If, as quoted from the ASL A SURFERS GUIDE TO FIN THEORY fin guide , “Twin-fins and thrusters are designed to be ridden rail to rail to exploit the drive created by the foils of the outside fins. The centre fin provides stability - it doesn’t drive the board. A larger centre fin on a thruster will increase stability but reduce drive.” Then why do think the original Simon Anderson tri-fin boards where named “Thrusters” ? The boards had quite large center fins. but the thrust must have been very obvious to the rider. Am i misinterpreting drive and thrust ?
Hi sandy,
Try this experiment, you will need a thruster with removeable Fins.
Remove centre fin and, (if you like) ,replace rail fins with the largest fins you have.
You will find drive is reduced, at least thats what I've noticed.
The rail fins maybe generating drive but the centre fin gives it direction.
If it looks like it works and it feels like it works then it works.
The terminology is just imprecise. The difference is between what happens when a thruster rider pumps off the rail fins and rail, and what happens when a single fin rider trims using the rail and center fin. To me, the latter is drive, as in passive lateral drive down the line given by the hull and stabilizing fin, where the former is thrust. But you see thruster riders use the word drive for the pumping kine all the time.
So on a thruster with a big center fin, the big fin will interfere with the rider pumping rapidly off the rail fins, “tightens” the board up.
The quote is using the terms drive and thrust interchangeably.
Thrust is very misunderstood by many. In a nutshell…
In a turn, you push with your feet perpendicular to the stringer, from the inside of the turn, to the outside of the turn. The response of the board is a CLEARLY PERCEIVABLE MODULATED acceleration towards the nose. The harder you push the tail sideways, the faster you accelerate forward.
It is possible to get a lot of thrust with one fin, but it needs to be angled relative to the stringer. The single RFS (rotating fin) boards could be pumped for thrust, easily, and once you found the sweet spot there is no other way to ride them.
On the three fin boards, there is a balance between the rear fins tendency to want to keep the board straight, and the rail fins tendency to want to turn the nose into the turn. This balance allows the thruster to have a stable position in which thrust is easily created on rail if the fin config and bottom contours and rails and tail shape are made to complement the fins. If you make the rear fin bigger, the board will still generate thrust, but it will not turn as well (thrust in bigger circles). Smaller rear, smaller circles for the thrust. Fine tuning the size of the rear fin for waves will definitely let you surf better.
But a rear fin matched in size/template to the rail fins works pretty well over a pretty wide range of conditions.
HTH.
“In a turn, you push with your feet perpendicular to the stringer, from the inside of the turn, to the outside of the turn. The response of the board is a CLEARLY PERCEIVABLE MODULATED acceleration towards the nose. The harder you push the tail sideways, the faster you accelerate forward.”
Wow! Free energy! Perpetual motion!
For me, going from a small rear fin to one similar size to the side fins made my boards get to the lip faster and - more importantly, down from the lip. I don’t know if you call it drive or thrust, but I was getting to places I couldn’t get to before. When I drop the rear fin size to try to loosen up a gunny board, I just end up with skatey turns. Maybe the fin research needs to qualify what’s meant by bigger?
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"In a turn, you push with your feet perpendicular to the stringer, from the inside of the turn, to the outside of the turn. The response of the board is a CLEARLY PERCEIVABLE MODULATED acceleration towards the nose. The harder you push the tail sideways, the faster you accelerate forward."Wow! Free energy! Perpetual motion!
Neither.
Simply a re-direction of efforts. You push down toward shore, the board moves through its arc line faster. Application of this principle is the most important contribution to performance surfing - controlling and generating appropriate speed.
You push sideways, the board accelerates towards the nose. You feel the force coming from the rail fin. This is thrust.
I push towards shore a few times in this video, and you can see the “squirt”. But this is a rotating single, the 3 fins are much better (as are fixed three fin boards).
DB, you don’t accelerate through turns unless you’re heading downhill or youre adding muscle power to a pump. Conservation of energy 101.
In mechanics, conservation of energy is usually stated as E = T + V. Ah, but you were talking about turning downhill. (?)
Thanks for posting that video, Dave. It really illustrates the point well.
Pat
If, as quoted from the ASL A SURFERS GUIDE TO FIN THEORY fin guide , “Twin-fins and thrusters are designed to be ridden rail to rail to exploit the drive created by the foils of the outside fins. The centre fin provides stability - it doesn’t drive the board. A larger centre fin on a thruster will increase stability but reduce drive.”
Thats absurd. Most properly implemented fins create drive and that includes the center.
Dont know what ASL is, but whoever wrote that is dead wrong.
I suppose the next thing they’ll suggest is that sailboat keels are useless.
Do these people even surf?
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DB, you don't accelerate through turns unless you're heading downhill or youre adding muscle power to a pump. Conservation of energy 101.
In many turns, both criteria apply.
Craftee–That part is dumb–the center fin does add drive–you can also thrust with it in combination with the other control surfaces. If people would just differentiate between passive drive and active pumping arcing thrust using the rail fins, the way Simon Anderson meant to differentiate it with the new term.
Blakstah–You have some video of someone actually pumping out of a top turn? – link the clip. I’ve seen tons of shock-absorption compressions, but never someone actually squat-thrusting off the top.
ah, and some that don’t. Flowrider effect anyone. Fin as foil (airplane wing) or fin as keel (boat keel)…seems to be a fundamental discrepancy in fin theory here. Mr Casey are you there ?
The rail fin during a pump is like a hand sculling through a swim stroke, while the center fin is doing nothing like that.
Quote:
If, as quoted from the ASL A SURFERS GUIDE TO FIN THEORY fin guide , “Twin-fins and thrusters are designed to be ridden rail to rail to exploit the drive created by the foils of the outside fins. The centre fin provides stability - it doesn’t drive the board. A larger centre fin on a thruster will increase stability but reduce drive.”
Thats absurd. Most properly implemented fins create drive and that includes the center.
Dont know what ASL is, but whoever wrote that is dead wrong.
I suppose the next thing they’ll suggest is that sailboat keels are useless.
Do these people even surf?
you can download article at http://www.mytempdir.com/2022543
Blakestah,
I have a question concerning the following:
"Simply a re-direction of efforts. You push down toward shore, the board moves through its arc line faster. Application of this principle is the most important contribution to performance surfing - controlling and generating appropriate speed.
You push sideways, the board accelerates towards the nose. You feel the force coming from the rail fin. This is thrust.
I push towards shore a few times in this video, and you can see the “squirt”. But this is a rotating single, the 3 fins are much better (as are fixed three fin boards)."
Could it be that the ‘thrust’ felt is not actually a force forward, but a reduction of drag that allows for more board/rider acceleration relative to another fin setup/design that has more drag?
Also, I know that if you do reduce drag, the resultant ‘forward’ force vector grows, so it can be thought of as an increase in force ‘forward’. This begs the question:
Is all of this just semantically driven?
The way I understand it (which has a very good chance of being flawed) is that a side fin, with its toe-in and cant, is in a much more advantageous position relative to the water flow to generate lift in a turn than the rear fin. The rear fin allows the side fin to keep this advantageous position throughout the turn (keeps the board from over-rotating or ‘spinout’), and the end result is that less speed is lost with the change in direction than otherwise would have been lost without the side fin. The side fin allows you to change direction more efficiently (lose less speed doing so), and the rear fin allows the side fin to keep working.
The ‘pump’ (weighting/unweighting on the board), allows you to generate more speed than the wave would ordinarily give you. By ‘pushing’ downwards and toward shore, you are essentially helping accelerate the board yourself by increasing the relative flow across the board/fin cluster. As the board accelerates across and starts going back up the wave, you save the energy gained by unweighting the board (light on your feet), and then use that saved energy by pushing again on the next ‘pump’, the net result is that you end up going faster across the wave than if you didn’t ‘pump’.
I may be way off base, but that’s the way it makes sense in my head.
JSS
"Could it be that the ‘thrust’ felt is not actually a force forward, but a reduction of drag that allows for more board/rider acceleration relative to another fin setup/design that has more drag? "
No, the acceleration is real. The application of force by the rider actually impacts the speed coming out of the turn. It is more than lean angle and fin configuration. By increasing, temporarily, the force on the high pressure side of the fin, and sliding it sideways through the water, you are FORCING that water to re-direct towards the tail of your surfboard. The equal and opposite reactive force is thrust. I like the analogy of a sailboat tacking 45 degrees into the wind. The keel and sail are oriented differently - with slightly different ways they re-direct the air and water. With the wind blowing in the right direction, the sailboat can move UPWIND, in an angled way.
I have a separate explanation here - that doesn’t go into the details of rider input. But you push perpendicular to the stringer, and the board will acceleration towards the nose.
http://www.blakestah.com/fins/truckexplain.html
I have found it difficult to get a good perception of thrust on longer boards or boards where the fins and rails and bottom contours don’t mesh together well. For me once a board gets down in the 6’4" or shorter range (especially shorter) and when the fin fore-aft spread drops to 7 inches, the thrust is usually quite nice. But I have seen good longboard riders stomp on their tail in just the right way to harness it, so it is not limited to shorter boards, probably just easier to find.
And, as on the single fin RFS boards with their version of thrust, once you find it, there is no other way to ride a 3 fin surfboard…
The rest of the comment I agree 100%…
Blakestah,
I see what you mean now.
Sandy Thorn,
The ASL download is OK, but it links to a real gem:
http://www.surffcs.com/documents/foil_education.pdf
That’s some good stuff, and decently explained as well. I wonder what other documents are in that directory?
JSS
“the acceleration is real. The application of force by the rider actually impacts the speed coming out of the turn. It is more than lean angle and fin configuration. By increasing, temporarily, the force on the high pressure side of the fin, and sliding it sideways through the water, you are FORCING that water to re-direct towards the tail of your surfboard. The equal and opposite reactive force is thrust.”
DB I wanna give this all a shot, as much for my own clarity as anything
By applying pressure to the rail, the fin penetrates deeper/countersteers (Google countersteering) for a millisecond owing to toe-in, starts the tailward rail rotating around it, thus enters positive AOA, attacks and then the standard Newtonian downwash off the rail fin translates what would have been going off the rail in a diagonal flow when the rail fin wasn’t in positive AOA into a more tailward flow, so you get more nose forward velocity. The rail fin, which a moment before was a more trimmed-out plane (in basic trim) or a resistant plane to water headed for release off the rail (sideslipping), is now an attacking wing directing flow, Newtonian style, more tailward, and the lift is helping the rail track and hold too.
and the center fin stabilizes the AOA of the rail fin and keeps the tail from rotating out of the face from the leverage axis of the rail fin. Result of the combination of those fins’ planes is you can push off it all, or trim off it all.
Your system zeroes out the outboard fin’s opposition. But thruster-pumpers want to feel that inboard rail fin attack immediately, don’t they?
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Your system zeroes out the outboard fin's opposition. But thruster-pumpers want to feel that inboard rail fin attack immediately, don't they?
The outboard fin loses some resistance…the inboard has a larger or equal positive AOA at all times.
When “thruster pumpers” ride surftrux they feel it is identical to a thruster going into a turn, with slightly better action in the second half of the turn than a thruster, usually they say it “releases” more nicely or “is faster in the second half of a turn”, FWIW.