Bottom finish

Here's one to stir things up a little: I have read many different descriptions of how to prepare a fast bottom, some of which are contradictory in terms of what degree of smoothness to attain. When my boat (fresh water) has not been cleaned in a week or two, it has a thin coating of real slick slime on it that can be wiped off with a bare hand or a sponge, but that won't come off when it is sailed, even in a strong breeze. It feels much slicker than the bottom itself after it has been cleaned. I have heard people say that it is the same slime that is on fish, but don't know anything about that. I've read of guys wiping down with soap, vaseline, and more exotic treatments to make the bottom faster, and wonder if a minimal coat of this natural slick growth might not be about the same - in other words, what if a guy cleaned the bottom a week before a race, rather than the day before?

the slime

I think you have to sponge it off.

On a few boats that I have crewed on, we'll put on goggles or a mask and swim around down below and sponge off the morning of the race.

I think the slime sets up a field of stiction or friction that slows the flow of water near the hull of the boat down.

Regards
Rich
Neegee
E-23
Monroe Harbor, Chicago

bottom prep

Bob,
Here is some of the basic fluid dynamics behind the bottom prep game. There are two forms of Drag that apply here. Form drag or pressure drag and skin friction or viscous drag. Form drag is due to the basic shape of the object in the fluid flow (the hull in our case). Streamlining decreases this drag. I am going to assume that you are not going to attempt to change the shape of your hull, so I will drop this topic and only say that this is by far the dominant type of drag as far as magnitude goes. But since we cannot do much to change this drag we play with the viscous drag factors hoping to mimimize the effort of passage through our chosen fluid.
Before I continue I need to say that this is a very over simplified explanation that may have errors because of those simplifications (enough of the disclaimers already). I'll do my best to be brief.
Viscous drag has to do with the flow of water at the boundary layer near the hulls surface. Drag is produced by wave making flow separation. With a very smooth surface at the low reynolds numbers that boats experience the flow is very laminar (smooth and layered). This type of flow is very delicate and as the velocity increases it seperates easily from the hull forming drag producing waves. With a roughened surface the boundary layer is forced to become turbulent which is a much more robust flow that remains attached to the hull for significantly longer and thus produces less drag. This can be seen in a practical application in the dimples on a golf ball. A ball without the dimples would fly significantly less because of the higher drag.
Theory says that a surface that promotes turbulence would produce less drag. Of course as in all things there is a point where the roughness would begin to produce a much thicker boundary layer that would begin to affect the pressure or form drag.
That's the theory anyway. I know of sailors that lightly sand their bran new glossy bottom finish with hopes of producing a slicker bottom.
By the way, this is a perennial and controversial topic in many sailing forums. In other words, who knows. Personally, I think that one messy tack would defeat all my carful bottom prep by at least one order of magnitude. Thanks for listening.