Scrapers are great for reshaping. Even thick razors can be better than grinding. I use them to scrape down trailing edges. But a proper blade from a thicknesser sounds like it would be a killer. Thanks for sharing Mike.
Alby is pretty much spot on with rebasing pointer fins at 45deg leading edge angle to make a weed fin. They need to be stiff to start with otherwise they will lose a lot of lift since the flex will appear as twist to the water once the blade is raked (hard to visualise). Also for the following reasons...
Mike the effect of rake is summarised by simple sweep theory. A raked blade is like a plane wing flying with a sideslip. The sideslip component does not contribute to lift. The chordwise velocity is reduced by cos(rake). So a raked fin needs more surface area to produce the same amount of lift as an upright fin at a given angle of attack and speed. Thats why older thick slalom fins which were developed for lower speeds are often the best things to use for weed fins. And why you often feel like you are going sideways at low speeds with a weed fin that isnt as big as a table tennis table.
Sorry to dumb things down but I'm a little confused.
I'm an aircraft engineer and my definition of chord is measured from trailing edge to leading edge in the direction of flow. See picture. So if I where to rebox a standard slalom fin at 45 deg that would effectivly increase the chord length, changing the chord/thickness ratio. In effect this would make the standard slalom fin too thin in section because you are increasing chord length but not thickness?
Is this where the hacksaw comes in?
Good news is contrary to what most people think we aircraft engineers are still living in the dark ages so I happen to have a few bearing scrapers on hand for shaping
Ian,
Yes, that's exactly why the advice is to choose the thickest slalom fin you can find to rebox at 45deg.
I think the hacksaw comes in, to change the angle at the base of the fin, and possibly to cut it down, to speed fin size.
I thought the definition of chord, is the line at right angles to the foil's center line.
Just done some googling and looks like I was wrong. Seems the chord is parallel to flow, as you say Ian.
Unless the foil is undergoing extremes (stall, separation etc) decreasing the % thickness does not significantly reduce the lift.
To avoid confusion, when I said "The chordwise velocity is reduced by cos(rake)" I meant relative to an unraked orientation, not the direction of travel (which is technically the chord as Ian pointed out). The forward speed is comprised of the chordwise and spanwise(sideslip) flow components when the blade is raked, when looking at it from an unraked orientation.
Why bother with this way of thinking about it, instead of just analysing it directly with rake and thinner profile?
Lift force is approximately proportional to surface area for a given planform shape (and rake).
So when you rake a fin, why does the lift disappear if the surface area is essentially the same, and the reduced % thickness is not reducing the lift? A 27cm weed fin has a lot less lift and drag than its original 36cm pointer in the same conditions despite having the same surface area and physical thickness.
The answer lies in simple sweep theory.
Like a lot of things you can look at it in many ways and come up with the same answer.
I googled "simple sweep theory" and found a description of an experiment where they slide a 100 metre long foil left and right (transversely) through a nicely fitting slot in a 1 metre wide wind tunnel. The measured lift from the 1 m section of foil in the tunnel is the same regardless of whether the foil is moving or stationary. That didn't surprise me. Did it surprise you? Apart from a bit of boundary layer disturbance the foil can't tell that the wing is sliding sideways.
BUT! say the aeronauts, the airflow over the wing is now twice as fast but the lift is the same ie. Simple sweep theory, it's the chordwise component that produces lift.
But surely if you use that revised value of airspeed you've got to revise the angle of attack. Simple geometry. If you take the path of the air molecules over the wing the angle of attack is reduced by that same cos of some angle. So of course lift is the same. Maybe not so "of course" - lift going up with the square of speed confuses things a bit.
I suspect the slip-sliding aeronaut who reduces his lift by the cos of the angle of side slip is taking his angle of attack from his artificial horizon. The angle of attack of his wings is less than this by the cos of the angle of sideslip.
I think there is another reason weed fins are less efficient. A straight fin can flex and still hold an angle of attack. The "real" aspect ratio of weed fins is lower than it looks. Might be other secondary effects going on.
From the designer of XFoil (he had a better explanation somewhere on the web but I can't find it anymore)
Newsgroups: sci.aeronautics
From: drela@athena.mit.edu (Mark Drela)
Subject: Re: Swept Wings Questions
Date: Fri, 29 Mar 1996 02:03:54 GMT
In article <rddDos6qD.IuC@netcom.com>, Dirk Lorek
<dirk.lorek@mailbox.swipnet.se> writes:
|> 1) It is my understanding that a backward swept wing is inferior to a
|> straigt one on subsonic aircraft because it produces lesser lift.
|> OTOH, a bkwd swept wing delays compessibility in supersonic flight. My
|> question (provided that I'm right of course) is at what point
|> (speed/altitude ?, very rough) a bkwd swept wings advantage can be
|> regarded as superior in comparision with its disadvantage.
Sweeping a wing makes sense only if you are up against the Mach number
limit, and want to fly faster, as with a jetliner. It doesn't make
sense if you want to fly higher, as with the U-2, or if Mach is of no
concern, such as with a General Aviation aircraft.
The airfoils on a swept wing behave as though they were flying at a
reduced speed, reduced Mach number, and reduced dynamic pressure.
effective speed = V cos(L)
effective Mach = M cos(L)
effective q = 0.5 rho V^2 [cos(L)]^2
where L is the sweep angle, and V and M are the airplane's speed and
Mach.
Imagine a straight-wing airplane flying at its maximum Mach number.
As you sweep the wing in flight from 0 to L degrees, the available
lift drops by a factor of [cos(L)]^2, and the Mach compressibility
effects on the wing's airfoils decrease (weaker shocks, etc.). You
then increase the speed by a factor of 1/cos(L), so that the effective
dynamic pressure and lift are increased back to their original levels.
The effective Mach is also increased back to its original level. In
effect, you haven't done anything to the wing's lift or
compressibility effects, but the airplane is now flying faster!
In reality, this isn't a complete freebie, since the skin friction
drag has increased by a factor of [1/cos(L)]^2 -- the wing skin
friction isn't affected by sweep very much, and feels the full brunt
of the real dynamic pressure increase, just like the rest of the
airplane. So the overall L/D will typically decrease from the sweep.
An airliner depends on the higher speed to more than compensate for
the lower L/D and give better overall range (the product V x L/D is
what appears in the range equation). And of course flying faster
gives faster revenue stream for the airlines.
If you repeat the above sweep exercise for a U-2, you find that you
haven't gained any additional altitude capability, although you may
have gained some range. Since ceiling is paramount for the U-2, it
doesn't have sweep.
Sweep also doesn't make sense on slower piston and turboprop
airplanes. In general, if Mach number is not a speed-limiting factor,
it makes more sense to get more speed by reducing the wing area.
|> 2) What is the advantage/disadvantage of a forward swept wing ?
To first order, sweeping the wing forward is aerodynamically the same
as sweeping if back:
cos(-L) = cos(L)
There are also secondary compressibility effects having to do with
taper which tend to favor forward sweep.
However, the forward swept wing has much more severe structural
problems which tend to overwhelm any minor aero advantages.
Mark Drela First Law of Aviation:
MIT Aero & Astro "Takeoff is optional, landing is compulsory"
Getting way of topic here but I disagree. Density has little to do with compressability.
Edit : Can we get back to talking about fins
loving it!! can we have some wind now please sweep or no sweep we aint going no where without it !
)
Back to the subject about the best of the shelf speedy I received my new choco speedweed 24 yesterday, just looking at it and thought it looked a bit like my techtonicc falcon F1, layed it over top and the outline is exactly the same just racked back. coincidence !! will take a couple pics tommorrow and post. 
looks like it will do the job for me. 
I should get to try it out friday.
R&D can mean two things For some its "Research and Development", and for many in WS industry it means "Ripoff and Duplicate". "But its OK because everyone does it" 
There are many "coincidences" in windsurfing products.
pics of the fins,,,
off to work for my last aevo shift and the seabreeze is 20 knots already.
Yes that sounds right. A while back i was chasing a choco speed weed. The agent told me he didn't have any but he could rebox a choco slalom for for me as all they have done is rake them at 45deg
A few of us have been doing that for a while now
Hi
I've tried to design a weed fin with the Eppler E836 profile.
With a base chord length of 160 mm the fin will become 20,2 mm thick which gives 12,6 %
Even though I also believe in thick weed fins, this seems just a little too much !?
By measuring some of my old weed fins I can see that they are between 14-17 mm thick.
Any good profiles with the thickest point closer to the leading edge ?
I've found this page with various profiles:
a190754.free.fr/PROFILES.PHP3
quick question
I just completed my new spewed fin but need to get it reboxed
is there a calculation in bringing the fin forward?
I see a few about 10-12mm set forward in the box and the new Vector looks closer to 20mm?
the pic is what i think LOOKS good and makes SOME sense.
from what i can see from my other fins
comments welcome
cheers
for the correct longitudinal position try to match the centre of effort [COE] of the weed fin with the COE of a "normal" slalom fin.
a rough estimation would be to find the COE on the two fins by measuring ~42% along the span from the base and about 25% back from the leading edge.
match those two points in a line perpendicular to the bottom of the board.
that should be close enough for what you are trying to achieve.
Boogie
I really don't like setting a weed fin forward in the base.
There are 2 potential problems.
1 damage to board from that hard pointy bit in front of the box. This will happen if the base is the slightest bit loose in the box. Even if the base is an exact fit, it just takes a bit of forward pressure on the fin when you pull it out, to ding the bottom of the board.
2 collecting weed if there's a gap. This will happen if the base is slightest bit tight in the box.
How many tuttle bases have you seen that are an exact fit?
Using a softer material there is a solution, but I still have my doubts.
shifting foot straps back can work as long as that doesn't sink the tail too much.
thanks boogie very nice to info from THE man ! thanks for sharing.
as usual decrepit food for thought , I did wonder how you got around that problem now i have the dilemma of what to do..
I really want to try it forward in the box as i run my straps and mast base as far back as pos as it is,
even just testing the other fin i still feel i need to pull really hard on the front foot to get the board at the right angle. I'm hoping this will solve the problem?
Had the first run on the choco weedie on Friday and was impressed (thanks for the tip Jarques, and second wind) ran it in my 2006 62litre F2missile with a KA Kobcept 5.0 and has a 38 on the dial and constantly running 37's which was great as was sailing in the middle of the lake, never let go once and went upwind well and the speed was great, but it also gybed fantastic allowing me to get 90% on the speed board, was running over weed no problem at all, only moved my base back 1cm and still had my straps fully forward. was sailing by myself as usual so took the safe option with the 5.0 I'm sure I would have had over 40 if I had the 5.8 up. very very Happy with it. the weed downwind of the spit was right to the top in big patches and just blasted through it. heres my tracks .
Rasmus, you can thin the E836 profile down to 9%. 9% is probably the most versatile thickness and is the one Decrepit put up in this post. There are many programs onl the net that will do this for you .
Decrepit builds his own boards and gets around the problem of LE forward of box on weedies by puttting the box ~2 cm forward.
Select to expand quotejimbob SA said... Had the first run on the choco weedie on Friday and was impressed (thanks for the tip Jarques, and second wind) ran it in my 2006 62litre F2missile with a KA Kobcept 5.0 and has a 38 on the dial and constantly running 37's which ....../quote]
fantastic sailing wish we had wind to have a go
what size fin are we talking with that board n sail combo
