Rampant 10-5 project

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Rampant 10-5 project

Postby Roy Stuart » Mon Jun 17, 2013 12:59 am

This one's a round tailed version of the Hotkurl 10-5. So far the deck and bottom panels have been made and the two fins:

Shown here is the initial foiling on the central planar cutaway fin which we call the 'Patu' fin because of its resemblance to a Maori war club. The thickness to chord length ratios are 13% through the base, and 8% through the tip area.

http://www.roystuart.biz/2013/06/rampant-surfboard-3-initial-fin-foiling.html

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Here is the Vort-X tunnel fin with scalloped trailing edge. Half pipe tunnel fins produce a beneficial flow rotation inside the the tunnel when experiencing lateral angles of attack ( which is the case most of the time). The purpose of the scalloped trailing edge is to induce counter rotating vortices in the flow outside the tunnel, so that when it meets the flow from inside it does so more smoothly and with less drag. Slight channels are a consequence of the sine curve shape of the trailing edge, these channels are used only on the outside of the fin to prevent non beneficial vortices occurring on the inside flow. Viewed from the tail this results in an undulating trailing edge line. approximately 16 hours of hand foiling has been applied to get the fin to this stage. The thickness to chord length ratio of the tunnel is approximately 8%.

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Re: Rampant 10-5 project

Postby defever » Mon Jun 17, 2013 7:50 pm

Very interesting Mr Stuart.

Can you explain in lay terms the function of tunnel fin please? I ask this because 1) it's unusual, 2) I knew a friend who had a longboard with a turbo tunnel fin back in the early 00s, but I don't see any of them around any more.

I lost you at "flow rotation inside the tunnel", but I think I understand when you mean lateral angles of attack (i.e. riding along the face of a wave?). Is the rotation you mention on the longitudinal axis of the board (i.e. drawing a line from nose-tail and rails tilting sideways? I'm not good at describing this)?

And did you sell your recently finished $1.47million Hot Kurl?

Thanks for your post, different from other post, as always.
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Re: Rampant 10-5 project

Postby Roy Stuart » Tue Jun 18, 2013 12:28 am

Double post.
Last edited by Roy Stuart on Tue Jun 18, 2013 12:29 am, edited 1 time in total.
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Re: Rampant 10-5 project

Postby Roy Stuart » Tue Jun 18, 2013 12:28 am

Thanks dfever.

The rotation is as you suggest, around the longitudinal axis, whenever the board is trimming or turning i.e. whenever it isn't heading at 90 degrees to the wave front. It doesn't happen with whole tunnels.

The tunnel provides lift at any angle of attack.. up down or sideways and anywhere in between in response to angle of attack in any direction. One of the main reasons why I use them is that they provide increased and more efficient planing lift in the tail without the use of a wide tail. There are in my opinion advantages in efficiency and in handling and control over a wide range of speeds using a narrower tail in combination with the tunnel.

With a wide tail the force required to get the board on the rail increases dramatically as speed increases, this changes the handling characteristics in response to changes in speed. The tunnel equipped tail behaves consistently at all speeds. The half pipe tunnel offers virtually zero resistance to rail to rail roll, as it simply rotates around the fore and aft axis of rotation of the board.

In addition the tunnel produces a fulcrum at the tail which vastly improves a board's ability to flex under load. Planing hulls have a high pressure area just behind the leading edge of the wetted surface area, but the pressure drops off quickly further aft and at the tail is is only a fraction of what it is further forward. So even a wide tailed board ( without horizontal fin area) has only one effective fulcrum, making it almost impossible to get flex under load from the rider. The tunnel's horizontal lifting area is below the bottom so it lifts in a higher pressure flow with a higher angle of attack, it also provides lift more efficiently than the hull because it has greater span efficiency. It can also be set up with a few degrees of positive lift against the hull, further increasing the force which it produces. So, with a tunnel on the tail the board responds flex wise to rider force in much the same way that a board does in a land flex test when it is suspended at nose and tail. Conventional fin setups don't do this effectively ( some conventional setups have a small amount of horizontal lifting area but this area is almost always quite far forward on the hull).

Tunnels also produce relatively high lift at very low angles of attack and at low speeds, so they give lots of drive as soon as the board catches the wave and also a precise lift and drive response to subtle direction changes.

Tunnels have the lowest induced drag of any fin type, this equals more lift and a better lift/drag ratio. They do have high wetted surface area, and since wetted surface area predominates at high speed whereas induced drag predominates at low speed, they are theoretically at a disadvantage at high speed. I haven't found that to be the case however as the lift which the tunnel provides increases by the square with greater speed, reducing load on the hull for lower hull drag and enabling hull wetted surface area to be reduced, thereby controlling to some extent the amount of wetted surface area used at any given speed.

The first time I tested a tunnel fin I was sold on them immediately. I put a four inch diameter tunnel on a favourite 91'" pintail which I'd been riding a single fin for a couple of years. The result was excellent, and serendipitously coincided with a rare 20 minutes of wedge tubes at a place called Shark Alley. handling was perfect, unlike all the flat planar 'hydrofoil' wings which I'd been testing.

Tunnels setups do have to be designed in harmony with the tail rocker though as the tail rocker affects the angle of lift... too much tail rocker and they can pull the tail down. Although the tunnel fin can be set at a higher angle of attack in the tail to overcome this only a small amount can be used as otherwise the exit area of the tunnel is too much greater than the entry area... this causes drag. Torturing the shape of the tunnel into an ellipse at the trailing edge can overcome this but results in 'toe out' on the tunnel sides, so it is easier to just get the rocker right.

Turbo tunnels are a different beast, I've tried 'tunnel on a stick' fins and they are basically rubbish. The axis of rotation of the tunnel is misaligned with the axis of rotation of the surfboard, this greatly increases rail to rail resistance. Also the turbo tunnel is set so that it pulls the tail down rather than up, this creates a lot of drag. a further problem with the 'turbo' is that the tunnel exit is much smaller than the tunnel entrance, again this creates drag, rather like towing a bucket with a hole in it.
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Re: Rampant 10-5 project

Postby defever » Tue Jun 18, 2013 8:42 am

Thanks Roy, I think I get that.

What happens if you change 1) the diameter 2) depth (i.e. the surface area, or canopy?), 3) thickness (same concept as the board rails?), and 4) canter (narrower at the leading entry and wider at the trailing end, rather than the other way round like turbo tunnel fin) of the tunnel?

And is the central planar fin only for stability or has it got another function?

I guess these will all depend on how the tail of the board (in fact the whole board?) is shaped or where it is going to be surfed...?
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Re: Rampant 10-5 project

Postby Roy Stuart » Sun Aug 11, 2013 6:08 pm

All five panels sanded ready for lamination:

http://www.roystuart.biz/2013/08/rampan ... e-and.html

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Re: Rampant 10-5 project

Postby Roy Stuart » Fri Aug 16, 2013 8:23 am

Red glop and wood lamination time!

More here, with video: http://www.roystuart.biz/2013/08/rampan ... art-8.html

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Re: Rampant 10-5 project

Postby blue_pig » Fri Aug 16, 2013 9:54 am

Feature red I like a lot - where will you be weight wise on this? I do like the outline shape on this.
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Re: Rampant 10-5 project

Postby Roy Stuart » Fri Aug 16, 2013 11:16 am

Hi Blue Pig she'll be just under 30 pounds ( it depends upon how much I shape away) at 98 litres of volume. The planshape will look even nicer once cut out as there's an inch plus extra hanging out all around at present. Tomorrow it's brick unstacking and stacking time again for the next panel, I'm using a 'slow' hardener which even in winter is only giving 20 minutes of pot life so we have to look lively on the day.
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Re: Rampant 10-5 project

Postby Roy Stuart » Fri Aug 30, 2013 10:40 pm

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Re: Rampant 10-5 project

Postby Roy Stuart » Mon Sep 02, 2013 3:51 am

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Re: Rampant 10-5 project

Postby Roy Stuart » Sun Sep 15, 2013 8:21 pm

Still shaping, and thinking about concave hulls...

http://www.roystuart.biz/2013/09/rampan ... ncave.html

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