Blistered, dimpled, smooth--Which is faster?
 

After reading the thread on the Donzi and the blister problem I was wondering which surface is faster?

1)Blister-- and the tiny little bumps
2)Dimpled-- and tiny little dents like on a golf ball (less wind resistance and more distance)
3)Smooth--speaks for itself
4)Waxed smooth--some say is slower
5)Bottom paint--rough surface, slower

What do you think? Has any manufact tried different hull surface textures and how they effect speed?

Wouldnt it seem that the dimpled golf ball with less wind resistance is kinda like the step bottom with less water resistance.

And without a step, like you said, smooth, it speaks for it self. I have also heard some say wax is slower, but then you have "Hydrocoat"(sp), but does it really work?

And you would think there would be a simple answer to this.

When hit properly, the spin of a golf ball with the dimpled surface gains altitude giving it farther distance.
The bottom of a boat is not spinning (aside of a few that go too fast into a turn) and dimples create more drag.
Wax also creates drag.
speed gain???.........Wet sand the bottom of your boat with 800 grit paper.

I think the boat with the most horsepower will go the fastest!:)

Sand the bottom of my high dollar boat for, maybe, 1 MPH!!! I don't think so.

:eek: :D

The bottom of my wake board is dimpled:D Sand the bottom of a good hull I don't think so eather:cool:

Ok guys do not shoot me but the blow boaters sand to a perfectly smooth finish. They go up to 4000 grit on the sand paper and make it perfectly smooth. If you ever get a chance to take a look at a high dollar racer thake a look. The finish is nothing short of amazing.

Jon

I recall from my Fluid Mechanics course (don't quote me here, I fell asleep alot in that class :) ) that a dimpled golf ball travels farther because the dimples allow the air flow around the ball to become turbulent, rather than laminar, resulting in less drag. My only fear about creating turbulent flow underneath a boat would be that the flow of water would be aerated, and that situation might cause a handling nightmare.

As for Hydrocoat, there was a thread on OSO earlier this year about this product. I seem to recall that wax slows a boat down because the waxed surface creates more drag because you have the water surface flowing against the wax layer. The Hydrocoat product is supposed to allow the hull to pick up a layer of water, so that you have water flowing against water, reducing drag and increasing speed slightly.

Fluid Mech guru's, please correct me if I'm way out of line on this one :)

Racing_fan, you're really close with your statement. The dimples work twofold on a golfball. One, they do disturb the airflow and allow a greater boundary layer to form around the ball. This allows some air to stick to the ball creating air-to-air drag, which is much less then air-to-solid drag. Secondly, as DJD eluded to, it is this same adherance that allows the ball to generate greater lift while spinning and thus allowing is a longer range as it counteracts gravity pulling it to return to earth.

How, this translates to boats?...Well, a lot of paralells can be drawn and this also applies to aircraft. Next time you are flying near a wing seat, notice the little triangles along the leading edge of the wing right along the spar(also the center of lift). These are called vortex generators and that's exactly what they do. Turbulate the air and disturb laminar flow. Once again, this reduces the boundary layer drag. The motion of the air is not always paralell to the surface, some of it is rotating and away from the surface. The vectoral flow being in different directions changes the components of drag and ultimately reduces the sum of all drag.

Same goes with boats. You want a certain amount of the water, on the order of a few microns, to adhere to the hull. This eliminates water-to-solid drag and induces water-to-water drag. It's bordering on hysrostatic loading(like a film of oil on a piston), and helps lower the friction. Theoretically, this would lend itself well to sanding or some sort of irregularity. However, the reynolds number and density of water is orders of magnitude different than air and boating velocities are realms apart, relatively speaking. To notice the same kinds effects as aircraft, boats would have to be in the 400-500mph range.

Like most racers will tell you, the speed cotes, snake oils and sanding will barely account for a single mph...Worth it, not really?

Laminar flow is a good thing, allowing the boundary layer of water to remain attached to the bottom of the hull, which in turn reduces drag. Golf ball dimples are effective due to the round shape of the ball. As air passes over the ball at it's maximum diameter, the boundary layer tends to be pulled away from the ball due to the significant pressure differential. Dimples keep the boundary layer attached to the ball further back, reducing overall drag.

Consider the shape of a bullet. Very different from a golf ball. I don't recall ever seeing a dimpled bullet. Anyway, we're talking about boat hulls. Laminar flow is difficult to maintain, and it is difficult to design a surface that will maintain laminar flow in a resistive medium (water). Flow of water over the hull will turn turbulent very quickly. Of course, this means sanding a stepped hull beyond the first step is a waste of time as the flow is already turbulent.

My $0.02