Good question(s).
Have you seen any good skim boarders turn on a shorebreak wave?
They turn on a dime, without fins.
You can make a surfboard turn without fins, but youll loose lateral hold without them.
That lateral hold is everything.
The board bottom banks off water.
The board rail is like a big keel.
The fin makes all this possible.
my very simple $0.02
A great picture which shows why thrusters and other boards with toed in fins are so slow.
Your trux are one solution, another is to avoid toe in altogether !
I was just getting to that… here’s the drawing…
The outside rail fin only contributes lots of drag and a '‘righting’ or ‘stabilizing’ force (like the single fin above) to end the turn (unless it is out of the water).
The inside rail fin not only tries to stabilize like the single fin example above, but also has a force component in the direction of the board’s nose (Thrust?).
The Inside rail fin is working at a small AOA (in the drawing), so to maximize lift, an asymmetrical foil is used (flat on the inside).
The rear fin stabilizes like the single fin example above, but in doing so, keeps the inside rail fin at a smaller AOA so it is more efficient at generating lift with less drag, and creating a forward force (Thrust?).
I think I have this right, it is now making sense…
So, Blakestah’s Surf Trux take away the outside rail fin’s contribution to drag, like so…
So, with the Trux, you take away lots of the drag from the outside fin, you get relatively more ‘thrust’, and the board comes around faster (no outside fin drag to impart a rotational force counter to the turn), once the rotating fins have aligned to the water flow.
I know this is posted on blakestah’s website, but it didn’t make sense to me until now…
JSS
One question, though. Are the Trux’s outside fins symmetrically foiled?
Yes Max, except that this statement isn’t quite right:
The inside rail fin not only tries to stabilize like the single fin example above, but also has a force component in the direction of the board’s nose (Thrust?).
The problem with that statement is that it implies that the singlefin does not produce forwards thrust, when in fact it does. ( well to be fair you might not be implying any such thing but i mention it because it is a mistake which Blakestah has previously made )
The mistake is in assuming that thrust is defined in relation to the stringer line and the nose of the surfboard, and then deducing that because there is no angle between the stringer line (or centreline) and te fin that therefore no forwards thrust can be produced.
In reality, it is the angle of the fin to the rail line which determines forward thrust during a turn, not the angle in relation to the centreline. . . the angle in relation to the centreline is completely irrelevant. Because singlefins are set at an angle to the rail, they pruduce forwards drive, ot thrust, in the same way that the inside rail fin of the thruster does.
The main difference between the thruster side fin and thhe single is that the thruster sidefin can produce drive even on a parallel (or relatively parallel) planshape, whereas for reasond of symmetry a singlefin’s angle of lift depends entirely on the the angle of rail convergence.
I think I see what you’re getting at, but could you put what you are saying into a drawing/picture (if you have time)? I am unfortunately a visual learner, hence all the drawings above…
JSS
Hi Max, yes can do, but perhaps try this while I’m doing it:
Check out your own pictures, and compare the singlefin pictures with the thruster pictures, you will see that in both cases the fins are producing lift which is forwards in relation to the rail line. If the fin and the rail are parallel then this won’t occur, but as long as there is an angle between them it will.
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So increase the camber, generate more “turning lift” when the thruster is on rail (the fin becomes more vertical in the wqater) and put the outside rail fin into stall earlier? (but the outside fin leaves the water earlier, too)
I also think that the key part of turn from fins is that the fins pull the tail into the water (especially well-foiled fin tips), allowing the rail/rocker line to operate. Insufficient fin tip and foiling means the fins operate purely as rudders so the board needs to turn flatter or risk tail skip out. More important in front foot surfing or when the fins are moved off the tail to further up the board.
I got other things from this pic…
That this is a diff angle to the hand-trail is just due to the angle that the pic was taken.
The area required to plane at speed is 1/3 - 1/4 of the boards total area.
The water contact line is so close to the outside fin that its trail is ventilation not cavitation. Esp as the plume is enormous and appears to be coming from the root of the fin not the tip.
To the right of the water contact line is a lot of spray bounced away from the hull, this shows that the hull angle is creating a lot of resistance, and therefore boards could go a lot faster if this resistance was addressed.
As every drag surface is a control surface, wouldnt fins that rotate to reduce drag, also reduce the control ?
As every drag surface is a control surface, wouldnt fins that rotate to reduce drag, also reduce the control ?
I do not think every drag surface is a control surface. If I could make outside rail fins completely disappear during turns it would be even better.
But when you go straight on a board with SurfTrux, the board feels extremely skatey, much more so than a thruster. That’s why I named it SurfTrux, just like a longskate feels loose and fast when you go straight and controlled in turns, the SurfTrux feels loose and fast when you go straight and controlled in turns.
In truth, I think the diff between SurfTrux and a thruster, when on rail, is positive and noticeable to a sensitive rider, but not so big you are likely to get it on one or two sessions. The diff when you are in between rails or going straight is huge.
Hi,
Nice thread! But, after reading the posts, I noticed nobody mentioned the fact of that the outside rail fin works a little out of water. The visual effect we can see could be the result of an air/water mixture while carving the board. In this case, the fin angle adjustment could really matter? What do you think?
After reading the posts, I noticed nobody mentioned the fact of that the outside rail fin works a little out of water. The visual effect we can see could be the result of an air/water mixture while carving the board. In this case, the fin angle adjustment could really matter? What do you think?
I know it matters because I can feel it in my feet.
My guess is that you could easily halve the size of the outside rail fin wake by adjusting the toe-in (or toe-out) via rotation.
I think of the outside rail fin as a “blocker” during turns. Its position and toe act to prevent you from turning more than anything else.
You are probably also aware of this if you have ever gone from a soft turn to a hard turn. You can watch pro surfers do this intentionally. They stay on a slow turn, then tilt the board to lift the outside rail fin completely out of the water, and their speed noticeably increases.
blakestah,
At first I was really shocked to read that–I guess I know what you’re saying, but you should clarify–your last statement there is an over-simplification with regard to the physics and what does the work of accelerating. (ie Simply letting off the brakes doesn’t accelerate a car.) Any actual acceleration during a turn is relative to the vector vs. gravity. Deceleration is much the dominant phenom in a turn.
Thanks Blakestah, now I understand. Regards.
Blakestah, have you tested your surf trux on twinfins and quads? Just curious as I’m making some.
blakestah,At first I was really shocked to read that–I guess I know what you’re saying, but you should clarify–your last statement there is an over-simplification with regard to the physics and what does the work of accelerating. (ie Simply letting off the brakes doesn’t accelerate a car.) Any actual acceleration during a turn is relative to the vector vs. gravity. Deceleration is much the dominant phenom in a turn.
Surfers on an inclined plane during a turn receive a nearly constant source of force. They reach an equilibrium in which the drag matches the input force from the wave, and travel at constant speed.
Then, they pump with the outside rail fin out of the water. Despite increasing drag from the hull itself and putting the inside rail fin and rear fin at larger AOAs, they visibly accelerate.
I’ve seen Curren do this on video many times, and recall watching multiple surfers at Trestles contest 2005 do the same thing.
Keep the outside rail fin down and stay at low speed. Tilt it out of the water to speed up and attack the wave. Its a stall tactic to help time the pitching lip with the attack.
Blakestah, have you tested your surf trux on twinfins and quads? Just curious as I'm making some.
No, I have not.
I am sure it will offer SOME performance advantages for each, and that the process of dialing in the fin templates and positions on the board would take some effort.
Quote:blakestah,
At first I was really shocked to read that–I guess I know what you’re saying, but you should clarify–your last statement there is an over-simplification with regard to the physics and what does the work of accelerating. (ie Simply letting off the brakes doesn’t accelerate a car.) Any actual acceleration during a turn is relative to the vector vs. gravity. Deceleration is much the dominant phenom in a turn.
Then, they pump with the outside rail fin out of the water. Despite increasing drag from the hull itself and putting the inside rail fin and rear fin at larger AOAs, they visibly accelerate.
Keep the outside rail fin down and stay at low speed. Tilt it out of the water to speed up and attack the wave. Its a stall tactic to help time the pitching lip with the attack.
I know what you’re talking about. The std. thruster tightening turn toward the wave out of a bottom turn. A tightening line toward an approaching wave looks faster. No actual acceleration there, if you think about it. What would be doing the WORK of acceleration out on the flat?
A tightening line cutback on the face is a radical braking maneuver, and the fin comes out there too.
A decreasing radius top turn is a braking maneuver too until the nose aligns with gravity and accelerates out.
A caveman’s view:
What my body says to me when it’s on a surfboard is my camera.
I have surfed boards at my meager level of ability with a wide variety of fins and fin combinations and am in Nathan Fletcher’s camp when it comes to fin set-ups on the boards of the present day and how they facilitate maneuverability, performance and create board speed. This is to say that the center fin is little more than a parachute. It is the brake on a thruster surfboard. IMHO the amount of drive gained from it is negligable.
As board’s rail engagement is increased, the fins on the lower side of the board become less of an interference and the engaged fin or fins channel a laminar flow that complements the boards projected arc performance, speed and “The Flow” are promoted. Naturally the less turbulence coming off the trailing edges makes for a faster ride as turblence is the result of drag.
Boards of today are becoming more centripital. One shaper does one little tweak and another does another one and boards get better and better. I collaborate on a few shapes tweak a few fins. It’s a joy (-;
The day of the deep veed single fin board that carved deeply into the wave face and sat in the pocket isn’t gone. That design still works fine and is prefered by many nostalgia worshipers (the slow pose will never go out of fashion as it is at the heart of surfing) but because this type of surfcraft runs more deeply in the water and is more of a rounded conformation this (in the water) type of a design cannot maneuver at the speeds that designs with less wetted surface can. Boards with single fins depend on having a bottom surface along with a rail surface that engages the wave. The center fin does give these boards their drive but when it comes to get up and go they fall sort in the high performance curve. Add rail fins as was done some 40 years ago to begin with and board contours along with their foils change to accomodate the new driving forces attached to them. Boards have progress through a stage where the rail fins were the draging force to the place where the center has now taken that role. The evolution continues and here, chipping on my little piece of flint, it’s good to be a part of it.
No Worries, Rich
The other cool thing, if you look REAL close, is that the inside rail fin re-directs the water towards the tail ONLY until it crosses the rail (compare the angle from the rear of the fin base to the wash at the rail, and the angle of the wash off the rail). This is why if you mount the inside rail fin too far out, you lose drive.
Wow, I’ve never thought of the water moving that close to perpendicular to the rail. And the surfer is not going slow, that’s for sure. on the other hand there might be no other surfspot in the world where the water is moving faster in the upward direction. Interesting discussion.
One thing I don’t get when looking at these pictures is why twin fins and quads feels like they have less drag than a thruster when going straight. Wouldn’t the outside fin on a twin create the same amount of drag as with the thruster? Confused…
