Hi guys, apologies if this has been posted before and it’s not intended to be a quad v thruster debate but I’m really interested in this stuff and want to understand more about the theory of different fin set ups.
Firstly
my quad is faster down the line than my thruster (at least when not
pumping rail/rail) and the only real explanation I hear is that the
centre fin is causing all the drag and with a Quad both fins are working
together on the rail.
The doesn’t really help me visualise what is occurring at all really! Any help here?
Surprised no-one's given you an answer so far, so here's my take on it:
In my imagination, fins provide direction and propulsion to a surfboard. Boards move, because waves push them, if a board has fins, then the wave pushes against these. To a certain extent - the greater the number of fins you have, the greater the push - or thrust if you prefer!
However, your fins are moving in a liquid medium so that drag also comes into play. On a quad the forward rail fin provides 'draft' to your reaward rail fin - so the board benefits from extra push, more than it suffers from extra drag.
This happens on a thruster too, but to a lesser extent because of the the greater 'offset' in position between rail and centre fin.
Of course - none of my theorising above is backed up by physics or experimental data - hence the caveat 'in my imagination'!
I'd be very interested in what Herb. Griffin or any of the 'Bills' have to say here - feel free to shoot me down in flames.....
Yep that makes sense to me. Having raced
motorcycles competitively when I was younger, I understand the benefits of
slipstreaming. I guess the next question would be why split and stagger the fins
at all and not just go for a twin? I’m guessing people prefer the hold of a quad
which bring me to the next question…why does a quad hold more than a twin with
the same fin area?..or is that more of a myth?
These are my opinions and there are many generalizations here so please take that into account. I hope you’ve tried all of these fin configurations to make sense of it all.
First the single fin is pivoty Most of the drive coming from #1 wave positioning, #2 rail and finally #3 fin. The best type of board to learn how to read a wave with.
Second the twin fin. All drive down the line speed and drive Little pivot. Drive coming from #1 fins #2 wave position and #3 rail.
The thruster has a good mix of drive and pivot for most surfing done nowadays. equal balence fins, positioning and rail.
Quads are all drive with four fins being on the rail without the drag of a center fin. (I think that pivot equals drag to a certain extent. The drag aspect can be defeated with volume though. That’s why I think quad and twin fin boards can be surfed a bit shorter.) The fin position and template of a quad make the board good or bad. If it’s good it has the same manuverability of a thruster with a bit more speed due to not having the drag of a center fin. The speed comes from a keel that is broken into a more thruster like template. So thruster front fins and then smaller rear fins that are still near the rail like a twin but closer to the center like a thruster. Best of both worlds to some!
Twins are down the line the fastest, Thrusters are the most manuverable but with a loss of speed, Quads are just as or nearly as manuverable as thrusters while maintaining speed through turns
Rant based on my experience. Probably didn’t answer the question but my two cents. Btw I own and have made and have surfed on all of the above mentioned fin configurations.
insane’, your description of the different set ups makes sense to me, thanks for the explaination.
I haven’t had the chance to ride a twin yet but I recently had a go on a quad for the first time and compared to the thruster I found it to feel looser on the first wave or so but once I got used to that feeling I found it very solid and as you say it didn’t loose any speed in cut backs etc compared to the thruster (6’4" x 20 1/2 x 2 3/4) where I usually bog down (that’s just me though ).
I’ve only been surfing for about 4 years now but I’ve never really got the thruster dialed in to get the most out of it. I change boards a lot (long boards, mini mals etc) and they are all different. I find, for me at least, that I need a really good wave with some juice to get the most out of the thruster otherwise it seems to bog down (drag) in turns or trim. I know this is only my style and not the board (I’m 43 and like the carving style of surfing more than the progressive/pumping style) but I found the quad to be quite easy to ride and turn and be pretty fast down the line, I can’t wait to try a good fish! I like singles too but again I don’t think my limited experience does them justice.
I was talking to my shaper mate and he wasn’t too keen on quads but then again, he grew up on the thruster and is quite an accomplished surfer and knows exactly how they work and how to get the most out of them, drag and speed is not an issue for him or others with the experience.
For me personally I think I’ll build a quad for my next board, maybe when I can really throw that around I’ll move back onto the thruster.
Of course, changing your fins around a bit could make a huge difference, I’d like to try a smaller centre fin with lager side fins (I think KS does this a fair bit) and see how that goes, just might loosen it up/reduce the drag enough to make the difference.
[img_assist|nid=1063018|title=fins|desc=|link=none|align=left|width=640|height=480]This board gave me the opportunity to definitively discern the difference between a thruster and quad, all other factors held constant. Based on my experience with this board in chest to head high hollow beachbreak surf, my theory is that more fin area in general allows the board to capture more energy, translating that into speed, but sacrificing that looseness a thruster provides that allows you to push the tail around with your back foot. This is pretty basic stuff, but I’d like to suggest another theory…
The toe-in of a single foiled fin puts the low pressure area on the outside of the fin slightly ahead of the center of the fin. This provides some “thrust” in the true sense of the word. Putting TWO fins on the rail toed in doubles that thrust force. In addition, the cant of the fins provides double the lift, allowing the board to plane higher and go faster. As we all know, too much toe in results in excessive drag, and cancels out the thrust.
The double foiled trailer on a thruster provides no significant lift or thrust… only directional stability with minimal drag. That drag translates into control.[img_assist|nid=1063017|title=convertible|desc=|link=none|align=left|width=640|height=480]
After thinking about this a bit more, I’m guessing that the quad is working more like a jib/mainsail combination where the jib aids the flow over the main sail which reduces stall (i.e. drag) which is particularly helpful in not so good conditions where any loss of speed ruins your ride (or like me who hasn’t mastered the art of keeping a thruster ‘on the boil’ yet). Having this smooth, faster flow could also be reducing the stall on the front fin due to it drawing the water smoothly over it as well.
With this theory the quad has more of a tendency to direct the flow of water out the back smoothly regardless of your angle of attack (to a point of course) whereas with a thruster, if your angle of the board in a turn is not in tune with the optimum angle of attack with your fins you could be creating stall therefore drag.
After thinking about this a bit more, I'm guessing that the quad is working more like a jib/mainsail combination where the jib aids the flow over the main sail which reduces stall (i.e. drag)
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McD, You move to the head of the class. The original idea for the thruster was to give more traction to a wider tail twin fin style Board. In The process of creating the Thruster design thats to Mccoy we also got the modren surfboard shape. The shape and bottom contor have more to do with over all speed then fins. Fins by them selves can not make a Board go faster. What you can do is reduce some of the negative aspects that fins have on speed.
“The shape and bottom contor have more to do with over all speed then
fins. Fins by them selves can not make a Board go faster. What you can
do is reduce some of the negative aspects that fins have on speed.”
This makes good sense but I can’t help wondering if you can’t gain just a little speed from the fins. I used to think that fins created drag only and I was heading down the road of the single fin until I road a quad. I couldn’t believe how it held speed in turns and had speed down the line so I searched around about quads which lead me to study how sailing works. It’s incredible that a boat can power along “into” the wind using a good trimmed jib and mainsail.
This site has some good info for those interested -> http://www.sailtheory.com/wrongtheory.html
Of course, to gain any speed the board would have to be on the perfect angle of attack for the flow to be perfect through the fins for this to happen and I guess this is the area where toe in and cant play a big part for the type of surf you have. This perfect AOA would most likely be when in trim or when in say a bottom turn around a section of foam with the latter being more important in junk or fast surf I’d imagine (the power of the wave and plan shape/rails can help with any shortfalls when in trim).
).