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Dr. David Fleming
10-20-2010, 12:49 PM
I am assuming that clean gel coated bottom maybe with wax is the fastest hull surface for Donzi?

Not a lot of discussion of bottom treatment and related speed here. In Michigan here folks are using a clear not sanding product for surface treatment called "Scatt" and claim a faster surface but it is flat finish. Any opinions out there before I make a mistake?

zelatore
10-20-2010, 12:59 PM
Well you see, it depends on what type of oil you're running and what prop you have....


:popcorn:

VetteLT193
10-20-2010, 01:17 PM
technically a matte / flat whatever surface is better than shiny. you can go too far however, so think wet sanded finish, not rough.

But, in our world as pleasure boaters, it won't make any real difference relative to all the other stuff. i.e., your time is better spent elsewhere.

gcarter
10-20-2010, 02:13 PM
What Bob said w/one difference;
a sharp corner at the transom/bottom corner is a good thing.
It would be hard to quantify, but larger boats will appreciate 2-3 MPH increase in speed.
BTW, I sanded the TR bottom w/360 (I think). Looks flat.
This is akin to placing vortex generators on the after part of aircraft wings...basically it rids the boat of boundary layer.
A smooth bottom and wax is about the worst you can do.

maddad
10-20-2010, 02:15 PM
I remember this one guy who had a special wax......

gcarter
10-20-2010, 02:25 PM
i remember this one guy who had a special wax......

lol!

gcarter
10-20-2010, 05:02 PM
I think it would be interesting to sand the bottom of a 22 w/something like 80 grit, first in a longitudinal direction and test, and then sand in a transverse direction, repeat tests.

Here's what I did on the TR;

http://www.donzi.net/forums/attachment.php?attachmentid=45519&d=1241273227

http://www.donzi.net/forums/attachment.php?attachmentid=45520&d=1241273227]

joseph m. hahnl
10-20-2010, 05:31 PM
Apparently you want water to stick to the hull so you get water to water contact:boggled:. This ceates less friction than water to wax contact:yes:. I guess with the flat finish it retains the water to hull contact better:crossfing:.

Barry Eller
10-20-2010, 06:00 PM
In nature...a sharks skin is rough...creates a buffer/turbulence against the water and makes them faster. On a boat it traps a small amount of air and reduces drag/friction.

tmdog
10-20-2010, 07:53 PM
In nature...a sharks skin is rough...creates a buffer/turbulence against the water and makes them faster. On a boat it traps a small amount of air and reduces drag/friction.


We have a winner.:kingme:

The Hedgehog
10-20-2010, 09:18 PM
We have a winner.:kingme:

Yes, the blow boaters wet sand their hulls. So do some of the race boaters. it adds a boundary layer. It works. It also attracts water scum. I will trade the .0005 mph for a clean hull.

Hp = speed!

Ghost
10-20-2010, 10:11 PM
I'm guessing a dirty hull is more noticeable than the speed difference, for speeds under 75 mph.











Hey, I said I was guessing.

Eplebnista
10-20-2010, 11:37 PM
Porpoise skin - really pricey but makes the tree huggers crazy...

Pismo
10-21-2010, 06:47 AM
I concur, a sanded bottom is fastest.

MOP
10-21-2010, 07:12 AM
Many of you know I have been pitching that a smooth bottom is slow, I know from my own racing experience. Some have read my posts and below is some input from an engineer I copied from another site

I would not wax the pad. Let me qualify this in that I have no data to back this up, but I am an engineer with some fluid dynamics background. Here is why I say not to wax the pad. When fluid flows over the surface at relatively low speeds, (speeds that a typical boat would run are relatively low compared to many other applications such as air over a jetliner wing), it flows in what is called a laminar flow pattern. Essentially it flows smoothly and evenly over the surface. This smooth flow has a particular drag coefficient. When the flow reaches a high enough speed the flow pattern changes to what is known as turbulent flow.
Under turbulent flow conditions the drag coefficient is significantly reduced as compared to the drag coefficient under laminar flow conditions. Here is where waxing comes in: Turbulence at the surface/fluid interface can be induced at lower velocities by roughness of the surface. In other words, a smooth surface would facilitate a smoother flow pattern and thus higher drag than a rough surface. As JB mentioned one of the best practical examples of this effect is golf balls. The first solid golf balls were smooth with no dimples. It did not take the pros of that day long to figure out that a ball that had been hit several time, scuffing up the surface, would fly farther than a new ball right out of the box. Pros began to use their practice rounds to hit all of the balls that they would use in the actual tournament rounds to scuff them up before the tournament began. Once the manufacturers understood what the pros were doing they began to produce balls with the pre-made "scuffs" and thus the birth of the modern dimpled golf ball.

DonziJon
10-21-2010, 11:36 AM
Just to muddy up the water, the Navys heavyweight Mark 48 Torpedo is painted with GLOSS green enamel. The nose is black rubber with a finish like an inner tube.

Rumor has it, in the final phase of the run to the target, it cruises at a little over 63 MPH. (I saw this on the internet) :wink:

BTW: The Navy fires and retrieves these weapons (w/o warheads) all the time during practice. They float to the surface and are picked up by surface ships or boats.

JustMercMe
10-21-2010, 05:39 PM
Been sanding bottoms on the smaller go fast outboards for years....Seems to work. Might try this also.

http://www.e-performancemarine.com/index.php?act=viewProd&productId=423

Been seeing a lot of this stuff lately on the pads and along the bottom of drag boats. A lot of the ODBA racers swear by it.:wink:

Donzi_Dude
10-24-2010, 06:24 AM
As JB mentioned one of the best practical examples of this effect is golf balls. The first solid golf balls were smooth with no dimples. It did not take the pros of that day long to figure out that a ball that had been hit several time, scuffing up the surface, would fly farther than a new ball right out of the box. Pros began to use their practice rounds to hit all of the balls that they would use in the actual tournament rounds to scuff them up before the tournament began. Once the manufacturers understood what the pros were doing they began to produce balls with the pre-made "scuffs" and thus the birth of the modern dimpled golf ball.[/COLOR]


so we have the answer. smooth-bad, sanded-better, dimpled-best!


:kingme:


given enough speed/horsepower of course.