You're getting there, but there is a loooong way to go.
Raking reduces the drag but also the lift. But you can compensate by going faster :-))
But this isn't a total freeby, since the skin friction drag has increased by a factor of [1/cos(L)]^2 -- the foil skin friction isn't affected by sweep very much and increases as the real dynamic pressure increases so the overall L/D will typically decrease from the sweep.
But this is more technical than you need to get. At Hardies you need a raked fin for the weed. Unless you are planning to do 45kn no need to worry about the cavitaion effects of sweep or thickness consideration. As Elmo found, all you need is a good slalom fin and some spare time and you can make your own.
Remember rake reduces the lift so you need to increase the area a bit to compensate. I think Elmo's was a 32 at least.
well I'd love to do 45kn but with a PB of just under 35kn at your spot, don't think that will happen in a hurry at hardy's.
I won't be converting an existing fin, I'll be starting from scratch.
I'm experimenting with western red cedar and carbon in a light custom box.
What I'd really like to know is how low profile to make the fin, I'm sure lower profile than normal will work better at hardy's because most of the weed problem is growing on the bottom.
The first time I discovered the inlet 3 years ago I sailed at Kite beach which as you know is a carpet. I used a 18cm deep biconvex razor sharp LE foil with a base of 15cm and a LE rake over 50 degrees. The crabs were ducking for cover!
Didn't go any slower or faster than a proper foil. I think I did a 29.8 NM (or was it 28.9?) which wasn't bad then as the world record was still 33.94 by Bjorn.. but we have all come a long way since then.
Just a few things. Low aspect foils can develope high lift but at extreme angles. But really are only workable if the LE is raked quite a bit. Have you seen fotos of jet fighters at extreme AoA and the vortexs coming from the LE? It is those vortexs (vortices?) that keep the flow attached and allow for high lift. Rounding the tip also helps. Basically elliptical is a good way to go but harder to make.
But this high lift at large angles isn't very practicle..have you tried to sail a board going 20 degrees sideways. Rail trip city! And of course at high angle you'll get cavitation at the speed you want to go at.
Still the main problem was not the performance but the difficulty getting back up wind. Off the wind the 18cm was fine and as solid as but coming back the centre of effort was too close to the bottom of the board and the windward rail was digging in. On a narrower board it would have been OK.
definitely swings and round about stuff, guess I'll just try a few things and see what happens.
It would certainly be nice to get back upwind!
Used my 26cm FO Weedeater on my new Carbonart sp44 yesterday and for the first time I actually managed to get a run. On the second run I suffered tailwalk at a relatively low speed of 28 knots. Is this a technique issue caused by not holding the board flat or is the fin too big. My other boards are a Hypersonic 105 and S-Type 105 both of which are much wider in the tail and tend to hold themselves flat without concerted effort. Also how do you start a narrow tail board in shallow water. I am sailing in 40cm deep water and as soon as I put my back foot on the tail sinks and the fin sticks into the mud. Bring on the 22 cm speed weed.
Frant, the CA44 has tail kick to allow it to sail on the tail at speed 'cause that is how Chris likes to sail...with the board free. So you will need make sure you keep mast foot pressure on or it will get away from you. But it's relative to what you are used to.... Roo prefers his CA44 to the 47 which sails flatter.
At speed the small tail will come into its own particularly on flatter courses like SP where a wider tail will force the nose down engaging too much board and slowing you down.
quote:
Originally posted by yoyo
Frant, the CA44 has tail kick to allow it to sail on the tail at speed 'cause that is how Chtis likes to sail...with the board free. So you will need make sure you keep mast foot pressure on or it will get away from you. But it's relative to what you are used to.... Roo prefers his CA44 to the 47 which sails flatter.
At speed the small tail will come into its own particularly on flatter courses like SP where a wider tail will force the nose down engaging too much board and slowing you down.
My reference to flat was athwartships, not flat in the sense of fore and aft trim but hopefully I can convert this information into knots
quote:
Originally posted by frant
Used my 26cm FO Weedeater on my new Carbonart sp44 yesterday and for the first time I actually managed to get a run. On the second run I suffered tailwalk at a relatively low speed of 28 knots. Is this a technique issue caused by not holding the board flat or is the fin too big. My other boards are a Hypersonic 105 and S-Type 105 both of which are much wider in the tail and tend to hold themselves flat without concerted effort. Also how do you start a narrow tail board in shallow water. I am sailing in 40cm deep water and as soon as I put my back foot on the tail sinks and the fin sticks into the mud. Bring on the 22 cm speed weed.
Have another look at Chris Lockwoods speed fins. They are raked almost as much as most weed fins and may do the job. They are certainly capable of 45+ knots and Chris, Tony Wynhoven and myself have proved.
Pic of 23cm symmetrical next to Starboard IS50 fin. 23cm (Curtis sr6b slalom type foil racked back). I like Chris's version better! :-)
www.seabreeze.com.au/gallery/gallery.asp?imageid=5997
Of course, at the moment the Lockwood fins are about as common as rocking horse do do......
Opps. Forgot to say that the printing on the proto fin was a stuff up. Should have read Lockwood/McDougall designs!
I think Chris said the 1/4 chord rake was 30 with the leading edge being 35. Have you had a chance to try the black Lockwood fins Andrew?
Do you think the finish/performance is identical to the earlier grey ones? Obviously I'm sure that was AMacs intention but I remember Chris commenting that his molds for the new design were a bit rougher than he hoped and needed a fair bit of finishing.
They look almost the same as my C3 X1 speed fins which i purchased two years ago.
Hey Yoyo,
Don't take this the wrong way, and I know you're all technical and stuff, but I really struggle to take someone seriously who can't spell "chord".
Y'know?
Select to expand quote
Of course, at the moment the Lockwood fins are about as common as rocking horse do do......
You wouldn't be teasing us Daffy......."these are great but you can't have one"
It was late and I was tired. Is "Y'know" a spelling mistake or some americanism you picked up somewhere?
Choco, I have a C3 X fin and I can tell you Chris' fins are nothing like them. Much thicker, much longer base with more taper and completely different profile.
Hi Pete. As I understand it, the black fins were in the same batch as Chris's grey ones. I think Chris may have carefully hand finished his though. ;-)
As I said, Tony, Myself and AMac have PB's on the Black ones.
These are VERY different from the C3 fins and work differently as well. As Pete said they are thicker and asymmetrical. They drop into their 'drag bucket' very easily and have lots of lift for their size and can be loaded up heavily at less than optimum angle and still go very fast. Not quite as low drag/fast on a tight reach as the TM45v7 I don't think but seem as fast broader. From what I understand, the C3's are more optimized for broad running with less load than either of the above fins but have definitely got the runs on the board in the right conditions.
AMac is still working on getting a few more of these fins but even when he does numbers will be quite restricted. They are precision CNC machined in super quality G10 by an absolute craftsman whose time is very limited and expensive.
quote:
Originally posted by sailquik
Have another look at Chris Lockwoods speed fins. They are raked almost as much as most weed fins and may do the job. --
www.seabreeze.com.au/gallery/gallery.asp?imageid=5997
I have tried to take such pics but they just don't show up much at all. Maybe I'm not a good enough photographer.....
Chris's assy fins are fairly subtle, Mal's a bit more assy but there is some similarity in the basic foil shape to my eye.
Best to get your eyes on them yourself when Chris gets back....
Looking at rake angle, I wonder what we can deduce from these pics of '80's and early '90's speed fins?:
www.seabreeze.com.au/gallery/gallery.asp?imageid=6044
Mike,
Chris and Mal's fins are so slippery because they have a gradual , smooth acceleration of the flow over the front of the foil.
The more refined (in regards to laminar flow) the entry the more critical it is to get it EXACTLY right otherwise it will have drag numbers worse than a naca 00 series foil.
Unless you are planning to have a CAD CNC milled foil you'd be better going for a more conventional section.
I posted a photo of my fins,the trailing edge of the C3 is so sharp you could shave with it no joke.
I posted a photo of my fins,the trailing edge of the C3 is so sharp you could shave with it no joke.
www.seabreeze.com.au/gallery/gallery.asp?imageid=6047
quote:
Originally posted by yoyo
Mike,
-----
Unless you are planning to have a CAD CNC milled foil you'd be better going for a more conventional section.
What sort of shaping are you using? When using Xfoil Chris, Mal and myself have been able to get section drag number down quite alot going for our own inversed designs which ended up with max thicknesses back about midfoil. But I found if you move the thickness position a few percent forward or back you find the sectional drag can be much worse.
The further forward the thickness the less critical this effect. Boogies C3 used a modified Eppler design with the thickness at about 38.6%. It's pretty slick at most AoA. His X2 is 7.5% thick at the base (5% at the tip) but on investigation with Xfoil I found that the thicker X1 surprisingly has virtually the same sectional drag but is much stiffer and is probably the better fin overall especially for someone who wants to use a longer swept blade.
The drag difference of the complete foils is much less than the sectional differences as these are fairly low aspect wings. (sectional drag numbers are for infinite span)
The main advantage of Chris' and Mal's is the way they manage the peak suction curve to minimize the Cpmin and reduce and delay the onset of cavitation at mid 40 knot speeds. This is not a prob at mid thirties.
Thanks for the detail Dave, I'm still very much on a learning curve with this Technical/mathematical approach. I'm a shaper, not an engineer.
So in answer to this-----
quote:
Originally posted by yoyo
What sort of shaping are you using?
Some designs work. It may be worth copying the ones which do. I have a couple of old FinWorks foils, you know the grey VTX ones, which still work well.
