Door control from the wheelhouse
The idea of a pair of trawl doors that can be adjusted at the turn of a dial has been extensively experimented with.
Rather than shifting attachment points and backstrops, dial-controlled door operation has been at the back of many people’s minds over the years – and there have been numerous experiments and trials carried out to get this to work.
It has taken a while, and finally the first commercially-produced controllable pelagic trawl doors are available, produced by Esbjerg company MLD, an acronym that stands simply for Multi-Level Door. According to MLD’s sales manager Jan Allan Müller, the doors have been through a long process of testing and this extended trial period to be sure that every bug had been ironed out was part of the company’s strategy from the outset, with the first pair tested on board pelagic vessel Fagraberg, owned by Faroese company Framherji.
He said that Fagraberg’s skipper Øssur Hentze has been very positive about the MLD doors, having used then across the vessel’s various fisheries ranging from targeting mackerel close to the surface to towing for blue whiting during the difficult spring fishery west of the British Isles when bad weather, heavy fishing on dense marks and strong currents all come together to put pressure on any fishing operation.
The MLD doors operate with controllable flaps that can be adjusted while the gear is in the water, opening or closing the upper and lower flaps to keep the gear square and the trawl fishing at its most efficient.
From the outside they don’t look to be hugely different to many other trawl door designs on the market, but the difference can be seen behind the flaps and World Fishing took a look at the workings of the MLD doors at this year’s DanFish exhibition where they were given their first public showing.
Each upper and lower section has a hydraulic mechanism that adjusts the flap, with an electronic control system and an integral computer that talks to the catching vessel’s wheelhouse via an acoustic link – carefully configured with its own IP address to ensure that once there are more of these systems in the water, that one trawler’s signals cannot interfere with another’s gear.
Batteries in the doors are charged via a charging port while the doors are in the gallows and the lithium batteries have a 36 hour lifetime between charges, depending on how much adjustment is made while towing.
Alongside the mechanics of the MLD doors is a control system that makes full use of the integral depth and pressure sensors, providing the skipper with a set of possibilities that aren’t an option with conventional doors. The MLD Autodepth feature allows the skipper to simply punch the required depth into the control system and once the doors are shot away, they stabilise at that depth, with a pre-set tolerance for automatic adjustment that brings them back to the required depth if they drop or rise.
Even in a turn, this means that the trawl doors remain at their pre-set depth, keeping the gear fully open as the catching vessel comes around. Under normal circumstances, the door on the inside of the turn would drop down and the outer door would rise, but this doesn’t happen with the MLD doors that compensate automatically to keep them at the required depth.
“These are good pelagic doors, with excellent CF/CL properties in their own right,” Jan Allan Müller said. “If there’s a problem with the control system, the flaps close, the doors default to a zero setting and they can just be used as conventional doors to finish a trip.”
Through the trial period on board Fagraberg there have been a few problems – but not with the doors themselves. At one point Fagraberg had an engine problem at the height of the summer mackerel season that kept them from fishing for a week, but they still completed the season ahead of any other Faroese pelagic vessel.
Jan Allan Müller said that opening all of the flaps has the potential to increase spread by 40%, and according the skipper Øssur Hentze, the doors are easy to work with and hang comfortably in the gallows.
The level of technology in the MLD doors means that they don’t cost roughly four to five times more than conventional doors, although MLD is certain that this high initial cost is offset by higher efficiency and better fishing.
“We tell people that these doors will improve fishing capacity by 5%, which is a cautious figure,” he said.
The doors attracted plenty of interest at the DanFish exhibition, with a crowd around the MLD display throughout the three-day event, and Jan Allan Müller said that they are following up on options for several customers. A second set of MLD doors is already on the way, built for another Faroese vessel, the former Asbjørn, now Nýborg, and the next step for MLD is to take its experience with controllable pelagic trawl doors to the demersal sector.
MLD has a semi-pelagic door on the drawing board, and according to the company, this draws on the technologies they already have, so the development time for these is going to be shorter. These are aimed at trawlers towing demersal gear with the doors flow off the ground. The thinking is for these doors to be fitted with both downward- and forward-seeking sensors to identify obstacles up to 200 metres ahead while also tracking the seabed below, with the flaps adjusted automatically to ensure that the doors remain a set height from the ground.
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