Designing a Canard

..."The depth of the trawl is controlled by changing the length of the warps or the towing speed."....

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/trawl-nets

Maybe you could build your own Boaty McBoatface modified to be towed and just have remote trim control...

Just need some actuators...

https://commons.erau.edu/cgi/viewcontent.cgi?article=1417&context=edt

That's a great link.Lots of good info.

Well...the Winged apparatus looks interesting...Aluminum, poly board, steel, etc.??

Well it seems to me the smaller the wing the heavier it would need to be, and visa versa the lighter the wing the larger the plane surface would need to be to offset the weight of the net, basically you need enough water force exerted for proper steering capability and that would vary according to weight....So there's a lot of math involved, and some testing of design necessary with tweaking here and there....Then there's the cost of materials to consider, marine brass may be the best material but be prohibitively expensive, and a costly loss would be painful....So all things considered probably the cheapest materials is the way to go....Probably marine plastic...maybe fiberglass it to improve rigidity...

Referring to the previous thread and comments there, if the tow connection to the Cannard is above and behind the centre of gravity, then the canard will be inclined to point "down". I suggest the net be tethered to the canard above and behind the tether point for the tow line. The level of drag (dependant on boat speed) on this will cause the canard point to raise.

If that canard had horizontal fins, then you have down force dependant on boat speed. There would then be an equilibrium determined by line played out and boat speed to provide a depth. Ironically, to bring it "up" would require faster boat speed, thus increased drag on net pulling the rear of canard down. (Or else slow down and winch it up against the down-force.)

Oh well, that's what I would do with a cable drawn plough behind a tractor.

So...would their be any chance of a design from one of you guys? Nothing detailed...just a an idea of Length x width x height....angle of the dihedral wings?

We would need more information to make those calculations, like what is the bollard pull at the boat, what is the static drag of the empty net and the difference when the net is full. Temptation resisted to play the point for more data any further my friend.

The thought process comes up with the following proportions.

Length 1.5m cylindrical, though RHS might also be suited.

Density around 1.5 to 2.0.

Wings parallel to shaft around 0.4m from front of canard. (The angel of the shaft will provide the "drive" down or up)

Connection of pull to boat 1.0m from front of canard, 10% of diameter below C/L of canard.

Connection to net pull 1.3m from front of canard and 10% of diameter above C/L

You might want a trace line from front of canard to the tow line to limit the absolute dive angle to be achieved.

For higher density, the tow line and net line would need to be brought forward closer to the Centre of gravity so it doesn't become nose heavy.

Really? ha ha ha

You know our motto....Go Big or Go Home...

Here is an idea. I don't know if it exists or is practical...

It would seem to me that you want the canard to fly through the water with the net dragged behind at the same depth. The forces on the net would cause it to follow the canard at the same depth.

The boat would tow the canard with a line fastened slightly forward of its center of pressure so that the canard is free to pitch up or down. The canard has control surfaces which are controlled by the hydrostatic pressure. It would be configured to pitch down if the depth is too shallow and pitch up if the depth is too deep, thus flying at a predetermined depth.

The canard probably should have dihedral and central ballast weight in the center to maintain an upright orientation.

Just an idea...

I would think that the same principles for airplane flight would apply in water.You would need a fixed angle "wing" with trailing control surfaces for adjusting "altitude" plus a vertical tail for stabilization. The control surfaces could be manually operated via cables or servos.Probably cables would be more rugged in that type service.The cables could be adjusted by servo powered winches however.Do you have feedback of the depth of net and bottom of ocean? If so,a controller could be designed to keep it at a desired level. The adjustable height would allow different depths without much of an adjustment, just as a plane changes altitude..and the height could be adjusted for changes in speed and load.With enough data points,this could be programmed to automatically adjust for variations in load and speed. I do not have enough technical knowledge of your nets,weight empty,full,speed,etc to offer any drawings,but I can advise on programming a controller if given enough information on the variables. The closer the canard is to the ship,the more downward force will be required to keep the net at the desired level,so it is a balancing act.

I would think the device would be more like a submarine than an airplane, with buoyancy control chambers that could be flooded to add depth and purged to reduce depth...then the control surface movement would be minimal...

Maybe a full on model submarine... The Flying Wing Submarine.......but I sense we are exceeding the budget...oops

https://engel-modellbau.eu/shop/en/Submarine-Kits/AKULA-II/AKULA-II-Model-Submarine-SET.html

There are ways to control buoyancy without ballast or air. There are some autonomous survey submarines that record depth,temp,salinity,etc. for scientific research without ballast tanks and an on board air supply. They control buoyancy by lengthening and shortening the length of the vessel. They surface when the batteries are low and recharge via solar cells. They are constantly diving and surfacing and sending info via sat link. I do not know what type of seals they are using,but I would presume an accordion-like structure between the 2 sections. Only a slight movement is required to influence displacement. Perhaps a canard using the same principles could be designed. Another means of controlling lift is to change speed according to requirements. A pressure(depth ) sensor could provide feedback for control,and could perform autonomously.

Edit to above post: Integrated ultrasonic transmit/receive circuits are cheap and plentiful,and could be used to prevent bottom collisions of the canard.Modifications will probably required for underwater operation,but that is to be expected.

The same general principles apply insofar as lift surfaces but the forces involved are a magnitude higher in water,so less surface area is required. Of course,drag,turbulence,etc. is also increased in water,so it is a math juggling act to get everything balanced. I think the field for underwater canard research is wide open.I am sure all of the world navies have plenty of info but is is held very securely.

I agree with the airplane flight principles, but would simplify the concept to avoid separate control surfaces. I envisage a swept-wing design with aerofoil and dihedral inverted with respect to their airborne counterparts. The whole structure is a control surface, using two lines to adjust the pitch attitude. There remains the problem of gauging depth, which could be calibrated against the towing speed and the angle at which the towlines disappear into the water.

Would this simple mechanical solution be subject to underwater corkscrewing?

If you have hundreds or thousands of feet of tow line, you could never use two lines to control the pitch attitude of the canard, for two reasons: the lines have some stretch, and the hydrodynamic force on the lines causes them to not be taut but bowed by the force of moving through the water.

I'm still thinking it should be designed like an airplane, with an elevator controlling the wing angle-of-attack and an "autopilot" to fly it at a commanded depth, sensed by hydrostatic pressure.

The lift for airplanes is based on the compressibility of air/gas. Water doesn't compress, and travel thru it is very different. I'd think that the blunt leading edge of an airplane's wing would put excessive drag on the submerged object traveling thru the water without actually providing any lift. In water, to get lift, the pressure below the "wing" (plane) is more significant than the "vacuum" above it. And, a sharp leading edge would keep down drag, without affecting the lift.

It seems to me that working with buoyancy-operated planes instead of heavy mechanical-operated planes would be better suited for a light net. The OP stated that massive weights should be avoided. I don't know what kind of load/weight the net will carry. But, I'm sure it it doesn't have to be heavy-duty like for an airplane or sub. Besides, if it's made right, It could even be collapsible when not in use for transport or storage.

The lift for airplanes is based on the compressibility of air/gas. Water doesn't compress, and travel thru it is very different. I'd think that the blunt leading edge of an airplane's wing would put excessive drag on the submerged object traveling thru the water without actually providing any lift. In water, to get lift, the pressure below the "wing" (plane) is more significant than the "vacuum" above it. And, a sharp leading edge would keep down drag, without affecting the lift.

It seems to me that working with buoyancy-operated planes instead of heavy mechanical-operated planes would be better suited for a light net. The OP stated that massive weights should be avoided. I don't know what kind of load/weight the net will carry. But, I'm sure it it doesn't have to be heavy-duty like for an airplane or sub. Besides, if it's made right, It could even be collapsible when not in use for transport or storage.

Wings work very well under water. Compressibility is not necessary. A wing deflects the airflow around it. The change in momentum (mass x velocity) of the water results in a force that provides lift to the wing - conservation of momentum.

Hydrofoils are alive and well.

The US Navy's XCH-4, with hydrofoils clearly lifting the hull out of the water

https://en.wikipedia.org/wiki/Hydrofoil

Whatever you call it, air-planes and water-planes are different. Wings/planes/foils work in the medium they were designed for. I never said otherwise. But, aerodynamics and hydrodynamics are different. Hydroplanes won't work for aircraft, and airplanes won't work (very well) for watercraft (even if the control surfaces have the same name).

Hydroplanes are more like a vertical rudder. A rudder needs to go left/right; a hydroplane needs to rise/dive. An aircraft doesn't really need to dive; the aircraft can do that without the planes.

A passive cylinder with ram, akin to a shock absorber, to control the "aileron" on the canard wing, but with a piston exposed to the water presser at whatever depth. The area behind the piston would be pressurized, with air while on the boat, pre deployment, to a predetermined pressure extending the shaft connected to the piston, which would cause the canard to dive until it reached the desired depth when the water pressure would force the piston back against the preset air pressure causing the canard to level out. The depth could also be actively controlled via a air pipe from boat to the cylinder.

....but they want the depth to be variable...

Can you have a second line running in parallel with your main tow line, which could be used as a pitch control on a wing? You could then descend to the required depth (measured by pressure sensor if you want fancy, or maybe tow line angle) and level out.

That could happen.

I'd like to pursuit something along these lines due to my lack of engineering skills

A. WHAT ANGLE BETWEEN THE CANARDS

B. How long of a wing

C. How wide is the wing

D How far back from the nose cone are the wings

E. How long of body and how big in diameter

F. Tow points...how many and how to space

https://www.militaryaerospace.com/unmanned/article/16715557/navy-researchers-choose-underwater-glider-uuv-sensors-from-aml-oceanographic

Might pick up some design pointers here....

https://uncw.edu/uvp/documents/gliderusersmanual.pdf

Something I have used when shad or sturgeon fishing is a round weighted disk with a rod attached at the center of the disk and perpendicular to the disk. The end of the rod not attached to the disk is connected to the line. it ends up looking like a mushroom with line connection at the base of the mushroom stem.

When put in moving water, the disk goes down and pulling on the line inclines the disk so that the current pulls the disk down. Since the rod and disk are symmetric around the rod axis, the disk is not prone to rotating. The only variables would be to set the disk diameter for the amount of down pull, weight of the disk to bias the disk down and the length of the rod and connecting line to determine the disk stability.

Something to start with.....copy, paste, add, return...

Hello,

Once you get a prototype, you could mock it up inside a tank (or pool) with the "net" end fixed and by measuring tension in net line and tow line you will be able to see the attitude of the canard and be able to provide the towing vessel with the tension versus attitude relationship for that canard style.

If it works how I think it might per my previous post, there might even be the means for the canard to come to surface at a slightly slower speed using the net drag to raise the point of the canard with no "moving" components.

The longer the loaded line,the lower the net will tend to go.To achieve a predetermined depth,over a varying speed range,adjustment will be required to the speed or length of line or both. The depth will also be affected by the amount of catch in the net and sea conditions. What is the industry using on most of their rigs?No sense reinventing the wheel.I am sure there is a mature technology that addresses the same problems as you. Good luck.

I think the current method is to control the depth by a combination of the amount of line paid out and the boat speed, the same principle as flying a kite. There may be transponders on the net that report depth.

"Fish Operation

A single boat midwater otter trawl is towed by a single boat. Midwater trawling is a well-aimed fishing activity. The fish shoals are located by sonar (but some midwater trawling can also be carried out on slightly scattered fish staying in a given layer of water) and the vessel is towing in the direction of the shoal. The fishing depth level, which is controlled by the net sounder, is regulated by the length of the warps and/or the towing speed."

http://www.fao.org/fishery/geartype/309/en

Just found this link: ========================================================================================================================================================================================================================------------------================https://www.kongsberg.com/maritime/products/deck-machinery-and-cranes/deck-machinery/fishing-vessels/net-sounder-winches-fishing/------------------=============================================================================================================================================

https://www.kongsberg.com/maritime/Systems/marine-sensors-and-robotics/

Constant distance controllable trawl doors....by speed for width and up and down tow angle...