Why is bottom paint slow??
 

I used to think the answer was obvious: because it isn't as smooth. As an engineer, I should have thought that one through, but I've recently been told by several sources that you GAIN speed by roughing up the gel coat, which makes sense from a fluids perspective.

In fact someone advised waxing the bottom and NOT wiping the excess off to help protect the bottom from fouling. They said it wouldn't affect top speed. I tried it and it didn't hurt top speed as advised.

So why is bottom paint slow?

parasitic drag. but not to the extent that some people claim. it is the boundry layer flow that people talk about when they talk about " roughing up" the surface... why highly polishing an intake manifold interior surface is not the best way to go.

if you break it down to its elements you have two or three things going on.

surface friction and drag. this slows you down in exactly the manner that you would intuititively expect. smoother is better

shape and flow. this is drag associated with the way a viscous fluid moves across a non viscous surface... a wing , a hull or anything like that. in this case , generally , keeping the flow attached and non turbulant is a lower drag solution than a mirror finish that doesn't... that is to say if the shape promotes separation, then that will promote turbulance at the boundry layer which will both cost you energy and increase drag.

drag never sleeps.

and when you are talking about speed it is cubed. that means that the tinyest increase in drag has a huge effect on speed. and thats why it take BIG hp to go just a little bit faster once you get going 60 mph or more.

i believe a mirror finish would be a faster finsh on a boat. it always has been on the cars and airplanes simply because of the scale. i have used different surface finsh to reduce drag on various individual components by surface finish that were particularly draggy by virtue of their shape. a golf ball is the classic example of a shape whose drag is dramatically reduced by surface finish... but on a hull i would consider that a case where thats a solution on a broad flat surface at low angles of attack as opposed to a rotating sphere.

3m developed a hull finish for the americas cup boats about 20 years ago that reduced the drag very specifically in the speed range that they operated and even that was on the order of 1/10 of a percent but increased the drag at speeds higher than what they operated at.

Breaking the shine from the bottom finish reduces surface tension. If you take two materials, regardless of what they are, finish them to a micron-level surface finish and touch them together, they will have a molecular attraction to each other. An easy example where you can see this is with a glass of water. Fill a glass partway up and look through it. At the surface you'll see a small portion of water that is literally climbing up the side (meniscus). That's surface tension. Multiply that by however many square feet the bottom of your hull represents...

We used to have a surface plate here in the shop that was lapped to an exceptionally smooth surface. If you sat a gauge block on it, you couldn't pick it up. You had to slide it to the edge and off of the plate.

roughing up the bottom I doubt will do any good, if you really want to make a diff you would need the bottom to look like a golf ball surface, as that would keep the bottom airated, not real practicle but would work, the bottom paint itself just has lots of drag....Rob

What kind of surface does a submarine have?

I am sure the Navy has done some testing on this issue.

Chris

All of the above replys are exactly what I am saying, and still don't answer the question. If it's true that roughing up the bottom finish reduces surface tension (very much could be true), then why does bottom paint slow us down?

But if that is a myth, then leaving the wax on the bottom of my boat (very rough) should have really slowed me down (it didn't). Slightly rough should either be good or bad.

As the one post mentioned, the Americas Cup teams were looking for a surface finish at very specific speeds and conditions. What applys to a car in air definately does not apply for a boat in water.

Also, if slightly rouhg is bad, why can't bottom paint be applyed and sanded smooth?

It seems to be a matter of opinion whether roughing up the surface of the gel coat helps or hurts, but it seems to be completly undisputed common knowledge that bottom paint hurts speed.

Still not making sense.

There's a pretty significant difference between breaking the surface tension by sanding the finish with 320 grit and the hydrodynamic drag presented by the very rough finish of most bottom paints. A smooth but not shiny finish breaks the tension. The surface presented by anti-fouling paint actually grabs at the water as it passes over. If you painted a bottom with anti-fouling and then blocked it down level and to an equivalent surface texture, there would be no speed difference.

To the golfball dimple thing- aerodynamics is dramatically diferent than hydrodynamics.

And on Navy subs- when you have a reactor-powered turbine powerplant, a scant difference in surface drag is fairly easily overcome.

Sailboats are a displacement hull. The smoother they are the better they move through the water.

Our boats are planing hulls and need to get free of the water. Sanding gelcoat with 400 grit sand paper to releive surface tension is much different than bottom paint applied with a roller or brush. Can bottom paint be sanded down with 400 grit and achieve the same results....I don't know. Bottom paint is so taboo I don't think anyone is brave enough to try. Plus it would be very labor intensive.

The golf ball dimple has been discussed and thought to create suction due to the size of the dimple.

roughing up the bottom I doubt will do any good, mmmmm Ya think ???? try some 120 grit or speed coat and see if ya still got DRAG

Here is an explanation about wettability: http://www.rcboataholic.com/faq/hull_finish.htm

Don't know if it is true though.