Side Fin Physics

[retrothis]

@$1

With that statement, you have set your feet on the path of knowledge.     The relative flow is alway nose to tail, in line with the stringer.  I NEVER toe in side fins on 3 fin systems.     There are ‘‘secret’’ tweeks, that I will not go into here, but you have a good initial grip on the topic.     Toe in, or toe out, is a guarantee of increased drag, slowing the boards max speed.    My experience with 3 fin setups began in 1964, so I’m no stranger to them.   I will not give up speed for maneuverability.    I require both.

Hello Bill

Just out of interest, are all 3 fins double foiled?

 

[ghettorat]

Man its like listening to a pompous ignoramus, not seeing, feeling, and experiencing the value of toed in fins, and properly foiled single-side, side fins.  I can’t say I understand the physics, but I understand the ride.  Hey, why do they toe the front wheels in on cars?  Maybe there is something to it you guys are all missing.  Oh but its faster, but what about releasing, and loading on a turn?  What’s a single side foil do on a loaded turn?  Oh bigger fins, create drag, but they also create drive.  I may be older, but I don’t want to have one gear and go straight.  Feckless arguments, and for Reynold’s number, I would have to ask Sally, or Lauren before I give it to you.

[stoneburner]

To date, I have seen no evidence to suggest anthropomorphic perceptions can override the laws of physics.

Gravtitational acceleration and centrifugal acceleration. Drag and lift. Hydrodynamics and fluid dynamics.

Frames of Reference … Relativity … Vectors

[ghettorat]

I’ve been blinded by science, the new religion of the day, and if my understanding of History is somewhat correct, Man by invoking the Supreme Law of Science, has created as much havoc - no more than the scourges of religious zealots who did so in the name of their god.

[stoneburner]

I worship neither religion nor science.  Science is a tool.  Religion is social control.

Is the surfboard moving?  Or is your mind moving.

Don’t confuse mind with awareness.

[BgSurfer]

“In a rear wheel drive car, increased front toe in provides greater straight-line stability at the cost of some sluggishness of turning response. The wear on the tyres is marginally increased as the tires are under slight side slip conditions.”

Increased wear from side slip is from increased friction (drag).

[obproud]

@$1

Obproud & Stoneburner - If I understand your points about the flow of water, I think you are talking about something similar to “Apparent Wind theory” in sailing, where you have the True Wind blowing over the boat and then the “Induced Wind” which is created by the boat moving forwards through the air. The Apparent Wind is a vector of those two and is what you actually set your sails by.

Yep.

 

@$1

I’ve attached a diagram of this theory, altered for a surfboard on a wave. In Figure 1 the board is travelling across the wave with the “True Flow” of water up the face of the wave at 90 degrees to the direction of travel. If we assume that the board and the wave are moving at about the same speed (in reality the board would be moving faster) then the “Apparent Flow” would be at 45 degrees to the direction of travel, and the the angle of attack would be perfect for generating lift. In principle, I can see how this theory could work and make sense, but I have a few problems in my head, maybe you can help me understand:

1. If this is correct it means the effectiveness of your fins is directly related to your direction of travel on the wave. If you drop down the face of the wave, the “True Flow” and “Induced Flow” are in the same direction, and we still have a negative angle of attack (Fig. 2). While if the board is turned up the face of the wave the “True Flow” and “Induced Flow” are opposing, and would cancel each other out leaving no water flowing over the fins (fig. 3).

 

You’ve got it. For the most part the only time boards with toed in fins have a negative AoA is when the board is moving opposite the flow of the water in the wave… or if it is within that range defined by the amount of toe.

 

@$1

2. For this theory to be correct, the board would have to be sliding sideways on the face of the wave. On a boat the air flows uninterrupted over the deck, allowing the sail to affect the flow and create lift. However the rail of the board is designed to grip and re-direct the flow of water behind it, so that a board angled at 90 degrees across the face of a wave, will gently rise up and off the back (and we all know how it feels when the rail actually slips on a steep face) By my understanding, the use of concaves on the bottom of a board is to re-direct as much water out the back of the board as possible, this would mean that the flow past the fins is running nose to tail. Maybe I’m wrong, and despite the best efforts of rail, concave and fins, most of the “true flow” up the face of the wave does pass under the board and out the other side, in which case there is still the first problem that the effectiveness of the fin would be altered by your angle to the wave face. Again maybe this is the case, and as with a sailing boat, the fastest direction of travel is the equivilent of a “Beam Reach” when the Apparent flow is just right for your fin set-up.

Yes, the hull redirects the flow and changes the AoA relative to the fins. But remember this is happening in 3 dimesions, and there is a (sort of) field of curving flow. The flow closest to the hull is turned the most and further from the hull it is turned less. IMHO, the tips of fins encounter a higher AoA than the base (because the flow closest to the hull is turned more).

 

The toe of the fins allows the fins to maintain an effective AoA (still creating maximum lift) even when the board has passed the bottom of the arc of a bottom turn. Without toe, the fins would start creating less lift once the board reached the bottom of the bottom turn arc, right at the point when maximum fin lift is necessary (when you want to lift back up the wave). And visa versa, on a top turn, a board with toe creates less drag after the top of the arc of a top turn, relative to a board with no toe.

 

IMO, fin toe (along with cant and bottom concave) loads the board design with “built in” AoA, which amplifies rider input in the direction of the desired turn… kind of like power stearing.

 

[TaylorO]

Well stated Ob

[hans]

@$1

Hans & Jameskiri - Firstly,
and this is a little off topic, but I have to disagree with you both on
the issue of pumping. I have spent many hours breaking down slow motion
footage and I promise you that at no point is the board “Pushed
sideways down the wave face”. Pumping, if done properly, should be a
series of smooth, linked top & bottom turns, combined with a
compression and expansion of the surfers body. The whole point is to
keep the fins and rail engaged in the wave to generate forward speed,
and so if the board slid sideways at any point it would destroy any
forward momentum. If you doubt me, try going frame-frame on a video and
watch the tail of the board, it will never move sideways.

Hi Harrington,

It
is not possible to see the sideway slip, there is no way to see how the
water of the wave exactly moves. The water moves from bottom to the top
of the wave, so the downward sideway slip can even appear as a
resultant upward movement on the wave. It is very subtle.

I have
no scientific proof for this, but my experience tells me that toe-in is
good for small waves and that for big waves no toe-in is desired.

I
agree on your description of good pumping, but while you expand your
body, you put more downward force on your board. This downward force
gets converted in a forward lift due to the angle of attack, I totally
agree that the pumping is a smooth linking of top and bottom turns. Draw your velocity triangles and you will see what I mean.

[BgSurfer]

Basically, the board is moving down the wave, up the wave and parallel to the wave.  But the board is continously moving perpendicular to the wave front and towards the shore – even while traveling parallel to the face of the wave.  The wave form is moving forward but the water is not, except for the limited forward water movement when the wave breaks.

[thrailkill]

Never lose sight of the fact that the FIN has multi functions.    It is your keel, your rudder, and your wing.     All at the same time.   

[TaylorO]

Good point Bill -

I feel like I’ve forgotten more about fin science than is in this thread.  Once upon a time there was a thread with some well explained math about the forces on a fin, and that was at a static given moment.  So far the scientific consensus seems to be no one has expended the time involved to make the adaquate software.  Heck, we know the best weather programs get it wrong some times, and decrease quickly in accuracy as the prediction goes out.   Just sayin’ that’s some complex figurin’.

Scientific explanations aside - there seems to be something to say for the historical, emperical state of fin use.

Started out here blowin’ this horn - Dale S. started telling me over 30 years ago, and it hasn’t changed, “Surfcraft design is a balance of speed and control.”  This holds for fin toe too.

 

[halcyon]

Many words appear above.

IMHO three are most apropos: wing & sail, & rudder.

Fins placed and foiled with these definitions in mind

have the most postive effect on performance.

To watch a bird in flight is to begin to understand what

rail fins are about or so it seems in my limited scope.

 

Aloha, Rich

 

 

[newschoolblue]

@$1

Never lose sight of the fact that the FIN has multi functions.    It is your keel, your rudder, and your wing.     All at the same time.   

Very well-said, and it is for this reason that I personally think your side-by-side asymetrical fins set parallel to the stringer in place of a single fin (do you have a name for this setup?  I hope so cause if not, I’m gonna call it a twingle-fin.) makes tremendous sense to me, even though I have yet to try it.

However, in response to the original question, my take is this: single-foiled side fins on performance-oriented (maneuver-oriented) boards are towed in because they are intended to be ridden in such a manner that the water almost never hits them head-on, in a straight line, parallel to the stringer.

 

 

[thrailkill]

Newschoolblue,

Twingle is a name that somewhat describes the fin setup.     Another Swaylocker made the same suggestion, several years ago.    Thank you, for your comment.

[surfiber]

fin connoisseurs can probably make use of this resource : http://jeb.biologists.org/content/213/12/2038.full

 

• RESEARCH ARTICLE

doi: 10.1242/​jeb.040162June 15, 2010 J Exp Biol 213, 2038-2047.

Numerical simulation of a pectoral fin during labriform swimming

1. Kourosh Shoele and 
2. Qiang Zhu*

 

[Dan_Johnston]

ok, for a simpleton, in building an 8’6" mini mal 23"x3.5" very pinched rails and V running though the tail. The buyer wants a tri-fin setup. He is about 200 pounds, 6’ guy. suggestion on what FCS fins I should use and what toe-in number?We have always done single fin/fin box installations.

 

Much thanks.

dan

Blind Dog Surfboards

[Michael_Frey]

I found a good introduction to the topic on the website of a windsurfing fin manufacturer:

http://www.mauiultrafins.eu/index.php?option=com_content&view=article&id=132&Itemid=127&lang=en

 

Also I have written down some own thoughts on the problem of toe-in-angle in the blog of my company hamamasurf (scroll down for english text). Please note the literature I mention at the end of the text if you do not already know it.
http://hamamasurf.blogspot.de/2012/01/frequently-asked-questions.html

 

The fins I have developed with respect to my understanding of the topic and the findings I have made in scientific literature will be available through www.hamamasurf.com in a couple of days.

 

regards,

Michael Frey
Germany

[Michael_Frey]

What I forgot: I have been working in the wind tunnel facility of a University here for a while and just recently asked my boss if I can use the tunnel for own experiments. It is not sure yet, but maybe I get some tunnel-time later in the year for experiments with my hamama-fins and a popular big-brand fin as a benchmark. If this ever happens I will let you know the results.