Filleting a delta

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boardsurfr said..
>>>>- but one could envision different fin boxes that leave a bit of room for a fillet.

Now you've got me thinking, traditional boxes just don't suit leading edge projections. But with varying projection lengths and widths it's hard to figure out how to accommodate the variety.
So the standard box would have a recess the size of the biggest fin, and the smaller fins would include a plate to match the box recess.
Would that work????
Could still have a standard tuttle or powerbox base, just with a recess, 1 or 2 mm deep. Not ideal for a standard fin, but it would still work.
And the fins are going to take up a lot of room.

Can I be bothered modifying a board?? We'll see

As has been mentioned... a few percent reduction in drag, would be beneficial. Particularly for speedsailing where a few percent is everything. Obviously... "we need more analysis" specifically at windsurfing speeds, ie: 15 m/s - 30m/s (30-60 kn). But they did do analysis at windsurfing speeds -> 7m/s is about 14kn. **

It is pretty clear to me graph 12 shows - for planing speeds where our AoA is a few degrees at most - fillets are less effective. Once the AoA increases to wavesailing levels during a bottom-turn, then there is significant beneficial lift produced (graph 14).

** Granted that is about the minimum speed for planing -> but if you look at the graphs, the faster you go or the less AoA, results in less benefit.

That said, our own real-world testing appears to show a beneficial effect for some other types of sailing too:

a) When we sail in shallow water or weedy areas, we are using the end-plate-effect to increase the effective-length of the fin (as per other studies) and reduce tip-vortices. Since there are now less turbulence as a whole coming from the fin, the turbulence at the junction is a bigger percentage of that.

b) On heavily raked fins, we tend to sail with more AoA than vertical fins (eg: delta's are notorious for spinout due to *either* too much AoA or not enough AoA). The graphs show a beneficial effect of a fillet for larger AoA. **

Since we mention fin-length ... I would be surprised if a fillet added anything that you could feel for length-choice we make. I would expect a bigger impact [ of the fillet ] would apply to the total-projected-area (so surface-area + AoA) - that may alter fin choice.

Lets ignore "wave drag"... we are planing, so most of that isn't relevant as it applies to the transition phase from non-planing. Lets also ignore sail/sailor drag -> air is about x700 less dense than water, so if for no other reason the drag effect is about that multiple lower (there is also a differing opinion on how much effect a sailor-drag is significant). So let focus on the three main sources of water drag, 1) board contact and depth/impression 2) fin size/area 3) AoA.

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sailquik said..
Their figures of only about 3% overall reduction of drag (at surfboard speeds) is misleading for windsurfers. The opening paragraphs of the paper describe the 'degree' of planing of sufboards and it is much more in the planing/displacement transition stage than a high speed windsurfing board (not always a wave board though). The hull drag contribution is much higher in this transition zone, decreasing the contribution of the lower fin drag to overall drag. So fillets should have more (much more?) benefit to overall drag on a speedy windsurfer.

Ignore wave-drag. The faster a windsurfer goes, the less AoA we want... so much so that we use cambered fins and want AoA to be negative (or at least as close to zero as possible). The graphs show that the fillet is less effective at small AoA.

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boardseurfr said..
Interesting article. Figure 11 shows that the fin drag could be reduced by about 15%. The only source that I know that estimates drag contributions at high speed is at the Maui Ultra Fins web site; it has the fin contributing about 10% of the total drag, while the board contributes about 50% (mauiultrafins.com/wp-content/uploads/2015/10/How_fast_can_we_go_25_04_2017.edited-1.pdf). I have no clue if the number is correct, but it's clear that board, sail, and sailor also contribute drag.

As I started with - lets ignore sail/sailor, because I dont think there is as much sailor drag relative to everything else. I will qualify that with - at world-record speeds - a couple of 1% benefits all add up. But if the air-drag-reduction of some contraption, results in less board/rig control -> then it isn't worth spending effort on [ at this point in time ].

And I use history as an example here -> ever since speed sailing existed, people have been wearing special suits to make them go faster - with bugger all benefit. But... if we make the water smoother, we go much faster - for lots of reasons... eg: a) because we have more control over the gear, so more efficiency b) better/cleaner water-flow over the fin, so we can reduce its size or AoA.

=> given how complicated the field of knowledge is, we should just focus our grey-matter on the dominant sources of drag.

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mathew said..
Whomever thinks Delta's are awesome, needs to test out a few more fins...!

I also suspect - given that say a Delta is such a terribly hard fin to sail fast - the fillet is acting as a spinout-recovery mechanism. I haven't tried using one, so I dont know...

One might also say that whomever thinks a Delta is not awesome in it's intended use, needs to test a few!

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mathew said..
The faster a windsurfer goes, the less AoA we want... so much so that we use cambered fins and want AoA to be negative (or at least as close to zero as possible). The graphs show that the fillet is less effective at small AoA.

It is not 'what we want' , it is just the result of going faster.
All normal fins are 'cambered' ??
AOA is never zero or negative on a symmetrical fin when generating lift. It must be positive to generate lift by definition. otherwise it is only making drag It may be possible on an asymmetrical fin, but probably not normally.

The graphs show that fillets are less effective at small AOA only at the speeds and fin shapes tested. It is probably dangerous to extrapolate that to our environment without additional relevant data. And only 'less' effective anyhow.

No wind or surf up at Gero' Mat?

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sailquik said..
>>>where there is a strong feeling from a number of sailors that something positive is going on, especially with regards to spinout resistance (Often contributed to by a high AOA situation).

It's not just spinout resistance, the fin feels more positive, there's something there for the back foot to push against.
Before I filleted a couple of my fins they felt a bit like the rear screw was loose and air was being sucked down, killing the lift, and then spinning out if pushed too hard. After filleting, the fins felt rock solid, they can still spin out without warning in heavy chop, but are easy to recover.

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decrepit said..
Can I be bothered modifying a board?? We'll see

While you're at it, make the tail transparent, like Lessacher did once, and film what's going on

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sailquik said..

mathew said..
b) On heavily raked fins, we tend to sail with more AoA than vertical fins (eg: delta's are notorious for spinout due to *either* too much AoA or not enough AoA). The graphs show a beneficial effect of a fillet for larger AoA. **

Since we mention fin-length ... I would be surprised if a fillet added anything that you could feel for length-choice we make. I would expect a bigger impact [ of the fillet ] would apply to the total-projected-area (so surface-area + AoA) - that may alter fin choice.

Can you elborate on your points here please?

"b) either too much AOA or not enough" ? Huh

and the whole next paragraph? I don't see what you are getting at.

If you use a Delta, there is a fine line between pushing too hard on the fin, and not pushing enough... try to unload the fin, spinout... try to load it up, spinout.... there is some porridge for those that sail it just right.

The point was - everyone is talking like fin-length it king wrt the fillet - I suspect that aspect isn't even measurable. We should be talking about fin-area ... and that is because ...and dare I say it (because I haven't seen this idea anywhere) ... I think some type of "area rule" applies to fins+body within water [ ref: en.wikipedia.org/wiki/Area_rule ]

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sailquik said..
The graphs show that fillets are less effective at small AOA only at the speeds and fin shapes tested. It is probably dangerous to extrapolate that to our environment without additional relevant data. And only 'less' effective anyhow.

Words are hard - which is why some confusion with my statement of AoA.... ie: the fin can have negative AoA (ie centreline of the fin) and yet still have positive lift, due to camber (the non-centre-of-lift). I didn't say all fins have non-zero camber.

And similarly....words are hard -> that word "dangerous" means something different - but I understand the implication. In this case, while the surfing-fillet-data is very limited in the grand scheme of things -> it is good enough that an extrapolation is a reasonable thing to do. Indeed there might be something that the data doesn't show, like early-onset-cavitation due to accelerated water-flow - but given what we already know about fin-surface-preparation, accelerated-negative pressure gradients on the trailing edge, etc, -> it also seems reasonable that early onset cavitation, is somewhat unlikely for speeds under 40 kn { aka just a tripling of the graph to 20m/s } - more than that speed, is anyone's guess.

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mathew said..

ClausF said..
In very weedy flat water spots the classic MUF Delta work as its supposed to - and as long as its not too choppy I don't find it difficult to sail fast with at all (Vmax 2sec: 33.3knots)..

33kn isn't fast for a Delta, or pretty much any fin ( given not-too-choppy water ).

For example, 37kn 2-sec using a 66cm formula fin, [ ref: gps-speedsurfing.com/default.aspx?mnu=user&val=198098&uid=930&spotid=22 ]

But... increase the choppyness (is that even a word?), and 33kn is downright near impossible, for either the Delta or that Deboichet 66.

The point is, we cannot say "Delta is fast" without also running the same tests using other fins. Which is why the statement of "It's not just spinout resistance, the fin feels more positive, there's something there for the back foot to push against." is so interesting, because if Delta's can be made to have more control, then the extra production complexity might be worth the effort.

Ok, - my statement was not meant to scientific, what I meant was that the Delta is performing well in very weedy (and hence flat-) waters of course.. www.gps-speedsurfing.com/?mnu=user&val=107268&uid=2678

A more scientific approach would, as you say, be a 1:1 speed-tests with some upright fins that have the same surface areas.. Or better perhaps, to do multiple test on drag and other important parameters under standardized conditions in a laboratory..

But for an amateur surfer who tries to be as fast as possible for the giving conditions (!) and to have fun, the most important thing is often to be able to surf in all conditions, and the Delta makes that possible when classic weedfins can't cope with to much weed.

If the fillet can better the control of the Delta then why not, it will be more fun to use.

I have already described my experiences with the "classic" Delta under the Delta XT-50 forum.

I'm looking to get into some weed guys!! So I took your lead and had a go at some filleting....but not sure my handy work is up to scratch.

i thought I'd take the easy option and use aqua knead it, as suggested by a previous poster.

it went on ok, see result below






But when I started to sand it, I noticed an edge was starting to crack, it looked a bit soft. I think my mistake was adding some knead it to a section I'd already done a few minutes before. It must have been going off so the second bit didn't bond properly.




So I'm not sure how robust this is going to be especially when trying to pry a fin out of a fin box.

would it help is I add a layer of resin / Q cell mixture on top. Or should I just break that part off and fix with the resin/q cell. All suggestions welcome.

i also have another fin for my 115L / 73cm which I use with an 8m. This fin looks a monster at 43cm (no good for shallow water) so I was thinking about cutting it down and then maybe fattening up the leading edge with some carbon. Just to give it more stiffness and lift. I've overlaid the delta below just to give you perspective on size / area.
What do you experts think? Is it worth doing

+1 what Stu said.
I wouldn't wreck that makani fin, too valuable for an experiment. Get a 28 fangy fin for $100 and convert it like I have on the show us your fin thread. The 26 woody I made from a 28 works a treat with the 7.9 and 115 slalom board. Far better than any delta in a bit of chop and goes upwind great.

Thanks, I'll go and find some glass to add to my fillet.

Where do I get a fangy?

28 cm might be fine for me, I'm not sure my skill level is up to grafting the wood into the fin without consequences.

I have it on good authority that a fangy fin is better than a filleted delta, and much better than an unfilleted delta.

why are you using fins with this overlapping ?here in Germany everyone who had such a fin was very disappointed (including me) and we all put them away.
The reason was that it is very hard to prepare the fin that it fits perfectly into your board. Mostly there is 0.1mm space between board and the top of the fin, where weed gets in and kills the performance.

Don't you have similar problems ?

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WST said..
why are you using fins with this overlapping ?here in Germany everyone who had such a fin was very disappointed (including me) and we all put them away.
The reason was that it is very hard to prepare the fin that it fits perfectly into your board. Mostly there is 0.1mm space between board and the top of the fin, where weed gets in and kills the performance.

Don't you have similar problems ?

The cast aluminium versions have a 0.5mm overbuild on the leading edge of the weed scoop. This allows for some rocker, but ideally carefully hand-filing of the fillet fitting surface should allow a very tight fit, without exerting too much pressure on the board deck. My test - if dental floss fits underneath, it's a too loose fit. In my experience, the less thickness of the any gasket applied to the the fillet, the better, but in some cases a gasket it is a must. The photo below is not specifically of the fit, but gives you an idea of the tickness of the fillet at the peripheral. Tapping softly with a wood mallet and soft wood end grain tip will mould the malleable alumiunium edge even closer to the board surface if there are any high points.