Tunnel fin/annular wing talk

Hey Roy, It's Rich. I'm a fan of fin lift too. I always semifoil my twinners and like a lot of angle, with the idea to keep the tail snug into the wave at speed. I get the rail-to-rail advantage, so now I'm wondering about that bottom section of a tunnel fin. If that surface is foiled to produce lift, does that not tend to lift the tail out of the water? And what of the cost of that lift in terms of drag? Like I said, I am just curious...

<p>Hey Roy, It's Rich. I needed to test out how posting to a group started by someone else worked from a non-admin perspective. I'm also really curious about those tunnel fins too so figured this was a good opportunity.</p> <p>I'm a fan of fin lift too. I always semifoil my twinners and like a lot of angle, with the idea to keep the tail snug into the wave at speed. I get the rail-to-rail advantage, so now I'm wondering about that bottom section of a tunnel fin. If that surface is foiled to produce lift, does that not tend to lift the tail out of the water? And what of the cost of that lift in terms of drag?</p> <p>Like I said, I am just curious...</p>

Hi Rich, Thanks for the questions Regarding the middle section of the tunnel, it will produce upwards lift no matter how it's foiled, We've had good results with double foiled sections and also with single foiled ( nearly flat on the inside or 'up' side) as well. Because the tunnel is an enclosed system, pressure is more evenly distributed inside the tunnel than it would be with a planar non enclosed wing. For that reason it's easiest ( in my opinion ) to understand and visualise the lift via the Newtonian method, i.e. redirection of water flow and mass, rather than Bernoulli's system of pressure differences, although both systems ultimately give the same answers. Whether or not it lifts the tail out of the water depends upon the angle the fin is set up at. For example If the tunnel fin is set up parallel to the bottom, then it's running at the same angle of attack as the bottom in the fin area. So, one might expect the fin not to lift the bottom upwards. In fact it does lift the bottom though, because of the difference in the lift characteristics of the planing surface of the bottom and the tunnel. The tunnel produces more lift at lower angles of attack than the planing surface in the tail and thus lifts the board in the tail area even when running parallel to the bottom. Adding a positive angle of lift between the tunnel and the surfboard planing surface in the fin area increases the force lifting the board. One might expect that the tunnel, acting as an underwater lifting foil, will just keep on lifting the tail upwards if it generates enough lift to do so, in other words jacking the tail out of the water. What happens in reality is that as the tail lifts slightly, the fore and aft pitch angle of the surfboard changes. . . the surfboard pitches forward. Because the tunnel is fixed to the bottom of the board this automatically lowers the angle of attack of the tunnel. Thus a state of equilibrium is reached where the tail is riding slightly higher than it would be without the tunnel, and the tunnel is taking some of the load. The system ( if adjusted correctly ) becomes self tending. This self tending aspect doesn't completely determine the angle of attack of the surfboard because rider mass can be redistributed to alter the trim of the board. Thus the rider can trim the board nose up and the tunnel and planing surfaces working together will lift the board out of the water, in much the same way as a surfboard usually behaves when trimmed nose up, with some added lift from the tunnel. When trimmed nose up the efffect is not to lift the tail up more than at low angles of trim, what happens is that the whole board is projected forwards and upwards in the direction in which it is pointing. . .. this is because the tunnel and the surfboard tail have a fixed relationship to each other. The self tending aspect is greater on longer and/or heavier boards, but is still there on lighter boards. As far as lift vs drag goes, the tunnel has a better lift/drag ratio than the surfboard planing surface because it is effectively a wing of infinite span. Tunnels also have low induced drag. So, whenever the tunnel does part of the job of producing the necessary lift which is usually done by the planing surface of the board, there is a drag advantage to be had. According to some tunnel analyses tunnels are better at low speeds because they have high surface area, ( skin friction drag due to surafce area predominates at high speed and induced drag at low speed) Such analyses make a couple of misleading assumptions, one is mileading because it doesn't apply to the surfboard application of tunnels and is based on the areonautical assumption that the wing must provide all the necessary lift ( not the case with surfboard fins ), the other is an anomaly in the way annular wing area is measured when comparing lift/drag characteristics with planar wings.. Testing identical boards with and without tunnels it's immediately apparent that the tunnel equipped boards are faster as well as haveing more drive and hold . I'm not claiming that tunnel finned boards will always be the fastest in all situations, as there are other ways of achieving low drag, but they certainly work and are a force to be reckoned with.

#Baron_Surfboard project part 5: The tunnel fin

Here's the tunnel fin for the #Baron_surfboard taking shape.

Because this tunnel fin setup also has a planar fin in the form of a bulb tipped flex fin, it won't have to handle high angles of attack, so we are able to use a very thin fin for low drag, just as we did with the original D 11-9 prototype shown below:

http://www.youtube.com/watch?v=WQ6M9GY1XC0

The tunnel is only 6mm thick, and has a chord ratio of 6% which is very low.

Because the tunnel is so thin we are making it from 22 layers of thick brown paper and resin, it's the same paper which we use for drawing plans. Paper and resin is a sort of low tech carbon fibre laminate and is immensely strong.

The tunnel mould is a piece of plumbing pipe of 200mm diameter, we,e added some poster pint to tint the glue lines

Tunnel fin and cutaway D fin continued. . .

Time to clean up the finished panel, make a tunnel finned sized slice and do some fin foiling for the tunnel plus one combination. . . .

The 'lightbulb' 5-11 tunnel finned hollow wooden surfboard

Finished !It's fast, handles well, and the fin holds in nicely.

The 'Lightbulb' hollow wooden sidecut surfboard

Another wide ultra thin shortboard. The 'Lightbulb' is a cousin to the 'Gobstopper' being an inch and a half narrower at 27 wide, and a tad shorter. The Lightbulb has a wider tail and subtle sidecut in the planshape.Construction is 6 layer parallel profile, with double laminated paulownia deck and double diagonal internal frames. The rails are a continuous round 50/50 in cross section, except for the squaretail.She's flat in the tail rocker with a prounounced nose lift, and will soon sport a big tunnel fin with leading edge channels.Here's the laminating process, showing the double diagonal frames and the z axis ties which join the frames.

More ultimate wave tools Makaha

Flexibility in this board is incredible, if it is laid carefully deck down on the floor ( supported nose and tail ) the slight 'bump' makes it take on a visible rhythm for a couple of seconds, it just sits there bouncing softly up and down it is SO sensitive to any movement !The whole board is pre tensioned like a drum. It has about 2 inches of flex on a land test using only a fairly gentle push with the hand.I know what that means, it means that it's a slingshot !When the rider weights and unweights the board it responds with a gentle twang, this motion drives the tail up and down and transfers the energy through the underwater foil of the tunnel. . . . . . into forwards motion.The board literally comes aliveA luxury surfboard with perfect suspension like a Roll Royce.it's an ultimate experience !

Tunnel finned 'Squidfish'

Squidfish is based on a slightly stretched version of the 'Duke' 9'3" hull .A suggestion: Gaze upon the unique physical reality of this dream wooden surfboard sculpture, refresh the mind and absorb a hint of the fantastic gift of Squidfish glide.VORT-X annular wing tunnel fin mesh:VORT-X tunnel flow simulation at 24 degrees angle of attack:

Spitfire-Tunnel fin for the 12-9 Makaha

The fins are shown here with the main Spitfire cutaway fin sitting in its slot and the tunnel fin placed in position, it will actually be a bout 3/8ths of an inch lower once set in to the board.There are 73 pieces of paulownia in the tunnel fin, which is a two layer lamination with a wool/polyester blend cloth in between the laminations. The fin is glassed on the outside with bamboo cloth and the inside is resin coated.Six laminations of paulownia are used in the main spitfire fin, with 36 pieces in total. This cutaway fin has developed from the original spitfire fin to the point where it is hardly recognisable as a spitfire fin, but we still use the name as it is an upright planform fin using ellipitical leading and trailing edges.

Bamboo cloth on the tunnel fin

Wooden tunnel fin with bamboo cloth part 1:Here I am sewing the bamboo cloth on to the outside of the fin with cotton. The bamboo cloth is knitted and it gets stretched over the fin under tension. I started with a piece about half the size of the surface to be covered which was rather ambitious but the cloth stretched on quite easily, although a fair bit of tension was needed.Epoxy resin time:The bamboo cloth absorbs resin well and wets out readily, if more resin is applied the cloth absorbs it and bulks up, which suits me in this case as I want a reasonably thick cloth and resin layer to allow some further fairing and foiling of the fin. This attribute of wetting out with a small amount of resin but bulking up with more resin makes the cloth versatile.Clarity is remarkably good, I don't mind cloths which partially obscure the grain, however so far this one looks to be almost as clear as glass.Tomorrow the inside of the tunnel will be sheathed the same way.

Tunnel fin for the 12'9" Makaha

The Makaha woody has a 10 inch deep singlefin with a 5 inch diameter tunnel fin tucked under a cutaway in the main fin. Here we have the tunnel fin laminated and partly foiled, it is made of 73 narrow strips laminated together in two layers over a cylindrical mould, with a layer of cloth in between. The resin/cloth layer is harder than the wood and is there to provide a hard layer so that the trailing edge can be finely foiled.Here is the fin setup, in reality the main fin will be set down two more inches into the board. It's an incredibly good fin setup which we have been using for the past 5 years, no other fin arrangement that I have tried has anything like the drive, speed and control that this one has. It is powerful and free running, just the cat's whiskers !