around
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Re: Will a light weight rider be faster than a heavy weight?

Postby Floating around » Sat Apr 10, 2021 6:16 pm

BayAreaKite wrote:
Sat Apr 10, 2021 3:40 pm
Bigger boats are generally faster because they have a longer waterline and can therefore generate more forward thrust under sail. However sailboats operate under displacement theories and foils are lifting bodies and should therefore be compared to airplanes. Heavier planes require more lift which results in more drag. There are two components of drag, lift-induced and parasitic. All things being equal, two riders on the same foils, kite, skill level, tack... the lighter will always be faster due to having less lift-induced drag.
Bigger boats are generally faster because their longer waterline gives them a higher “hull speed”. Hull speed is basically how fast you can go until you make a big enough bow wave and the trough behind it that the boat essentially falls in to a hole that it has made itself! You then need to push up this self made hole to get “on the plane” this take a huge amount of power to push over the hump.

One thing to consider re foils is that essentially everyone is on the same size foil. You can add more power with a heavier rider and bigger kite but the wing and mast are the same, especially now with allot of people using the IKO registered kit.
Edit, That would be IKA not IKO.
Last edited by Floating around on Sat Apr 10, 2021 8:35 pm, edited 1 time in total.
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plummet
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Re: Will a light weight rider be faster than a heavy weight?

Postby plummet » Sat Apr 10, 2021 7:01 pm

I say kiting is one of the sports where size doesnt really matter. Spec the gear to suit your weight and smash it out.
With foiling there is so much rider input required translates to speed weight difference will be insignificant if specced with weight appropriate gear.

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jumptheshark
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Re: Will a light weight rider be faster than a heavy weight?

Postby jumptheshark » Sat Apr 10, 2021 7:46 pm

Talking out my ass, but I’d suspect light conditions to favour smaller riders and as conditions get bigger the benefits shift up the weight scale, to a point beyond which extra weight no longer in ropes the strength component and you get diminished returns and even drop off.

How big if Florian Grubber?
How big is Johnny Heineken?

BayAreaKite
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Re: Will a light weight rider be faster than a heavy weight?

Postby BayAreaKite » Sat Apr 10, 2021 10:48 pm

Thanks for explaining this @floating around. It’s amazing how much my 40’ sailboat gets pushed to leeward when on a tight reach (I don’t have a super deep draft or big keel) so I assumed part of it had to do with waterline length. Moving forward in a straight line, I never really understood why waterline length mattered. Now I do!

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Re: Will a light weight rider be faster than a heavy weight?

Postby grtlakes » Sun Apr 11, 2021 1:01 am

Bay AreaKite there is a cool formula that gives theoretical hull speed 1.35 X Square root of LWL. (Length of waterline)
Eg
25’ THS is 6.75 kts
36’ THS IS 8.1 kts
Your 40’ if shear lines 8.5 kts

BayAreaKite
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Re: Will a light weight rider be faster than a heavy weight?

Postby BayAreaKite » Mon Apr 12, 2021 5:04 am

I'm aware of the formula, but to be honest I never really understand the layman's explanation for it:) I'm a structural engineer so fluids has always been a bit of a mystery for me. If you want to geek out over hydrofoil equations and models, check out my partner's site here: http://www.tspeer.com He helped me optimize the section of Project Cedrus. More impressively, he designed the rigid wing sails for the BMW ORACLE America's cup trimaran and then all the hydrofoils for the following Oracle Racing during the SF Cup.
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