Fin theory,tom,halycon and others

General Discussion
181

Hello Guys, I am a fin nut and have been working for some years on the hydrofoil principle, and on bulb fins.

I have built boards with bulb fins and tunnels and was surprised and excited to see others working along the same lines. I put bulbs on fins to reduce tip turbulence. Bulbs are able to reduce tip turbulence when it is most likely to happen: namely when the tip is moving laterally and has more water pressure on one side than the other. The fatter foil proportion of the bulb is less likely to cavitate, or less likely to cavitate catastrophically.  

About hydrofoils.  Horizontal fin surfaces are very difficult to control if they are placed far below the surface of the board. We had interesting results a few years back with large horizontal wings built onto singlefins, but there were severe handling problems due to    

                                           1) The depth of the wing. When the wing starts to lift it will keep on doing so until it reaches the surface of the water (assuming it is directed that way) As it does this it jacks the tail out of the water and control is lost. 

                                           2) The straight wing. A straight horizontal wing will twist the board unexpectedly in the tube because water flowing from one side of the wing to the other will suddenly press on the top or bottom surface, twisting the board. 

  We have solved these problems completely using a half pipe tunnel which is attached to the bottom of the board, not the fin (therefore controlling the distance it can lift) and which can't twist the board due to the circular shape. A nice spinoff of this fin is that it has no tip and therefore no tip turbulence (the old 'annular' wing principle). It works like a charm.It is very fast, has tremendous drive, amazing holding power and is absolutely viceless. So far we have done several hybrid versions with a single in front of the foil, and a "pure' tunnel. The tunnel offers good lateral resistance once it is moving fast, but can be sideslipped at low speed (fun). The hybrid version doesn't sideslip at all. I have some pictures and videos of the boards moving fast. Some pictures are on my site at <a href="http://www.woodensurfboardbuilder.com/" class="bb-url">www.woodensurfboardbuilder.com</a> There is a lot more information I can offer on how to set these things up. Roy 'bowser' Stewart


https://swaylocks7stage.s3.us-east-2.amazonaws.com/s3fs-public/1000770_foilboardpic13#.jpg

https://swaylocks7stage.s3.us-east-2.amazonaws.com/s3fs-public/1000771_foilboardpics%20086#.jpg

182

interesting stuff. Have you tried this fin set up on short boards, or anything with hard rails? What do you build your semi-circular fins of? Do they have a foil shape, and if so, is the foil on the whole (er, half) fin? Or just on the vertical sections?

183

Hello Keith,

Yes we have tried the tunnel setup on shorter boards. My son James has a 7’5" with a tunnel which he is riding presently. He likes it very much, in fact he broke the foil off accidently recently and was pretty quick at getting one back on there again. It does seem to be better to have a bit of weight in the board as they are sensitive to fore and aft trim.

The tunnels can be made any way that works, I started by laying up paper and epoxy over a piece of plumbing pipe which is great but being a man in a hurry now I just use the plumbing pipe. You can foil it on the outside and leave the inside flat, it also works with the inside foiled. I don’t have much to say about rails except that I always use a continuous round rail and that I think that the setup will work with any kind of rail. There are a couple of little tricks to setting them up so if you get that far just ask.

Roy

Here’s a picture of James with his 7’5" on the right (board with foil) The board on the left with Patrick has no foil but I have another one identical with the tunnel. The singlefin version is mellow, smooth, and quite fast, thetunnel version is a rocket.

https://swaylocks7stage.s3.us-east-2.amazonaws.com/s3fs-public/1000772_scan0001#.jpg

184

Hello Tomatdaum,

Do you remember a product called ‘friction free’ which was supposed to work (I think) by reducing the drag between boundary layers. I used to put detergent on the bottom of centreboard racing boats and surfboards. These days I use coconut oil on the bottom of my boards and it definitely makes them faster, provided that the water is warm enough to allow the oil to dissolve (about 20 degrees). Is this something to do with a reduction in viscosity or surface tension ? It isn’t a good idea to put too much on the rails as it makes the board hard to hold on to. Roy

185

Hellow droptrow, I had one of those Bobby Owens kind of fins made up back in the 70’s it was ok except it’s a bit of a seaweed catcher. Roy

186

Hello Halcyon,

I noticed a reference to a speed of 35 knots in a recent posting of yours. Isn’t it time that we started actually measuring speed so that we can test designs and have a lot of fun outdoing each other in the process? It’s perfectly easy with a small g.p.s. unit. There’s a little one on the market called a Garmin "Gecko’ which is also cheap and well store your top speed for the session.

I spend a bit of time making claims that my boards are the fastest and so on but it would obviously be a good idea to back it up with some figures. I haven’t got a gps yet as my wallet is busy buying wetsuits, computer gear and boardbuilding materials but hope to get one soon.

Regarding the fastest surfing speed possible, it is true that given zero friction, a surfer will keep accelerating until the speed of light is approached. This is true no matter how small the wave is. As speed increases you simply wind up a higher glide ratio. The potential for high speed on small waves is tremendous. In the meantime I will post a video clip of some small wave speed surfing (soon)Roy

187

Guys

Just a little suggestion when you post a pic of a fin and the text regarding that fin is about camber depth and position etc

How about we all use one of those little gadgets that you can buy down at the local hardware store that copy shapes

They just consist of two metal plates with lots of pin each side

Put up against the fin a there is the camber

regards mike

188

interesting test method and results near the bottom of the page…

http://www.aae.uiuc.edu/m-selig/uiuc_lsat.html

check out this this link…wait for the entire page to load and look at pic of the foils tested…there are foils in there that look awefully like standard thruster fins as well as lots of curved foils…all tested and available for your research…

http://members.cox.net/hstokely/soartech.htm

http://www.aae.uiuc.edu/m-selig/pd/manifest.html

“Often, NACA airfoils are used for these applications, but as compared with airfoils that could be designed today, many of the NACA airfoils (which were designed decades ago mostly by trial and error) are inferior”

“Other topics of interest include the effects of turbulators and contour accuracy. Are boundary layer trips simply “repairs” to otherwise bad airfoils, or can trips be integrated with the airfoil and result in improvements”

Turbulators 101

http://www.mh-aerotools.de/airfoils/turbulat.htm

189

roy stewart certainly threw something different into the ring…

i had to spend some time digesting the whole concept ,yea interesting,i think the combination of the boards your build along with those fins is a unique package…

roy! do you ride more conventional style boards as well?? or are you only working along the lines of what you showed us???

why i ask is , what sort of comparison can you give between what you build and what we would term the more conventional fin set up and board, or any comparison really between anything we may be familiar with???

meecrafty comes up with the killer links as usual…

im going to make a comment and im curious if we all agree on it as it leads to a theory i have ,and id like to explain it further down the track once i see what sort of response i get to this next comment…

if we look at a flat sided typical wing/ surfboard sidefin foil…

the region of low pressure on the curved side can be moved in relation to the foil ,by simply moving the camber forward or back…

so if we move our camber forward we move the region of low pressure closer to the front of the fin,if we move the thickest part of the foil back we now move the low pressure region further back…

and in relation to that comment the region on a fin creating the most lift will be the area in which theres the greatest potential pressure difference between both sides…

both those comments relate to the theory i have …

so would we consider those comments to be correct for low speed foils???

or is there another equation im overlooking???(discount rake and template design, just a vertical looking wing for now) like the ones in the pic im posting…

regards

BERT

190

Hey Bert,

I think the first thing we have to do is distinguish what is refered to by camber and cant and where the word camber fits into the fin-foil picture. When it comes to tires on cars they mean about the same thing but with fins it believe it’s a different story. This English language can be very confusing at time.

I’ll leave the clarification of the definitions to Blakestad, Tom or others who are better versed in expressing theory than I.

I have some input on the fins and statements just above here but I feel it best to save them in the interest of clarity.

Keeping our term definitions in order is a most important part of these sorts of assessments.

Off to work, Rich

P.S. I made the attached about three and a half years ago and abandoned the idea as a viable template for rail fins. Ask me why ~

191

Camber refers to curved surfaces. So, its use to refer to the foiling is appropriate.

Cant refers to slant or angular deviation. I prefer to use cant, and toe-in, to describe the two components of angular deviation, or slant, for fins. The use of toe-in is completely unambiguous.

Template is outline.

Why did you stop with the upright symmetrical templates?

192

Q ~ Why did you stop with the upright symmetrical templates?

A ~ They’re gutless, by that I mean they don’t give you any acceleration during the process of turning.

Vertical fins IMHO. Depend on the board rails to do all the real work of turning. They feel to me as though they only act as directional stabilizers. However I think that if a fin like this is outlined and foiled right it approaches being the fastest of all fins in a straight line.

I have reasons for believing this that I don’t have time to articulate right now.

Off to work, Rich

193

Hello Bert,

The differences you are seeing between what I am building and what is ‘conventional’ are all changes which make the boards easier to ride. Therefore, as a surfer you are already instinctively familiar with what these changes look like.

Regards, Roy

PS But on a practical level are you saying something along the lines of 'Interesting, but what what can I do about it?" ? Quite clearly there needs to be a practical way of evaluating any interesting board theory yourself, I mean to say that the acid test is actual surfing.

If what you are interested in is tunnel fins then I suggest that you can park one behind the main fin on any single finned board and go and have a blast. A heavy longboard is the easiest to use as you want to be able to control fore and aft trim and a bit of weight in the nose helps with that. Also a board with a reasonable dose of rocker helps as then you have a bigger range of fore and aft trim options.

Tunnel fins can be made out of a piece of plumbing pipe, foiled on the outside. The most reliable way to set them up is exactly parallel to the bottom of the board. If you want to make life exciting put a tiny bit of lift angle in, by keeping the trailing edge a millimetre further from the bottom than the leading edge. This will make you go even faster but there are some little quirks.

I am building tunnels all the time so can easily make you a carbon fibre one if you like, I will be offering them for sale on my site soon. Size-wise four, six, or eight inch diameter pipes all work well (translating into two, three, or four inch deep half pipes)

194
Quote:
Q ~ Why did you stop with the upright symmetrical templates?

A ~ They’re gutless, by that I mean they don’t give you any acceleration during the process of turning.

What happens when you enlarge them by making them deeper or longer in chord length and also deeper so that the total fin area is comparable to a standard fin? Most gutless fins can be given guts by increasing chord length and/or depth until they have enough drive - the question is what do they feel like once you give them enough guts, but have the same basic template shape?

I somewhat wonder if chord lengths and/or total area aren’t the most important variables (in a template - not talking about foiling here), and that once those are accounted for, the differences between templates are more minor.

Or, perhaps, the experiment didn’t make it that far…

195

more links to come…

196

Hey Dave,

Rather than go to all the trouble of making a series of thruster sets I just built a series of singles 8.0" deep and surfed them all on the same board that surfs pretty well with a fin that size. The overall surface area of the fins was very similar. The vertical fin was just about worthless unless I put it all the way back on the tail of the board and even then although the board went very fast in a straight line I had no ability to climb the wave face or change my trim line effectively, It was more like aiming a dart than steering the board for the power of the wave. Even the small profiled very flexible 8.0" Greenough Stage IV was an improvement and a “Mental” template at 7.5" surfed well. That was enough for me. I’m not going to persue vertical templates except for tow-in surfing, paddle in on waves in excess of 20 feet, wind surfing or other high speed fin applications and someone else will be riding them. That kind of stuff is way out of my scope as a surfer.

One needs a lot of speed and a completely different kind of bottom and rail configuration than what we see on most modern surfboards for this type of fin to be effective. It might work in small waves but I’d have to build or get someone to build a board to suit a vertical fin or set-up. It would be a very interesting combination of the new and the old and the guy I’d need to talk to before I took the project on would be Geoff McCoy and he’s 12,000 miles away. At this point in time I have to confess that I don’t think many surfers would be very excited about how it performed, but I’ve been wrong before.

Certainly everything has it’s place. Everything one does to a surboard including the fins or fins changes the whole package.

Constants are just as hard to find on the wave face as they are on surfcraft, thus I struggle on learning as I go.

Mahalo, Rich


197

Hay Rich

Just a point from my little book of theory (take it or leave it i dont mind)

If you go upright you absolutly cannot go with a tip shape like that,you must have some area there

Think of the what the object of the fin is, to create a pressure difference, the result of which is lift .

For that you need an area where the fluid paths are separated,for that you need distance relitive to the direction of flow

Now look back at that tip and picture what is happening at an upright tip like that.

Also as I have said before if you go upright ,you need different foil camber position and depth relitive to what you have as the norm on a raked fin

Regards Mike

regards Mike

198

Bert

please see my post to Rich regarding this plan shape,it works if you have the area of a spitfire wing. On a foil the small size that we are talking about it has serious problems

Cheers Mike

199

Hey Mike,

Yep it’s a spitfire wing. Just like Bert’s rail fins.

What kind of an upright template do you suggest?

Are we still taking the same amount of surface area?

Respectfully, Rich

200

Halcyon

Yes,a point or sharpish round with small area of a fin is only good with rake,where it gains area or flow length

On a upright fin it is just a point.where it has no or very little area or flow length which equales to no pressure difference so no lift just drag

With the small area of a fin I think you need a more trunkated

tip

Ideally you gain this buy rounding the leading edge back and keeping the trailing edge straighter

Most small R/C gliders are a good example of this.ones with points or small eliptical tips generally dont fly that well , the exception being when you add heaps or diheadral or poly headral

Also the shorter the cord length the thicker the foil needs to be to get the pressure difference working

One idea I have played with before and will persue again very soon is.

Make a flat profile with carbon around 4mm thick now cut to a plan you think will work ,taper its front and back thickness .Then using microballoons if using epoxy or Q cell if polyester

try different foil cambers

Doing this it is very easy to change the foils ,dont discard a profile untill you have tried all the different thicknesses and foil positions

If any one finds improvment post its plan and camber

With all of us we can move through the range far quicker than an individual

can

The difficult task with or fins is the small relitive size ,area and aspect ratio

there is so little aspect that you have to make what you do have totally work at providing pressure difference

Cheers Mike