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PaipoBoard with Hydrofoil
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batboard



Joined: 25 Jun 2005
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PostPosted: Sat Jun 25, 2005 10:01 am    Post subject: PaipoBoard with Hydrofoil Reply with quote

Ok, a few questions...
a) Is it possible to put a hydrofoil onto a paipo board ?
b) Will it work on paddle out waves
c) Is there anyone out there who can build such a board
d) Is it worth doing anyhow ?
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Poobah
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Joined: 09 Jan 2004
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PostPosted: Sat Jun 25, 2005 12:15 pm    Post subject: Reply with quote

Here's one

http://www.rodndtube.com/paipo/hydrofoil/paipo_hydrofoil.shtml

and another

http://www.iav.com/%7Esponge/stuff/gilbert.htm
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batboard



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PostPosted: Sat Jun 25, 2005 12:19 pm    Post subject: Reply with quote

You wouldn't have an email for Terry Hendricks would you ?
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Poobah
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PostPosted: Sat Jun 25, 2005 11:30 pm    Post subject: Reply with quote

Try sending a personal message (PM button) to MTbarrels on this forum. Hopefully he's still connected here.
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MTbarrels
Tray Rider


Joined: 10 Jan 2004
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PostPosted: Sun Jun 26, 2005 8:06 am    Post subject: Reply with quote

Poobah wrote:
Here's one

http://www.rodndtube.com/paipo/hydrofoil/paipo_hydrofoil.shtml

and another

http://www.iav.com/%7Esponge/stuff/gilbert.htm


...and another:

http://www.rodndtube.com/paipo/GaylordMiller_Paipo_Foil.html

(the original hydrofoil paipo board, as far as I've been able to tell).
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Poobah
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PostPosted: Sun Jun 26, 2005 12:49 pm    Post subject: Reply with quote

Thanks. I was looking for that page, too.
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Poobah
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PostPosted: Sun Jun 26, 2005 11:41 pm    Post subject: Reply with quote



The front foil is described on one of Rod's pages:

The forward foil is a ventilated surface-piercing foil. It's functions are to provide some roll stability, assist in turning, and maintain the flight elevation within a narrow range of values.

Terry,
would you please explain "ventilated?" I'm also curious about the reverse thickness of the front foil. I've read that canard foils are sharper on the leading edge for manueverability, but don't understand why. Would you please give us an explanation for the layperson?
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MTbarrels
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PostPosted: Mon Jun 27, 2005 1:02 pm    Post subject: Reply with quote

Poobah wrote:
...<snipped>... would you please explain "ventilated?" I'm also curious about the reverse thickness of the front foil. I've read that canard foils are sharper on the leading edge for manueverability, but don't understand why. Would you please give us an explanation for the layperson?


Just so we all use the same words to mean the same thing, here's the terminology that I use (and as used in naval architecture):

When a foil (e.g. fin, hydrofoil, etc.) is submerged in water and generating lift, there is a low pressure side and a high pressure side to the foil. At high speeds, there may be locations on the low pressure side of the foil where the pressure is below the vapor pressure of the water. In that case, the water "boils" and creates a cavity of water vapor in that area. When this occurs, the foil is said to be "cavitating".

Alternatively, if the foil is close to the sea surface, the pressure may become sufficiently low so as to cause air to be sucked down to the low pressure side of the foil (typically tangentially along a surface-piercing foil, or along a strut connecting the foil to the hull in the case of a fully-submerged foil). This forms a cavity of air over the low pressure area of the foil and the foil is said to be "ventilated".

Either of these processes results in a reduction in the lift force generated by the foil. In fact, if the gaseous cavity extends over the whole upper surface of the foil, the foil is functioning essentially as a planing surface.

Another mechanism that results in a reduction in the lift force is if the foil "stalls"--i.e. the smooth flow of the fluid (e.g. water) over the upper side of the foil "detaches" from the low pressure surface leaving a region in which there is a vortex-like flow between the surface of the foil and the region of smooth flow over the foil. In this case, the foil is experiencing "separated flow" (or it is "stalled"). It differs from ventilated flow or cavitation in that no gaseous cavity is formed.

[An aside: "Cavitation" is often used in the surfing community to represent a situation (with regard to fins) that is more correctly described either as "ventilation" (air cavity formed) or as "stalled", or "separated flow" (no gaseous cavity formed]

When traversing across the face of a wave, only the very tip of the forward foil (on the wave side of the board) on my hydrofoil paipo board is in contact with water--and since the face of the wave is sloped, this tip "sticks", or penetrates, into the face of the wave. Thus it is a "surface-piercing" (versus a "fully submerged") foil.

The wetted length (effective "span") of this foil is typically from 0 inches to 3-1/2 inches (depending on the skill of the rider). The chord of the foil (distance from leading edge to trailing edge) is 7 inches. The short span, compared with this chord of the foil, and the fact that the foil penetrates into the face of the wave, means that it is easy for air to be sucked over the upper face of the foil by the low pressure that would otherwise exist there. Hence the forward foil functions most of the time (i.e. at normal operating speeds) as a "ventilated surface-piercing" foil.

[An aside: once the ventilation takes place, the pressure on the upper surface of the foil is essentially atmospheric pressure.]

[Another aside: to further complicate things, sometimes some air passages are incorporated into a foil so as to assist, or ensure, that a foil is ventilated--but this ventilation system is not a requirement for a foil to become "ventilated".]

When cavitation takes place, the heat of vaporization is exchanged between the water vapor and the water. This can lead to pockets or regions of super-cooled water vapor which ultimately abruptly condense back to water, and large, very localized changes in pressure can occur when this takes place. If this occurs at some downstream point on the face of the foil, this can lead to erosion of the foil material and premature structural failure of the the foil.

To cope with this problem, foils used where cavitation is likely to be present are often designed to promote the formation of the cavity of water vapor in such a way that the cavity extends to, or downstream from, the trailing edge of the foil. In that case, the spikes of pressure change do not occur on the foil, and erosion is avoided or minimized. Foils designed to accomplish this are often referred to as "super-cavitating" foils.

One means used to accomplish this is to have a sharp leading edge on a foil that operates at an angle-of-attack in excess of about 5-6 degrees. The upper surface then curves downward to help avoid the upper surface of the cavity from coming in contact with the upper surface of the foil. The lower (high pressure) side of the foil is concave and also curves downward, resulting in a cross-section that resembles one-half of a cresent shape. It is this downward curving lower face that imparts downward momentum to the water passing under it and thus generates the lift produced by the foil.

However, since it is unlikely that the hydrofoil paipo board will ever go fast enough to produce cavitation, this is not the reason for using a super-cavitating type foil for the forward foil on my board.

One of the reasons has to do with rider comfort. Suppose this semi-crescent shaped bottom surface were replaced by a simple inclined plate. Now where the surface of the water first contacts the plate, there are very high pressures generated (proportional to the square of the velocity). This contributes to the lift force generated by the foil, but also causes a thin layer of water to reverse direction and form a sheet of spray moving both forward and upward (along the plate). Since the rider's face is not far from, and positioned behind the leading edge of the forward foil, in some circumstances this can result in a substantial volume of spray/water striking the rider in the face (sort of like looking into a fire hose). This can be very distracting to the rider!! However, with a curved (concave) lower surface chosen such that the tangent to this surface at the leading edge is "horizontal" (i.e. parallel to the longitudinal axis of the board), the forward moving spray will still be moving forward, but it will not have any upward component to this motion, and thus it's effects will take place below the hull of the board an the rider.

A (moderately) sharp leading edge--as in some super-cavitating foil designs--is desirable for a similar reason: a blunt leading edge can also result in the generation of spray (from where the foil penetrates the wave face) that is thrown into the face of the rider.

There is yet another possible reason for the choice of this foil cross-section. Richard Miller, who has designed a successful hydrofoil-based windsurfer, notes that when ventilation is possible, it may not be a steady condition, but rather that it can oscillate between normal (attached) flow, and ventilated flow in an unpredictable manner. But that transition is accompanied by a doubling of the lift generated by the foil if the change is from ventilated flow to attached flow (and one-half the lift if the flow becomes ventilated). This can cause substantial control and ride problems for the rider. Thus he argues that choosing a foil cross-section that ensures that once the foil becomes ventilated it will remain ventilated avoids the problem of random changes in the lift generated by the foil. A sharp leading edge, operating at an angle of attack of 5-7 degrees or more, and a convex, downward trending upper face on the foil, are examples of means of promoting ventilated flow.

[An aside: however, in view of the low aspect ratio (=foil-span/foil-chord) of my front surface-piercing foil, the foil is likely to be ventilated most of the time even with a relatively blunt leading edge--but as noted above, the sharp leading edge has spray control advantages as well]

I have never heard that a sharp leading edge on a canard foil leads to increased maneuverability (and off-hand, I don't see why that might be the case). More likely, I would guess, is that with a canard aircraft one wants the canard foil to stall before the main foil to assist in spin recovery--and this condition can be facilitated by using (within limits) a sharper leading edge for the canard foil than for the main foil. This would not apply to the hydrofoil paipo board, however.

Hope this helps.
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Poobah
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PostPosted: Mon Jun 27, 2005 2:08 pm    Post subject: Reply with quote

Thanks for that, and sorry about my poor choice of words. Control and stability, not manueverability. Here's something on Rich Miller's sailboard for those of you that liked the sound of that idea.

http://www.foils.org/miller.htm

P.S. Is he by any chance closely related to Gaylord Miller?
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MTbarrels
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PostPosted: Mon Jun 27, 2005 3:22 pm    Post subject: Reply with quote

Poobah wrote:
...<snipped>... Here's something on Rich Miller's sailboard for those of you that liked the sound of that idea.

http://www.foils.org/miller.htm

P.S. Is he by any chance closely related to Gaylord Miller?


I don't have the remotest idea...but I'd bet that he isn't.
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rodndtube
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PostPosted: Mon Jun 27, 2005 6:00 pm    Post subject: Reply with quote

Poobah wrote:
Thanks. I was looking for that page, too. In my search I came across an interesting brief mention...

Steve Mosher: I basically started out as a Southern California beach kid. I got involved in making surfboards pretty early on. Then I made hydrofoil belly boards, doing things a little differently. As I look back on it, I was playing with plastics long ago.

I was playing my Fender bass with various people around Eugene Ore., and I just got bored with it. So I started screwing around, trying to make myself a neck that I would like to play. And it got out of hand.

The iterview was at
http://www.stick.com/interviews/mosher_01_02/

Any of you know/knew Steve Mosher?


Poobah, are you thinking of making contact with Steve Mosher?
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Poobah
Dolphin Glider


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PostPosted: Tue Jun 28, 2005 10:41 pm    Post subject: Reply with quote

Sure. Why not? Hopefully Steve's not too busy to share with us.
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batboard



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PostPosted: Thu Jun 30, 2005 1:15 pm    Post subject: Reply with quote

you know, i've been around the houses with this, and am at the last lap already. All I need to find know is someone who will build the actual hydrofoil fins, have sourced a bord shaper. Have had a lot of opinions, general view, possible danger to rider or swimmers from the fins.

Anyway, if y'all know someone who could make the fin, we might be on.

Best
Bat Board
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Poobah
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PostPosted: Thu Jun 30, 2005 2:03 pm    Post subject: Reply with quote

I'm not sure how much MTBarrels can tell us about construction and shapes online. Perhaps a little more privately using the Personal Message system on this forum.

I say worry less about the impact on swimmers, and worry more about support struts rupturing through the deck, etc.
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batboard



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PostPosted: Thu Jun 30, 2005 3:00 pm    Post subject: Reply with quote

Poobah wrote:
worry more about support struts rupturing through the deck, etc.


ten four on that.
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