fin placement, wherelse?

General Discussion
1

 whether you’re in the ‘my-surfboard-is-a-planing-hull’ or the ‘my-surfboard-is-a-displacement-hull’ camp, the finned board you’re riding (or shaping) will inevitably keep the fin right in the tail area. all because that’s where blake stuck his skegs on his boards. but today boards being made bear little resemblance to blake’s ‘kookboxes’. different materials, different design parameters, different in everything except in one respect : the bouyancy necessary to support the surfer. so how come fins are still stuck on the tail ?

as surfinghandbook.com reports, "Legend has it that (Blake) asked a speedboat skipper about the skeg on his boat, and was told it was to help stabilize the boat during hard turns. One day Blake noticed an abandoned boat on the beach. He took the keel off, modified it, and attached it to his surfboard. At first he didn’t like the sensation, but once he caught his first wave, he was hooked.

“When I first went to the Islands, they used wide-tailed boards and they used to spin out on a steep, critical slide,” he said. “I figured it would be easy to correct that problem, just add something – a keel. Finally, I put a fin on the board and it worked fine. It was a shallow fin, about 4″ deep and a foot long. It took ten years for that thing to catch on and then the boards kept getting lighter and smaller and [then] the fin became more effective for steering.” 

steering, he says. today it’s referred to as ‘turning’ while using newfangled terms like ‘center of gravity’,‘center of balance’, ‘fulcrum’, ‘pivot point’ and whatnot. regardless whether you’re riding a single, a twinner, a thruster, a quad or a bonzer, these dang skegs are still stuck on the tail! WHY?

why not elsewhere underneath, like under the midpoint of your stance? or under the thickest/widest section of your board? or a shaka ahead of your front foot? ever tried riding boards that you or some other crazy shaper made in such way?

tell your story here. with or without moral lessons. there might be some lesson here somewhere, hmm !

; )

2

wow! 50 views, ZERO confessions  " )  to this day the closest “deviation-from-normal-fin-position” to be found online is this :

at nearly 2’ from tail tip, that fin would probably be under the rear foot with front foot further forward than usual. comments? reactions? objections? admissions? anything further forward? speak up, fellas!

3

It’s an interesting idea. Think of something like a thruster or quad setup, keeping the trailing fin or fins back at the tail and moving the front fins up toward the center point of the board, maybe to the midpoint of your stance, or at least, to the midpoint of the planing area when the board is at speed. You’d probably get better hold up against the face of the wave in hollow down the line sections, because the center of resistance to sliding would be centered under you instead of way back behind you. You’d have no directional control in this configuration at all though, without the trailing fin(s), pressure on wither foot would just incite you to spin. Adding back some tail fins would give you a bit of directional stability and give you something to push off of with your back foot.

 

If you think about the way pros surf on high-performance surfboards, you can actually see a hint of this already happening, not because the fins have moved forward, but because they’re feet have moved back. You’ll generally see pros surfing with their back foot right on the tail block, so that the back fin on their thruster is essentially under their rear foot, and their front fins are placed between their feet, but not anywhere near the midpoint. Less accomplished surfers I think tend to stand further forward, with the fins placed relatively farther back and therefore in a position more resistant to turning.

 

Also note that sailboats (and sailboards) have this configuration, with a keel or centerboard centered under the main force acting sideways on the vehicle (which is to say, the keel is under the mast). If you moved the centerboard on a sailboat back to where the front fins on a thruster are, the boat would be completely incapable of going in any direction but dead downwind. Accordingly, if you moved the rudder up off the stern, you’d have almost no directional control.

 

Who wants to build a normal shortboard shape, but stick a longboard fin box along the centerline right in the middle of where the surfers feet would go, and then put either a normal thruster trailing fin on it, or quad rears, in the location they’d normally go. Using longboard fins up front, in a standard longboard box would give you plenty of adjustability up front. It might work pretty well.

4

Even a cursory study of fluid dynamics and naval architecture will give one a good sense of why and where surfboard fins are places as they are.

Comeonman (-;

Consider that with surboard fins less is more: The more efficient fin placement and foil of a fin is the less fin surface area is required. Thus more effective fins allow one to surf faster and manuever more freely along the wave face.

No Worries, Rich

5

finally, an indication of (some) validity. that’s precisely how i’d been thinking about it, except i couldn’t figure out why nobody’s been toying with the concept just yet when it’s pretty obvious that the purpose of rear placement is to prevent spinouts, which is what blake saw after trying it himself.

to begin with, blake’s recognition of a boat keel as a turn-stabilizer for a surfboard (in the same place, at the tail) appears to disregard the difference of propulsion in boats & boards. while boats have engines to drive them, boards rely on waves only-- a single one in fact. so if improved board designs today have led to maneuvers & lines on a wave not previously doable on older, differently-designed boards, how come the fin cluster is still at the tail ? would a further-forward center fin lead to more fluid, alaia-like rides on finned boards?

let’s say a wavegrinder fin instead, either behind or ahead of the front foot, with bonzer side bites (the smaller pair) attached a la simmons’ fin setup at the tail, close to the rails. workable? or better? how different would it ride?

heya rich,

re naval architecture for planing objects like surfboards, it appears simmons got it right himself. however for those belonging to the ‘my-surfboard-is-a-displacement-hull’ camp, they may disagree. and nobody seems to have picked up on the shark porn shot i pasted above, but the intention was to illustrate where fishes/sharks/whales/dolphins’ primary propulsion system is placed-- the tail.

for primary directional stability, it’s the dorsal fin.  check out where it’s placed :


always at midsection. see why it keeps me wondering about alternative (better) center fin locations?

6

A Surfboard has three sources of propulsion the wave, rider, & fin/fins.

An alia has little or no subsurface flow resistance so it will trim
rocket fast but cannot be driving to creat speed, but then why try to
create speed on a board that goes along the wave will little or no drag;
here the issue is trying to slow down.

As the rider moves forward he has less ability to drive the board and relys more on the board and fins to do the work of propulsion, which must as with effectively swimming creatures of the water come from aft. The surfboard depends on fins for directional stability and for propulsion. Because propulsion has to come from aft the fins have to be there. Fin more centrally located on the board provide less propulsion than those located closer to the rail. Propulsion is effected more greatly by engagement athwart ships i.e. What mono hull can compete with a catamaran? So it is the same with surboards, put the fins in an effective combination closer to the rail at well choosen distance from the tail with proper toe-in, cant, size, and foil and you maximize board performance.

Surfers on boards without center fins have a similar experience to those on alias, that is the finding a way to slow down. The maneuvability provided by fins allow one to do this more affectively by climbing the wave face (slowing as a resuit) but driving off the top and accelerating in that process rather than have a center fin to stop and turn on as we see with thrusters. IThe opportunities to swing up to 2 o’clock are special ons. t is in a way a most poetic experience: that need indeed to bleed speed & the resultant freedom to surf more of the wave face radically.

 

I’ve gone “Mental” ~ help!

 

Rich

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7

I think that pretty much all of your logic is wrong, there.

 

A surfboard does have three sources of propulsion: wave, gravity, rider. The wave pulls you toward the beach, gravity pulls you down the face, and the rider can pump the board to use his own weight to increase the effect of gravity and wave speed. Fins don’t propel a board any more than wings propel an airplane. They generate no force in any direction except as a counter to force exerted by one of these other three influences.

“As the rider moves forward he has less ability to drive the board”

As far as I can tell, this is only because the fins are at the tail, and so he can no longer push off of them. If the fins were moved forward, you could pump from further forward. There is absolutely no reason that the propulsion “must come from aft”. Yes, it does in fish, but so what? It doesn’t in, say, penguins which propel themselves primarily with their wings, like all birds. Or surfers who are paddling surfboards. Or boats with trolling motors mounted on the front. Or most airplanes. You can put the propulsion for any type of vehicle nearly anywhere and make it work.

 

“The surfboard depends on fins for directional stability and for propulsion. Because propulsion has to come from aft the fins have to be there.”

The bit about directional stability is true but is irrelevant to the point you’re making. The rest of your argument here is based on two separate faulty premises, one being that fins provide propulsion, and the other being that propulsion has to come from aft. Your comparison to an alaia even makes this point stronger – if the fins provide propulsion, why does removing them make an alaia faster? If “the fins have to be there” (with “there” being aft), how does a board with no fins manage at all?

 

 

“Fin more centrally located on the board provide less propulsion than those located closer to the rail. Propulsion is effected more greatly by engagement athwart ships.”

even if fins did provide propulsion (which they don’t), there’s absolutely no reason this point would be true. This would be like saying engines provide more propulsion when mounted on a plane’s wings than when mounted in the fuselage. It’s nonsensical.

 

 

“What mono hull can compete with a catamaran?”

This is a completely false comparison. Catamarans are fast for two reasons. One is because with no ballast in the keel, they are much lighter than monohulls and therefore have a lot less mass to drag around. The second is because they can hold more sail area into the wind because they heel less. They heel less simply because they are wide. If you were to push on the top of the mast of a monohull, the whole boat would tip along with it, because the hull at the base of the mast is narrow. If you were to push on the top of the mast of the catamaran, it would tip much less, because the hull way out to the far side of the mast is holding it up. This means that more of the sail stays held up in the wind.

Besides, the sail on both boats, where the power from the wind is being harnessed, is on the centerline. Do you actually mean to imply that a catamaran is faster because the centerboards are farther apart? Many catamarans don’t even have centerboards. Certainly if the centerboard is what makes the boat fast, a monohull with a 6ft deep keel would be faster than a catamaran with none at all, right? But that’s not how sailboats work.

By your logic you should also put the mast at the transom of the boat, since “propulsion must come from aft”. Or maybe you want to put the keel at the back of the boat. I really can’t tell.

 

I can’t even follow your last paragraph. I have never known a surfer to worry that his board was too fast and he can’t figure out a way to slow it down. If anything, everyone I know wants to go faster. It seems to be also pretty objectively true that the world’s fastest surfers go pretty close to the same speed on boards with fins on the centerline and without. Watch Kelly Slater surf on one of his thrusters and one of his quads. You can’t even tell which he’s on until he puts it on a rail and the fins come out of the water. Maybe one or the other is a little bit faster, but certainly not such that it’s obvious.

8

hi rich,

as i haven’t tried twin fin boards yet nor alaias, i can only agree with your analysis on how they ride. however,

if fins are also to be considered as sources of propulsion, how come the transition from beginner (going straight) to intermediate (bottom turns, cutbacks) to pro (aerials, fancier stuff) requires mastery in using the entire body to shift one’s weight to manipulate board direction? 

IMO fins belong to board maneuverability ‘influencers’ / enablers such as concaves, vee, dome, rail foil, wings, bumps, tail shape. yes?

9

how about right at midship, on the rail or maybe 1" off, and either toed parrallel to the stringer or slightly out for a big wave gun.

would it help clinging to the wall and not tracking to the bottom without the inefficiencies of a round rail?

10

what size fins, chrisp?

11

“Fins don’t propel a board any more than wings propel an airplane”  your words. Here we depart: Take the rail fins off the board and it won’t accelerate. All theory aside: I can’t remember when this was better illustrated than when Mark Richards and Cheyne Horan contested for the world championship four years running as I recall. Cheyne’s boards had center fins and he surfed deep inside the wave. His boards wouldn’t project out into the wave face well. Mark was on a twin fin most of the time and surfed all over the wave face up and down it and looked like some kind of big gull on the wave. Watch the films and tell me that fins don’t propel the board. If you still see things that way find. I’m not the one to change your mind. Another practical and primary example of “fin drive” is an experiment my good friend Nick Palandrani did a few years back wake surfing. He took the same board with flat sided fins on it very similar in shape and template and surfed behind the boat and was able to keep up but couldn’t drop back out of the steeper wave face and then catch back up with the board. He then took a set of my helically foiled fins and was able to drop way back off the stern of the board and still swim the board for speed and a smaller wave face and catch back up to the boat. I rest my case on this point.

 

Another point if you move fins forward ona given board the tail will slide all over the place and it jus spin out. The fins work inconjuction with bottom, rail, and outline contours in the after section of the board.

 

Airplanes and surfboard will never correlate directly so don’t try and draw these sort of direct theoretical comparisons. They are misleading and off target.

 

My example of catamarans is simply to demonstrate that drive comes off the rail the outside of the vessel regardless whether it is a surfboard or a boat.

Certainly center boards on a mono hull don’t make sailing craft go faster. They just facilitate a higher angle of attack.

As with aircraft, there is some correleation between surboard and boat performance but the direct azimuth doesn’t meet all points of the compass here.

There correlation with birds and seabeasts is also applicable but making comparisons must be tempered by experience.

I have met lots of surfers who want to be able to slow their boards down. The reason that thrusters are surfed so prominently on the world stage is not only that the accelerate rapidly but the decelerate just a rapidly because the have a center fin. Another point of fact is the the big wave paddle in boards that we see a Mavericks have to dampen speed somewhat in order to be managable. There are many nuances in surfboard design the understand better as part of my process.

It may not be obvious  that Kelly’s quads are faster than his thrusters, but ask any surfer which will trim faster and hold it’s speed through the turning arc better.I for one sure as hell see a huge difference in board perfromance generated by fins.

 

In the end it’s all what you feel happen under your feet and how you and your board meld with the wave.

Yo SKI, what kind of stick is your daily driver?

 

 

12

howdy rich, i think T means propulsion sources as absolutes, hence wave, gravity, rider.

all the other elements under the board, on the tail and the rails work in conjunction as ‘influencers’, if you will, of direction that the rider himself controls by using his body to shift weight. 

as to fin drive, your example mentions foiling which, as fin shapers would agree, is what makes them either work, if appropriately-foiled or fail, if not. 

in that case how about the alternative setup ventured earlier, a wavegrinder at mid-board with bonzer side bites at the tail rails set up a la simmons?  

or something like this pair, moved mid-board :

with a double-foiled shallow center fin, say only 3" or 4" deep, at the tail. doable?

13

 

I agree that the fins help top accelerate the board, but not because they provide propulsion. The propulsion's coming mostly from gravity, but a good surfer can take advantage of gravity and get himself a lot of extra speed. He uses his legs (whole body really, but easiest to visualize with his legs) to "lift" himself through turns. Come into a bottom turn low and crouched, you effectively maximize the length of your drop and therefore the speed you get by ending up lower at the bottom of the wave, then you extend your legs through a bottom turn, which lets you push yourself partly back up the wave face. then you crouch again going up the face, which is free distance travelled because you're not fighting gravity, you're just pulling your legs up underneath you and letting your board ride up to meet you. Then he pushes himself through a top turn, extending his legs, lifting himself up to get him in a position for the maximum length of a drop into another bottom turn, and therefore the most speed. "pumping" and "driving" are basically just quick repetition of this same technique: You left gravity build your speed as it continually pulls you down the face, and you use your body to keep climbing back up the face. If you're good at it, you can do it faster than drag slows you down, and you can build some incredible speed.

 

But the only reason it works is because you can "lift" yourself back up the wave. The reason you can do this is because you can push off your fins. If you have no fins, instead of lifting yourself up the face of the wave, the board slides out under you, so in that respect, the more fin area (up to a point) you have to push against, the better you can pull off this maneuver and build up speed.

 

The fins here are analogous to the tires on the car. They don't provide the power, the engine (surer's legs) does, but they're necessary to actually get the power transmitted to the ground so you can actually use it. The fins provide "grip" so the surfer can push himself up the face of the wave.

Too small of fins (or tires) and you start sliding all over the place and can't get the grip you need. Interestingly, NASA actually had this problem with experimental airplanes in the 60's. They put tiny wings on them because they were designed to go super fast, but then at high altitudes, where the air was thin, the pilot could crank a hard turn (which is the equivalent of a surfer leaning into his back foot for a turn), and the wings didn't have enough "grip" to actually get the plane pointing in the new direction. The tail would simply slip right around to the front and the plane would go into an uncontrolled spin. Test pilots were killed this way.

This same phenomena is one of the attractions of the alaia, the touchy tail that gives it a glidy/slipepry feel, but spins out easily on you, because it's got no fins. You see other boards do it too, but not as much. It seems fairly common if you try to push hard through turns on a twin, for example. Pros can also get their thrusters to do it just because of how much force they can apply to the tail, but that's their thing and they make a show of pushing their boards just out of control enough so that they know how to get it back again. I actually remember watching Dane Reynolds launch an air at last year's Lowers contest. He landed in the whitewater and came out standing up, *after cutting his head by hitting it on his board*. I have no idea how you could cut your head on your board and still pull a maneuver, but he did.

 

Anyway, bringing the the fins forward will increase the tendency of the board to spin, *if you continue to drive it off the tail like a thruster*. It seems like it could work though, if you put some stabilizer fins on the back, and surfed it with your weight further forward. It seems like you could get better hold and drive pumping off your front foot with a setup like this, as opposed to using your back foot on a more standard board. You'd still want those trailing fins though or you'd never make it through a bottom turn or cutback without spinning out.

And my daily driver board is a 5'10" thruster. Two different ones actually, one wider and flatter for smaller days. But they're both short thrusters. They're the orange and white boards in this picture: http://www.flickr.com/photos/tylerkaraszewski/5019177716/

 
14

thank you, tyler. man, you nailed it!

i remember one thread awhile back where some guys were saying ‘nobody surfs off the front foot’ while on another, someone made the wise observation that there’s actually no single pivot point on a board (i.e. tail), since all the other variables on the rails & bottom can be used to control direction. that and the alaia/finless threads kept me mulling about the possibility that a center fin shifted further forward will work, as long as tail slide is controlled by other means.

your thoughts on this clears whatever doubts i had going in. the challenge now is, how different will mid-board fin placement be on shortboards & longboards? under the midpoint of stance? or under the thickest/widest section of the board? or a shaka ahead/back of front foot? with what matching rear small-fin/cluster setup to prevent spinouts? or maybe round-ish tail rails to provide grip?

15

You don’t want to move that primary fin too far forward. Any fin on a surfboard is going to act like like the tail of a weathervane, which is to say it’s going to want to point the board into the flow of water under the board, with the center of mass on the board out in front of it. The closer you move the fin to the center of mass (which is pretty much right between the surfer’s feet), the less directional stability it will give you (it will still keep the board from sliding laterally, but it wont help keep the nose pointed the right direction).

As soon as the fin moves forward of the center of mass, it’ll make the board super squirrelly because it’ll try to flip the board around backwards, so that it’s the new tail (imagine if you cut the tail off the weathervane and moved it in front of the pivot point and reattached it, the whole thing would just flip around backwards).

So at the very furthest forward, you want it right between the surfer’s feet. That’s why I suggested a longboard fin box on a shortboard. You know pretty much where you’re going to be standing, and the fin box has some adjustability built in so you can slide it back a little if you need to. On a longboard, it’d be harder to tell, because you walk around a lot. If you move it real far forward, you wont be able to cross-step back to the tail and dig it in and do a cutback, because as soon as you get behind that fin, the board’s going to try to spin on you.

The tail fins will supply some directional stability, but if you make the forward fin the big one, which is what I’m assuming, it will still overwhelm the influence of the small trailers if you move it too far forward. I also think bonzer side runners might be too small. I was thinking more the size of quad trailers or a standard thruster trailer.

16

brilliant, got it!

this means i can now aspire to ride/build at shortboard-ish lengths hehe i’ll just have to deal with the added overall thickness from the reduction in length, while keeping the rails thin.

with the setup you suggested, would the LB finbox be aft, or fore of the rocker ‘peak’? or dead center?

(i was confusing rocker depth with the widest/thickest section of the board which by definition is at its maximum width, and not necessarily at the rocker’s dead center)

 

1 Like
17

[quote="$1"]

[quote="$1"] how about right at midship, on the rail or maybe 1" off, and either toed parrallel to the stringer or slightly out for a big wave gun. would it help clinging to the wall and not tracking to the bottom without the inefficiencies of a round rail? [/quote]

what size fins, chrisp?

[/quote]

i was thinking like little bonzer runner type fins... whaddaya think?

18

A board like this isn’t meant to be surfed off the tail, so you don’t have to try to set it up like a standard shortboard where your back foot is real close to the tailblock. In fact, with the small trailer fins, you might even benefit from having a little more tail protruding behind your rear foot. You could get away with making the board a bit longer this way, as long as you can predict where you’re going to end up standing on it. Let’s say you build this board as a 6’6". I normally surf pretty short boards (my regular boards are 5’10" and I’m currently building a 5’5"), so that’s long for me. Assume your rear foot is supposed to sit one foot off the tail, and your stance is 2.5 feet wide. This puts your front foot 3.5 feet from the tail, which is just forward of the center of the board. Your center of mass, if you’re standing straight, is going to be right between your feet, which is 2 feet, 3 inches off the tail. I’d center the fin box there, and then put the trailers way back on the corners, like a Simmons-style board. You’ll of course have to adjust all these numbers if you’re making your board a different size or your stance is different or you stand closer to/farther from the tail. Once you build it and put the fin in, start in the front of the box and move it back a little at a time if the board feels uncontrollable.

As far as relationship to rocker goes, the flattest spot in the rocker (which is where I’m guessing you’re calling the “peak”) for most boards tends to be pretty close to under the front foot, so I’d guess that the fin would be mounted behind there. This depends on what rocker template you use and how you measure your rocker though.

And if you build this board and you end up hating it, don’t blame me. Just because I think it might work is no guarantee, and I’ve never built one like it before. :wink: It’s definitely not going to surf like a normal shortboard, that’s for sure. This is all speculative. I’d guess it’ll hold a high line on a steep, down-the-line wave pretty good, though. If you do build it, post pictures and a ride report.

19

tyler might have the right answer for that

cheers,

20

truth is i’ve never ridden a shortboard/thruster setup, none of the rentals where i surf north of manila have sufficient buoyancy for my build so i literally cut my teeth on single-fin concaved guns & LBs from 9’ to about 8’6"-ish.

that’s why i drew up my first (now due for final glassing) self-made HWS LB-gun hybrid at 8’8" x 19.25", comfy as i am with the size/volume (except it’s now 9’ x 22.50" in final form, don’t ask how it grew 4" from start to finish or ended up weighing approximately 20 kgs in 2 years hehe)

so going shorter i thought the simmons might be it, with maybe a quad setup. but after thinking about mid-board fin possibilities, i figured i’d ride it the way i’m used to (and not struggle with a typical twin or quad setup) with my front foot closer to mid-board as in the LBs i liked, hence the center fin adjustment/transfer.

i’m looking at something like 6’ or thereabouts (i’m 5’7"). given there’s hardly any rocker-arc on a simmons save for a slight nose kick, will adding center fin mid-board to a quad or classic twin-fin simmons setup (likely) make it fly or fail ?

(and no, i’m not gonna charge overhead barrels just yet. i think at 40 i still have the balls for it but it’s not yet time hehe)

cheers,