Which Has More "Spreading" Power?

As no theoretical model exists for such an arrangement, particularly one where the channel walls are undefined, the best thing to do would be to build it and try it.

Purely a guess from this old cat.
But I'd go for the louvre arrangement with the muliple planks.
My gut feel is it would give a more laminar flow, consistent, even, controlled flow and more of the energy (drag) would be deflecting the water and thus the planks rather than creating random turbulance behind it.
But heck what do I know? Cat don't like water
Certainly my comments would apply if we were dealing with airflow.
Damn you can never find an aeronautics engineer when you need one.
Del

Although the flat face area of the two arrangements is the same, the four-plank scenario has an additional three faces at 90deg from the flat surfaces on the leading side, and an additional three faces at the same angle on the trailing side, which is going to introduce more drag and turbulence compared to the single plank arrangement.

The distance from the arrangement to the channel walls is going to have an effect, as is the distance between the planks in the 4-plank arrangement, and the speed of the fluid impinging on them in both arrangements.

In the limit, increasing the number of planks to the same width as the thickness means that there will be no difference between the front face dimension and the side face dimension, so the thing becomes a simple bar-screen, and the only thrust it will experience is the drag in the direction of the Stagnant Streamline; there will be no side thrust at all. So the thickness of the planks is also of interest.

If the pieces are fully immersed, the arrangment of the bracketry holding them onto the feature that is to receive the side thrust needs also to be evaluated.

Why are these things being made of rectangular section and not a hydrofoil section instead?

Google "boundary layer effect" and "bridge scour" for more bedtime reading towards this topic.

Someone with more understanding of fluid mechanics than me might have a better explanation - but I'm thinking in terms of swimming: If I try to propel myself through the water with my fingers splayed open, I get very little 'pull' from my arm stroke; but if I close my fingers together the 'pull' is significantly better.

Actually, if you check some of the internet sites devoted to swimming techniques, there have been studies done on the effect of finger spacing on the swimmer's "catch and pull" of the water. It was found that the most effective pull was with the fingers slightly spread apart, less than a full finger's thickness. Yes, a cupped hand does more than widely spread fingers, but the ideal is someplace in between. Here is a link:

https://io9.com/5933306/science-settles-some-decades+old-debates-about-the-best-way-to-swim

The planks would probably have less turbulence drag.....

Even with their rectangular cross-section?

I would angle the ends.....but say this is traveling through the water at 3-5 knots, once you reach full spread you are then dragging the structure, minimizing the turbulence is important....I would probably just go with a stainless steel single plate and vent it near the leading edge....

With a 1" separation, the wide plank wins. The logic for this is that the 2nd, 3rd and 4th planks are shielded by the previous one so there is less total width, less water pushed aside. There is a further effect from having them so close together; while pressure builds up on the front, pressure is lowered on the back if there is room, I think that 1" is not sufficient for the full effect.

ding! Ding! DING!

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Winner! Winner! Chicken dinner!

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Also, even if the flat plates were not stacked, the effectiveness would still be reduced.

JavaFoil

You don't see many airplanes with more than one stacked wing any more.

There must be a reason for that.

Go with the single piece wing. Much simpler and easier to do. One piece instead of six.

Ah, but then you don't see many airplanes at low speed, underwater catching fish

Del

Baaaaaaaaaadddd kitty.

Here are a couple.

Sad, really.

Check out foilsim and javafoil

These are free, fun, and informative simulations for questions just like yours.

both will flex and eventually snap, but your question is really more about surface area, vs. rigidity. are you building a wave power generator?

I can't belive you actually took the time to run a projection haha

In my opinion, nick name is one of the most valuable resources CR4 has for in-depth technical analysis, and advice.

use the term or metaphor of your choice, I'm still surprised you took the time to run it

May I make 2 remarks:

- my comment was Nr 17 your text indicates that Lyn did the simulation which is wrong.

- you used the word "metaphor" at the wrong place, "term" was enough, for the right usage of "metaphor" may I warmly recommend to read the meaning description in wikipedia or any other English dictionary

And as last comment I have a high consideration of those as "netmaker" who try to survive in a complex world where they need competent help. Further more I consider that we -at Cr4- have a responsibility with respect to the OP and should give correct answers qualitatively and quantitatively when we feel that the need is real.

What if the 4 plates are moved apart sothey have the same frontal area as the single?
Del
(scampers off to hide)

THANK EVERYONE FOR THE INPUT.

I have been using the SOLID PLANK arrangement for years now and , no they do not break . They are used on the surface to open a special net ( Mamou Trawl).

I need them to be light and inexpensive as this allows for more low budgeted universities to use them. Built in aluminum or lightb steel they are too heavy and require a machine shop. My wooden design is made for under $200 with steel ballast and float tank.

What I am curious about is if I can make a configuration in the same slotted design as my steel pelagic doors only without the dihedral effect and all the bells and whistles.

After all, I JUST want to remain on the surface, at under 3 knots and spread a 12 meter wide x 4 meter net ( 12mm mesh). Drag is a very serious factor but the gear has worked well under most conditions.

Why work on something that is already working? The answer is that I am asked for bigger gear and smaller mesh ( more drag). I just want to look at some options. I need to increase spread when I venture into the smaller mesh (2mm, 4mm and 6mm ).

Any suggestion using either of the two sketches with minimal adaptions???

Thank you all for your time and as always, it is greatly appreciated.

'....Any suggestion using either of the two sketches with minimal adaptions???....'

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The low hanging fruit is the AOA. Large increases in 'spreading force' along with reduction in drag are almost certainly available by simply reducing the angle of attack from the current 45° off parallel to something around 15°.

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You can do a very simple simulation in with the free Nasa program FoilSim, in probably less than 5 minutes total.

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Maybe Nickname might even be so kind as to run a more sophisticated simulation.

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Either should that cutting the AoA in half from the current 45°. should yeild a large reduction in drag and large increase in lift for flat plates under water at the speeds under discussion.

I have a margin of 35 to 45º for the 48" doors to work on a 12 meter net. Less than 35 and the spread of the net decreases greatly, but speed picks up. In trawl terms its called 'walking the dog' as the doors simply trail behind the boat like a dog on a leash.

To increase spread with an angle in the 30º range, we have to speed up considerably and that is difficult for some of the scientists to do with the smaller boats they work from.

Thank you for the comments though. I have a unique situation in these gears as they are designed for smaller boats than your standard commercial vessels. They have limited hp, no winches and only 1 or two extra crew.

Thank you again for any input.

That is perplexing.

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I am probably misunderstanding something about the set up.

Is the angle measured from parallel to the direction of flow, or is it being held constant with regard to the tow line?

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If the angle is with respect to the flow, there should be much more spreading force with angles less than 45°, even if it doesn't initially spread as fast (kind of analogous to lower gearing...).

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If on the other hand the angle is with respect to the tow line, I can see that with a little spread, the angle with respect to flow would be reduced to near zero....

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so.... the angle is with respect to the tow line.... or am I still off-base?

You are correct with this.

So, at typical full spread, your included angle between the tow lines is about how much?

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Around 70°-75° with the spreaders angled at 45° off the towline? Is that close?

Another way to improve the 'spread' would be to use a plate with a higher aspect ratio...

..say 96 x 12 instead of 48 x 24...

If those dimensions make it unwieldy... perhaps it could have discrete hinges and locks?

The picture shows the specific lift and drag forces you have only to multiply by the density ,the area and the velocity squared and you obtain the forces. As one sees there is a point where the lift starts to decrease and the drag continues to increase. This point is at about 35°. It thus NOT and advantage to increase the angle over 35° since the drag becomes more and more important and there is a loss of lift.Values are for a flat plate in water.

Good stuff! Thank you.

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Any way you can enlarge the image?

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I'm also curious about a couple things on the graph:

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1. I don't see any of the loss of lift I expected to see in the mid-lower angles that would correspond to stalling. Is that not relevant in these conditions?

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2. What is the bizarre sudden drop in drag at about 43 degrees?

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If you have the time/inclination I'd really appreciate being schooled a little on what I'm misunderstanding here.

You'd probably be better off for the real life situation by simply grinding some simple curves on the single plank. Generally, long thin blades stall out easily, and weirdly. Since your spreader boards can be asymmetrical, no problem. Try putting end caps on the side with leakage, like the upturned tips on modern airplanes.

If the distance between plates is 3" the picture is different:

One sees that the flow is less disturbed. But The problem is more complex: the deflector has to be maintained in the right position with respect to flow.

If the PLANKS are spread further than 1", will that help in the spreading power?

By 'up-turning' the end with leakage, do you mean curving the backside to slant away from the main body of the gear? I could conceivably add on a an 'end cap' of some light gauge aluminum to each plank????

My deep water pelagic doors have curved steel segments) and are dihedral. They spread quite easily at almost any speed. They are just too heavy and too expensive for these simple projects sampling surface fishes. However, I understand what they are doing . Trying to accomplish the same thing with wooden planks may just be outside the realm of possibility.

we may be stuck with just using what we have.

Any suggestions are still welcomed.

since you are interested in the reaction force to flow of water presented against an area, I conclude that the one that presents the greatest normal area to the flow (the sheet of plywood), will move with the most force.

I can attest to that James, In my younger days I worked construction. Doesn't take any calculations at all to realize that a single sheet is the way to go. Just hold one up to the wind and compare the force with holding your slatted design to the same wind.

Just don't do this on a roof. Plywood makes a crappy parachute.

Thanks. That provided a moment of "lightness" this afternoon.

Not totally true it depends on the "over-lap" of the plates. If the distance between two plates is big enough then the effect of 2 can add. And then 2 is more than one or the surfaces can be reduced for each plate but the effect stays the same as for one plate.

.....so if I over lap the planks by spreading them further apart, I might pick up some additional spread?

Hi Netmaker

Your multi-plate design ain't going fly well. Staggering the planks a bit more (like the modern trawl doors do) will give you better results.

You can get a good indication of Cl and Cd values from this document. Look at the values of the Flat Wooden and the Hydrofin Multifoil doors.

It might also provide some clarity to you other non-fishing types (No offense intended!!)

That was a pretty comprehensive report.

The solid Mullet doors have been doing ok so far but now we are going into larger and/or smaller mesh.

I understand the curvature and dihedral effect.The Pelagic doors I build for my midwater nets are 6 panel, cambered and both top and bottom segments are tilted on an angle. The plates are 3" apart...the door spreads excellent from around 10 fathoms to 350 fathoms deep with a 15 meter net. But they weigh too much for a hand operation surface gear. Even at the epipelagic setting they require a 3 knot speed.

This surface gear works on fish under 50mm in length and can only be pulled effectively by these small skiffs, at under 3 knots. The pelagic design stalls at around 2 knots + or-.

With this research gear I just need a little more kick to open the net and I need to keep it floating, simple and limited in size as these researchers are working from small skiffs and usually hand operating the gear. 48x24 is the absolute max I can get away with. I just thought I'd throw out some ideas and see what this gang came up with.

They are a smart lot.

Thank you again for the information and I hope you all have had a good year so far with work? Its been good down here too ...small scale and low key but interesting. Take care and stay away from MEGA SHARK!