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.

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.

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.

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

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?

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

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.

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

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, 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

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.

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?