Boundary Layer Texture?

Tom et al. ,
Good information and consistent with my limited experience.
However,, irrespective of racing rules, the objective of all this stems from a two questions..,
1) is there a vacuum, friction, or what ever along or about the stern sections that drags up, or pulls up a stern wave ( Creating drag) ? ( more than the Stern wave resulting from displacement replacement. )
and,
2) is there a simple, easy to maintain means to break the vacuum or friction in the area of the stern end ( if it exists) ?
Extended staring at the bubbling and rolling over of the wake of cut off sterns of small craft, it kinda looks like there may be. I understand Leo's highest tech references are of mixed conclusions.
For the fun of thinking about such esoteric things, say there is ,, and if so, can it be dissipated in some cost effective manner?
To try to do so, I'm thinking about locating holes in a lip at the stern WL to aerate water as it leaves the final surface.. See Sketch.
Hypothetically cogitating, I wonder if it will break the phantom vacuum, or does it simply move the release point forward?
Bill PKS

 

Bill,

What you are talking about is achieved by the 3M drag reducing film. The problem is that it frankly is a gain only achievable in very limited conditions. Not conditions in the scientific sense since it works at all speeds and hull shapes (I believe), but in the real world sense that these micro scratches are so easily fouled by any type of marine growth. To just wish this limitation away is to disregard the reason the already available technology is not used. For the sake of argument though if we started with a highly polished copper clad vessel on which this copper was etched with these micro scratches the copper should be able to retard growth significantly...

As for the air bubble option, it has been shown to work scientifically, but at the same time causes the prop to aerate much faster causeing a decrease in performance equal to the gain. There may be some gain from only aerating the water behind the props, but given this only decreases drag I would imagine the gain from such a limited friction reduction over such a small part of the hull would be minimal.

The other option I could envision would be to discharge water into the boats wake at a critical point to try and reduce the creatin of the stern wave, but 1) I don't know how you would carry enough water to be meaningfull, 2) It seems very expensive and complicated, and 3) I doubt the gains would be outweighed by the extra weight of the discharge water.

Basically I keep thinking that the nano technology acoting is really the best tech going, but the problems involved in its current deployment aer very significant and haven't been even close to solved. Though I don't know if anyone is even working on it at the moment.

 

Capt. G.

What do you think about the little bubble holes shown in the previous PDF?
( Is the drawing understandable ??)

Bill PKS

 

Ever notice how smooth a golf ball is? I think you all are missing the point. Smooth is good, but I think carefully placed vortex inducing bumps and dimples will pull the flow closer to the hull.

 

dolphin skin has been studied a lot for its low drag.

biomimetic.pbworks.com/f/The+myth+and+reality+ofFish.pdf

Seems to be covered with tiny 'stalks' or 'papillae'.

Whole subject seem complex and dolphins may be doing all sorts of things with their skin as they swim.

 

At one time I believe Goodrich was selling a ship coating called NO Foul.

Installed with Epoxy as glue on the ships bottom,

It looked like rubber sheeting that a scuba diver would use but stank.
It contained mercury so it was banned for all but military vessels.

Their claim was a good percentage fuel reduction (5% or 10% ??, was too many years ago) .

Anyone playing with this style of covering today?

FF

 

Not really. 3M's riblet film reduces drag by dampening the vortical structures in the turbulent boundary layer. The flow is attached to the surface, and the drag reduction comes about by reduced skin friction. That is an entirely different drag reduction mechanism than is being discussed in this thread.

What Bill is wondering is whether or not he can affect the wave drag (and possibly the pressure drag) by reducing the size of the stern wave through introducing air between the hull and the water.

I suspect the answer is, "no," but I don't have any quantitative results to back that up. I don't know what the pressures are near the stern. I suspect the pressures are higher than atmospheric, for a couple of reasons.

The boundary condition at the surface of the water is atmospheric pressure. That is a big part in determining just where the water surface is. Since the stern is immersed below the level of the stern wave, the pressure should be increasing from the waterline toward the centerline of the hull, and the hull should be pushing down on the wave, even if the wave is elevated above the mean water level. In other words, the wave is elevated because there is pressure underneath pushing it up. I don't see why there would be suction on the hull that is pulling the water up.

If the surface of the hull is sloping up toward the stern, as it is in classic yachts with significant overhangs, positive pressure on the stern actually has a forward component. Eliminating this pressure would increase the drag instead of reducing the drag. If the hull is sloping down toward the stern (picture a powerboat struggling to get on the plane), then there is an aft component due to positive pressure that adds to the drag. So again, if the stern wave is coming up to the hull, I suspect that is a good thing and one wouldn't want to prevent it if one could.

 

Tom,
Good thoughts.
I can't quite get my mind around the issue, but I believe it may be more important for displacement forms and speeds, as I believe bernoulli has a significant impact on creating drag by pulling the hull downward, and wavemaking..
( This is suggested by the bailing scupper in the bottom of a Sunfish that vacuum's water from the foot well at over 5 or 6 kts.)
While bernoulli is mostly from around the apex? of the camber to the stern, it would seem to be difficult to modify the effect in most areas.
BUT if there is a vacuum along the trailing surface, there may be a possibility to diminish that in the area of the stern.
Think I'll hang over the stern of some boats for the next while and soak my head.
Bill PKS

 

It's not quite that simple.

Waves created near the stern could cancel those made upstream, thereby reducing the overall wave resistance or, at other speeds, they could re-inforce upstream waves and increase the resistance.

Leo.

 

Leo,
It would seem to me that making a wave of any sort, any where requires power.
Consequently, wouldn't the objective be to reduce wavemaking where ever possible?
Notwithstanding, it would seem reasonable that various displacement hull forms result in wavemaking locations that are more in harmony with a frequency for a speed, therewith minimizing power,,,, to a point.
So, wouldn't further reduction of wavemaking anywhere be a benefit?
All in all, I can't see how reducing a Stern wave, by some means, would amplify a bow wave?
(Not considering depressing the bow and lifting the stern,, or vice versa.)
Bill PKS

 

Not if your objective is to reduce the wave resistance.

The bow wave can be reduced or "amplified" when it passes the stern depending on whether it is in phase, or out of phase, with the waves created by the stern.

Start with a very simple case. Think of the wave at the bow as a sine wave (starting with a crest) and continuing far downstream, and the stern wave as a sine wave (starting with a trough). Now add them together. For some hull lengths the two sine waves will completely cancel out behind the stern which is what you want. For other lengths the resultant wave behind the stern will be twice the height of the individual wave trains, and the wave resistance will be a maximum.

You should be able to do this in a spreadsheet like Excel.

Cheers,
Leo.

 

Leo,

Oh ,, I see what you are referring to,
but isn't this simply neutralizing or amplifying waves after they are made?
Wouldn't the power needed to make the wave by and at the hull, in the first place, be unabated by what happens where the waves intersect out beyond the hull?
( Maybe this would be an example.. Noise wave cancelation tech. at the ear requires power to cancel the original noise wave, so there are two power inputs.
or another example .. Ocean Wind Waves can cross, pile up, or meld, but the original energy that created the waves is unaffected and undiminished. )

Bill PKS.

 

No, it doesn't work that way with wave resistance.
Sometimes you just have to do the math in science

Leo.

 

Yes!

Yes, submarines.

-Tom

 

Query probably my physics is flawed but the way I immediately think of this is....

Isn't the energy in the creation of the wave in the first place?
Whether they cancel out or not subsequently has little to do with wave making resistance since you don't get the energy back when the waves cancel.

You make a low wash vessel but how does cancelling a subsequently generated wave make the vessel more efficient. Whether its in the near or the far field.

You need to interfere with the wave creation in the first instance as in bulbous ships bows.