Waxing the bottom of the boat

Breezer · Outlaw Joined: 17 Jan 2003 Posts: 246 City: Austin

Is it neccessary? If so, how do you wax the spots that sit on the trailor?

Swass · Guest

No. You don't, unless you have a lift.

Waxing the hull slows the boat down anyway.

Geoff Standish · Addict Joined: 13 Jan 2003 Posts: 843 City: Calgary

Why would waxing it slow it down? If anything it should make it glide through the water faster/easier? My one buddy waxed the bottom of his old boat and said he gained 4 MPH in top speed.

It's probably not neccessary but it isn't a bad idea. Even if you miss the spots around the trailor rails.

Swass · Guest

Care to place a wager?

Grouch · Posted: Apr 24, 2003 7:38 am Post subject:

I would believe Swass, that guy knows everything. Very Happy

Jeff Royal · Outlaw Joined: 12 Jan 2003 Posts: 125

I'd like to hear the explaination....not that I doubt.........just I would think the opposite.

Swass · Guest

Grouch is a wize old sage.

I'll let the debate rage a bit before I lay some fluid dynamics on you.

One caveat: I'm assuming the boat is new.

No takers?

Jeff Royal · Outlaw Joined: 12 Jan 2003 Posts: 125

I'm not willing to bet on it......sounds like you know what you're talking about Swass.

Troy · Criminal Joined: 16 Apr 2003 Posts: 79 City: Nor Cal

Jet Ski racers scuff up the bottom of their hulls so they go faster. The explanation I got was that the turbulence caused by the scuffing holds a cushion of water near the hull and water slips past water better than it does fiberglass. I don't know if thats true but it's what I've been told.

noneya · Addict Joined: 13 Jan 2003 Posts: 796 City: Roxboro, NC

Laminar flow vs turbulent flow seperation charecteristics?

Swass · Guest

Noneya is hot on the trail.

RYDOG · Outlaw Joined: 13 Jan 2003 Posts: 163 City: NOR CAL

Not this again... I rememeber this discussion from a long time ago.

Swass · Guest

Welll, that's never happened before, has it?

Geoff Standish · Addict Joined: 13 Jan 2003 Posts: 843 City: Calgary

I'm not doubting swass either, he's helped me quite afew times before. I am just confused.

Waxing the bottom of the boat would let the water slide with less friction. I don't think that is in dispute. But now that you mention laminar flow (I took a basic fluid dynamics course but nothing too in depth) I dont think that you want the water to slide over the hull. That would disrupt the water and you would lose speed!

I copied this off a website from some University proff, and it makes sense now.

"Wax tends to disrupt the boundary layer. I believe it is better to have the water adhere to the surface of the boat and have the water slide over the adhered layer like a shear layer. This will give a good boundary layer and lengthen laminar flow."

I think that there would be a point where it could get too abrasive (Like my buddies boat) and you would lose speed.

Thanks for making me think about it...

Sound Illusions · Posted: Apr 24, 2003 10:46 am Post subject:

Ok Im a FREAK" I wax the bottom of my boat! I don't care if it goe's slower. It looks nice and cleans up easyer. We all put so much stuff in our boat's weight, stereo, people, It seems kinda foolish that ones reply to some one asking about waxing the bottom of the boat would slow it down would even be a comment that would matter fact or not. I would think that the bad effect's of waxing (if it does indeed slow's down the boat ) by how much in our application (wakeboarding)? would NOT exceed the benifits waxing could bring. If I was to be in the market for a new boat and I looked underneath the boat and It was nice and shiny that says way more to me than "I didnt want to slow down my Boat" so i didn't wax it. I think this is a answer where to much information can hurt you. "Im in Ignoriante Bliss".
WAX ON BUDDY Very Happy

OttoNP · Addict Joined: 13 Jan 2003 Posts: 848 City: MI

I don't care enough to figure it out for a boat but here's an example about golf balls.

In anything involving fluids there is something called the Reynolds number, which is basically a ratio of inertia forces to viscous forces. The Reynolds number gives you an idea if the flow is laminor or turbulent. Laminor is like nice flow and turbulent is all crazy (common sense). Laminor flow will separate off a golf ball differently than turbulent flow. Laminor flow separates earlier and causes more drag than turbulent flow, so since there is less drag and the ball will go farther, which is why golf balls have dimples.

Ok, now I'm curious since a golf ball is a sphere in air and we're talking about a plate in water, also I'm guessing that the difference in roughness won't change the flow from laminar to turbulent. My initial thought is that since it's a plate and the flow is probably turbulent, it will be faster if you wax it, but we'll see...

The Reynolds number for my boat at 20 MPH is 5.04E7 and at 58 MPH it is 1.46E8. At a Reynolds number of aroud 3E5 the flow becomes turbulent, so the flow is most definitely turbulent in both these cases.

So, for a smooth plate the drag coefficient would be 2.35E-3 at 20 MPH and 2.02E-3 at 58 MPH. A rough plate would have a drag coefficient of 3.52E-3. A plate twice as rough would have a drag coefficient of 4.01E-3.

So, as you can see, your better waxing it.

Swass,
I would like some of that Dale's Pale Ale that is only in CO sent out my way...

Thanks,
Nick
________
Free kmart gift cards

noneya · Addict Joined: 13 Jan 2003 Posts: 796 City: Roxboro, NC

Yes but Otto, what did you use for your reference length?? I think you calculated this at the back of the boat, which would be turbulent, the ideal situation is to have laminar all the way to the trailing edge and then have turbulent (car spoilers are an example). In real life the flow will start as laminar and then transition to turbulent.

Oh yeah, I'm looking at this as a physics problem only, I would wax my boats bottom if I wasnt so lazy.

k2_mn · Posted: Apr 24, 2003 1:17 pm Post subject:

Otto,

What in the hell do you do for a living?

Nor*Cal · Ladies Man Joined: 12 Jan 2003 Posts: 9479 City: Sac

I am by no means an engineer nor have I taken any classes in this but...

Racing sailboats go through a great deal of trouble to fair their hulls finishing with a 1500 grit wet sanding.

Swimmers recently broke new records by wearing full body suits that trap water against the body.

Both those are general statements and I'm not well versed in this but Otto aren't you assuming that a streamlined boat hull is a equated to a Bluff plane of a golf ball?

Don't ream me just a question...

Wake upPp · Soul Rider Joined: 11 Feb 2003 Posts: 338 City: NOR-CAL

Swass, we're waiting...

dba4life · Outlaw Joined: 20 Feb 2003 Posts: 129

I guess I am taking "wax the bottom of the boat" off my things to do list.

Thanks for the info.

Phaeton · Addict Joined: 12 Jan 2003 Posts: 572 City: Redding

Waxing would fill in the voids making the bottom of the hull smooth.
In water that means drag. Graphite and Teflon seem to work the best to reduce the drag.

Nor*Cal · Ladies Man Joined: 12 Jan 2003 Posts: 9479 City: Sac

3M has teflon waxes right?

OttoNP · Addict Joined: 13 Jan 2003 Posts: 848 City: MI

Noneya,

The transition from laminer to turbulent is pretty quick on a flat plate. Also, with a Reynolds number that high turbulent flow would be expected pretty quick. Even if the flow was laminar that doesn't mean that a rough surface is better.

k2,

I'm a Mechanical Engineer

Nor*Cal

What you said,

"Racing sailboats go through a great deal of trouble to fair their hulls finishing with a 1500 grit wet sanding." supports waxing, 1500 grit wet sanding will make a surface incredible smooth, possibly smoother than waxing. By waxing you are just filling in all the tiny bumps and making the surface smoother and the same is accomplish by sanding with a high grit sand paper. By wet sanding you make the surface even smoother. If rougher is better why don't they use 25 grit dry paper? Also, swimmers also try to make their surface as smooth as possible, you don't see swimmers with hairy legs to roughing up the surface of their legs...ha...ha. Also, a golf ball comparision isn't appropriate at all since water is only on one side, I treated it as if it where a flat plane on water, which I think would be good assumption especially when a boat is on plane. This is how many fluid text books analyze boats.

You can never say that smooth or rough is always better, it depends on Reynold's number, the roughness, and the flow around the object. Under different conditions a smoother sphere would have less drag than a golf ball.

Nick
________
Chevrolet greenbrier

OttoNP · Addict Joined: 13 Jan 2003 Posts: 848 City: MI

Noneya,

If the flow could be made to be laminar instead of turbulent, the drag coeff. would be 1.87 E-4 at 20 MPH and even less at 58MPH, both of these numbers are significantly less than the drag caused by turbulent flow. This would be another reason in favor of waxing. The only reason rougher is better on a golf ball is because of the way the flow separates around the sphere. There is two types of friction for a golf ball, the friction which only accounts for a small part and the drag from the separation of the flow behind the ball.

None of it is too bad, let me know if you guys want to see the calculations I did...

Nick
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MUSCULAR DYSTROPHY ADVICE

Swass · Guest

It's a surface tension and laminar flow thing. It's all just an exercise in academics, folks. We're not talking about an appreciable difference in speed. Will waxing a new hull slow it down? Yes. Will you notice it? No.

The original question was, "is it necessary?" And, of course, the answer is "no." Why would it be "necessary?"

Now, about the guy who claimed a 4 mph increase: Since you mentioned an OLD boat, then yes, that is technically possible, but I suspect the majority of the increase was due to other factors.

For the record, I WAX MINE TOO! I don't do the bottom, but I wax the sides because it's easier to keep clean.

Leggester · Posted: Apr 25, 2003 6:05 am Post subject:

Swass/Otto are correct. Jet ski racers actually lightly sand the bottom of the hulls.

OttoNP · Addict Joined: 13 Jan 2003 Posts: 848 City: MI

I agree you won't be able to tell, but I'm sticking with the calculations I did and saying that waxing will speed it up, the smoother your hull the better. The calculations I did take the surface tension into consideration.

I want my Dale's Pale Ale...

Nick
________
COACH PURSES

Swass · Guest

Yeah? Well, I want bigger 'nads and raw talent. Wanna trade?

That's just it, Otto - there's a point at which it becomes TOO smooth.

OttoNP · Addict Joined: 13 Jan 2003 Posts: 848 City: MI

What are you basing this on? All the equations I've got say that for a plate with flow on one side the smoother the better and that laminer flow has less drag. Once you have flow on both sides separation becomes an issue, (golfball). But even then that is only under specific conditions. The dimpled golf ball has more air friction drag than a smooth golf ball, but, the way the airflow separates off the ball is different. The dimple's cause the flow to be turbulent and the air goes farther behind the golf ball before separating. The smooth ball has an earlier separation and this causes more drag. So for both cases you have air drag plus separation drag. The air drag is more for a dimpled golf ball but the separation drag is more for the smoother golfball. The conditions a golf ball faces are such that the separation drag is reduced enough to make the dimpled golf ball have less drag overall. These don't apply to boats.

How about I send some MI beer your way and you send the Dale's Pale Ale up? Man that's good stuff!

Nick

Leggester · Posted: Apr 25, 2003 9:24 am Post subject:

Otto, try basing it on the racers. Everyone lightly sands the hull to create the tiny air bubbles.

Coefficient of friction, IIRC. You need to actually break surface tension or you get a suction cup.

NAW · Posted: Apr 25, 2003 10:17 am Post subject:

I usually bring the boat to my work and lift it off the trailer with an overhead crane and wax it.....I'm a neat freak like that.

I thank all for the acedemic/fluid dynamics lesson....it really validated my decision of years ago to pursue a degree in Marketing Wink

OttoNP · Addict Joined: 13 Jan 2003 Posts: 848 City: MI

The below except is from a racing webpage, it centers on making the hull as smooth as possible, first by sanding and then by waxing, let's see your source...

I'm thinking it is just rumors and confusion, a surface wet sanded with 1500 grit sandpaper is way smoother than our boats if we just waxed them and this is what racers do. When you wax the boat you make it smoother by filling in the tiny holes. The has the same end effect as sanding with a fine grit sandpaper except that sanding will leave the surface more permanently smooth since the wax can come off. This is probably why racers wet sand with 1500 grit sandpaper instead of waxing. If they truly desired a rough surface they'd use like 50 grit dry sanding.

"Smooth and fair surface on the hull, centerboard and rudder. Fair is achieved by block sanding which was easy on Pilikia with over 1/4 inch gel coat and may be accomplished on later boats by local filling and sanding. The new boats shouldn?t require this. Smooth is achieved by sanding with progressively finer grit to 600 and the polishing with rubbing compound (not applicable to boats with anti-fouling paint). Put the greatest effort into the front third of all the underwater surfaces. There are proponents of leaving the surface wet-sanded with 500 or 600. I find this a high maintenance surface as it gets dirty easy. Pilikia is polished and I use Turtle Wax to clean and polish the bottom."
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TOLEDO TRANSMISSION

$-[]0[][]V[][]0-$ · Criminal Joined: 12 Jan 2003 Posts: 99 City: san jose

Waxing the bottom will increase your top speed, dont listen to swass.LESS resistance.Manufacturers recommend waxing the underside of the boat for best performance,but what do they know.

Dragonlady8 · Guest

Uhuh oh........them there are some fightin' words Shocked

You just hope Swass doesn't see your comment.