Active Drag Reduction?

Greetings, new to the forum.

In surveying the design space for human powered boats, I note the majority of drag in an optimized hull (e.g. long and slim) is viscous drag. I note much research on active boundary layer control in aerodynamic applications, but little such research for ships. Yes, there is the air lubrication research for giant ships... but I can't seem to find anything for small craft.

Are there any viable technologies under investigation in this space? The only item I've found for smaller craft are some interesting (and difficult to implement) ideas around having a moving surface (conveyor belt) line the hull.

Regards

 

3M riblet film might be on your list. Just google 3M riblet film boats and you can see their miniature ridges that look like a phonograph record (hope you are old enough to know what those are) have been used in aviation, racing yachts, and some other boating applications.

Kiss-Cote polymer may be in your category

We have seen some super hydrophobic coatings for marine drives said to reduce drag

gary

 

Welcome to the Forum!
Yes, there has been a lot of research and development into active skin (as different from form) drag reduction starting about 1968 and continuing to the present.
Yes, some of that is available in the open literature; google "active drag reduction".
Yes, it is still ongoing as a major research and development topic. There was just an article in the JSR "Turbulent Skin Friction Reduction through the Application of Superhydrophobic Coatings to a Towed Submerged SUBOFF Body ".

 

Specifically about air lubrication: not only in giant ships - you'll find experimentations in small fast powerboats as well, like Beneteau's Airstep Hullform - Professional BoatBuilder Magazine https://www.proboat.com/2018/11/beneteaus-airstep-hullform/

But implementing such a thing in a slim and slow human powered craft sounds very difficult to me - your nice air cushion will always tend to bubble out at the sides.

 

Polymer injection into the boundary layer can substantially reduce the skin friction drag. In 2010, BMW Oracle Racing experimented with polymer injection on the BOR90 trimaran. It was found to give the yacht a 3 kt improvement in Vmg upwind and down, in light winds and heavy. However the system was removed from the boat the weekend before racing began in order to lighten the yachts. They wanted the 250 kg more than they wanted the 3 kt of Vmg.

The BOR90 also used riblets on its amas. 3M had to create a new film with finer grooves just for this boat because of the higher speeds compared to the 12 Metre boats.

Both polymer injection and riblets are normally banned in the Racing Rules of Sailing, but those rules were waived by the defending club in the 33rd America's Cup match.

 

Approaching 40 years ago at Oklahoma State I assisted with a study of long chain polymer drag reduction. We stirred up different batches (different polymers and different concentrations) in a large tub with an oar so we did not break the chains. The fluid was then pumped by air pressure (again so we did not break the chains) through each of several different diameters of pipe to determine pressure loss over a considerable length of pipe. Results were plotted and compared against normal water being pumped through the pipes. Not sure whatever became of it. Has always made me a little interested in long chain polymer injection. I still have the charts around here somewhere.

 

I remember reading about the Beneteau system and I can understand why it helps with a planing hull.Similarly the multi-stepped hull forms make sense as long as they are operating in their optimal speed range.I can't quite convince myself that air injection and displacement hulls would work;yes I understand the air might reduce the friction-I also would expect it to reduce the density of the water that supports the hull and to require the boat to be immersed a little more to compensate.From the thread title I did wonder whether there might be some form of elastic bow bulb under consideration,maybe linked to the log or GPS sensor and capable of tweaking the topology of the bulb to work at reducing the drag.Perhaps an inverted version of the Concorde nose to work at varying draughts.

 

A lot of boundary layer control is associated with lifting surfaces, and especially in preventing flow separation. This is not relevant to displacement boat hulls, but is relevant to sails and underwater foils (keel, rudder etc).

 

I don't believe it would work on a displacement hull. Research with hydrofoils has shown that flow separation is necessary for ventilation, and air that is injected onto a hydrofoil is simply swept away. All of the air injection hulls that I've seen have a step that results in flow separation and the air is injected into the separated area. Simply injecting air from a smooth displacement hull is likely to just result in a stream of bubbles.

 

Concur, based upon the experiments I have seen. Best way forward for active control of the boundary layer is to manipulate the viscosity "near" the hull...but right now that requires a lot of other things.

 

That has been my experience as well. We've been involved in quite a number of air and polymer drag reduction experiments and schemes over the years. The only one I've seen that worked well and over the long term was a fast crew boat that had a step added that roughly followed the planing surface boundary, and air was injected over the entire width of the step using roots blowers powered by a small diesel engine. I don't recall off the top of my head what the speed improvement was but I remember it being quite significant.

On the other hand, we were involved in some "air cavity" and "air lubrication" projects that resulted in no improvement/benefit at all.

 

Is anyone familiar with any expirimental ionizing of a hull to achieve active drag reduction? I understand that the leading edge of the wings on the new stealth bombers are ionized to help reduce drag and therefore noise.

 

Source? A huge amount of power would be needed to "ionize" the amount of air flowing over a bomber wing. A wild guess, an only a wild guess, is there may be some sort of charged panels along the leading edge to reduce radar reflections. But I have not idea what the physics might be.

 

I don't have a good source, it was something I read years ago. I originally read it in some book about alien anti-gravity technology, but then I remember seeing a legitimate article about the practice of ionizing the leading edge of the stealth bomber, but not because it created a gravity well, but because it reduces friction.

Here's a related article about ion drives.
MIT engineers fly first-ever plane with no moving parts https://news.mit.edu/2018/first-ionic-wind-plane-no-moving-parts-1121

 

It's called magnitohydrodynamic (MHD) propulsion.
Physics de Pristine: Magnetohydrodynamic Propulsion (MHD) http://physics-depristine.blogspot.com/2012/01/magnetohydrodynamic-propulsion-mhd.html?m=1


Chinese whispers: China shows off magnetic propulsion engine for ultra-silent subs, ships https://www.theregister.com/2017/10/27/chinese_navy_silent_magnetic_propulsion/


I believe this is what the Philadelphia Experiment was trying to achieve, not time travel or invisibility. Tesla had written about electric ship propulsion. Laying magnetic coils against the hull could produce a repulsive force on the ions of seawater.