The King's Ride

A tunnel fin overview.

2013-06-18T12:25:00.001+12:00

Tunnel fins provide 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.

'Rampant' surfboard 3: Initial fin foiling completed.

2013-06-17T11:04:00.000+12:00

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.

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%.

Recent Rampant 10-5 posts:
http://www.roystuart.biz/2013/06/rampant-surfboard-2-planar-cutaway-fin.html
http://www.roystuart.biz/2013/06/fcuk-work-rampant-surfboard-1-lets-make.html

RAMPANT surfboard 2: Planar cutaway fin panel

2013-06-11T21:31:00.001+12:00

Four layers of heart kahikatea wood being laminated into a panel for the Rampant 10-5 surfboard's central planar cutaway fin.

Next Rampant 10-5 post:
http://www.roystuart.biz/2013/06/rampant-surfboard-3-initial-fin-foiling.html

Previous rampant 10-5 post:
http://www.roystuart.biz/2013/06/fcuk-work-rampant-surfboard-1-lets-make.html

'RAMPANT surfboard 1: Let's make tunnel fin panel...

2013-06-09T10:07:00.001+12:00

Making a wooden tunnel fin for the new 10-5 'Rampant' surfboard. Heart Kahikatea is being laminated in two layers over a 6" pipe mould, with an internal layer of 6 ounce fibreglass. Subscribe to follow Roy's creation of this unique one of a kind surfboard.

Next Rampant 10-5 posts:
http://www.roystuart.biz/2013/06/rampant-surfboard-2-planar-cutaway-fin.html
http://www.roystuart.biz/2013/06/rampant-surfboard-3-initial-fin-foiling.html

The Dream...

2013-06-07T09:18:00.001+12:00

Wood and water...

A day in the life

2013-05-31T17:57:00.002+12:00

Roy in the shop and at the Point... produced by Dave Duffin and Kurt Oram.

Roy Stuart. Surfer. from Kurt Oram on Vimeo.

Art and desire...

2013-05-29T23:15:00.001+12:00

"The truth about creators of art is that no matter what anyone says, they pretty much are self-motivated and it's something they have to do.

I've sat in poetry writing groups at college. I've been to film reviews in film school. And I've been involved with creating things, most of my life, and most of the time displaying them.

And the truth about creators of art is that no matter what anyone says, they pretty much are self-motivated and it's something they have to do.

Why do they have to do it?

That's where it gets tricky.Maybe they'll find a gene someday that forces people to become artists. It's not an intelligent thing to do. The phrase "starving artist" didn't appear from thin air.

In other words, one thing I've noticed, over the years, is that no matter what you say to an artist, you won't be able to stop them. You just won't.

Either they enjoy it too much, or it helps keep them sane. Maybe they don't care if you like their work or not. Maybe they receive praise and enjoy that. But praise or not, money or not, there simply is no way to squash the artist anymore than you could catch a lion with your bare hands.

That desire to keep going is part of the whole equation. Or I should say, that desire to keep going while the mantle is upon you — that is part of the artist's personality"

Dave Beckerman

https://plus.google.com/u/0/+DaveBeckerman/posts

Hotkurl: Million dollar ride...

2013-05-25T12:07:00.001+12:00

2013 Hotkurl 10-5 by Roy Stuart:

HOTKURL Teardrop planshape perfection

Voluptuous curve synergy

'Kurl' in Feanorian script 23 ct gold tail detail

Vort-X tunnel & spitfire cutaway fin horsepower

Concave hull with tucked rail edge

The Dream

HOTKURL Displacement tailed planing hull in action

Polished stainless steel vent and 23 carat gold leaf

Empress wood, stainless steel and gold

1.47 million USD

Displacement tailed planing hulls 101: Tunnel or single fin?

2013-05-24T14:24:00.002+12:00

Displacement pintails when correctly designed allow the tail of the board to sink during turns and stalling moves, greatly increasing the versatility, control and balance of longboards. When force from the rider is released the tail lifts again automatically due to a combination of planing and buoyant lift.

It is vital for such tails to have sufficient thickness to allow both a low drag shape ( with soft catch free rails ) when submerged at higher angles of attack and to provide sufficient volume for the required buoyant lift. A narrow low area tail is also needed both for clean underwater running and in order to reduce rail to rail resistance from athwartships planing force leverage.

We have used this principle with great success both on single finned longboards and those also equipped with tunnel fins. The single finned boards use hull based lift only, with a higher proportion of buoyant lift over planing lift. The tails on these singles are easier to sink and have a corresponding softer spring back as the tail lifts, all else being equal. Because of this the ratio between the force required to roll the board from rail to rail and the force required for fore and aft trim is altered in comparison with the tunnel finned tail.

Here's a single finned displacement tailed board, the Ghost 12, sinking the tail during a bottom turn, the aft 6 feet of board is beneath the wave surface, this is achieved with moderate pressure from the rear foot and most importantly from a central riding position which allows excellent balance and gives the tail more leverage during spring back:

A reduction in weight on the rear foot will bring the board back into down the line trim in a 'self tending' fashion, here's the Ghost 12 again:

Dragon 13-9 with twice the weight and nearly two feet more in length utilising the displacement tail. Greater weight increases the force required to lower the tail since the longer heavier nose has a stronger counterbalancing effect. The longer tail also has more leverage, which reduces the depth to which the tail submerges.This is appropriate for the Dragon since it draws longer lines.

In contrast here's the Makaha 12-9 which has a four inch diameter tunnel fin. The planing lift from the tail is greatly increased, the effect is somewhat like stronger or tighter rear suspension on a vehicle:

Both options work admirably and the advantage which the system has over standard longboards can't be over stated.

Unveiling the incredible Hotkurl: Polished stainless steel vent and 23 carat gold leaf

2013-05-24T09:19:00.005+12:00

Hand operated stainless steel vent with 23 ct gold leaf.

Unveiling the incredible Hotkurl: Vort-X tunnel plus spitfire cutaway fin horsepower

2013-05-23T23:30:00.002+12:00

High lift virtual rocker fins on the Hotkurl 10-5. Nine inch cutaway Spitfire with four inch diameter half pipe Vort-X tunnel, fed by a full length single hull concave, with displacement pintail trailing.

Hotkurl Vort-X tunnel plus cutaway Spitfire fins.

Unveiling the Incredible Hotkurl: Voluptuous curve synergy.

2013-05-23T15:36:00.003+12:00

Beauty via pure geometrical design conceived and made manifest from first principles. Hotkurl 10-5 lies in wait for her prey.

Hotkurl 10-5 surfboard curve synergy

Hotkurl 10-5 ready to ride...

2013-05-21T09:12:00.002+12:00

More pictures coming soon

Making the Hotkurl part 11

2013-05-11T19:35:00.003+12:00

Empress wood in amber: resin coating and gilding the Hotkurl wooden surfboard.

Hotkurl surfboard Feanorian 'Curl' script 23 ct gold

Hotkurl 10-5 surfboard 1.47 million dollars

Quantum Mass

2013-05-09T22:24:00.002+12:00

A Weapon of Mass Velocity....

Making the Hotkurl part 10

2013-05-08T15:06:00.001+12:00

Resin coating in progress, note the concave and tucked under edge, shaping has taken a few weeks...

Making the Hotkurl part 9

2013-05-06T21:39:00.002+12:00

Shaping continues...

Roy: can a traditional stance be used on your surfboards?

2013-04-27T18:07:00.001+12:00

Any stance can be used, I use both a relatively forward stance and a sideways stance. My stance is as traditional as any other, and it's not uncommon at present either.

The key is to work within the 'sweet' area, which is more concentrated and central than on mainstream longboards. Longer heavier boards which are well balanced having a more central riding position tend to favour a narrow stance also.

Steve you'll find your own way, if your current methods need to change slightly that will happen naturally and you won't necessarily stand the way I do. I've seen a lot of people ride the Ghost 13 which is a similar board and there were different styles used.

Leroy Grannis:

Tom Blake:

Duke:

Roy:

Bob Simmons:

Update on the 13 foot 'Future Primitive' Ghost Olo tour.

2013-04-26T12:20:00.002+12:00

The broken surf board is now in the hands of Brad Tucker who is taking on the major challenge of rebuilding her. Thanks to Huck and Riebhoff for transporting the project.

From Brad:

"Thanks again for the hand off Huck! Get me that address again so I can send you a Paulownia thank you card.
Roy, I knew in advance I'd be stoked to see this and I was right. Beautiful work and not just aesthetically.
I can see I'm going to learn a lot from this project.
This board was built in 06 or 07 correct? My first impression is this board lasted long and held up amazingly well considering it's age, where it's traveled and the size of waves it's been on.
Other than the big hole in the middle it's in amazing shape. All the internal structure is still tightly glued together. No mold or soft wood that I can find. The outer surface is smooth as if it was finished up last week. No warped or cupped planks. Am I correct this board is unsealed inside and unglassed outside? Wow! Pretty sure that's what I'm seeing.
Anyway working on this project is going to improve my own work I can already tell. This is a treasure box of board building technique to learn from.

Huck and I were making the hand off at the Malibu surf parking lot yesterday. At least 4 or 5 complete strangers walked up and knew exactly what it was. Pretty cool. One guy said he recognized the board from out in the water so he paddled in to check it out. We let them all check it out, pick it up. Big BIG grins all around.

I'll push to get it done over Summer and take lots of pics."

STOKED!
Brad
WSS

Donald B with the Ghost 13 surfboard in Hawaii, November 2008

Eyes on the prize

2013-04-20T16:04:00.001+12:00

Makaha 12-9 at the Point

'Makaha' means 'fierce' in the Hawaiian language, here's the Chinese character for 'fierce'.

Plain D Blues

2013-04-18T18:35:00.000+12:00

The Gilded Dragon 13-9 gliding some unbroken swells through to the inside section at the 'Blowhole' at Mount Maunganui. She covers a lot of distance with ease, both on the outside swell and through the inside.

5am Deal

2013-04-16T23:14:00.003+12:00

Earl 10-6 at Mount Maunganui's main Beach, New Zealand.

Making the Hotkurl part 8

2013-04-16T16:20:00.001+12:00

Shaping continues on the Hotkurl 10-5

Does surfboard flex occur under the rider's front foot?

2013-04-15T18:59:00.001+12:00

Not usually. The reason for this is that water pressure is greatest right under the front foot, and rapidly reduces as it goes aft towards the tail. Water pressure under the front foot is many times greater than at the tail. The high pressure area is like the support used during a land test. Place that support under the front foot and apply as much force as you like and there won't be any flex.

To make the board flex pressure needs to be applied near the fore and aft middle of the wetted surface area, and the tail needs to have a high pressure zone... this is like pushing down on the middle of a board which is supported at the ends.

There are two problems with this:

One is that most boards have no high pressure area at the tail... the tail has very low pressure. With only one high pressure area the situation is like a land test with the board supported at only one end... pushing down in the ;middle' will only depress the tail without any flex. Canted fins or horizontal wings can provide higher pressure in the tail but typical cant is very small and the canted fins are placed fairly far forward which shortens the lever arm and drastically reduces the chance of flex.

The second is that when pressure is placed in the middle of the wetted surface area this moves the high pressure area aft as the waterline length decreases ( because the board raises its nose.)

Another issue is that even assuming that there is a high pressure area at the tail, if the lever arm ( length of board behind the rider's foot) is short then the board won't flex significantly. The amount of flex is greatly increased when there's a long lever arm, for any given force. This also means that a riding position which is right at the tail when turning ( for example on a mal) gives a very short lever arm.

.

Long tails are ideal for surffboard flex.

Here's Savitsky's drawing showing typical planing hull for and aft pressure distribution ( the curved line above the hull). As can be seen the high pressure zone is where the rider's front foot is usually. Moving the weight back moves the high pressure zone back as well.

Making the Hotkurl part 7

2013-04-05T17:52:00.002+13:00

Hotkurl 10-5 with first rail facets shaped:

The King's Ride: Roy Stuart Surfboards

Hotkurl 10-5 Roy Stuart Surfboards