Quote:
HalSose,
You can’t use Joel Tudor’s name to criticise what I do!
Go suck a lemon!
Thanks Keith!
To all…
what is/are the typical angle(s) of attack on the inside fin (thruster) during typical bottom turning or pumping?
From the tests I have run, drive/thrust/punch does not come from fin lift…it comes primarily from lateral leverage and water flow redirection from the inside face of the fin (on turns water flows from the waterline towards the inside face of the inside fin at shallow to very steep angles depending on how hard you turn).
One of the key advantages of TOE-IN is that the angle redirects flow a bit outward towards the rail to facilitate turning. There’s nothing technically wrong with zero Toe-in if all you care for is FORWARD drive. However, since many of us would rather turn than trim in a straight line, the advantages of Toe-in become obvious.
Part of the problem has been the lack of Toe-in adjustability with today’s most common fin systems. The EDGE system is a giant leap forward…unfortunately I have not tried them…YET. I know Greg Loehr swears by them on his travel boards.
“On any board if you move your weight back and lift the nose the board will stall because this reduces trim speed”
Actually, you’re increasing pressure drag when you bury the back half of the board in the water. Stall relates to lift. Pressure drag is much more critical than skin friction drag (hence why flatter rockers go faster on equal bottom area boards).
BTW, pressure drag from the board bottom is one of the key elements to forward drive/pump…pushing off water has tremendeous benefits. Pudding? stand next to a small board or skimboard and try to push the board straight down into the water with your hands/arms…mucho resistance…and lateral leverage when surfing. A thurster’s extra fins is what allows one to exploit board pressure induced drive.
To some, and you know who you are, thrusters are a drag. But if you read between the physical lines you begin to understand the many tremendeous benefits from such a simple three fin setup.
Stall and washout? I think stall is a bit over-emphisized, mainly because I just don’t think fin foil lift is a major benefit…you get more tail lift from the toe-in drag)…I have no opinion on washout.
my $0.02
Regards,
Dave
lots of interesting comments here but I’m only going to touch on one (lack of time at the moment). Here’s a suggested semi-empirical test on fin foil lift and whether or not it matters.
Remove and reverse your left and right thruster fins. Put the left one one the right side and the right one on the left side (i.e. with the lift going the opposite direction).
go surfing.
Report back the results.
I’ve done that (reversed right and left foiled fins).
Whereas there is some loss of drive, the board is still remarkably surfable.
The effects are small, indeed, compared to changing the side fins so they are not toed in at all. Or, if they are toed across the nose (instead of someplace between 0.25" on a 4.5" line and toed to the nose).
Keith, Dave, All…
I like your test method…I thought of the same but only using center fins instead (symm foils).
Let me clarify…I think fin foil matters, the question is: to what degree does fin foil lift matter? (particularly when one better understands what the water is doing under the board)
I think tail lift has much more impact on a board’s performance… that comes mostly from toe-in/rake pressure drag. Foil/cant lift is there too but likely has less effect on tail lift (again pressure drag will dominate). A Blakestah said, toe-in has gobs of effect…
After reading this link in detail some of this phenomenon became obvious to me
http://www.coachesinfo.com/category/swimming/146/
Dave…Im glad youve done that test (u-da-man)…your test results do not surprise me at all…the loss of drive is likely from the fin’s water flow exhaust direction caused by the foil on the inside (exhaust flow more towards the rail than out the rear). Perhaps also a slowing of exhaust flow along the curved side of the fin. My guess would have been that the board rode ok. Foil has an affect but it is likely minor when compared to other fin characteristics. Additionally considering the thinness of today’s most popular fins, I think FOILING its more about hydrodynamic-streamlining (to reduce pressure drag and reduce turbulence) and less about lifting.
Considering water flow and AOA, would a symm foil on the sides stall less?
Hi Keith
Re washout I think you got it the wrong way round
Washout has no effect on the leading edge height it only effects the trailing edge
Get a assametrical model glider wing put it flat on the ground so that the root flat side is on the ground
The leading edge will be in contact with the ground all the way to the tip
The trailing edge will gradually lift off the ground so at the tip it will be well clear
You can do the same with your thruster side fin if you put it flat side down on a flat surface and the leading edge at the tip is clear of the ground,then you should maybe throw it away a it will stall at the tip very early and help induce more drag
If it flays flat the whole way then every thing should happen lift/stall etc at much the same time
If the trailing edge lifts clear then the fin should stall from the base up gradually and therefor not induce drag as abruptly
thats my 50 cents worth
regards Mike
I’m glad someone read all that stuff!! what I was trying to describe was the relative positions of TE to LE. Yes in wings the LE usually is straight, only the TE is twisted. Sorry, it appears that I described wash-in (generally a BAD thing)instead of wash-out. Well, it WAS somewhere between 4 and 5 in the a.m., I’m surprised I only made one mistake!
Washout: “leading edge higher than the trailing edge” should be
“trailing edge higher than the leading edge”
My Bad, as they say.
Hi Roy
As you say, as speed increases the amount of water displaced decreases, however this will only happen in flat water going straight
If you are going along on a wave your board will not be flat in the water one rail will be more immersed than the other and the water plan shape under the board would not be symmetrical so the fins if both are in the water would not be operating at the same angle of attack
This is the reason the thruster side fins work well, the wave side fin will allways have the best optimal AOA where as the other fin , the one away from the wave face is on the side of the board that is displacing less water and so will be at or nearto a 0 angle of attack
Of course as soon as you wiggle ,turn or whatever all this changes again
I would speculate that side fin could work just as well with symmetrical foils as long as the leading edge radius was correct
The high speed cavatation of fins in a straight line could happen as you say though the cavation will start from the leading edge inside face
However I would think that it would be self leveling as each side drag would yaw the board this would then equilize the AOA ,this could happen in series from side to side and any drag will slow the board down ,which will then also return things to normal
This changes if you are plumiting down a verical face where the extra does not slow you down, in which case I would suggest you stop worrying about theory and start just plain panicking
For us average guys surfing average waves our speeds are not that extreme so our fin work in the speed range quite well
I think that it is generally agreed that the smaller the wave the more the toe in for fins
and the lower the boards utimate top end controlable speed
If you are one of the few who ride monster waves that I find give me nightmares, than
every thing changes,the speeds the wave face angles etc
Theory says that they should have less toe in as there turns are much wider and the straights much longer,as I say I havnt been there but I would be interested to hear from someone that has
Regards Mike
I’ve ridden twins and twinzers like that. You take off, and they just are not happy until you lay it on a rail. The decreased drag when you lay it on a rail is real easy to notice.
These are sub 6 foot boards toed to the nose, or more.
Yep ive got a five foot eleven kevlar board thats 11 1/2" accross the tail it has my sloted biplane fins in a twin setup
Hugely fast in small stuff and in larger head high and over stuff its still great but gets
stiffer the faster you go ,which is kind of handy to keep it under control
Hopless in bumpy stuff though
Mike
Strangely enough big wave boards with reduced toe are tracky and a pain in the rear.
Big wave boards with normal toe on their rail fins turn nicely.
You can even go among big wave riders, board to board. The ones that look like they have too much toe - the riders dig their turning power. The ones with reduced rail fin toe turn like a pain in the rear.
I’ve thought the difference in toe from the side to rear thruster fin served as a control device. Rapid increase in rear fin drag tends to prevent the rail fin from a stall. So getting the difference in angle right is important for stability on a rail.
Going straight, well, thrusters were never designed to go straight.
Thrusters, going straight, have fins separated fore-aft, and starboard-port. Typically the fore-aft separation is 7 inches, more or less, on a shortboard, but can be two inches more on a gun. I suspect this relates to the tracky nature when fins are not adequately toed.
Blakestah
Is it the same with big wave paddle in boards as compared to much the smaller tow in boards ?
I dunno about tow-in boards. I’ve heard different things, but you’d be better off talking to someone who makes or rides them.
Keith, your mention of airplane modellers and your discourse on basic aerodynamics brought back many memmories for me as I did a lot of designing, building and flying of RC gliders in my younger days.
I was always a little wary of claims of aerodynamic effects on surfboard fins as a lot of what I saw in setting surfboard fins went against the grain of what I saw and had experienced as aerodynamic efficiency.
Here are some of my physical observations of AEROdynamics, how they relate to surfboard HYDROdynamics I am just tossing the ideas in the air:
The Clark Y (flat bottommed) foil which closely resembles that of the current foil on surfboard fins was one of those foils that never did anything particularly well , it was a general foil used largely because of its ease of construction.
It was generally set up for relatively steady slow speed flight at an angle of incidence of + 1 - 3 degrees. In sboard terms this would be toe out which we all know is a no no. The Clark Y foil has a zero lift co efficient at about -3 degrees incidence which I believe roughly relates to the current toe in used by many.
One of the visible characteristics of this foil was that it didnt like running at -ve incidence angle (AOA) such as in a prolonged shallow dive (higher speed , more drag). The tendency was for the aircraft to “balloon” ie dramatically increase its angle of attack , bleed off speed until it either stalled or was corrected and resumed its normal trimmed flight. When a surfboard goes on rail and lifts the outboard fin this may be allowing the inboard fin to follow its tendency to balloon as the board follows its rail and changes its direction which happens to co incide with a favourable change of the angle of incidence for that foil. So the foil shape may have some contribution to the drive of a multi fin board, however, why not use a lower cambered bi convex foil that will better tolerate the -ve AOA that these fins initially are set at. You may get a little less ‘zing’ in your turns but you will probably also have a little less drag at other times.
To be honest my G5 FCS dont even look like an airfoil, there is no gradual entry from the LE to the bottom of the section, just a sharp edge, I would have thought that with these fins running with toe in there must be an enormous amount of unnecessary turbulence on the inside of the fins when running straight. Even a Clark Y foil has a subtle tuck under the leading edge.
Andrew
Hey Andrew, good comments.
My point really was not so much that aerodynamics and hydrodynamics are exactly the same, just that there is a lot of vague terminology being used in the surfing world when well defined terms are available, that would perhaps result in better communication if everybody used them. Eg chord, thickness, camber (how many surfers/shapers do you hear talking about their fin camber?)
aloha,
Keith
Andrew, thrusters are designed around what happens in a classic bottom turn - board on inboard rail, two fins engaged, driving through a turn.
Although it may be clear that there is a compromise position to be taken wrt how the fins handle when going straight, and how they turn, if it doesn’t come through a bottom turn well, it will not win acclaim.
Also, particularly close to the hull, water is displaced outward as it moves rearward. So, the negative 3 degree AOA of the rail fins is actually closer to a real zero near the fin base. Near the tip, though, it is still negative.
Roy,
Your reply is tardy,
so consequently I lack the passion
to play with you.
But heres one all your own,
rearrange the letters in my name and
spell your favorite word.
I see you’ve stopped self-promoting
for a while.
Good Boy.
roy pisses me off !!!..
regards
BERT
ps
ill be back another time with some practical comments on this thread…
hopefully by then roy may have learned to build rather than tear down…
no apologies this time roy…
you do realise the more comments you make , the more insight you give everybody into your level of understanding and experience…
to quote a famous 20th century cult icon…“better it is to remain silent, than to speak and remove all doubt”
sorry for getting off the subject, but im sick of seeing good quality threads and subjects being hijacked …
keith,meecrafty,sabs,blakestah,halycon,some fine comments from all you guys,
ive been over this thread a few times now…
to keep it on the subject…i think stall is relative to surfboards…
but washout is not an issue in surfing, but before im 100% on my last comment…
can anyone give an example of a wing that is twisted so the leading edge is lower at the tip???
the only real area where i can see washout is relative or a problem , would be the propeller of a plane ,because the tip is cutting through the air at a far greater speed than the base, the leading edge needs to have less angle of attack near the tip …coz stalling is also relative to speed …you could have the same angle of attack on a wing/fin ,but at low speeds it will still function,where as at higher speeds it will stall…
so is there anyone qualified enough to clarify this aspect ,
if my theories are correct we could forget the term washout in relation to our subject ,and stick with stall…
back to halycons original post and his questions/comments…
Quote:
“On any board if you move your weight back and lift the nose the board will stall because this reduces trim speed”
Actually, you’re increasing pressure drag when you bury the back half of the board in the water. Stall relates to lift. Pressure drag is much more critical than skin friction drag (hence why flatter rockers go faster on equal bottom area boards).
meecrafty’s response was relavent…
to add that ,
halycon, i can see how you could make this conclusion…we may make an assumption about a certain fin set up …but we only came to that conclusion because of the board the fins were in …we have to be careful not to make hard and fast statements based on our experiences…coz that can influence any future decision we make about design , coz we can find our fundamental basic design principals may be flawed…
i know where your at coz ive grappled with the same subject before…
ill establish some basic laws that everyone will agree with and the rest will answer itself…
a larger fin/wing has more lift …true statement…
[=1]now halycons statement
Quote:
On any board if you move your weight back and lift the nose the board will stall because this reduces trim speed, however there’s a thing called fin stall too.
[ 3][u][/]
Rail fins that are to big tend to create this feeling when the board is triming in a straight line.[/u] The will even make a board track back and forth from rail to rail because the alternately over power the board.
this statement is both true and false???
its relavent to tail rocker from the front of the front fins to the tail…
also fin cant plays a major role as well …
as our cant or side fin size increase it automatically wants to pull the nose up as we move forward…if were running tail rocker and the nose pulls up , the board then experiences pressure drag due to the extra load placed down on the tail…
but on the other hand if were running a flat tail as the nose pulls up we actually increase in speed coz now theres load being put on a flat part of the board
so a larger side fin with more cant and a flater tail will actually give more speed and squirt as we pull onto the back foot giving drive and speed out of turns…
but a larger side fin combined with more tail rocker will actually slow down when we pull onto the back foot…
plus fin stall may also happen when our camber /thickest part of the foil is to far from the leading edge or is to thin in general, or our leading edge is to sharp…
when you look at your fins down the bottom line of your board you cant see much exposed inside edge ,but when you drop the nose of your board down , you can see more exposed inside fin area…the same happens when were surfing , lifting the nose or standing a little futher back on the tail will automatically increase angle of attack…with aspects of the foil like those mentioned above , fin stall will start to happen ,and we experience induced drag…
Quote:
Rail fins that are to big tend to create this feeling when the board is triming in a straight line. The will even make a board track back and forth from rail to rail because the alternately over power the board.[/]
this statement is true in relation to pulling your board back and forth from rail to rail…
its a matter of dialing in how much you want it to pull from rail to rail…
for small waves its a good thing, allowing you to work your board with ease gaining more speed …
however at higher speeds you can find your board doing things you dont want it to and over responding to your every subtle body movement…even going off in a direction you dont want …reducing toe in here also helps …it will make the fins less responsive so it will only go in the direction you want ,well and truly after your body weight is committed in a certain direction…
Quote:
When a board washes out it seems to me we’re talking about its inability to hold a line, side slipping or skating are very closely related to this term. Fins wash out too. Sometimes when you try and drive off them you just dont get any punch, like when a single is too far forward for the size of the waves you’re riding.
Clarifications ~ ?
this is just basic stall due to , to much load, to much speed, not enough camber,not enough rake (which resists stall)…or a handful of other related problems…
like i said above im not sure if the term washout belongs in here…???
if anyone is more familiar it would be great to get a proper clarification…
im with keith and everyone for that matter , to get all our terminololgy in sync so were all clear about the subject matter…
clear communication is the key to progress…
ok thats all the clarification ive got time for now on that particular subject…
regards
BERT
ps just got back from a trip away …found some real isolated spots ,didnt even see another surfer for 5 days on this one stretch of coast…put some pics in …
found some sick untouched territory…
