600 Grit to Smooth Poly Yak Bottoms?

We used to use 600 grit paper to smooth the bottoms of our poly windsurfers about 20 yrs ago as the “theory” back then said the 600 finish would grip a thin layer of water and lube the surface contact. Any real science in this or a better grit/other to smooth out the bottoms & reduce drag on poly yaks? Thanks

303 makes them
go fast like the wind!

No comment on speed gains for poly
but if wetsanding them or any plastic the best results are obtained by working through various grits keeping each ( sand scratches ) perpendicular to each other finishing up with finest running along flow.

Long ago, at MIT, the coach had us
wet-sanding the shells based on evidence from the naval engineering department that the effect might be helpful at the speeds 8 oared shells could attain. I’m sure coachie wouldn’t have bothered if the science hadn’t been there, but we couldn’t detect any improvement. I wonder if anyone sands today’s composite shells? This effect may be more useful at planing speeds achieved with lots of horsepower.

Incidentally, this issue usually arises
when someone wonders if a boat hull that is all scratched, cut, and fuzzy has lost some of its ease-of-paddling. For this more marked difference, I tend to side with Eric, because I “smoothed” two composite hulled whitewater boats that had been scratched and fuzzy, and both boats felt somewhat easier to paddle, both in a straight line and even when drawing sideways over strong crosscurrents.

However, that smoothing took some money and a lot of effort, and I’m not sure I would attempt to smooth a worn hull on a poly boat. To do it, useful material would have to be scraped and sanded away, and I think the end result would not justify the effort. It is possible to suppress and partially smooth some hacked-up areas on poly boats using a hot iron, but I don’t know whether that might set up hidden stresses in the hull due to different degrees of heating and cooling.

You have to be Greg Barton to get a
measurable difference in a composite.Plastic-go paddle.

Drag reduction
Well, the no-slip condition in fluid flow says that every surface drags a layer of fluid around with it, rough or smooth. This is well-documented experimentally. The idea of roughening to reduce drag is based on the idea of modifying the velocity gradient in the flow at the hull surface. A reduced velocity gradient means less skin friction, and hence a reduction in drag.

However, in thickening the boundary layer to alter the velocity gradient, you are effectively dragging a thicker layer of water along with your boat while paddling. Water is heavy, so this will require more power to accelerate that thicker layer of water up to your boat speed. These are competing effects, obviously - obtaining significant advantage by this method seems farfetched. Depending on how long ago this theory was advanced, it may be out of date by now.

The theory also ignores turbulence, unsteady flow, etc. My suggestion is to try for the smoothest hull you can get. If you want to go faster, buy a boat that’s been designed using hydrodynamic principles to have reduced drag.