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types of hydro foil finishes

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Created by junglejim1971 6 months ago, 21 Dec 2021
junglejim1971
VIC, 124 posts
21 Dec 2021 6:16AM
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Hey all Merry Christmas to you all,

Im just looking for opinions on the pros and cons of matt foil finishes verses shining smooth finishes i have been told that wings finished with a wet sand 800 grit works for foil racers produced the best result but i would have thought that a foil so smooth that its like a mirror and then waxed to repel water is better but i have been told differently and given no explanation why, so i have sent an email asking for an explanation and in the mean time im asking you guys

I thought moses foils are shiny smooth finish and axis foils are mat finish

i ride axis


James.

Kamikuza
QLD, 6367 posts
21 Dec 2021 7:30PM
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Read somewhere that the finish affects speed, and that the racer boys are going with matt finishes.

I wasn't really paying much attention though so don't trust me

My Axis foils gradually get shiny at the tips though, guessing the water is polishing them more as they move faster...

junglejim1971
VIC, 124 posts
22 Dec 2021 9:02AM
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yep I've been told the same by Axis, apparently polished wing repel the water which is not the best for racers they want water to stick to the foil not repel it, there must be some balance between drag and flow.

As for my mast I have the axis red mast which is polished by comparison to the foils and i assume I don't need water sticking to it but I'm sure the carbon masts are finished in the same matt finish as the foils so that would not be a good thing for racing ?

I'm just a hack so to me it doesn't matter either way i like a polished foil and mast as i can see the condition better.

derek72
WA, 39 posts
22 Dec 2021 10:48AM
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I'm not sure exactly how the science works but by sanding most racers are looking primarily to avoid ventilation and also to increase "grip" and stability so you can push harder and achieve higher speeds. If the foil is not matte finish there will be more chance of ventilation, often caused by air traveling down the mast or from a wing-tip near the surface sucking air down a channel and causing the wings to destabilise which is a constant battle for racers. I guess the finish and sanding are more about how air and water stick the surface and avoiding a surface that allows air to travel along the surface in bubbles but with good fast grippy water flow. Newer race foils also have a shape to their wing tips that repells air rather than sucks it down the wings which means you can now turn with an exposed wing-tip where wiht older generation foils that would have caused a ventilation.

With my moses 633 I ended up sanding the bottom of front wing and rear wing and kept the top all shiny and painted, I'm not even sure if it helps, sometimes I find the 633 ventilates heaps and other times not at all. Usually its about how hard you are pushing it upwind.

junglejim1971
VIC, 124 posts
22 Dec 2021 1:55PM
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Its a pretty good thought Re sanding the bottom only I'm sure there are stats on that somewhere and im sure i never would have thought of that a compromise . I think I will go back to the matt finish for the foils at some point i will just see if there is any noticeable difference to me after all I'm just a hack lol if i cant notice anything i will keep the polish so i can easier see for areas on the foil that need attention.

Kamikuza
QLD, 6367 posts
22 Dec 2021 8:40PM
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junglejim1971 said..
yep I've been told the same by Axis, apparently polished wing repel the water which is not the best for racers they want water to stick to the foil not repel it, there must be some balance between drag and flow.

As for my mast I have the axis red mast which is polished by comparison to the foils and i assume I don't need water sticking to it but I'm sure the carbon masts are finished in the same matt finish as the foils so that would not be a good thing for racing ?

I'm just a hack so to me it doesn't matter either way i like a polished foil and mast as i can see the condition better.


Carbon or aluminum mast? I would have thought the aluminum anodizing is more of a matte finish myself... maybe satin

Mine have worn the anodizing thin at the LE and have funny patterns up and down it. I remember the racer guys also freaking out when people touch their masts on the beach because greasy fingers or something, so I give it a quick wipe with my towel...

junglejim1971
VIC, 124 posts
23 Dec 2021 8:41AM
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my masts are all ally I can see how racers don't want finger prints on their gear takes time to clean, i don't even want people touching the waxed surface on my car lol

dachopper
WA, 1645 posts
23 Dec 2021 5:17PM
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I did quite a bit of" looking into" this area, which got me 33 kts on an aluminium mast the manufacturer told me would be maxed out around 25....

All I can say is reducing drag is king, and there are "coatings" which are banned, that effectively do that. In reality all the coatings do, is lay down particles that are a particular size, because it the surface is covered with particules of the " right " size, the foil becomes super hydrophobic. Interestingly, that should mean that between the particles there is an air gap that water doesn't touch, and so the water only touches the edge of the cliffs in either side so reductiong in drag = reduction in boundary layer thickness.

To complicate matters, the correct particle size to do this depends on a bunch of factors - one of which is the pressure from the water.

I tried something like, 1200 on the leading edge, reducing to 800 in the max camber section, as the pressure inverts, then 400 on the trailing edge. 400 is also better for avoiding cavitation moving around the mast ( where ever the surface normally touches the mast. ) Sand from front to back, only 2 or 3 passes necessary. You will feel a difference here.

junglejim1971
VIC, 124 posts
24 Dec 2021 3:47PM
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sounds interesting. Im sure axis dont go to that length on their foils a pic or 2 of this result would be cool if you can do it?

dachopper
WA, 1645 posts
24 Dec 2021 3:03PM
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Here is the article that gave me the idea

iopscience.iop.org/article/10.1088/0022-3727/43/4/045301

No pics sorry - but you can't see anything in the pics either, just looks like another mast / foil.

AndrewUA
WA, 6 posts
24 Dec 2021 4:39PM
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Hi there. Without getting into physics of the process... rough surface will be better option as liquid will flow around moving object much closer to laminar state. (Less little whirlpools that actually take some energy from moving object) Similar to shark skin which is pretty much like 100 grit sandpaper when you touch it. Same principle humans now using for plane paint to reduce fuel consumption. Glosssy finish... Looks more expensive :)
Cheers

ActionSportsWA
WA, 855 posts
19 Jan 2022 10:51AM
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Hi JunhgleJim,

The finish on a surface moving through water determines the drag. For your average Joe, any finish will be fine, rough, matt, satin or gloss, makes little difference. Racers don't sand with 800 grit, that's too rough. I finish my foil with 2000 and sometimes 3000 grit. These are finishing grades of paper.

Using finishing grade paper smooths out the surface and makes it shiny but not really glossy, it still leaves micro scratches which allow the water to adhere to the surface. High speed foils that have a high differential pressure from one side to the other, like a foil racer pushing hard upwind, create a very low pressure on the upwind side of the foil. If the foil is glossy or has an oily fingerprint on the low pressure side which repels the adherence of water to the surface, it invites the water to detach in the spot affected. If there is any imperfection in the mast or a water disturbance, or a tiny piece of weed which can allow a rapid change in pressure or disturbance in the delicate nature of the laminar flow of water at speed passing over the foil and mast, air can flow down the mast to fill the void. This is known as a ventilation.

The ventilation can be a microsecond "kick" from the foil as it re-acquires water adherence, or can be a totally wild full on ventilation leading to an instant high speed crash. The events I'm talking about are only when pushing the limits at high speeds. Tis isn't a worry for most foilers, even at 30 knots. I have ridden aluminium mast foils with rough finish to 30+ knots without ventilation. The Alu masts flex and are less stable at really high speeds.

So to answer question about gloss, satin or matt? Choose the one you like the look of... unless you are racing

DM

dachopper
WA, 1645 posts
24 Jan 2022 3:25PM
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junglejim1971 said..
sounds interesting. I'm sure axis don't go to that length on their foils a pic or 2 of this result would be cool if you can do it?


I think there is polish, and then there is polish. All polishes are different.

My 2 cents, I think you will find that all racers sand in the direction of water travel. By doing this, they are in effect creating a micro groove in the foil, kind of like a micro " rudder" because water in contact with the groove will have some propensity to flow along the groove, since going across the groove, is higher drag and requires more effort from the water. Hence where they talk about " gripping " the water, they are referring to the tendency of the water to follow the same path they have used the sandpaper.

Polish is typically done randomly in circles, so there is no " grooved" path for the water to flow down, and perhaps this allows the foil to slip sideways to the flow more easily causing a feeling of less stability.

On the sandpaper grit size that you need, at the surface of the water with zero change in pressure that is not in the high or low pressure area of a foil ) the grit that reduces drag the most is around 400. On the leading edge of the foil, up until the point of maximum camber, the pressure is higher here, so the gritt size needed must be smaller. Plus on top of this, as you go deeper, then the optimal gritt size needed also increases.

For me, I used around 400 on the trailing part of the mast ( back 1/3, then around 800 at the point of maximum thickness then high grit 1500 or so for the leading edge and where most compression occurs).

Then you use the same mentality for the wings underwater, except they need a little high gritt due to increased water pressure.
Best to experiment with your setup, we are not talking sand the mast into oblivion, just 1 or 2 light passes with wet and dry, and plenty of water is enough to make the grooves.

dachopper
WA, 1645 posts
24 Jan 2022 3:39PM
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A little Data to back up the science:
www.researchgate.net/publication/230958162_A_novel_and_inexpensive_technique_for_creating_superhydrophobic_surfaces_using_Teflon_and_sandpaper


books.google.co.uk/books?id=mhm4DwAAQBAJ&pg=PA255&lpg=PA255&dq=do+boats+with+high+contact+angles+have+less+drag&source=bl&ots=-20dYH6VGo&sig=ACfU3U3MOzncRIGb7GTnRQhzGlKxPVAxyA&hl=en&sa=X&ved=2ahUKEwjniZLF8cn1AhWRB2MBHZHtCxcQ6AF6BAgxEAM#v=onepage&q=do%20boats%20with%20high%20contact%20angles%20have%20less%20drag&f=false
And another one here, this one is more interesting because it includes a case study of a hydrofoil, and shows where you need to sand that profile to gain 26 % friction drag reduction and 19% pressure drag reduction. What it doesn't say, is what grit you need to use, to make it superhydrophobic.

ActionSportsWA
WA, 855 posts
8 Feb 2022 10:21AM
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Hey Dachopper,

For most freeride application of foiling, hydrophobic finish is fine and will reduce drag, but greater than 30 knots, you need the water to adhere to the surface otherwise it leaves the mast very vulnerable to ventilation. One greasy fingerprint from a previous application of sunscreen can seriously inhibit the ability of a fast foil mast to function correctly, and can even make it un-rideable as has been evidenced by some riders on the pro tour in the past. Once the foil is sanded and windexed to remove grease and oil, we will not touch the surface again until after racing.

On slower moving vessels or foils, hydrophobic solutions can dramatically release friction to allow boats to plane earlier and faster. I used to apply hydrophobic solution similar to rainex to the bottom of my VJ dinghy hull, it works great.

DM

warwickl
NSW, 1893 posts
8 Feb 2022 8:15PM
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The biggest drag my foil has is me

Fishdude
170 posts
12 Feb 2022 1:24AM
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derek72 said..
With my moses 633 I ended up sanding the bottom of front wing and rear wing and kept the top all shiny and painted, I'm not even sure if it helps, sometimes I find the 633 ventilates heaps and other times not at all. Usually its about how hard you are pushing it upwind.

The ventilation happens on the low pressure side, (basically stalling your wing or mast) so I'm not sure why you would sand the bottom high pressure side of your Front wing? Even on the top, only the "down hill side" is low pressure.

I'm also puzzled about the the direction of sanding, why would someone sand with the flow, or even 40 degrees to the flow? from what ive read it's Ribs that reduce drag and improve surface contact and they are perpendicular to flow not with the flow. Sanding with the flow seems like it would only create valleys that would increase surface water contact area. The right size perpendicular valleys or ribs allow the moving water to skip over some of the surface and keep water attached like the dimples on a golf ball. Seems to me that dachopper has a approach but i don't know enough on this subject.

The boat sailors seem to agree a 0.005mm surface as being good, which is said to be achieved by 400 sand paper.

My only concern is to sand the mast enough to minimize chances of venting while winging which is the only problem I've experienced. I'm going with Dachopper sanding on my mast. If I ever have venting on my wings ill look into at this some more, but for now 3000 grit perpendicular on all foil surfaces will be good until I learn more. I'm sure the billion dollar race cup boat crew have this all figured out maybe they'll share LOL

dachopper
WA, 1645 posts
12 Feb 2022 10:14PM
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Fishdude said..

derek72 said..
With my moses 633 I ended up sanding the bottom of front wing and rear wing and kept the top all shiny and painted, I'm not even sure if it helps, sometimes I find the 633 ventilates heaps and other times not at all. Usually its about how hard you are pushing it upwind.


The ventilation happens on the low pressure side, (basically stalling your wing or mast) so I'm not sure why you would sand the bottom high pressure side of your Front wing? Even on the top, only the "down hill side" is low pressure.

I'm also puzzled about the the direction of sanding, why would someone sand with the flow, or even 40 degrees to the flow? from what ive read it's Ribs that reduce drag and improve surface contact and they are perpendicular to flow not with the flow. Sanding with the flow seems like it would only create valleys that would increase surface water contact area. The right size perpendicular valleys or ribs allow the moving water to skip over some of the surface and keep water attached like the dimples on a golf ball. Seems to me that dachopper has a approach but i don't know enough on this subject.

The boat sailors seem to agree a 0.005mm surface as being good, which is said to be achieved by 400 sand paper.

My only concern is to sand the mast enough to minimize chances of venting while winging which is the only problem I've experienced. I'm going with Dachopper sanding on my mast. If I ever have venting on my wings ill look into at this some more, but for now 3000 grit perpendicular on all foil surfaces will be good until I learn more. I'm sure the billion dollar race cup boat crew have this all figured out maybe they'll share LOL


From what I have looked at, around 400 gritt is very good around the water / air barrier zone. It is superhydrophobic, and prevents onset of ventilation, I don't buy the " gripping the water" argument. I bet they are making the mast and foil superhydrophobic with the grooves in direction of travel providing the stability.


I don't recomment sanding 90 degrees to the flow.... have you ever seen an aircraft with features on it that are 90 degrees to the flow?
If you did, the only place I would do it, is whereve on the foil there was an area where the water ventilated.... ie not the leading edge to point of max thickness..... it would be behind that somewhere.

I had a shiny finished front race wing, and anodised aluminium mast, that I did my first sand job on it.... and it wasn't particularly noisy or unstable before except near the limit. After I sanded the entire thing ( mast and the front and rear foil ) under foot, it felt like ..... sort of like a violin. Like there was 5 or 10 noises it was making.... but I couldn't hear them, only feel this slight , tiny , tiny vibration under foot.

That pushed me from 28 kts, to 33 kts. Before when 28 kts was semi out of control, not it's in control and around 31 - 33 kts was the new out of control..... I havn't sanded is for a looooong time now, and all I could manage these days is around 27 / 28 again. That's ,multiple session on dead flat water going flat out onthe edge of instablilty

Fishdude
170 posts
14 Feb 2022 5:02AM
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dachopper, I certainly have more questions than answers on this subject for sure. I'm just trying to sort all this out the best I can.

The way I understand it is we want two things, reduce drag, and prevent ventilation.
Drag has two categories, Surface Fiction, and Pressure Drag or (low pressure drag).
From what a gather pressure drag has the MUCH bigger impact. Both on speed and ventilation and this is all about low pressure side of things.
Surface fiction is the bigger puzzle to me. I really don't get why the super smooth causes drag. From what i read it's super tiny air bubbles sticking to the smooth surface and causing a bunch of extra turbulence on hulls that stay submerged.

Reducing Pressure drag is the reason dimpled golf ball travels twice as far as a smooth golf ball. The dimples keep the air flow close to the surface longer than a smooth surface does. The golf ball dimple (and before dimples, crisscross grooves which looks like a very course sand paper finish) ADDS surface resistance to thhe front of the golf ball that slows down the ball, but the reduced pressure drag on the trailing edge way more than makes up for it. The dimple keep the trailing air flow closer. This is what I believe our goal is with sanding our foils and masts in the low pressure zone in particular for preventing ventilation.

The "ribs" like the dimples I referred to are about reducing pressure drag. I don't think most aircrafts have to worry about pressure drag except on the wings IF it approaches a stall angle/speed. Aircraft have a few other ways of handling flow separation (stall). But some aircraft do use tabs (vortex generators) before the low pressure area to "energize" the flow stream to better adhere the low pressure section of the wing.

Those parts I think I'm understanding. I think I'm starting to see how a 40 degree sanding direction could be better than 90 and even a crosshatch pattern seems like it could be good. (180 i still don't understand.)

The more confusing thing is why we call this sanded surface hydrophobic?
We are sanding to get rid of beading water, and then call the surface Hydrophobic? A Lotus leaf is a Superhydrophobic surface, where the water beads so much it keeps a almost spherical shape. It seems to me that we are going for the opposite, a more hydrophilic surface. Wanting the water flow to hover just above the surface. id think a superhydrophobic on the board that you want to separate from water would be good, like on a planning high speed boat hull. But on moving submerged surfaces we want water tension to go away, want the water to spread out like if we had soap on it which is hydrophilic right?
It seems if we wanted a Superhydrophobic surface, we could achieve that fairly easily with polishing and Nano coating... i think on a wing board bottom that might be a good thing just not on the foil/mast.

Seems like a lot of things going on to understand

dachopper
WA, 1645 posts
14 Feb 2022 1:26PM
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There has been quite a lot of literature on Superhydrophobic surfaces... It turns out it is more to do with the surface roughness than the chemical composition of the surface. There has been a load of experiments done, because as it also turns out, the size of the Gritt in a lot of retail sandpaper, also happens to be the perfect size that the surface roughness needs to be to make a surface superhydrophobic ( SH ). SH surfaces reduce the contact between the water and the surface, therefore decreasing skin friction drag, which also decreases the boundary layer thickness moving more towards laminar flow, which also reduces pressuring incuded drag.

Interestingly this paper here books.google.co.uk/books?id=mhm4DwAAQBAJ&pg=PA255&lpg=PA255&dq=do+boats+with+high+contact+angles+have+less+drag&source=bl&ots=-20dYH6

They did an experiment with a hydrofoil at 4 degrees angle of attack, and found the best place to coat the upper and lower surface to minimise drag. They achieved a 26 % reduction in friction , and 19.6% reduction in pressure drag, and they only coated the top back half after max camber, The front / top 1/4, and the bottom around the point of max camber / thickness.

That was done to avoid the cost of creating an entire object that was SH, as a lot of people use coatings that they have to renew.

Tell you what...I'll do some testing on my foil this week and let you know the outcome. I always find that is the best place to get answers.

dachopper
WA, 1645 posts
15 Feb 2022 10:57PM
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well....... I don't have answers..... just more questions.

Max speed twice over two days pre - sanding was 20.8 kts
I then tried 600 -> 1500 grit - 23.3 kts
Then today only 600 gritt -> 26.6 kts

Conditions identical for the 1500 and 600 test ( 8 - 9 kt average )
for the pre-sanding it was 10 kt average , and 8-9 kts.

Just got my sandpaper kit, so I'll go up somewhere between 2000-3000 for the next one.

Fishdude
170 posts
16 Feb 2022 8:03AM
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dachopper said..
well....... I don't have answers..... just more questions.

Max speed twice over two days pre - sanding was 20.8 kts
I then tried 600 -> 1500 grit - 23.3 kts
Then today only 600 gritt -> 26.6 kts

Conditions identical for the 1500 and 600 test ( 8 - 9 kt average )
for the pre-sanding it was 10 kt average , and 8-9 kts.

Just got my sandpaper kit, so I'll go up somewhere between 2000-3000 for the next one.



Very Nice, and that's sanding 600 with the flow.

FWIW If you want to try a true SuperHydrophobic surface, you could just treat it with the 2 part spray called "Never Wet". I don't think it last long, but it would be interesting just to see what happens.
I suspect treating your mast with Never Wet will cause ventilation issues but who knows.

1st minute this video is good, the rest not so much.

TomoT
VIC, 1 posts
15 May 2022 9:26AM
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Hi all, very interesting chat/read!

Did you guys test your theories and did anyone try never wet on their foil, board or hand wing?

dachopper
WA, 1645 posts
22 Jun 2022 12:08AM
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Interesting, I read this on the alpine foil page, about their matt vs gloss finished carbon foils

"AlpineFoil is the only manufacturer in the world to apply an elaborate finishing process on its masts and wings. Generally, most manufacturers apply only light sanding on the mast at the exit of the mold. Unfortunately, carbon fiber, because of its crosses, has much micro porosity harmful to the good flow of water on the profile, which creates multiple turbulences. To solve this problem, AlpineFoil fills these porosities in its manufacture by applying multiple successive layers of varnish. Between each layer, sanding is necessary: we return to the original profile but the porosities are gradually filled. To finish, on masts with gloss option, after the last coat of varnish, an AlpineFoil operator performs manual water sanding with P2000 and P3000. He then administers a final polishing with P16.000 with a polishing paste. The matte versions do not have this particular high-performance finish that refines the design of the product but also and especially its glide."



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"types of hydro foil finishes" started by junglejim1971