Boundary Layer Texture?

Leo,
Well then..
Any observations of aeration at a Cut off Stern?
It would seem to me that inducing ( low viscosity) air along the stern sections or at the cutoff would diminish any possible drag, suction, adhesion or whatever that might be present in (high viscosity) water.
No.. I'm not going to go crazy and try to install bubbliers or some other such mess.
I'm just trying to get my mind around a concept of how water leaves the stern.
Clearly, there are lots of opinions.
Bill PKS

 

There are some photos of experiments performed by Maki, Doctors, Beck, and (I think) Troesch that show the bubbly flow below the critical transom draft Froude number. There are also photos of the flow once it is fully-ventilated.

There are many papers on estimating transom stern drag and wave profiles. Search for L.J. Doctors, Kevin Maki, Robert Beck and Armin Troesch.

Odd Faltinsen recently came up with an estimate of the profile of the wave behind the transom, but Maki et all showed it wasn't all that good. Hoyte Raven published something recently too on modelling the flow behind a backward-facing step. Maki used the CFD profram "Fluent" to model the flow with mixed results.

There's plenty to get lost in once you track down all the papers.

Happy searching!
Leo.

 

Paul,
So you see the stern wave as being pushed up as the return of the water flows to fill the hole created by the previous displacement?
It does seem reasonable that long overhangs create a longer WL for displacement hulls allowing faster hull speeds vs. the wave trap, as the hull is pulled down into the water by Bernoulli , but I also wonder to what extent Bernoulli creates a suction in a flow, particularly at the end of the surface (the counter, if any ) .
(( I know it's out of kilter, but I'm trying to ignore all handicap rules and only deal with real world hull dynamics.))
Bill PKS.
( The word texture may be a misnomer, but the fellow has built boards for many many east coast champions over a long time.)

 

Leo,
Sounds like "mixed results" is the soup de jour..
Bill PKS

 

The pressures are due to the shape of the hull, and the quality of the surface has no direct effect. The surface only gets into it indirectly, as it affects the boundary layer development and the boundary layer thickness then effectively changes the shape and the pressures. But these are small effects.

Texturing (or not) of the hull surface is not going to change the height of the stern wave.

 

Bill,

My experience is heavy boats, both power and sail, produce big stern waves. The stern wave can be reduced by making the boat lighter or longer. That's about the extent of my hydrodynamic knowledge regarding the formation of stern waves. The hydrodynamicist is always the smartest person in the room at project meetings, and just thinking about their presentations makes my head hurt.

I can tell you that surfboard shapers are notoriously bad at being able to quantify the hydrodynamic characteristics of their designs (myself included). Surfboard design is best done by experimentation and measuring performance through the soles of one's feet. It's a big stretch to relate what works on a surfboard to what works on a boat.

 

Tom,
Sorry ,, I have mixed subjects of hull Form and Surface .
Anyway,, it seems to me that the stern wave results from expenditure of considerable energy,, so I want to reduce that as much as possible.
I kind'a figured out how I wanted the entry, body and stern shaped in an earlier thread.
( Rightly or wrongly, the stern to be cut off on a Deadrise angle at WL of some desired angle of heel.

But I have noticed on cut off transoms that a bubbling up or rolling over of the water occurs at low speeds even from sharp edges ( windsurfers and small sail boats where you can see it) , so I was wondering if this is pulled up by some release of surface tension, friction, suction or what,, as ( if you balance the stern right) it doesn't seem to be a stern wave caused by displacement?

So now,, Leo's comments have gotten me even more perplexed.

Bill PKS

 

Paul,
That may be an advantage surfboard builders have,, Low cost allows lots of experimentation, and surely the Feel thru your toes is better than feel thru a computer model.
I know I GREATLY prefer a tiller to a wheel, because I can feel the rudder.
Absolutely no doubt that ANY Rudder angle is more drag, and in small boats , less rudder or board down is less drag.
Bill PKS.

 

There has actually been a good bit of research into films and textures that reduce surface drag, but from what I have read (and I am no expert) the practical limitations for these things really make their application impossible.

For racers they are just flat out illegal in most instances since most one design classes, major regattas, and even minor ones outlaw them. (Everything from local bearcan regattas to the AC). Secondly the way they work is by having very very small 'scratches' in the hull act to disrupt water flow, or make is faster not sure which. These 'scratches' though are quickly fouled by marine growth, and cant be replicated in normal bottom paint. So you have to at a minimum dry sail the boats they are applied to.

Finally the best of them are incredibly expensive, costing over $1,000 a gallon. Which is certainly within the budget for some boats, but not even the biggest are willing to pay this much for a coating that only lasts 4-5 days.

Most recently Dogzilla did use a specially manufactured film from 3M that they claim added significantly to the speed of the boat, but again they have the budget to pay round the clock divers to keep it clean. But 3M does manufacture this stuff for use on airplanes and wind-turbines, so it is out there in the market place though in limited quantities.

Try a Google search for "3M Drag Reduction Riblet Film"

For cites and more information see:

http://www.nasa.gov/centers/langley/news/factsheets/Riblets.html (coating used by Stars and Stripes)

http://forum.sailingscuttlebutt.com/cgi-bin/gforum.cgi?post=8414 (Nano-Tech application)

 

Tiny Bubbles

Cptn' G.

Wow/// That's something .
It would seem that "nano" coatings are very effective..
and expensive,
and high maintenance.
So , if it does work, I wonder if the effect can be created mechanically in a small area ..
For instance... ( Just dreaming) I wonder what would happen if//
- some pattern of small or large holes were drilled thru a lip horizontal to the WL some distance from the aft end on the stern ?
or
- grooves or slots were cut into the stern at W/L ?
The idea would be to aerate the very end of the water moving past the stern at WL.
Would that aerate the water or add more surface and more drag?
Also, what would happen if air is somehow induced to the bottom surface, say like from a CB trunk.
Has anyone ever tried that?

Bill PKS

 

Yes, the air entrainment gambit has been done. Results negative. This has been a particularly attractive idea for power boaters, and air bubble generation has had extensive experimentation by many a brainstorming entrepeneur. Results negative again. I am not at all optimistic that we can find a way to violate the laws of physics but I think we'd ought to continue to explore possibilities.

 

Messabout,,
A Quick Reply..
I built a Jet Drive Power Boat with channels inboard P&S of each chine. (You Tube>Sea Skiff 22)
If I balance fore to aft trim just right, it entrains air thru the channels ( I can see the aerated water in the wake) .. Makes a good difference in speed.
I know it works. It's a matter of speed, angles, surfaces, and air volume.
( The angle shown in the You Tube clip is way high. Also, what I built can be improved upon. )
I expect the cost benefit for air injection under a Sail boat hull by some mechanical means at sailing speeds may be difficult to justify.
But, I wonder if you drilled some holes on a strong reverse angel in a Lip extending from the stern at the WL/// would that have any release effect along that terminal edge?? It would be sort'a like the bilge drain in the bottom of a sunfish that vacuums water out of the foot well. Instead this would pull air thru the holes. Simple, cheap, clean, low maintenance..
Or, does it just move the terminal edge of the stern forward ??
Ever heard of anything like this??
Bill PKS.

 

Racing Rules of Sailing 53: "A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer."

 

Drag reduction by air injection has been done successfully, but I suspect not just any hull is suitable for it. A barge would be a good candidate. A round-bottomed catamaran, probably not.

Here's one reference:

ELBING Brian R., WINKEL Eric S., LAY Keary A., CECCIO Steven L., DOWLING David R., PERLIN Marc, "Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction", Journal of Fluid Mechanics, 2008, vol. 612, pp. 201-236.

Abstract
To investigate the phenomena of skin-friction drag reduction in a turbulent boundary layer (TBL) at large scales and high Reynolds numbers, a set of experiments has been conducted at the US Navy's William B. Morgan Large Cavitation Channel (LCC). Drag reduction was achieved by injecting gas (air) from a line source through the wall of a nearly zero-pressure-gradient TBL that formed on a flat-plate test model that was either hydraulically smooth or fully rough. Two distinct drag-reduction phenomena were investigated; bubble drag reduction (BDR) and air-layer drag reduction (ALDR). The streamwise distribution of skin-friction drag reduction was monitored with six skin-friction balances at downstream-distance-based Reynolds numbers to 220 million and at test speeds to 20.0 m s-1. Near-wall bulk void fraction was measured at twelve streamwise locations with impedance probes, and near-wall (0 < Y < 5 mm) bubble populations were estimated with a bubble imaging system. The instrument suite was used to investigate the scaling of BDR and the requirements necessary to achieve ALDR. Results from the BDR experiments indicate that: significant drag reduction (>25 %) is limited to the first few metres downstream of injection; marginal improvement was possible with a porous-plate versus an open-slot injector design; BDR has negligible sensitivity to surface tension; bubble size is independent of surface tension downstream of injection; BDR is insensitive to boundary-layer thickness at the injection location; and no synergetic effect is observed with compound injection. Using these data, previous BDR scaling methods are investigated, but data collapse is observed only with the 'initial zone' scaling, which provides little information on downstream persistence of BDR. ALDR was investigated with a series of experiments that included a slow increase in the volumetric flux of air injected at free-stream speeds to 15.3 m s-1. These results indicated that there are three distinct regions associated with drag reduction with air injection: Region I, BDR; Region II, transition between BDR and ALDR; and Region III, ALDR. In addition, once ALDR was established: friction drag reduction in excess of 80 % was observed over the entire smooth model for speeds to 15.3 m s-1; the critical volumetric flux of air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR if the surface tension was decreased; stable air layers were formed at free-stream speeds to 12.5 m s-1 with the surface fully roughened (though approximately 50 % greater volumetric air flux was required); and ALDR was sensitive to the inflow conditions. The sensitivity to the inflow conditions can be mitigated by employing a small faired step (10 mm height in the experiment) that helps to create a fixed separation line.

 

Here's a more relevant report, Microbubbles: Drag Reduction Mechanism and Applicability to Ships. The authors report that drag was reduced by 3% on full scale trials of a 100m ship in Japan.

However, one of the problems encountered in the ship test was a reduction in propulsive efficiency due to the effect of the bubbles on the propeller. This resulted in wiping out all the gains from drag reduction when they tried to use a more extensive area of injection.

So drag reduction by air injection can work, and not just in the laboratory. However, it's not as simple as it looks! (It never is, alas.)