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Forums > Windsurfing General

Fin truths revealed pt2

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Created by Ian K > 9 months ago, 4 Oct 2011
Ian K
WA, 4120 posts
4 Oct 2011 8:19AM
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Having first demonstrated that you don't have to be all that concerned about getting your reboxed fin at 90 degrees -

www.seabreeze.com.au/forums/Windsurfing/Gps/Asymmetric-fin-ready-for-test-run/#640906

I moved on to see how critical the finish is to fin performance. This fin is a select medium weedie (30cm) that has been sheathed in carbon. The trailing edge has been extended. It's a decent sized weedie. It's an unfinished project, the carbon soaked up all the resin leaving the woven texture fully exposed, plus some discontinuities where the layers overlapped.

I'm planning to fair the profile, fill the weave, polish it up, but thought I'd take it out as is just to see if it would work. Gave it a quick sand of the worst high spots, mainly at the leading edge. I'd half expected to have to walk back. (Not a big problem in Lake Illawarra)

Ended up sailing all day with it. Top speed of 32.5 knots on a 64 wide freeride board with a nocam 5.7. Did an unofficial pb for the 1 hr. It cranks upwind.

It works better than it did as standard. It's a little prone to spin out at low speed but more secure as speed rises. The unmodified fin was the other way around.

Conclusions.

Finish isn't all that important up to the low 30s.
A weed fin must be stiff.
A little bit of weed caught on a fin is way worse than the worst finish.

(I will get around to finishing it, it should go even better)




FormulaNova
WA, 14893 posts
4 Oct 2011 8:44AM
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Ian I think you have invented the 'golf ball' of weed fins.

That's pretty chunky CF. You know you can buy it in different grades don't you

NR
WA, 517 posts
4 Oct 2011 9:17AM
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17 Down: Nile ?

barn
WA, 2960 posts
4 Oct 2011 1:48PM
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FormulaNova said...


That's pretty chunky CF. You know you can buy it in different grades don't you




This is 'Sputnik' grade. Every other grade is just for decorating gear sticks..

Although the above finnish does look utter rubbish, a Vac bag was apparently too much effort...

keef
NSW, 2016 posts
4 Oct 2011 11:45PM
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ian your on a winner i couldnt believe how that fin smokes to windward, are you sure you didnt do a copy of that weedy ive got in the van

Mark _australia
WA, 22821 posts
4 Oct 2011 9:25PM
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Keef - we could pass the hat around if you like?

slowboat
WA, 560 posts
4 Oct 2011 11:21PM
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just proves laminar flow can only give you so much... cos you aint gettin none with that finish

keef
NSW, 2016 posts
5 Oct 2011 10:39AM
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Mark _australia said...

Keef - we could pass the hat around if you like?

mark too late , its on its way to sandy point (speed week) donated by keef for the ulgliest board of the comp

Al Planet
TAS, 1546 posts
5 Oct 2011 11:46AM
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keef said...

Mark _australia said...

Keef - we could pass the hat around if you like?

mark too late , its on its way to sandy point (speed week) donated by keef for the ulgliest board of the comp


The van or the fin?

keef
NSW, 2016 posts
5 Oct 2011 1:01PM
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Al Planet said...

keef said...

Mark _australia said...

Keef - we could pass the hat around if you like?

mark too late , its on its way to sandy point (speed week) donated by keef for the ulgliest board of the comp


The van or the fin?


if i was down there AL i'd be comeing home with both the van and the fin

barn
WA, 2960 posts
5 Oct 2011 11:07AM
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Thought I'd throw my hat in the ring, with these Lake Illawarra weed fins..

12cm $3 non-identical twins.. The pins at the back are bits of nails..


This was an earlier Kneed-it weed.. I decided it was easier to build from scratch after this one.. Kneed-it is smelly and expensive..


yoyo
WA, 1646 posts
5 Oct 2011 2:19PM
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Ian, in regards to the tilted fin you tried earlier maybe the idea may work with a small fin at SP where it could be advantagous to have the extra bite coming back upwind with a 21.

Reminder of your observations...

On the starboard tack it is a little prone to spinout in chop, but once you adjust it's fine. On the gust where i did the "top speed" I was bouncing off the chop, getting a bit of air, but putting it back in the water OK.

Going back upwind beating into the chop on the "wrong" side it was very solid, didn't look like spinning out. The chop angles were wrong to try bearing away. And I'd thought it may not even plane on this tack!

Easy to trim on both tacks, wet between the footstaps, just shows how adaptable we are.

If you've got plenty of wind and worried about spin out - sailing on the "dig in" side might aid control?

Conclusions. If you are reboxing a fin and don't get it exactly 90 degrees - don't worry about it, there's nothing in it.

Likewise if the fin is a little loose in the box.


By the way I always thought turbulent flow was more important to Weedies..



slowboat
WA, 560 posts
6 Oct 2011 2:31PM
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yoyo said...

By the way I always thought turbulent flow was more important to Weedies..



I seem to recall a certain polished weed fin that nearly killed me at the end of a SP run by letting go at 43kts and instantly catching a rail- sending me through the rig (broken boom, sail trashed, mast snapped after half a run the next time it was used) ... was almost death by laminar separation. Roughed it up and it doesnt spin out any more.

OT but the cant on the fin in Ian's experiment is interesting. Initially I thought it would spit him off, but of course he would have just put more weight on the board instinctively to balance it... not surprised it was hard to notice that. On the off tack it wouldnt have mattered other than maybe sucking the tail onto the water- which would aid in keeping it from ventilating I would guess => less prone to spinout. Dipping the windward rail has a similar effect.

oldie
VIC, 356 posts
9 Oct 2011 12:41AM
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Back to first principles, Ian! Fins that cavitate happily.
Sailrocket flipped on a wave at 50 knots today, but they remain chuffed.
"We have just been watching the camera which we positioned over the foil and it's fascinating watching the ventilated cavities form and expand down the foil."
"We all figured that this new foil should start to work properly at around 40-45 knots. We are there now. This foil has no top speed limits other than it's own structural limits. All the issues that limited the outright speed potential of boats like VESTAS Sailrocket 1, Hydroptere and Maquarie Innovations do not apply to this foil."

dinsdale
WA, 1227 posts
8 Oct 2011 10:30PM
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When I was racing RC model speedboats we used very sharp, knife edged, wedge shaped fins and rudders, just like the big boys, on real racing speedboats. These were all the go on boats doing well less than 50kts - about 35kts and up. Has anyone tried this sort of fin for high speed windsurfers?

oldie
VIC, 356 posts
9 Oct 2011 12:52PM
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They might have to be towed up to speed.

Stuthepirate
SA, 3590 posts
9 Oct 2011 12:51PM
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Quick Clues X-word
7 Across: Shoves = Pushes
11 Across: Tender = Soft
20 Across: Mouth Fluid = Saliva

dinsdale
WA, 1227 posts
9 Oct 2011 7:56PM
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oldie said...

They might have to be towed up to speed.

Why? There are plenty of people, in this thread also, talking bout speeds in excess of that. I'm talking about those seeking outright speed.

oldie
VIC, 356 posts
20 Oct 2011 11:05AM
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dinsdale said...

oldie said...

They might have to be towed up to speed.

Why? There are plenty of people, in this thread also, talking bout speeds in excess of that. I'm talking about those seeking outright speed.

I was contemplating how the drag of a "cavitator" fin (twice as much?) would frustrate the initial acceleration of the existing "one design" speed rigs.

Let's see... Sailrocket runs 22 square meter sail, but it is actually a tandem.
Might need to fly a wingsail, which it reminds me of.
Now, there is an opportunity for experiment ( and personal disablement )

My low speed experiments with canted fins and offset mast bases only showed great promise in strong currents, not often found on speed strips.

oldie
VIC, 356 posts
27 Oct 2011 5:52AM
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Today had a dramatic confirmation of a rule that I follow- Better to end up with a fin base slightly recessed rather than even protruding even a tiny amount.
My longboard homemade fin base was 3 mm recessed so I had screwed on a flat bit of plastic 3 cm by 15 cm to make it flush. After hitting a sand bar I thought that I had lost the fin, could not sail with any back foot pressure at all without a sideways excursion. Bit of a mystery as it looked completely undamaged and tight. but the plastic strip was now protruding a half mm at the back. Chucked it away...

oldie
VIC, 356 posts
27 Oct 2011 6:07AM
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P.S.
Sailrocket went back to conventioinal fin after getting stuck at the "50 Knot Barrier", even with thier cavitator. Might be a characteristic of breaking water surfacw tension at that speed. Maybe the hotter the water, the faster you can go, unless you fly a bit.
Heated speed strips are needed...



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"Fin truths revealed pt2" started by Ian K