Tiny bubbles

[Crikey]

I just read an article that stated the hull speed can increase when following in another boats wake, due to the many small bubbles in the water reducing the interface friction. Has anyone noticed this?
I always thought that the turbulent, disturbed water was having the opposite effect regardless of whether it seemed smoother or not.

Ross

[Tomfoolery]

Back in my small plane flying days, I remember the instructor telling me how sometimes float planes had a hard time taking off in glass-smooth water, so they'd skate around a big circle a couple of times to disturb the water, then make the takeoff run through it.

May be some truth to it.

[mastreb]

More likely any performance improvement is a case of sharing the drag through the water--similar to "drafting" in race cars, when two vehicles are within the same turbulent flow, they split the coefficient of drag between them.

You can calculate the efficiency either as a single hull whose length is the sum of the constituents (simplest) or by considering the 2nd boat to be a foil that decreases wake turbulence (and therefore drag) for the first boat, and the first boat as increasing the flow speed of the water for the second boat (and thereby reducing drag).

Bubbles do reduce surface friction simply by reducing the amount of water that is in contact with the hull by the area of the bubbles. I've seen something about a bubble generating machine used to reduce hull friction.

Perhaps hooking up a blower to the top of the daggerboard trunk and sealing it with duct tape will get you some speed

[bartmac]

You can buy these vortex generator stick ons for cars and similarly I saw these stickers applied to racing yachts with fine lines in them...laminar flow???

[DaveB]

Tiny bubbles in the wine..makes me happy. I get up speed on a 5 ft. wave going down wind and do see small bubbles and a rooster tail from rudders.
All I get from a powerboat stearn is hard control at the helm.
It is possible to ride the quarter wake of a powerboat but you need to stay at same speed as the wake.
It's like surfing but on a Mac.
Thats why Dolpins love to surf and wing out on a Trawler doing 8 knots.
Dave

I just read an article that stated the hull speed can increase when following in another boats wake, due to the many small bubbles in the water reducing the interface friction. Has anyone noticed this?
I always thought that the turbulent, disturbed water was having the opposite effect regardless of whether it seemed smoother or not.

Ross

[Crikey]

More likely any performance improvement is a case of sharing the drag through the water--similar to "drafting" in race cars, when two vehicles are within the same turbulent flow, they split the coefficient of drag between them.

You can calculate the efficiency either as a single hull whose length is the sum of the constituents (simplest) or by considering the 2nd boat to be a foil that decreases wake turbulence (and therefore drag) for the first boat, and the first boat as increasing the flow speed of the water for the second boat (and thereby reducing drag).

Bubbles do reduce surface friction simply by reducing the amount of water that is in contact with the hull by the area of the bubbles. I've seen something about a bubble generating machine used to reduce hull friction.

Perhaps hooking up a blower to the top of the daggerboard trunk and sealing it with duct tape will get you some speed

Exactly, and if it was electric and ran off the surplus current while running, and - ahem - it worked, could be a substantial boost at higher speeds. Throw in fuel economy to boot! At least it would keep the weight out of the dagger-board trunk.
I have to query your 'linkage' premise. Parasitic (or otherwise) draughting works because both vehicles are inside the boundary of a compressable air bubble. With incompressable water the only linkage between two hulls is the interaction of pressure waves, not suction. The hydro-drafting, if it occurs, would be based on turbulence at a distance; therefore friction related. I'm thinkin'....
Two hulls running side by side is hazardous due to the venturi occurring between them. Also turbulence related, I suppose.

R.

[Crikey]

You can buy these vortex generator stick ons for cars and similarly I saw these stickers applied to racing yachts with fine lines in them...laminar flow???

Yeah, I read that the old Americas Cup was both won and lost with a synthetic shark skin. Only problem was it liked to grow algae quickly if it was left in the water. Denticles.

[Crikey]

Tiny bubbles in the wine..makes me happy. I get up speed on a 5 ft. wave going down wind and do see small bubbles and a rooster tail from rudders.
All I get from a powerboat stearn is hard control at the helm.
It is possible to ride the quarter wake of a powerboat but you need to stay at same speed as the wake.
It's like surfing but on a Mac.
Thats why Dolpins love to surf and wing out on a Trawler doing 8 knots.
Dave

Make mine CastelVetro! No disagreement with uncertain steering whatsoever. Can't say I've ever surfed a powerboat wake intentionally though. From memory, while following somebody, things definitely seemed 'slipperyer'.
R.

[seahouse]

Fairly recently (might even have been at the last summer Olympics) bubbles adhering to the surface of the “skin suits” of Olympic swimmers improved their race times, and the suits were banned because of it. You could plainly see the silver bubbles stuck to their suits in underwater videos. But very few bubbles would actually adhere to the hull of your boat.

Power boats have stepped hulls, Beneteau has one with airducts that funnel air underneath to reduce friction, and it was quite a while ago a torpedo exceeded Mach 1 using bubbles as lubricant. Any springboard diver is aware of the great friction of a body through water, and I have always been amazed by the high speeds routinely generated by torpedoes. Modern large tankers (which are slow) use bubblers to reduce friction and fuel burn. So the presence of bubbles clearly reduces friction at both high and low speeds.

I can’t imagine that the bubbles made by another boat would be significant enough in quantity to play any noticeable role in reducing friction, unless maybe a Mac were caught in the wake of a tanker. Where did you read that, Crikey, I’d like to see it too. The conditions would have to be strictly controlled, I would expect.

I don’t think “drafting” as you would in a car on a race course, or peloton on a bicycle, translates practically to the water itself, if at all (please, someone correct me if I’m wrong on this- racers?) When you have an interface of water and air, most of the physics that applies to either one, or the other, go out the window – there is a whole new third set of rules. You might be able to find a sweet spot in the wake turbulence of a tanker (its “current”), but I think that would be about as wise (and illegal) as doing it behind a tanker on the highway.

Too many bubbles would have the opposite effect- reduced buoyancy would make the boat sit lower in the water and thus increase friction. As an extreme example- I’ve been caught in white water (rafting) and you go straight to the bottom and then wait to surface; “swimming” is useless (it was a beautiful experience- everything was white)

I’m guessing that the mass of bubbles is what cushions the fall for survivors of Niagara Falls (who miss the rocks).

The same effect has swallowed up ships in the Bermuda Triangle – though the bubbles that cancel buoyancy are generated from thawing fields of methyl hydrates on the ocean floor.

A much more significant and observable effect is to be had above the water level from the air – that’s what wins and loses races in sailing- maintaining a favourable position (and I’m not referring to the obvious “wind shadow”) relative to another boat so as to take advantage of the air flow around his boat to assist you, and hinder him at the same time.

You would benefit more by taking advantage of those air currents around another boat than by the bubbles generated by his wake.

- Brian.

[seahouse]

One thing I have done in the past, during rough conditions under power, is follow another larger power boat to take advantage of the “smoothing” effect of his wake over the surrounding rough water. I’m sure everyone has done this at one time or another.

There is a trail of bubbles at the core of that wake that I’ve found to be turbulent – you can feel random yaw on your boat if you follow directly in the middle of it, so follow off- centre to this. That air-entrained water also causes loss of your prop efficiency. (it’s less dense, and “pre-cavitated”).

That narrow trail of bubbles is also “clappy” (or “slappy”, pick one ) when you water ski in it. I can’t say that I can recall feeling any reduction of drag when skiing over it, though.

- Brian.

[seahouse]

Now it occurs to me (duh ) that in a powerboating context (Crikey, I was thinking in a sailing context), the bubble trail might be dense enough to have a practical use, if one powerboat were following another. Like if you were following a twin-screw boat putting out two through-hub exhaust bubble trails, you could line your prop up to be in the middle, and get some benefit

[kmclemore]

I wonder, though, if prop cavitation from the bubbles might not negate the effects of those 'slippery' bubbles?

[taime1]

Would those be the slow blue bubbles, or the faster white ones??

[bscott]

Some elite small boat yacht racers (Neil Pryde) claim that VC-17 produces a painted surface simulating shark skin that reduces water stiction to a fiberglass hull.

Bob

[dive4it]

Go to Alaska, all the guys with aluminum sleds purposely run the boats up on to the rocks to dimple the hull, "makes 'em go faster" is what I hear from all the guides. On the flip side (in regards to aircraft), dimple tape and vortex generators help the air to "stick" to the flying surfaces for better control and lift at much lower speeds. Not sure what's right or not but there is some merit to both. I recall seeing a sailing vessel with hydrofoils setting a speed record (many years ago) that was only able to obtain a certain speed and go no faster. It wasn't until they put an underwater camera near the foil that they figured out it was actually ripping the water so fast that it cavitated bubbles below the waterline and induced drag. A quick re-design of the foil shape and it pushed past the speeds easily (not that that has anything to do with this discussion).

JT