Painting Foils

Me and the missus had a long conversation about this the other night with my POV being that a rougher surface induces laminar flow and hers being that any laminar flow is killed by skin friction - actually it was more like hydrogen bonds or something but she a scientist and I'm an Contender sailor so I'd switched off long before I understood it.

The outcome was that a rough surface probably didn't help much and possibly absorbed more water anyway, so go with smooth. We breifly discussed polished surfaces by I ruled this out some months ago when I slid off a recently polished centreboard and threw a race with an inversion when I could have just about won it with a quick recovery .

Even if your foils are designed to run laminar - and not all are, Laser for instance - they will only do so over a tiny percentage of the surface unless the finish is absolutely perfect, ie polished to a mirror finish. The rough surface induces turbulent flow very near the leading edge, so the lower drag of the laminar flow section is lost. Under turbulent flow, the surface is "scrubbed" by the fluid in the boundary layer, and this is where the higher drag of turbulent flow comes from. It goes without saying that the rougher the surface, the higher the scrubbing forces will be, and therefore the higher the drag.

If you think about it, if you slide off your centreboard, the water will too! Intuitively, it's clear that a highly polished surface gives less drag than a rough one. The real world situation is complex to explain, but we don't need to explain it. Trust your intuition! If you want the absolute maximum performance from your foils, keep them scratch and ding free, highly polished and perfectly clean.

This is all based on my hazy memory of my Physics degree from 20 years ago, but I also dug out Bethwaites book, and he did loads of practical experiments that seem to suggest that the performance gain is much greater than I thought it was, so maybe it's worth going to all the trouble after all.

And in case you're wondering, it was work all this out or watch "I'm a Has-Been".

Neil

I still disagree, of course a matt finish foil is perfectly smooth, you still wet and dry it as much and put just as much work into it! its not completely matt, just not polished shiny. Where is Andy P to back me up....

Edited by tgruitt

I was horrified to find my rudder had tiny little pimples on it.  What am I doing wrong.  Clearly it needs a repaint?

Edited by Prince Buster