Interesting Jacobbohme wrote:There are a lot of contradicting statements on the net.
Last year I did some tests, with four different products.
-Wet sanding with grit 800-2000
-Sailkote from Mclube
-Polish (slipol and slipoline)
-Aquaspeed and Sport Polish from Holmenkol (nano technology)
My conclusion was:
Wet sanding was best.
The two stage high tech nano Holmenkol product was the worst. The foil was slow and very 'erratic'
My baseline is now 1200 grit wet sanding (warm water 2000, cold water 800).
I have been out foiling with 2000 grit, with sanding residue still visible after the foil session.
I recorded some video of my foil from underneath the board.
https://youtu.be/raPbVsGPwU4
The foil was wet sanded and behaved well (no stall or ventilation problems).
I could feel a bit of instability, but the right side is sometimes very 'white' (small LE damage).
I believe you can see the area (white) between boundary layer transition (laminar to turbulent) and reattachment.
This area gets bigger at higher speeds, which fits the theory.
Reynolds number is around 850000 (15° water, 100mm chord, and 20kn) for the front wing. Compared to yachts and planning dinghies, this is actually pretty low.
Anything below 1 million is normally considered 'low Reynolds number' with 'laminar separation bubbles' probably the biggest concern.
Theories
1) The smooth polished surface might delay laminar/turbulent transition (fast), but leading to laminar separation bubbles (slow and erratic).
2) Wet sanding is hydrophilic(high surface energy) as opposed to polishing which is hydrophobic (repels water). I am not sure it is related to drag.
I believe Olympic and America’s cup level sailing use wet sanding. They even wet sand the hulls, and in the past they even applied washing up liquid afterwards.
I can't find the theory or tests that explain why it is being done at higher Reynolds numbers (hulls and bigger foils).
CFD or boundary layer theory?
Anyway, if it was not banned in 'racing rules of sailing' future racing foils would probably use riblet film (like sharkskin and speedo swimsuits).
Jacob
Curious to WHY you think some are better or worse, feel or measured max speed or ?
Regarding #2, I actually, like you somewhat suggest, think that being hydrophilic or hydrophobic does not relate to drag.
But could be important when talking about planing surfaces or partly submerged hulls - so the idea that a "normal" board or windsurf board has less drag when wetsanded might be correct (or not), but can not necessarily or not at all be compared to a fully submerged wing IMO.
Somewhat connected maybe, but still not the same.
So many theories but very few facts apparently
The reynolds numbers are not extremely high no, but in aerodynamics it is between the low 20000 and up to 100000 things change hugely, and for 200-400000 the flow has stabilized
Maybe the difference is a balance between late turbulent transition (low drag) and avoiding erratic separation bubbles (high drag) ?
Which means it could also depend on the exact profile and the precision, and it is not black and white as said
PF
