Thanks to all for the nice comments, I really do appreciate them. And thanks for the likes and taking the time to visit my build log.
It has been a while since my last posting and I hope you are all well and staying safe. After finishing the trawl winch, I tired of working on the model and dropped it like a dirty sock. Now after several weeks away from all modeling, it’s calling me back.
Commercial fish trawling on its face seems like a static and unchanging technology, but it has evolved greatly over time. One of the most significant changes came with the invention of the “otter trawl".
Otter trawling was developed in England and came to America around 1910. It derived its name from the otter board which was the name given to a sheering device that was being used for “hook and line” lake fishing in Ireland. This rectangular wood board would skate laterally across the waters’ surface and away from the direction it was being pulled. To some, the water disturbance caused by the board must have called to mind the activities of an otter. In commercial fishing, two otter boards are used together below the surface of the water to horizontally hold open the mouth of a trawl net. Like underwater kites, the otter boards are setup to push outward, away from each other, as the hydrodynamic pressure of moving water acts upon them. This was a major advancement over the “beam” trawl, which as the name implies requires a beam of some sort to keep the mouth of the net spread open. This beam arrangement severely limited the size of the trawl a single boat could tow.
Otter boards used in ground fishing actually skid along the surface of the seabed. The noise this creates attracts fish and the turbidity helps to conceal the oncoming net. The fish become fatigued swimming out in front of the nets' cavernous mouth and eventually begin to fall back past a point of no return and end up in the “cod end”.
The graphic below is from Seafish with text that I added.
Otter boards are commonly referred to as “doors" and their design has been greatly improved over the years, but in the 1920s they were typically flat and made of wood.
The doors in a trawling system work in equilibrium with drag from the net and pull from the boat. The doors have to be the right size and weight for the trawl net gear used, but the available horsepower of the towing vessel limits the size of the doors and how much hydrodynamic force can be applied to them. The doors end up as the pivot point between competing forces and have to accommodate both. It has been estimated that the doors account for about 1/4 of the total drag on the vessel.
Because I have a boat of known horsepower, I can use the formula below from CIFT to determine the maximum square foot area of a single otter board based on tow HP.
This 45’ dragger is powered by a 100 HP engine, so...
S = 0.105(100) + 4 = 14.5 sq ft.
The rectangular ratio of 2 length x 1 height is commonly used to define the boards’ actual dimensions.
Doing the math and rounding up gives dimensions of 5’5” x 2’8”. This is surprisingly close to the ballpark dimensions I estimated from photo scaling.
W = 2.7P determines the optimum weight of a single wooden door based on tow HP. W=2.7(100) = 270lbs. Heavier perimeter strapping and a beefier iron skid plate can be added to increase the weight if required. So if I ever build a full-scale dragger in my back yard - I'll keep this in mind.
My drawings below are based on documents from the 1940s and any photos of trawl doors I could find. The two doors will be identical except that they are mirror images of each other.
I also draw part cutting and locating templates.
The wood part of the doors is made from 1/16” basswood, providing a 3" scale board thickness. The wood is a single piece scribed to imitate individual boards.
I’m using aluminum beverage can sidewall for the strapping and tin for the skid plates. This allows me to simulate seabed scraping by exposing the bright metal. I find this easier than trying to replicate bare metal with paint.
To create the section of strapping with the radius that follows the forward edge of the door, I use a laser printer to transfer that shape to the aluminum. I first separate out the pieces in CAD to be used as templates.
The image is printed on regular copy paper and ordinary cellophane tape is placed onto the templates. The sheet goes back into the printer and this time the image is transferred to the tape. The only reason for the first printing was to know where to place the tape.
I peel the tape from the paper and stick it to the metal and cut it out. The tape transparency allows me to take advantage of the clean straight edge and square corner of the material. Clear laser printer labels would work great for this if you happen to have some lying around.
All strapping and skid plates are glued on.
The rear rings are made up from thin slices of .120” OD brass tube, which are held captive by a loop of annealed .014" brass wire. The back plate is cut from 1:87 rivet plate material and the diameter of the rivets are approximately tiny. I’m pretending that the rivets are actually the heads of bolts.
The fore bridle is .032” brass rod and the rear chain is 15 links per inch. The paper clamps as you can see are yet untrimmed.
Channel iron is glued into place using the drawing templates as a guide and injection molded nut/bolts are added. The channel iron is styrene strip “I” beams with one side sanded off. The rear rings are placed and some first attempts at color is added.
The fore and rear bridles are placed along with the associated bolt plates. Thin CA is trickled down the chain links to help keep it from slumping. The rear ring plates are also glued on.
Holes are drilled around the perimeter of the strapping and square nut heads are glued in.
The wood was weathered with chalk and alcohol and the strapping is pigment over a base of flat black enamel. If I had to do this over, I would use blackened brass rather than the aluminum, as I’ve had to touch up the black enamel several times due to tiny flakes popping off from the constant handling. So once I finalized the color and was happy with the look, they were over sprayed with a clear flat coat to stabilize everything.
A couple of items were intentionally left out for simplicity but mostly because of laziness - flat head screws holding the skid plates on and the tiny bolts for the thin upper strapping.
I have never set foot on a commercial dragger, but building this model has given me a greater appreciation into how much skill and know-how is needed to bring in a catch of fresh fish. Describing what an otter board is and what it looks like is one thing, but being able to set it up and trim its attitude to work efficiently underwater is an art learned through years of experience and hard work.
Suddenly, I’m hungry for fresh baked haddock with a slightly browned topping of panko and parmesan . . .
Thanks for stopping by. Stay healthy.