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Blistered, dimpled, smooth--Which is faster?

Old 08-26-2002, 10:43 PM
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Yeah, freq, I was thinking that laminar was the "fast" flow and turbulent was the "slow" flow.

Seems that many, many NACA-shaped dmples were tested on an airfoil years back. The dimples kept pulling the boundary-layer air back down to the surface of the airfoil, instead of separating as would happen on a normal (smooth) airfoil. Since the boundary layer remained stuck to the airfoil all the way around to the trailing edge, there was no turbulent vacuum pulling at the rear of the airfoil. This, in turn, created a drastically more efficient surface, both capable of generating more lift AND being much easier to puch through the air.

Granted, this all applies to AIR, and differs a bit from water.

The airfoil was held back from vacuum produced by a negative pressure area behind the wing derived from the separation of the airflow.

A boat hull, correct me if I'm wrong, is held back NOT from vacuum, but from "hull-to-liquid" friction. With this in mind, it seems that a hull would be easier to push across water, if it touched LESS water. Steps do this.

If a hull is smooth, then you got a certain area of hull touching water. We still don't know if that area of hull moves easier through/across water if it is blistered, dimpled, waxed, or sanded.

A blueprinted hull has the "waves" filled and sanded, creating a more perfectly smooth surface. Makes a difference on little outboard dragboats, so I'm guessing that it also works on a bigger hull. This would imply that a smooth (perfectly smooth) surface is faster.

Wax slowing boats down seems odd. Wax causes water to "bead up" because it keeps the water from sticking to it. Just look at a water bead on a waxed surface. Wax keeps the water from chemically/molecularly adhering. Seems that this would be faster (I'm not suggesting that is the case, just saying that it seems so).

I'm open to more theories...
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Old 08-26-2002, 11:33 PM
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See you have to look at the blow boaters for this one. One mile per hour to us is no big deal. One mile per hour to them is HUGE. The more perfect that they can get the bottom on the boat they faster they go. The finner grit that they can sand it to the faster it will go. They used to sand with 600 grit and attempt to get the layer of water to stick to the boat. Now they are finding out that going with the much higher grit papers. IE 4000 grit and higher that they are getting more speed.

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Old 08-27-2002, 12:12 AM
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Here's a few thoughts that I jotted down on a napkin some time ago while whittling match sticks and swizzling a brew or twew.

Wake boards are dimpled. But is that so that they glide easier or stick better for more control. And do they actually work?

Many of the OB 1/4 milers directionally sand the last quarter or third of their hulls for less drag. Does it work? Me thinks yes. They spend a lot of their time working with solid numbers to back up their work and don't hesitate to abandon something that doesn't work!

Blow boaters are defiinately fanatics of some kink or another and more of them proportionatly ought to be slide rule and pocket protector freaks than we could EVER think of. So much can be said for their obsession with smooth bottoms (on second hand... I kind of like that thinking!) may have credence. I think many of them do go with the various bottom coatings.

Another thing about the bottom coatings if you search the info on one or two of their web sites. The largest gains are had in the lower 0 to 15 mph part of the speed/drag curves. Above that, the return diminishes rapidly.

I believe the problem with wax has to do with "surface tension" hence the "bubbling" effect in the car's hood demonstration. Another word would be "hydrophobic". The speed coatings all promote "hydrophylic" properties. IOW... water adheres in a flowing manner... back to the water-water drag vs. water-solid arguments.

Then the shapes of bullets. The old traditional shapes are square-based heels. The "hi-milage" versions are "Boat-Tails", which does bring us back to things nautical. Lately and especially in the 1000 yd benchrest and Hi-Power classes, VLD or Very Low Drag models are the kings of the target pits. They are very long for thier diameter with a rebated type of heel and are heavier than normal increasing their momentum. Additionally, these guys are coating their bullets with friction reducers such as Moly and Danzac. It reduces barrel fouling, barrel tension, and more importantly... gives the projectile a higher ballistic coeficient. Hell, it flies flatter as they say. But this is also air and 2500-4000 fps! BTW, anyone want to buy my custom built M40A-1 with it's 5.5x22 power Nightforce scope? My flintlocks are getting much more use these days!

Hey! And sense when do you look at the bottom of your hull more than once or twice a year. Probably when you're looking over a nice used boat to buy?! Why not sand on it? Labor is cheap when you're doing it yourself. If you don't want to do it, ask audiofn to do it. He loves to sand... Have DA, will travel, right man? Eh, maybe not. I think he's getting tired of that stuff!

When you get down to it, I like Cuda's response. MO POWER! But, sensability (and budget) dictates that I work on hull and setup rather than compression and cams. Damn, reality sucks. And besides that, all the coatings are relatively cheap. So again, why not? I can't even buy an exhaust tip for the price of a full coating!

Seriously though, this is an interesting subject and I've enjoyed the eloquent responses. Inquiring minds want to know. I am rather familiar with aircraft too and the various lift enhancing devices and drag reducing shapes. I break them for a living and formerly flew them for pleasure. Used to fix them, but that was much harder.

Now for my question which I have never seen posted.
If you sand the last portion of your pad bottom and slightly up (the usual wetted area), would it be better to leave it as sanded? Or coat it with a "Hydro-coat" on top of the roughed up area? Or leave the sanded area as sanded and coat the un-sanded forward portion of the hull?

Hey thanks ya'll!
Think I'll sit back and get out my Barlow...

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Old 08-27-2002, 12:23 AM
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Hey, I was serious about the Sniper rifle! For sale. Low hours - 400 bullets or so. 3/8" groups. Mint condition. $2400 obo.

Mullah guaranteed to go into my boat since I've got to keep it a while longer now instead of moving up next year. Thank you 9-11 and and my wifes pay cut at a large eastern airline!

Did I say reality sucks sometimes. Hey, I've still got a boat!
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Old 08-27-2002, 09:06 AM
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I wonder though if the sailboat results only apply to displacement hulls. A displacement hull in water is similar to a bullet in air. The turbulence aft of the stern will have a dramatic effect on a displacement hull while having negligible effect on a square-sterned planing hull.

Having lived the dragboat scene for a few years, I know that a "straightedge flat" running pad is worth mph. We sanded to 800 but the emphasis was on flat, flat, flat, flat. I'll buy into the 4000 grit theory as well.

Boattail bullets are once again to reduce the low pressure area behind the bullet.

I also understand the surface tension deal with wax, and although it goes against reflex thinking, I'll buy into the advantage of retaining a super-thin layer of water on the hull for speed.

Question is: is it better to have a 4000 grit super flat, super smooth hull with nothing OR a factory finish hull with a speedcoat OR somewhere inbetween?
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Old 08-27-2002, 09:40 AM
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The flow of water over a flat surface is a well documented phenomenon. Boat hulls are a little trickier, but some basic concepts apply. The beading wax analogy is always used - trouble is, the boats not moving! There will always be a boundary layer of water attached to the hull as it is moving through the water. I would think roughing up the surface would just increase the thickness of the boundary layer. In effect you are carrying more water with the hull.

On a side note, the P-51 Mustang was the first production aircraft with a wing designed for laminar flow, making it one of the fastest fighters during the war. Know what else it had - MORE POWER.
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Old 08-27-2002, 10:48 AM
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Default Hull Coating

I tried the Hydro Speed Kote product on my new 36 ft Cat. The product was put on accoording to instructions and took approx 3.5 hours to apply. No noticible speed difference, no softer landings, big fat $375.00 bill for product and application.

Waste of money. This is the second product I have tried that promised results by wax type applications. And the second tome I have been dissapointed.

Cat guys don't waste your time.

However, Some of the deep V guys said they had success with the product. maybe it works at slower speeds?

Just fantasy to me.

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Old 08-27-2002, 12:41 PM
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Default faster

Gel Coat OR Paint (imeron)
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Old 08-27-2002, 05:19 PM
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Ya see, I knew there were a few people who could explain the technical side of the dimple question! Bravo!!

Mike F of Outerlimits wet sanded the running surface of Wild Child when he raced it the season before I did. I'm not sure of the exact mph gain but before Key West last yr, he told us to sand it again (I never got the time).
I know there are quite a few other F2 boats and Super V's that do the same
Where TX2? T2X? when we need him?
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Old 08-27-2002, 06:24 PM
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If you goofballs must go faster...then I will give you a top secret quart of rain-x mixed with one quart of wd-40...mix them together and apply with a paint brush...two coats to the bottom of your hull.

Think about it....slippery and repells damage to your hull. For it to work, you must do it every time you go use your boat!!!! You boys wanted to know, so, I told you how to do it!!~!

DMan....and the D is for wD-40!!!!!!!!!!!
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