Pelagic vessels Høgaberg and Tróndur í Gøtu have been putting the new Twister trawl doors developed by Vónin through their paces for the last few months.
The development has been kept quiet as Vónin’s designers set out to improve on the performance of the Tornado pelagic doors that have been shown to be among the most efficient doors on the market, courtesy of the integral flow booster that has given them plenty of spreading power for their size. Now the technology has been refined still further and built into the new Twister doors, giving them a 15-17% improvement in spread compared to the Tornados.
“The Tornado doors were a big step in increased efficiency, so trawlers could spread their gear with smaller doors, burning less fuel,” said Vónin’s managing director Hjalmar Petersen.
“Our Twister doors are another major advance in that direction. Fishing gear development is a constant process and this never stops.”
While the development of the Twister doors uses elements of the work that went into the Tornado doors, including the principles of the flow booster, they have been largely engineered from scratch.
“The Twister doors are a big step forward from what we achieved with the Tornados, and those are very good doors in their own right. The CL/CD of the Twister doors is around 3.60, which varies slightly depending on the settings of the shutters. We have also reduced drag by producing these doors with a higher aspect ratio and carefully designed end plates, which further reduces turbulence around the doors. What we see is that these look to be the most flexible pelagic doors” Hjalmar Petersen said.
“We are very happy with the performance of these trawl doors and have been using them since last autumn on herring, mackerel and blue whiting fisheries. We have towed them deep and at the surface,” commented Høgaberg’s skipper Høgni Hansen, adding that Høgaberg’s 11m2 Twisters replaced their previous 13m2 doors, and spread the gear more effectively.
“The stability of these doors is excellent, as we can see that the trawl maintains its shape and full opening while turning.”
While the Tornado doors have five foils, this has been reduced to four for the Twisters, with a shutter arrangement that allows the flow rate between two of the foils to be adjusted. The test pairs that have been trialled on board Høgaberg and Tróndur í Gøtu, plus the first production sets of Twisters supplied to Icelandic pelagic vessels Vilhelm Thorsteinsson and Börkur, have shutters that are adjusted manually using a crank.
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.
“There is stable drag across the adjustment range and this is good for fuel consumption, regardless of the settings,” Hjalmar Petersen said, adding that the shutter arrangement of the Twister doors is a step on the way to active control, and the Twisters already is use are prepared for systems now in development to be fitted.
Vónin’s engineers have gone their own way in developing the doors for control from the wheelhouse – turning the Twisters into Smart Doors. Instead of a direct wireless link to the doors that requires a transducer and a constant link between the gear and the catching vessel, the focus has been on coming up with a smart system with sensors built into the doors that allows them to interrogate each other and sense the distance to the surface – and incorporating software that pre-programmes them top respond to changing conditions.
This planned active control system is expected to be ready for full-scale trials before the end of the year.
“The Active Control works from an app in a phone or tablet to connect to the doors so the settings can be selected while the doors are at the surface. The skipper has the option of selecting his own settings, or making a choice from a pre-set menu for a variety of fisheries and conditions. This can be for fishing in deep water, or for towing close to the surface, depending on the target species,” Hjalmar Petersen explained.
As the doors are shot away in their smart mode, the Active Control is programmed for the shutters to be adjusted for a maximum sinking rate until they reach the target depth, when the settings selected for the tow take over. This adjusts the doors to maintain position, keeping them at the set depth during a turn to ensure there is no loss of spread. As the doors communicate and the spread is registered, the Active Control also seeks to maintain door spread, adjusting the shutter settings incrementally during a tow to compensate for the increased drag of the gear as the codend fills. This function also adjusts each door automatically to offset the effects of a side current.
“The shutter system is a fantastic advantage of the Twister trawl doors.This allows the spreading force of the doors to be altered very simply,” Høgni Hansen said.
“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. So far these trawl doors are adjusted manually, but I am very much looking forward to when Vónin will fit the active control system.”