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sanding mast & foils - which grit to use

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StellaBlu
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Re: sanding mast & foils - which grit to use

Postby StellaBlu » Mon Mar 07, 2022 10:28 pm

kit3surfer wrote:
Mon Mar 07, 2022 10:00 pm
Thanks, and in case it becomes black dust, what is the consequence after stopping to sand of course. Will the carbon layer suck water and do I have to put on a kind of finish/spray afterwards or will it be also fine if I keep it like that?
Best case is you caught it in time and you just spray some additional clear coat and let it be. Worst case is that you have compromised the structural integrity and it needs a more involved repair.

I have learned that "less-is-more" when sanding expensive carbon/glass/kevlar/epoxy things... Reading between the lines, my sense is that advice might be too late?

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Re: sanding mast & foils - which grit to use

Postby evan » Mon Mar 07, 2022 10:37 pm

That's why you spray the entire foil with primer first. black looks best, but harder to spot if you sanded through the primer or not. Sand through the primer until you start to see the bare fibers abd stop there. Carbon/epoxy doesn't soak up water, so ok to sand down to bare fibers but obviously best to minimize that as much as possible because you don't want to alter the shape of the profile.

On moses/sabfoil be very careful not to sand through the glossy factory gel coat as that has very different properties to bare carbon, also possible to have a lot of tiny voids/bubbles under that gelcoat. If you do hit bare carbon it is best to remove the entire gelcoat and fill all the voids plus apply the primer before the final sanding pass. Quite time consuming hence the advice to be very careful sanding those foils with a gelcoat.

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Re: sanding mast & foils - which grit to use

Postby kit3surfer » Mon Mar 07, 2022 10:48 pm

StellaBlu wrote:
Mon Mar 07, 2022 10:28 pm
kit3surfer wrote:
Mon Mar 07, 2022 10:00 pm
Thanks, and in case it becomes black dust, what is the consequence after stopping to sand of course. Will the carbon layer suck water and do I have to put on a kind of finish/spray afterwards or will it be also fine if I keep it like that?
Best case is you caught it in time and you just spray some additional clear coat and let it be. Worst case is that you have compromised the structural integrity and it needs a more involved repair.

I have learned that "less-is-more" when sanding expensive carbon/glass/kevlar/epoxy things... Reading between the lines, my sense is that advice might be too late?
Thanks for your help. No, I hope it is not already too late. Wou,d it be possible to go through the carbon fiber with 1000 or 2000 grid paper when sanded with a machine/excentric?
As you see, my experiences with sanding are pretty basic and limited. Sorry for that many questions.

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Re: sanding mast & foils - which grit to use

Postby evan » Tue Mar 08, 2022 7:17 am

NEVER EVER use a sanding machine on your foil!

Take your time to gently sand it by hand and follow the profile and use a thin layer of primer to spot where you have or have not sanded your foil.

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Re: sanding mast & foils - which grit to use

Postby evan » Tue Mar 08, 2022 7:17 am

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Re: sanding mast & foils - which grit to use

Postby fluidity » Tue Mar 08, 2022 8:03 am

mede wrote:
Mon Mar 07, 2022 8:25 am
Hey everyone
I would like to sand my mast and foils, in order to avoid ventilation and increase speed.
Did my online research, but did not find consensus on what is the best grit to use at what places on the foil and mast.
Older posts seem to favor 1000 - 2000 p, newer ones rather 400 - 800 p.
Recommendations? I'm also interested in the reasoning behind those.
Many thanks in advance,
Mathias
Not sure about the race side but I've been making my own foils this last year and a half culminating with 3 new front wing designs and several new stabiliser designs, all axis black fuselage compatible and cast in carbon fibre this year. And with a model still to mould and another mould yet to cast carbon in. For me, I currently do a twill skin, minimum 2 layers unidirectional sandwiching a layer biaxial each side, with some odds and ends going into additional reinforcing around the mounting holes and middle section. Sanding with an orbital sander on 400 grit will deform small areas such as the leading edge, tips and trailing edge after not too long but on the flat areas it's going to take you an appreciable time to deliberately damage the surface significanly if you are sanding flat. With 800 grit and above, you could be sanding away for a very long time to cut through a layer. On my PLA plastic models I clean up the surface with 80 grit and higher, working my way up but the important thing is if not hand sanding, use an orbital sander. Start and stop it on the surface, DON'T let it build up speed spinning away from the surface, it will cut too fast then. Unless you are making your own foil fron scratch, NEVER use a belt sander or disk sander, they cut much too fast and need a lot of experience to control, especially on an already finished foil. Here in NZ, I can't get orbital sanding disks above 280 grit from hardware shops but auto shops have the finer grits available. I have nothing for my machine between 280 grit and 800 grit and the slowness of 800 grit is astonishing. I use a battery orbital sander and mostly wet sand, you do NOT want to breath carbon fibre dust, it's a nasty health hazard.

For my type of construction, the unidirectional carries most of the loads, compression at the bolts and over the top span and tension in the bottom skin. Double bias thickens the skins and provides warp resistance to the tips, the twill skin is to stop runs on the unidirectional, provide a pleasing apearance and keep everything underneath from splitting. You will find that the carbon fibre tow in the surface cloth is likely some 15 thicknesses + of carbon so it takes some time to sand through, but I do just this, tiding the "flash" left at the edges from my moulding process to complete the correct trailing edge shapes of my foils. You can get an idea of the amount of carbon in your construction(if there's no glass) by getting the volume from it's displacement and comparing that with it's weight. Googles give a density for prepreg carbon laminate of 1.5 x more dense than water so for example if your wing displaces 1 litre but only weighs 1kg then it's likely that a third of your wing is light weight foam core.

If you do sand through the epoxy into carbon(if there's a gel coat) then the appearance will change, look side on(even if you've polished it afterwards) and you will be able to see where the carbon is at the surface.
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Re: sanding mast & foils - which grit to use

Postby mede » Tue Mar 08, 2022 9:43 am

evan wrote:
Mon Mar 07, 2022 9:19 am
LE > TE
Submerged surfaces 2000 - 1200 - 800
Breaching surfaces 800 -1200 - 2000 (top mast, wingtips)
all cross sanded 45deg and use a spraypaint base coat before sanding to fill any imperfections.
The engineer part of my brain was thinking this over last night... :wink: :

Let's define optimal surface structure of the foil as the one that maximizes glide speed while avoiding ventilation.

Then
  • [2000 - 1200 - 800] makes sense under the condition that the optimal surface structure is a function of water flow acceleration.
  • [800 -1200 - 2000] for breaching surfaces doesn't make much sense, though...
First: Why would on the mast where it breaches the water pressure increase when moving to the rear? I cannot see how that would happen , hence I don't get this recommendation.

Second: the area of breaching is variable and will cover most likely more at least the top half of the mast, even with extreme canting angles (assuming a 100cm mast and a 60 cm foil). In chop, the breaching surface of the mast will most likely be up to 80% of the mast length.

Conclusion:
  1. if 800 is indeed the optimal grit for breaching surfaces, this means that a very large part of the mast (front side, rear side) should be sanded with 800 grit, with the part where profile is largest should be sanded with 1200. Only in the lowest part of the mast the front side should be sanded with 2000.
  2. the wings should be sanded with 800 at the tips, pressure side, and the rear, with 2000 at front part and 1200 on middle part of the suction side

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Re: sanding mast & foils - which grit to use

Postby Nelis » Thu Sep 22, 2022 2:05 pm

evan wrote:
Mon Mar 07, 2022 11:24 am
LE of the profile 2000 grit, middle 1200, TE 800 for least drag.

The reverse for least ventilation on the surface piercing profiles, some even do a single perpendicular pass with even coarser grit just on the first cm of the LE to reduce the ventilation even further.
Hi,

Sorry to drag up this old thread. My wings are in need of some TLC I think. It's a Levitaz 96cm carbon mast and Cruizer/RW240 setup. Symptoms are sudden loss of lift and a 'gurgl' sound when cornering hard, also sometimes during upwind course when a rolling swell passes. At those events I am pretty sure the foil is not actually breaching, yet I can sometimes suspect the event coming up, though I'm not sure why. It would be cool to gain some speed and stability, let's see.

The mast looks as good as new, but I will do a filling layer and sanding regardless. The wing and stab need filling scratches and sanding.

I went ahead and ordered colored putty, spray putty and sanding paper for the job.

I wanted to ask for some clarification on how to approach the transfer sections between grits on the wing, so I made a drawing for the front wing:
Foil tune grits.jpg
The stab should be treated as fully submerged, and the mast as fully breaching. Are my interpretations correct?

Many thanks in advance! I'll post pictures of the process afterwards.

Gr, Nelis.

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Re: sanding mast & foils - which grit to use

Postby evan » Thu Sep 22, 2022 9:08 pm

No need for a transition zone, also consider the bottom of the mast submerged.

In addition, don't nerd out too much over this on a slow hydrofoil that is never going to exceed 25kn, above that is the territory that a perfect finish becomes vital. And only if you ride powered with the foil angled as much as possible.

I won't say that it doesn't help, but it's already a big improvement if you get rid of all the scratches and give it a smooth matte finish of 800grit. You can always do the 1200+2000 pattern afterwards as a bonus finish.
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Re: sanding mast & foils - which grit to use

Postby mede » Fri Sep 23, 2022 2:35 am

Based on my own experiences here, I would also not overcomplicate.
Carefully sand everything with 600 or 800 grit. Make sure not to introduce irregularities when adding putty /filler.
Depending on the current finishing, you may not need filler to apply, if you sand the top coat.
Test for water adherence while sanding - if the water sticks uniformly to the foil and mast, you sanded enough. When the water still peels away, sand some more.
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