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.

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.

Bottom paint slows you down because it makes your boat look like a cruiser!:party-smiley-004:

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?

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.

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

Why would anyone paint the bottom of a performance boat.

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.

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.

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

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.

some super slick paints that may not hurt as much:rolleyes::drink:

Sometimes things in life are a compromise. I can't even imagine having to launch my boat every time I use it like most of you do. It's my summertime retreat (small one, yes) and I sleep on it often. No, I don't want a cruiser, as much of the enjoyment for me is walking out of a frustrating meeting, driving to the lake and taking a fast lap. Like PatM I've been pretty conscientious about pulling it every month or so to clean it.

I've heard a couple people talk about the gel coat absorbing moisture. One even quoted a couple hundred pounds??? if the boat is left in the water all season.

I'm doing some bottom work this fall and am using epoxy for the patches, so while I'm at it, I'm going to epoxy coat the bottom for water resistance. Would sure be nice to be able to do it with an anti-fouling paint instead, but I'd rather pull the boat to clean it than lose 5mph.

Pat: Any other info on the paint you used? Was it just normal Trilux bottom paint or something you had to order special?

The only paint you can use on a boat that's submerged is one rated for continual immersion. Epoxies, polys etc will come off like leaves in the Fall. Rustoleum 9100 has an immersion-rated activator, but I've never used it for something that was to be left in the water for long periods.

That's the first I've heard that. Had a very good conversation with a VERY experienced individual at WestSystems and they use straight epoxy for continual emmersion all the time, and have even run extensive tests for water absorbtion or delamination. From a theoretical perspective, water should have no effect at all on an epoxy coating.

We put a epoxy coat on the bottom of my Fountain hull in spring 2002 and the boat was kept in a well all summer for 4 of the past 7 years with no change in the epoxy bottom coat.
We rolled the paint on and tipped it to get a smooth final finish which isn't as smooth as the gel coat but very acceptable and much easier to maintain.
ed

No, it was not special order, however it was like twice the $$. Interlux Trilux is what I remember. It was paint recommended to me by the marina we kept our boat at. They used it on all the class racing sailboats and it was on the self. I remember it was vary thin and we had to put 3 coats on the first year then 1 coat each year after that. I remember it would rub off vary easily. I do not regret painting the bottom of that boat at all; it made maintaining the boat much easier.

More info:
http://www.jamestowndistributors.com...ne=1&page=GRID

I think it was the Trilux 33. It looks like they have others with Teflon

Do you remember if it was actually easier to clean off than gel coat? I would imagine I wouldn't have to worry about using acid to clean the bottom if it's epoxy coated. Unlike gel coat, a mild acid solution shouldn't effect epoxy.

I'll probably just try the epoxy for this year and see how I like it before I experiment with the Trilux. My intent is to spray it on and then sand it with 400grit blocks after I've blueprinted the bottom totally flat.

If you speak to any paint manufacturer and ask about water immersion, that's what they'll tell you. There are specific paints formulated for immersion. Regardless of how tough a particular paint is, if it's not immersion-rated, it probably won't stay on.

Bcarpman,
The bottom is easier to clean, seems some of the junk will wash off but any discoloration still needs acid which we use occasionally. Miy ull was painted with a Interlux epoxy product by a guy in Holland, Mi.
Remember epoxy is forever and Trilux isn't.
ed

They usually tell you that because they don't want to deal with some yacht owner putting it on and having issues. Plus, it comes down to the mixture that the painter uses. If the mix is correct, it will be fine. If it isn't correct, it's probably going to come off or at least bubble.

There have been a LOT of boats with Awlgrip or Imron on the bottom that have zero problems even staying in the water... IF they have been applied properly.

The theory that they can be submerged just fine is absolutely correct. A good product like Awlgrip/Imron is better in than gel coat in almost every way... the main one regarding water intrusion is porousness, or better put lack thereof.

Back to the topic at hand:
The bottom sanding thing is a load of BS in regards to any speed increase. Sailboat racers are the ones who started this by wet sanding their boats before a race. No actual increase has ever been seen... Most racers admit they do it because the other guy does it. The only thing it really ever did is play a mind game on the teams that didn't do it... it's tough to see a team wet sanding the bottom of a boat before the race when you aren't doing it.... it puts doubt in the other teams setups.

Hey,

What about speedkote? I have seen this stay on gelcoat that was totally immersed for 5 months every year. It only came off where it was abraded from the friction of use. Mostly on the edges of the chines. Kinda like how paint gets thin on an old muscle car that was lovingly waxed too aggressively over the years.

I was told it is incredibly toxic and requires fully ventilated masks to apply though.:cool:

Because its not made by Fountain!

ez question