Self 'lubricating' hull design

[Bergalia]

Many years ago I saw a piece on UK TV concerning a 'revolutionary' hull design which was claimed to be self-lubricating. It took the form of short overlapping pads - not unlike roof tiles - over lapping towards the stern. The idea being that air would be formed in the overlap grooves thus cutting down water friction. The film showed two motor craft of identical length, breadth and power, but only one had the 'tiled' bottom.
Over a given course the 'tiled' version certainly outpaced the standard hull.
Intrigued I built a 16-foot sailing dinghy with 'tiled' bottom. It's performance was remarkable (scared me sh+tless) - Unfortunately I had nothing similar to compare it to.
But the point is - does anyone know what happened to the original power boat - or its subsequent development ?

[D'ARTOIS]

On the America´s cuppers dating back at he Dennis Connors era, there were trials with a special type of skin hat was applied to the underwatership of those racing yacts in order to achieve higher speeds. Popular Science published an article about it but I can only recall the topic as the particulars are lost to me.
It was a kind of embossed vinyl that was glued against the hull.

[Wynand N]

Likewise, why do golf balls have little dents

[Bergalia]

Golf balls have little dents in them because I'm a lousy player....But thanks to you and D'Artois for follow up information.

[CT 249]

Originally Posted by Wynand N

Likewise, why do golf balls have little dents

Yes, but isn't it true that the Bruce Number of a golf ball is enormously different from the Bruce Number of a hull and therefore the two cannot be compared? I'm no expert but that's what I've read and the maths seemed compelling.

The idea of running air under a hull is far from new. It was tried in sailing dinghies (with pipes from the deck venting to the hull) in about 1976 and probably much earlier, and is still used on F2 windsurfers.

[usa2]

S & S '87 had "riblets" i think they were called. They were ridges that ran the length of the hull and fins.

[boogie]

hi guys,

there are plenty of ways to modify the flow over a body in order to reduce drag.
not all of them are practical though, due to things like cost or surface contamination.
in theory there is no difference between theory and practise
... in practise there is.


surface riblets

[usa2]

i would think that it adds wetted surface, which typically subtracts from performance.

[K4s]

I think the riblets where to trap a thin layer of water against the boats hull thus making a water on water interface resulting in lowering of friction.A bit like a high tech version of "rough" sanding the wetted surfaces of your hull.

[Tim B]

It seems to me that you're looking at a multi-stepped hull. I would expect it to perform very well at high speeds, but at speeds too low for cavitation (which is what's happening) expect loads of drag. Initiate cavitation though, and it'll accellerate like mad. It's the same "unsticking" problem as old seaplanes used to have.

Tim B.

[boogie]

hi guys,
does anyone out there have a "scientific" explanation why a very finely sanded surface should have less drag in water than a polished [but not waxed or siliconed] surface and under what circumstances.

as far as i know there is an allowable roughness depending on the thickness of the boundary layer, but why would it hurt to go better/finer than necessary.

the analogy of water sticking to the surface and thus water gliding on water seems a bit odd, as you basically need friction to make the water stick in te first place. or is it something else.



tim,

i think you might be confusing cavitation and ventilation here.

[K4s]

With regard to the water on water theory,Once the water touching the hull and causing the friction becomes "stuck in place" it now,for all intents can be considerd as part of the hull surface.
Now the friction between hull surface and water ,ie water to water is significantly reduced.
Absolutely no science behind these comments,but it seems as if it would work to me.
How big an advantage over a perfect smooth bottom this would be is pretty questionable on most boats,maybe the 100 footer greyhounds could quantify it but not me in my 26 footer.

[DSmith]

I just happened to be reading about this last night in Marchaj's 'Aerohydrodynamic of sailing'. He said that it depends on the thickness of the slow moving laminar layer close to the hull. This laminar layer is still present in turbulent flow. If the surface roughness pokes out through the laminar it will cause additional drag.

Polishing more than this should not affect the drag.... but I am happy to be corrected. Frank Bethwaite also swears by mirror polishing foils and the bottom of the boat to promote the length of laminar flow before transition.

[Asterix]

Looing on the net google has a lot of information on new technology using shark scale designs. I read years ago a shark skin vinyl covering on an areoplane gave both an increase in speed and fuel economy. Navy ships appear to be testing various types of coverings.
Regards Asterix

CT- "Bruce Number" for a golf ball??! Thats funny....