stevesxm

its not a matter of making sense. it is a matter of litteral and scientific fact. the dynamics are controled by fluid mechanics and the formulae and practice that define shape and flow.

they are not opinion. they are litteral fact.

when you say a hull in water is different then a car in air you are dead wrong. and as an engineer you have to remember your training and recognize the error of that statement.

the two are exactly the same and are defined by precisely the same mathematics. the constants change because the the mediums are different but what works in principle in air is what works in water or glycerine or anything else. it is wy CFD or Computational FLUID Dynamics is the gold standard for predicting shape and flow in EVERY situation where drag is an issue.

the SPECIFIC answer will be different. a golf balls best dimple shape and density in AIR will be a different shape and density in water or glycerine but the FACT that a dimpled surface will be better on a rotating sphere to reduce drag will ALWAYS be true.

the same is true on the boat. IF... and i would suggest it is a big IF you could demonstrate that on a low angle of attack large flat surface, the gain from ' aeration " from a rough surface is greater than the loss of energy from parasitic drag then you MIGHT make a case for a non mirror surface. i have never seen a study or an example where that is the case as a matter of fact as opposed to folklore.

and the empirical proof is all around you. the airforce would buff the wings of the b-52s to a high gloss to reduce drag. every wind tunnel ive ever worked in demonstrated that a smooth surface was better for drag than a rough surface.

as for bonding effect of sub micron surfaces, that is also a matter of fact but is not a relevant example in this case. the loads you are talking about here are shear loads.... the abilty of two surfaces to move parrallel to each other. NOT tensile which is what happens when you try to get your gage block off your surface plate.

you don't have to believe me. just go to your shape and flow textbooks that you must still have on your shelf and read the answers for yourself.

i reccomend FLUID DYNAMIC LIFT and FLUID DYNAMIC DRAG both written by Dr. Sighard F Hoerner which are widely considered to be the authoratative texts on the subject.

you make statements that " this should have slowed me down and didn't and that was done and i went faster " which in a word are nonsense. you aren't conducting repeatable scientific testing in any respect at all... a 10 degree change in air temp or water temp or fuel burn off or 1/10 of degree in drive angle would all be enough to make any seemingly meaningful observation completely unreliable... nevermind the error built into even the best gps systems...

the science defines in absolute terms what is true and what isn't. everything else is just folklore and bullsht.

ZXXX Donzi

Bottom paint slows you down because it makes your boat look like a cruiser!

Velocity Vector

iTrader: (4)

I can't see it slowing you down in a 60-70 mph boat but in the same sentence I could see it slowing you down in a 100-120 mph boat?

Chris Sunkin

Let's bring this discussion back to reality...

- Breaking the gloss on the hull bottom will give you a small amount of speed. It's been proven time and time again.

- Bottom paint is just that- paint. Neither the boat or the water can tell that it has a chemical in it that repels critters.

- Therefore, bottom paint, smoothed to an equivalent finish to a deglossed gel, should yield an equivalent speed.

Velocity Vector

iTrader: (4)

[QUOTE=Chris Sunkin;2711616]Let's bring this discussion back to reality...QUOTE]

Why would anyone paint the bottom of a performance boat.

Chris Sunkin

You see it. Guys buy them and leave them docked.

And yes, I've seen bottom paint knock 5 mph out of a 65 mph boat.

SDFever

Yup, 5-6mph on pretty much any boat. Standard complaint from any owner who wasn't informed on this before he asked his painter...

My experiences have NEVER involved someone with a 60mph boat let alone anything faster.

ezstriper

on the sub thing, big diff when totally under water vs we are trying to get away from it...and everything I have seen is about 5 mph loss on everything with std bottom paint..now they have some super slick paints that may not hurt as much, just don't know anybody who's tried them...Rob

Pat McPherson

From my experience, this is absolutely the case.

Like most of you, I have tried just about everything to make my boat go faster.
Just keep the bottom of a gelcoated boat clean and that’s it. Compounding and waxing the bottom smooth will not make the boat go faster. Been there and tried that.
As for the bottom paint-
A few years back we owned a 1978 28' Cig. The boat was/is a tank weighing in at over 8000lbs. She had twin 300HP/350s hooked to TRS drive so her top speed was a little over 60.
We kept her in the water because towing her was not practical for us.
The first year she sat with no bottom paint and we pulled her out every 3-4 weeks to clean the bottom.
The following year we did the unthinkable and painted the bottom with anti-fouling paint. We used a paint that was normally reserved for racing type sail boats. Trilux I think was the brand. Anyway this paint was vary smooth by comparison to the conventional stuff and it would also wear away keeping most of the scum from collecting.
OK I'm getting long winded here but I can honestly say the boat was not slower with the painted bottom, if anything the boat was faster because she stayed clean.

RunninHotRacing163.1

some super slick paints that may not hurt as much