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Small vs large foil board

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revhed
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Re: Small vs large foil board

Postby revhed » Fri Oct 27, 2017 8:10 pm

BWD wrote:
Fri Oct 27, 2017 6:02 pm
Riding on the surface, the curve for drag:velocity is steep until you are planing.
Once on plane, you can add speed while drag increases slower.
For foiling with most foils I think you should start to feel lift from the wing soon after planing (if no before), for a quick, smooth transition to the foil.
A big or efficient wing will start to give you major lift from 6-7 knots.
So you want a board that will stop "plowing" and start semi-planing (passing its wake) around 6 knots, so it will "let go" and not stick to the water.
This is determined by aspect ratio and surface area.
Typically the overall length:width ratio ends up being 3:1. Dynamically, with the board accelerating and nose up, the immersed board area has a shape more around 3:2.
This works out to boards shaped roughly like revhed's, which is not unlike the shape of a shortened paipo, a ketos skim, or any pocket board, really. To keep some floatation for starts and a scoop to help in touch-downs, the shape is usually 3:1-4:1.
How big should the area be?
For transition to planing on surface, roughly 70-80cm^2/kg rider weight. Less for more proficient, or highly powered riders maybe.
The surface loading is the same as found in surfing, regular directional kiting, etc.
This surface loading facilitates easy planing with low power, at a board size small enough to allow good "shortboard" handling and light weight.
This is why converting a good, compact outline surfboard for foiling can work, even if not ideal.

With pocket boards, you can reduce length as much as your stance allows, but unless relying ONLY of the foil for initial lift and acceleration, you want to preserve some width, because most of the lift in surface riding is developed at the front edge of the wake. So it's good to have width for acceleration and efficiency at low power. Also somewhere to put your feet!

What about volume? 10-12L seems enough to float almost any foil so it won't sink, but gives challenges starting in light wind. Double that and the sinking when starting will be slower. If the board has a wide shape, it will also sink more slowly, even if volume is not great. To me volume seems even more dependent on taste and specific use than shape, and definitely more than surface area, which seems to follow some basic hydrodynamic principles.
Such a joy to read such well understood concepts!
Still think as Bra cru has been testing for a while now is that board shape to get the desired lift off speed in ultralite conditions such as this,
BRCbottom.jpg
might help a lot?
And the combination of what Ram air kite, on what line length, on how wide a bar, using what board with ALL its possible parameters AND not to mention probably the MOST important element the F wing to get lower and lower wind speed KBHF flying...
interesting, If I was fortunate to have had SUPER rich friends in college that gave me BIG money to do R and D, I, we would hope to tell you all!
R H

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Peter_Frank
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Re: Small vs large foil board

Postby Peter_Frank » Fri Oct 27, 2017 8:22 pm

Precisely, an intellilgent design and sufficient size - the board shown from Bracuru is 155x55x7 cm, and a little sister a bit smaller for more wind :naughty:

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revhed
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Re: Small vs large foil board

Postby revhed » Fri Oct 27, 2017 9:07 pm

revhed wrote:
Fri Oct 27, 2017 8:10 pm
as Bra cru has been testing for a while now is that board shape
I have a strong feeling that 155x55x7 is WAY to big, I was referencing SHAPE as posted, but sure like his thinking ala speed boat hulls.
It would be SO easy to test given board designs by simply swapping EXACTLY same T bar configuration board to board, of course same kite, and feeling what works best!
R H

BraCuru
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Re: Small vs large foil board

Postby BraCuru » Sat Oct 28, 2017 12:07 pm

revhed wrote:
Fri Oct 27, 2017 8:10 pm
Still think as Bra cru has been testing for a while now is that board shape to get the desired lift off speed in ultralite conditions such as this,
might help a lot?
Long 4 years and I ended up in one model in the line: 150x55cm.
I might produce smaller one 145x50cm but as a custom model only.
The Hoover is designed for:
- beginners,
- anybody riding in light winds,
- anybody riding in unstable winds non thermal (Baltic).
- anybody not willing to posses a kite bigger than 12m.

A trick about Hoover is the bottom shape which distributes more volume on the rails than in the middle.
Revhed mentioned about thickness 7cm - I use it too but this misleading. The most fat part of the board is in the rails.
In the center line the board has between 1,5 up to 5cm with majority of the distance well below 3cm.
The SURFACE and light weight (3,3kg) lift the board up. Not so much volume.
This is catamaran idea which gives much more stability to a rider than on a classic volume foilboard.

This idea works well in quick lifting out of the water. Works great for learning tacking and jibing and sitting on a board while drifting.
The board has super direct ride once in the water. There is no yawing comparing with a classic foilboard.

Another advantage is that the bottom does not like the water ;) It bounces off the water much easier.
It might be important for beginners.

And the point is that (OK - nobody is going to believe again) in steady wind of 10kts (measured on a beach) I ride 7m inflatable kite. 10m kite in 7kts. 14m in 3kts.
My 2016 and 2017 wind statistics show (ish) that 60% I've been riding on a 10m kite, 20% on 7m and another 20% on 14m Apollo.
Going abroad to windy destinations I take big hoover and only two kites: 10 and 7m.
Having a classic shpae board I would need to add 2kts to my size limits and would ride 40% on a 14m kite - do not like it!

The boards are hand made. Waiting time is 2 months. Hope that any big name in the industry would copy this idea to provide it to more riders.
So far around 3 dozens of riders don't want to change this idea.
Honestly. Most of them are my Polish friends. Their opinion caused my decision to produce the 150x55cm only.

BTW.
the channel in the bottom is a bit wider at the nose and narrower at the tail. It improves a lifting effect a bit.

testing in windy Baltic ;)
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Re: Small vs large foil board

Postby flaps1111 » Sat Oct 28, 2017 1:37 pm

Yes, you are right nobody sailing can believe you that you can ride in 3 knots with a LEI kite ;-D

jaros
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Re: Small vs large foil board

Postby jaros » Sat Oct 28, 2017 7:03 pm

Get a medium size one, seriously. It will serve you well : )
Question for all - what size do you consider as medium?

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Peter_Frank
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Re: Small vs large foil board

Postby Peter_Frank » Sat Oct 28, 2017 7:51 pm

jaros wrote:
Sat Oct 28, 2017 7:03 pm
Get a medium size one, seriously. It will serve you well : )
Question for all - what size do you consider as medium?

Something 130-135 not too narrow is "Medium" in my definitions, but dont know what it is in others ?

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Re: Small vs large foil board

Postby bigtone667 » Sat Oct 28, 2017 8:13 pm

One of best light wind board types I ever used before I started foiling was a skim board. Wide, no rocker, no too long, hardly any volume.

Having tried a wide range of boards with my Zeeko Carver, I am back to using a skim board (naish hover 155) for super light wind. It gets me planing early and has very little swing weight.

If the wind is over 12 knots, I will use a shinn jackson (the chined edges and rocker improve recovery from hard touch downs and the swing weight is still okay).

I would be looking at at least two boards.

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Re: Small vs large foil board

Postby revhed » Sun Oct 29, 2017 9:43 am

BraCuru wrote:
Sat Oct 28, 2017 12:07 pm

There is no yawing comparing with a classic foilboard.

nobody is going to believe again) 14m in 3kts.
While as I posted I like your "speed boat" hull design not sure I would like a foil board that is not "loose" in yaw on the water?
We as many are finding that as well as a looser yaw with boards AND T bars, flatter wings, lends it self to quicker control of course much more skill needed!
And yes most, myself included, DO NOT believe you are foiling in 3 knots with 14m2 LEI!
If so how big of a F wing are you using please?
I am lucky to live in a hot bed of KBHF activity and have seen many times some of the best in the world and will say and post confidently that
unless sustained pumping NO ONE is foilboarding in 3 knots with a 14 LEI, maybe even ANY kite!
Almost forgot, GREAT PHOTOS!
R H

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Re: Small vs large foil board

Postby FrederikS » Sun Oct 29, 2017 12:03 pm

Medium is usually around 50 cm wide and 140 long. Small is 40ish wide and sub 140 long. Non of the windmeters will even measure 3 knots reliably.


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