Smart approach to trawl door technology

Faroese fishing gear supplier Vónin was a relative latecomer to the trawl door business, but has achieved some startling results in only a few years, having practically from first principles with its Storm and Tornado doors. Now the Twister doors are showing what they can do.

According to Vónin’s managing director Hjalmar Petersen, the Twister pelagic doors are a new design, but inevitably draw on the experience gained with the company’s Tornado doors.

‘The Tornado doors were a big step in increased efficiency, allowing pelagic vessels to work with smaller doors, burning less fuel – and the Twister doors are another major advance in that direction, which shows that fishing gear development is a constant process,’ he said.

Crucially, the Twister doors already in use in the Faroe Islands and Iceland are adjustable, and are prepared for a control system that will allow them to be adjusted remotely – and Vónin’s approach differs from that of other developers seeking to produce controllable doors.

The Twister doors, available in both straight and V configuration, have four foils, with shutters that can be adjusted to alter the flow rate of water through two of the channels between the foils. Currently this can be adjusted manually with a crank, but ideas are in development that will make the Twisters into Smart Doors that can be pre-programmed for particular operating modes depending on the fishery, and to respond to readings from the integral sensors in each door. These sensors will be able to interrogate each other, allowing the doors to ‘talk’ to each other.

‘One fantastic advantage of the Twister trawl doors is the shutter system, which allows the spreading force of the doors to be altered very simply,’ said Høgni Hansen, skipper of pelagic vessel Høgaberg, which has been trying out the Twisters.

‘One crewman can adjust the trawl doors from the setting for surface fishing to going deep in a matter of minutes with a straightforward adjustment of the shutters. So far these trawl doors have been adjusted manually, but I am very much looking forward to when Vónin will fit the active control system to the Twister doors.’

The planned active control system is expected to be in the water before the end of the year.

‘This uses an app in a phone or tablet to connect to the doors so they are adjusted at the surface,’ Hjalmar Petersen explained.

‘The skipper can make his own settings and can also choose from a pre-set menu for different fisheries, close to the surface, or deeper, depending on the target species.’

Once shot away, the Active Control takes over, initially steering doors downward to the target depth, before reverting to the chosen settings for spread and depth, adjusting automatically to maintain position.

The new doors are reported to have already performed well, are responsive and stable even in a sharp turn, but the Active Control will keep the doors at a required depth, so that there is no loss of spread in a turn as happens with passive doors as one drops and the other lifts.

The doors are also designed to adjust automatically during a tow, maintaining the same spread as the drag of the codend increases during a tow, which would normally bring the doors closer together. The same function also adjusts the doors to compensate for side currents.

‘We have been using the Twister doors since last autumn on herring, mackerel and blue whiting fisheries. We have towed them at depth and at the surface. I have to say that we are very happy with the performance of these trawl doors,’ he commented, adding that Høgaberg’s 11m² Twisters replaced their previous 13m² doors, and have more spreading force.

‘The stability of the trawl doors is excellent, as we can see that the trawl maintains its shape and full opening while turning,’ he said.

The doors already in use on board Faroese pelagic vessels Høgaberg and Tróndur í Gøtu and to Börkur and Vilhem Thorsteinsson in Iceland are already prepared for the Active Control system to be fitted, turning these into Smart Doors – without the need for wireless communication between ship and doors, and without needing an additional hull transducer to be fitted.

Aside from the potential offered by the Smart Doors option, the Twisters have already proved themselves. The flow booster that has made the Tornado doors so effective has been refined still further and incorporated into the new doors, which have been shown to have 15-17% more spreading force than the Tornados.

The shutters in both upper and lower sections of each door allow them to be adjusted to seek up or down, depending on the fishery, and there is a substantial adjustment range between shut and closed shutters – but without a significant change in the impressive CL/CD figures that the Twisters doors have been shown to have

‘The stable drag across the adjustment range is good for fuel consumption, regardless of the settings,’ Hjalmar Petersen said.

‘These doors are a very big leap from the Tornados – which are very good doors in their own right, and the CL/CD of the Twister doors is around 3.60, which varies slightly depending on the shutter settings. We have also reduced drag by producing these doors with a higher aspect ratio and carefully designed end plates, which further reduces turbulence. These look to be the most flexible pelagic doors and while we haven’t said much about the Twister doors yet, there is certainly interest in them.’