I'm late to the party, but please allow me to give my input.
Below I'll talk about top and bottom fin as if those are biplane wings, it just simplifies the explanation and corresponds to how the image is laid out.
This image perfectly explains why I believe the fins should be symmetric or even assymetric towards the outside instead of inside.
The idea behind having them foiled on the inside is to create a venturi, this venturi induces an extra low pressure region between the fins. But because both fins are identical to the inside (mirrored), the force on the inside surfaces is exactly the same for top and bottom fin, resulting in zero contribution to the total lift by the venturi. So the verturi only increases drag. (And then I even ignore the possibility of cavitation when the pressure become so low the water starts to form bubbles, this can happen with speed record windsurf fins, but probably wont be an issue here unless you make the venturi to extreme.)
A nice foil profile is needed to keep flow attached, that's why you see leading edge stall (the most abrupt type of stall) on the top fin, with a nice foiled top side it would not stall as fast and in case of trailing edge stall, it would stall much softer.
Talking about the bottom fin, the presence of the top fin helps in keeping the flow attached, this is known as the "biplane effect". Therefore, you can get away with a flatter top side of the bottom fin (reducing the venturi), although a symmetic profile would still keep the flow attached to the bottom fin longer.
The key to the thrailkill twin is that the top fin stalls earlier than the bottom fin, softening the total stall characteristic. However, this does not mean you have to make the top fin stall more abruptly because you'll counter the beneficial effect.
When the top fin stalls (as illustrated in the image), you still see there is lift generated by the combinanation. The downforce by the top fin, low-medium, is less than the upforce by the bottom fin, low-high. This is the thrailkill twin or "biplane effect" in practice.
Another crucial point is not to use flex fins. The purpose of flex is to soften the hard stall of thin profiled fins, this job is taken over by the biplane effect. Flex can interfere with the biplane effect in suboptimal ways.
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finFoil: free fin designing software
http://www.finfoil.io
https://www.instagram.com/finfoil/
https://app.finfoil.io
The easiest way to see if this set up works is ride one. The most common ride report (2) that I get is that this system is tried and then abandoned. If it worked well, you'd see more of them. The only ones I have seen in public were on boards that were made by Guru followers here on Sways.
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That which can be assorted without evidence was read in an illegal magazine.
The easiest way to see if this set up works is ride one. The most common ride report (2) that I get is that this system is tried and then abandoned. If it worked well, you'd see more of them. The only ones I have seen in public were on boards that were made by Guru followers here on Sways.
I agree. However designs are often abandoned because they have been used in combination with the wrong board, fins, ...
Often due to a lack of understanding on how to make it work.
A good example is the trifin on sailboards, which has been abandoned for years due to construction complexity and the struggle to make them work together, get it wrong and the board will drag like hell, and yes this is rider experience. But when you get the setup right it works fantastic. Nowadays the sailboard trifin is back but still only few really nail the setup. And guess what, it's those that understand the physics that are able to get them right. The witchcraft shaper is one of those who nails them and has kept doing them when everybody abandoned them and still he is the only one succeeding in making them work with large side fins, he understands the physics better than anyone: https://witchcraft.nu/boards/trifin/
Single fins are easy to build and hard to do wrong, thrusters on surfboards are also easy because you surf the rail (not flat like sailboards), quads are harder. The thrailkill twin is very hard to get right, that's why it's abandoned. If we keep doing the easy stuff, this sport would not progress.
like
1
finFoil: free fin designing software
http://www.finfoil.io
https://www.instagram.com/finfoil/
https://app.finfoil.io
Good observations. Not saying it can't work. Most any kind of fin setup can work. But if that setup isn't pretty revolutionary it is usually abanbonded. Neal Purchase and the Campbell Brothers have been pretty revolutionary. When I see two full sized fin boxes that close together with full sized fins installed. I see nothing but Drag. But I'm not gonna keep stating the obvious, nor am I going to buy into propaganda.
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That which can be assorted without evidence was read in an illegal magazine.
I think another way a design feature doesn't catch on is when the style of surfing its aimed at doesn't catch on. I watch Neal Purchase on his Duo and TO ME those lines he's drawing don't motivate me. It's like the Bonzers or the channel bottom thrusters; you need a heavy rear foot to take advantage of those features. Watching a thruster rider attempt to pump a single fin and then complain that they've got no drive is another example. Etc, etc.
Watching a thruster rider attempt to pump a single fin and then complain that they've got no drive is another example.
I know. It's comical, eh? Another reason many don't build them, is cost. Exra FU glass fin, approx. $65 dollars. Extra FU fin box, approx. $ 9 dollars.
There are other examples - Bonzers are mostly limited to the Campbell Bros and their associates, the Duo is limited to Neal Purchase Jr, the Zinger is limited to ACE/Eaton, just to name a couple.
I don't think popularity or the lack thereof indicates to functionality. Much of what sells is the result of aesthetics and marketing, including the effects of pro surfing.
Would the 3-fin setup been every bit as popular with novices who lack the skills to surf them if we had called it the tri-fin instead of the Thruster?
Hello Gdaddy; so what would you say to the thruster rider in that case? Not drive but more intrinsic speed?
Regarding design. A bonzer daily rider 5 10 that I used has 2700 waves ridden (yes I counted its) but has a contemporary shape not an old style shape. I am more of a light surfer than heavy back foot one but the board worked pretty good after the first 80 waves or so (rear foot too forward was the error) I think the guys doing fish do the same error, shaping old shapes and use keel fins...
I will try to see this guy Purchase surfing those boards but I assume old style shapes however, great surfer...but put those shapes under a normal guy and most will return to the contemporary shape with a thruster (or may be quad if the board has more area) set up saying that the "retro" stuff is just that: retro.
-------also I have been observing that the market for the thruster et all (read, CI; lost, Pizel etc) now are widening ALL the boards. Included toobs boards. Most between 19 1/2 to 21. Seems that the fellas are eating too much junky food.
Agreed on most points except that (IMO), the majority of surfers cannot do as well on a thruster as they can on a quad that's set up reasonably. And most noobs can't even make use of a thruster OR a quad.
I guess I hold the minority opinion on it because I think it should be illegal for shops to even stock entry level eggs and funboards with a thruster or a quad because the only people who can actually surf that setup as intended are surfers who have already spent a considerable amount of time on either a high performance longboard or the shortboards. Most noobs buying an egg are incapable of outsurfing a singlefin on it. Put a safety fin on it and they're good because the board is already doing most of the work. And that is not a bad thing for someone who's just starting out because surfing a singlefin is more forgiving with foot placement and stance and such.
But no, thrusters sell better because they're sexy and because that's what the pros surf. The noobs think they look cool while the people who can surf think it looks like the equivalent of wearing a gold chain over a Wal-Mart t-shirt.
Yes, and we should all wear masks. Or not.
That which can be assorted without evidence was read in an illegal magazine.
I'm late to the party, but please allow me to give my input.
Below I'll talk about top and bottom fin as if those are biplane wings, it just simplifies the explanation and corresponds to how the image is laid out.
pressures.jpg
This image perfectly explains why I believe the fins should be symmetric or even assymetric towards the outside instead of inside.
The idea behind having them foiled on the inside is to create a venturi, this venturi induces an extra low pressure region between the fins. But because both fins are identical to the inside (mirrored), the force on the inside surfaces is exactly the same for top and bottom fin, resulting in zero contribution to the total lift by the venturi. So the verturi only increases drag. (And then I even ignore the possibility of cavitation when the pressure become so low the water starts to form bubbles, this can happen with speed record windsurf fins, but probably wont be an issue here unless you make the venturi to extreme.)
A nice foil profile is needed to keep flow attached, that's why you see leading edge stall (the most abrupt type of stall) on the top fin, with a nice foiled top side it would not stall as fast and in case of trailing edge stall, it would stall much softer.
Talking about the bottom fin, the presence of the top fin helps in keeping the flow attached, this is known as the "biplane effect". Therefore, you can get away with a flatter top side of the bottom fin (reducing the venturi), although a symmetic profile would still keep the flow attached to the bottom fin longer.
The key to the thrailkill twin is that the top fin stalls earlier than the bottom fin, softening the total stall characteristic. However, this does not mean you have to make the top fin stall more abruptly because you'll counter the beneficial effect.
When the top fin stalls (as illustrated in the image), you still see there is lift generated by the combinanation. The downforce by the top fin, low-medium, is less than the upforce by the bottom fin, low-high. This is the thrailkill twin or "biplane effect" in practice.
Another crucial point is not to use flex fins. The purpose of flex is to soften the hard stall of thin profiled fins, this job is taken over by the biplane effect. Flex can interfere with the biplane effect in suboptimal ways.
finFoil: free fin designing software
http://www.finfoil.io
https://www.instagram.com/finfoil/
https://app.finfoil.io
The easiest way to see if this set up works is ride one. The most common ride report (2) that I get is that this system is tried and then abandoned. If it worked well, you'd see more of them. The only ones I have seen in public were on boards that were made by Guru followers here on Sways.
That which can be assorted without evidence was read in an illegal magazine.
I agree. However designs are often abandoned because they have been used in combination with the wrong board, fins, ...
Often due to a lack of understanding on how to make it work.
A good example is the trifin on sailboards, which has been abandoned for years due to construction complexity and the struggle to make them work together, get it wrong and the board will drag like hell, and yes this is rider experience. But when you get the setup right it works fantastic. Nowadays the sailboard trifin is back but still only few really nail the setup. And guess what, it's those that understand the physics that are able to get them right. The witchcraft shaper is one of those who nails them and has kept doing them when everybody abandoned them and still he is the only one succeeding in making them work with large side fins, he understands the physics better than anyone: https://witchcraft.nu/boards/trifin/
Single fins are easy to build and hard to do wrong, thrusters on surfboards are also easy because you surf the rail (not flat like sailboards), quads are harder. The thrailkill twin is very hard to get right, that's why it's abandoned. If we keep doing the easy stuff, this sport would not progress.
finFoil: free fin designing software
http://www.finfoil.io
https://www.instagram.com/finfoil/
https://app.finfoil.io
Good observations. Not saying it can't work. Most any kind of fin setup can work. But if that setup isn't pretty revolutionary it is usually abanbonded. Neal Purchase and the Campbell Brothers have been pretty revolutionary. When I see two full sized fin boxes that close together with full sized fins installed. I see nothing but Drag. But I'm not gonna keep stating the obvious, nor am I going to buy into propaganda.
That which can be assorted without evidence was read in an illegal magazine.
I think another way a design feature doesn't catch on is when the style of surfing its aimed at doesn't catch on. I watch Neal Purchase on his Duo and TO ME those lines he's drawing don't motivate me. It's like the Bonzers or the channel bottom thrusters; you need a heavy rear foot to take advantage of those features. Watching a thruster rider attempt to pump a single fin and then complain that they've got no drive is another example. Etc, etc.
There are other examples - Bonzers are mostly limited to the Campbell Bros and their associates, the Duo is limited to Neal Purchase Jr, the Zinger is limited to ACE/Eaton, just to name a couple.
I don't think popularity or the lack thereof indicates to functionality. Much of what sells is the result of aesthetics and marketing, including the effects of pro surfing.
Would the 3-fin setup been every bit as popular with novices who lack the skills to surf them if we had called it the tri-fin instead of the Thruster?
Hello Gdaddy; so what would you say to the thruster rider in that case? Not drive but more intrinsic speed?
Regarding design. A bonzer daily rider 5 10 that I used has 2700 waves ridden (yes I counted its) but has a contemporary shape not an old style shape. I am more of a light surfer than heavy back foot one but the board worked pretty good after the first 80 waves or so (rear foot too forward was the error) I think the guys doing fish do the same error, shaping old shapes and use keel fins...
I will try to see this guy Purchase surfing those boards but I assume old style shapes however, great surfer...but put those shapes under a normal guy and most will return to the contemporary shape with a thruster (or may be quad if the board has more area) set up saying that the "retro" stuff is just that: retro.
-------also I have been observing that the market for the thruster et all (read, CI; lost, Pizel etc) now are widening ALL the boards. Included toobs boards. Most between 19 1/2 to 21. Seems that the fellas are eating too much junky food.
Agreed on most points except that (IMO), the majority of surfers cannot do as well on a thruster as they can on a quad that's set up reasonably. And most noobs can't even make use of a thruster OR a quad.
I guess I hold the minority opinion on it because I think it should be illegal for shops to even stock entry level eggs and funboards with a thruster or a quad because the only people who can actually surf that setup as intended are surfers who have already spent a considerable amount of time on either a high performance longboard or the shortboards. Most noobs buying an egg are incapable of outsurfing a singlefin on it. Put a safety fin on it and they're good because the board is already doing most of the work. And that is not a bad thing for someone who's just starting out because surfing a singlefin is more forgiving with foot placement and stance and such.
But no, thrusters sell better because they're sexy and because that's what the pros surf. The noobs think they look cool while the people who can surf think it looks like the equivalent of wearing a gold chain over a Wal-Mart t-shirt.
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