Paddle-in hydrofoil board?

[fins]

@$1

Fins,

If we had a rectangle 5" x 14", the surface area would be 70 sq. in. – still need another 80 sq. inches.  Plus the curved tips make the area a bit less.  To get an area estimate, you just need to fit rectangles, triangles and half/full circles into the foil template you have in mind.

A 10" x 15" rectangle = 150 sq. in.  Your dimensions when multiplied must equal 150.  To get estimates, divide 150 by one of the dimensions you are thinking about and that will give you the other needed dimension. That is, 150 divided by 14 = 10.71" – a 10.71" x 14" rectangle.  By adding a tail stabilizer behind the wing, you can reduce the wing surface area by the amount of area in the tail stabilizer.

I do not believe strut dimensions will figure into the lift aspect.  We need Brett to guide us on strut design and properties.

It is worth it just for the experience and experimental journey.  More is learned.

thanks very much , mate !

Having it explained THAT way , certainly makes it MUCH EASIER for me to understand , cheers !!

so… the "square inch " area measurement DOESN’T include the base [strut] of the foil , you think ? … WHY would THAT be , I wonder ? I ask , because , with ‘normal’ [?!] fins , it is ALWAYS the total base x height measurements that would give the [?non-metric, too??] 'square inch ’ area dimensions , yes ?? … so …obvious question … WHY NOT , with a 'FOIL’S measurements , TOO ?

just curious

cheers

ben

[fins]

so… two [‘Wildy’ single fin setup ] ‘inline’ foils , in an 18" finbox , spaced apart like brett’s two blue winged ones , in his booger foil thread …THAT could work , eh ?? … You’ll never know , till you’ll give it a go , I reckon …

[stoneburner]

@$1

thanks very much , mate !

Having it explained THAT way , certainly makes it MUCH EASIER for me to understand , cheers !!

so… the "square inch " area measurement DOESN’T include the base [strut] of the foil , you think ? … WHY would THAT be , I wonder ? I ask , because , with ‘normal’ [?!] fins , it is ALWAYS the total base x height measurements that would give the [?non-metric, too??] 'square inch ’ area dimensions , yes ?? … so …obvious question … WHY NOT , with a 'FOIL’S measurements , TOO ?

just curious

cheers

ben

Only the horizontal foil surface generates lift – higher pressure on the bottom and lower pressure on the top, diagram below (Bernoulli).

Vertical strut and/or fin surfaces that are at 90 degree angles to the bottom of the board do not generate vertical (upward) lift.

Regarding two foils in one long box, you could just have 2, 12" boxes and adjust the front wing foil and rear stabilizer foil independently in separate boxes.

 

[Swied]

Has anyone done any testing to measure the coefficient of lift (CL) for their wing/foil configuration?

I used the lift equation that is presented on the following site to do a quick back of the envelope calculation for a hydrofoil surfboard.

http://www.grc.nasa.gov/WWW/K-12/WindTunnel/Activities/lift_formula.html

Here is a snippet from the site:

L = (1/2) d v^2 s CL
Where:

• **L** = Lift, which must equal the airplane's weight in pounds
• **d** = density of the air. This will change due to altitude. These values can be found in a I.C.A.O. Standard Atmosphere Table.
• **v** = velocity of an aircraft expressed in feet per second
• **s** = the wing area of an aircraft in square feet
• **CL** = Coefficient of lift , which is determined by the type of airfoil and angle of attack.

These are my calculations:

We are obviously dealing with water here, and will have to assume laminar flow.  Let's also ignore the air/water boundary interactions and ignore the buoyancy effects of the submerged elements.

Now, let's rearrange the formula to solve for the coefficient of lift (CL).

     CL = 2*L/(d*v^2*s)

Let's assume that our rider is 187 pounds (m = 85 kg).  

L is the force that will be required to keep the elevated surfer from sinking.  This is measured in kilogram meters per second per second.

     L = m*g = 85 kg * 9.8 m/s/s = 833 kg m/s/s

In this case d is the density of water, which is about 1000 kg per cubic meter.

     d = 1000 kg/m/m/m

Let's set the velocity of the rider with respect to the water at 10 mph (4.5 meters per second)

     v = 4.5 m/s

I'll use the same surface wing area as has been discussed above: 150 square inches (0.1 square meters)

     s = 0.1 m^2

Now we can calculate the required coefficient of lift (CL) that we will need in order to keep our surfer elevated.

     CL = 2*(833)/(1000*4.5^2*0.1)

     CL = 0.8

You can use the chart in exersice 1 of the NASA page, linked at the top of this post, to see the relationship between CL and the angle of attach (pitch angle).  In this case, a CL value of 0.8 would correspond to an angle of attach of 8 degrees.

The NASA site states that the CL is usually derived from experimental results.  Hence my question about whether anyone here has done any such experiments with their set-up?

[stoneburner]

…

[stoneburner]

 

I am operating on Brett’s 8-9 years of empirical observations that at walking speed (~ 3 mph) 2.5 sq. in. of foil will lift 1.0 kg of weight (including surfboard).  He also discovered early on that NACA profiles were not needed to get his results.  His final rule being as speed doubles lift quadruples.

My math is too rusty to quickly plug into and work all of those equations swied. But I think I should be able to generate enough information to build a functional hydrofoil with Brett’s observations and a little right triangle geometry/math.

I think there is a point at which the foil becomes a planing surface more than it is a foil.

I would love to see the conclusion of your mathematical progression though.  But for Ben, we need a rider weight of 60 kg.

 

[fins]

hi swied !

are you going to make a paddle in foil board , too ?

it would be interesting to see a few different ones , and compare notes / results ?? …

[fins]

" I used the lift equation that is presented on the following site to do a quick back of the envelope calculation for a hydrofoil surfboard."

is there some kind of equation that would help guide …

what length , width , thickness of test board ? …

And ditto , for the foil

And how / where would be the best place[ment] for the foil …

say , for a 60-85kg surfer [myself , alex , brett ] ,

on a waist to chest high mid tide 50 metre [at least !] long wave , with a bit of push ?

…To be able to arrive at some figures for those things , might help make it easier / ? quicker? / ? smoother? , in terms of giving some kind of ‘baseline’ to work from / around ?

Because , not everyone has the patience / desire / persistance / ??? [?madness?] , neccessary , to surf-test [apparently] 90 foils , over a 9 year period [?? ] … perhaps there can be an EASIER and more efficient way , yes ?

I really can’t help thinking , that if the foils had been tested on a SURFBOARD all those years ago , with [ last weeks’] success , waaay back then [2004???] , then …we would all have been much further down the ‘track’ , by now ??

It would be really interesting to be able to sit down with rush randle , laird hamilton , and buzzy kerbox , and see / hear what THEIR thoughts would be , on a paddle in shortboard foilboard …maybe The Surfers Journal could do an inteview with them on that one day ?? or ’ White Horses’ , or ‘Surfers Path’ … or , maybe …" Foam Symmetry" , eh ?? [ ‘Oldy’ , ARE you lurking here !!! ]

[stoneburner]

The tow-in hydrofoils seem to be mini-airplane shaped foils and are similar to those used for riding behind power boats in lakes/rivers.  It would be nice if there was a simple equation for hydrodynamic foils for surfboards  – for common/average daily use, they would likely be “low-velocity” hydrofoils.

I have a feeling that aerodynamic principles do not transfer equally/directly to hydrodynamic foils.  Board shape will have much less effect on performance than  foil design.  Except for paddling and take-off, the board is mostly just a riding platform for foil attachment.  Foil placement will be related to the rider, foil and surfboard center of gravity combined.

I think Brett’s empirical results have given us enough to start designing and building paddle-in hydrofoil refinements.

Here is another blast from the past:

http://www.swaylocks.com/node/1021283

This is the board I plan to offer up as a sacrifice on the altar of hydrofoil experimentation after I play around with a few foil shapes:

[Swied]

@$1

I am operating on Brett’s 8-9 years of empirical observations… His final rule being as speed doubles lift quadruples.

 

That is a pretty good observation. It follows right along with the lift equation that I posted above.

Here is the equation again:

 

          L = (1/2) d v^2 s CL** 

 

The density of water, the surface area of the foil, and the Coefficient of lift remain constant, so the equation can be simplified as…

 

          L = a * v^2

 

where L = lift force, a = constant, and v = velocity

Say you are going 3 mph, then L = a * 9.  If you double your speed to 6 mph, then L = a * 36.  There you have it: 36/9 = 4.  Doubling your speed quadrupals lift. 

 

 

[Swied]

@$1

hi swied !

are you going to make a paddle in foil board , too ?

it would be interesting to see a few different ones , and compare notes / results ?? …

 

I think building a paddle-in foil board would be a fun project.  Brett’s work is quite inspirational.  I want to understand the science behind it a little better before I start shaping, however.  I want to make sure that I don’t construct something with so much lift that it becomes a planing surface.  Likewise, I don’t want a submarine either.  I think my first project would be to build some sort of water flow tank that would enable me to measure the lift coefficient of various foil configurations.

Here is another question that has been nagging me.  How do you design a hydrofoil so that it will turn when you lean-over?  What are the key design elements that will allow self turning to happen?

[stoneburner]

@$1

Here is another question that has been nagging me.  How do you design a hydrofoil so that it will turn when you lean-over?  What are the key design elements that will allow self turning to happen?

I believe the foil should be self turning the same as a surfboard.  The foil lift vector is in the same direction as it was in its horizontal (parallel to water surface) position.  Centripetal/centrifugal forces factor in also.  Side slipping (tracking) must be controlled (fins?).  

[Swied]

Here is the board that I have decided to modify.

It is an XPS foam alia that I made to resemble Tom Wegener’s tuna model.  It rides ok, but I’m kind of over it now.  This should be a good platform for a hydrofoil: lengh 6’, and width 17".

[fins]

does it paddle okay ? does it catch rail ? [not much rocker there , by the looks !] …I ask , as you obviously will still need to be able to paddle in cleanly , with speed [same as with ‘any’ board ] , to catch the wave , before the foil[s] lift you up…

And what size / type / placement of foil[s] are you looking at , Swied ?

I still haven’t watched the dvd ‘the grizzly project’ that I was recommended , but the person thought I would get a ‘kick’ out of it [the 'maaaadddd scienccccce ’ aspect of it , I think he may mean ?]

[fins]

Hi again , 'Swied ’ ! Well, hopefully your 'water flow tank ’ science results , if they find out stuff that helps people design a ‘better’ foil , will be worth the effort. Good on ya for pursuing that !

Me ? …well, I’m just thinking along the lines of … 'let’s make it safe , not too heavy of an anchor under the board to paddle into waves , and try to make it the smallest , lightest , and strongest as possible that is needed , to get the job done well " . [ It is , after all , the Aussie tradition to abbreviate things , so why NOT , eh ?? ]…

cheers !
ben

[stoneburner]

Swied,

I like the idea of using an alaia as a hydrofoil test platform.  Simple and efficient.

[fins]