The Bluestream Type 22 trawl doors introduced by Thyborøn Trawldoor four years ago have been highly successful, but now the company has pushed development even further, while also preparing for full door control of the doors in the water.
The new Type 32 Bluestream doors now in use not only show a significant increase in efficiency, but these are also prepared for remote control functionality – a process that Thyborøn has had in development for some time, but which has been held up by the Covid pandemic.
“Our technical partners in Norway haven’t been able to travel during the pandemic, and that has really held us back on this,” said Thyborøn Trawldoor’s Henrik Andreassen, adding that the parallel developments they have been working on to produce the high-efficiency Type 32 doors and the control options promise a winning combination.
“The Type 22 doors have been extremely successful and we have shown that these are trawl doors with the best CL/CD on the market today, with satisfied customers around the world. Over the last couple of years we have put a lot of resources into research and development – both towards control while towing and to achieve even more effective designs.”
The Type 32 doors have been tested initially on board trawlers in Greenland and Iceland, followed by a 10m2 pair being supplied for further testing on board pelagic vessel Frank Bonefaas, replacing a 12m2 pair of Type 22 doors.
The first production doors have also reached customers, with Faroese pelagic vessel Jupiter heading for mackerel grounds in international water with its 12.50m2 Type 32 doors which were delivered in Kollafjørður, and Shetland pelagic vessel Antares called in Skagen for a 12.50m2 pair of doors.
Skipper Graeme Sandison said that the new doors replace the 15m2 pair of Type 20 doors Antares has been using for the last five years.
“These should give us the same spreading force as an 18m2 pair of Type 20s,” he said as Antares sailed from Skagen after a call at Karstensens for a paint job and some work on the tanks.
“We were looking for some more spreading power for the blue whiting. But first we’ll get them tried on herring,” he said.
“The look good and hang well at the stern. It’ll be interesting to see how we get on with them. But the Faroese skippers think these are good doors. We’re very impressed with the hydraulic foils. These make it very quick and simple to adjust the doors.”
Refinements to the hydrodynamics, working with CFD simulation and with wind tunnel and flume tank testing, have built on the success of the Type 22 doors and the Bluestream technology to develop something new, while working within the overall dimensions and door weight largely unchanged.
“The results have been better than we could have expected,” Henrik Andreassen said.
“These Type 32 doors have significantly more spreading force than the Type 22s – and those are the best on the market. The Type 32 doors have a deeper curve to the foils which forces a larger volume of water through the door. That makes them both more powerful and gives them even greater stability. This isn’t a small increase – this is a big step in terms of generating spread,” he said and added that the deeper profile of the doors, combined with the high tensile steel that Thyborøn Trawldoor uses, makes these doors even stronger than previous pelagic trawl doors, traded off against a slight increase in weight.
The Type 32 doors are tailor made for each customer and can be supplied either straight or in a V configuration, plus they can be rigged to either V or parallel bridles, and protective door keels can be added for towing close to the seabed.
Ready for full control
It has been a few years since Thyborøn introduced its Flipper option to trawl doors, providing skippers with the choice of altering the surface area of their doors by having these closed or open, but the technology has moved on rapidly in going from passive doors to trawl doors that are fully controllable during a tow, as well as adjusting the door parameters to fit the trawl gear.
“We’re very impressed with the hydraulic system,” said Graeme Sandison on Antares. “These are 12.50m2 doors, but we can adjust them down to 9.50m2 if that’s what we need.”
Despite Covid restrictions hampering co-operation with Thyborøn’s technical partners, progress has still been made and the company’s engineers have been developing a hydraulic system that allows the towing point to be adjusted remotely.
Combined with adjustable foils that are the latest evolution of the original Flipper concept, the aim is to offer complete control of the doors with adjustable flow through both lower and upper sections of each door, so that spread and positioning can be adjusted as required for the depth, target species and other parameters. This allows the same doors to be set for stability when working in deep water, and can also be re-set to be fully effective close to the surface.
According to Henrik Andreassen, the previous manual setup, which is still an option, requires the crew to get access to the doors to change the foil positions and lock them in place before the gear is shot away.
“This isn’t always easy on a moving deck in heavy weather, so we have introduced a hydraulic arrangement. To make an adjustment, the deck crew have to just connect a hydraulic hose, and a couple of pumps with a manual pump places the foils where they should be. It’s a lot safer, and quicker to adjust the doors this way,’ he said.
As well as being quicker and safer to make adjustments via a single easily accessible hydraulic connection point, this option also offers a much wider choice of setting, making fine adjustments possible.
“This is a step in the process,” he said.
“The aim is for this to be operated wirelessly, with the doors in the water. We can already make a range of adjustments to the doors by altering towing points and foils, and once we can do this remotely during a tow, we’ll have fully controllable trawl doors.”