Ulrik's blog
Ulrik in Nelson harbour
Experienced gear technologist Ulrik Jes Hansen from Denmark spent a year working with the New Zealand fishing industry. Exploring options to reduce the energy intensity of the catch sector was part of the joint energy efficiency programme between the Energy Efficiency and Conservation Authority (EECA) and the New Zealand Seafood Industry Council.
Education and training has been a big part of Ulrik's job. In his 26 years at the SINTEF North Sea Centre Flume Tank in Hirtshals, Denmark, he has viewed and tested almost every major net and door design and has over 800 trawl models in the archives.
We asked Ulrik to share his experience with us on this webpage. If you would like to ask Ulrik a question or comment on his postings please email ulrikhansenblog@seafood.co.nz. Questions and answers may be published on this page. Some questions may be edited to a generic issue.
More on trawl spread
Not long before I left New Zealand I got no less than three enquiries about door spread and the angle of attack of bridles and sweeps. There were two interesting things about this: firstly a few were from overseas readers, in Norway and the US; and secondly, one of them revealed surprising figures for the door spread.
Let's go back a step. The doors have two functions - to spread the warps enabling the trawl to open horizontally, and to help bring the trawl to the bottom, which it needs a certain weight to achieve. For those concerned with the possible impact on the bottom, I can say that most of the weight of the door is balanced by the upward lift in the towing warp and only small forces are left to create impact.
There might actually be a third function around the possible herding effect of the turbulence behind the trawl doors, but we do not know much about it. It would certainly make a lot of sense that turbulence might guide fish which are present between the doors down towards and into the net. Because the turbulence spreads like a cone after the doors, skippers would be wise to have a spread which let the bridles and sweeps closely follow the inside of the turbulence with no gap that the fish might escape into.
Bridles set up like that will normally take up an angle with a towing direction of 13 - 17 degrees. And that is exactly what most measurements on commercial trawls show. Underwater footage confirms that when the turbulence reaches the trawl net the sand cloud is sliding down just outside the wings.
You will therefore understand my surprise and frustration when a Norwegian skipper told me that extensive measurements with electronic instruments showed that his sweep angle was around 25 - 30 degrees! And he is not doing badly at all. In fact he is very successful.
The lesson is that we do not yet know everything about trawl doors, but it is not totally wrong to recommend a sweep angle of 13 - 17 degrees. After all that is what most experienced skippers end up with after years of trials.
Posted July 31st 2009 Comment
Trawl net testing
Q:
Dear Ulrik
Two months ago, I put on a new 150ft trawl net made out of Dyneema mesh. I have found the results to be very good, and my trevally and tarakihi catch has increased significantly.
I would like to know if there have been any flume tank experiments done on the nets? If so, where could I find a video or picture of the tests? What I think happens on the bottom may be different from what is actually going on.
Do you think there is another net design that would do the same job for me other than a scraper, which I have used for three years? It has worked OK, but when the fish get a bit lean we miss out totally.
Stephen
The true performance of a trawl is easily revealed in the flume tank
A:
Dear Stephen
Unfortunately, not many trawl types in New Zealand have been tested in flume tanks and documented. And even if they were, all the documents, pictures, videos would be the property of the maker of the net. It is best, therefore, if you contact the manufacturer of the net and ask if it has been tested and whether there is any documentation from the trials. I'm sure the company would provide you with this material, if it's available - isn't that why the trawl was tested, and found suitable for production?
There are probably several trawls which could be used in your fishery, but you should know that sometimes the same trawl can be rigged differently, giving a completely different performance in terms of height, spread, bottom contact and so on.
Altering your doors might give a better angle of attack to the sweeps and bridles for a new fishery. Slacking the upper or lower bridle could dramatically alter the height and bottom contact of the trawl and therefore also the species composition of your catch. Even minor changes to the hanging on the headline and footrope can improve fishing considerably.
A trawl is certainly a flexible piece of equipment, and is to be treated as such.
Ulrik
Posted December 10th 2008 Comment
Twin-trawl rig tested
Shooting away two trawls for the first time
Last week, in the middle of November, one of the medium-sized trawlers in Nelson was rigged for twin trawling. The whole system, except for the trawl warps and doors, was made up especially for the trials. That means that two small trawls were built and the clump, sweep and split-warp system were made to fit.
The vessel normally uses a 100ft to 120ft trawl with 80 meters of sweeps and 1.8m² modern trawl doors. For the twin rig we went out with conservative rigging - two 60-foot trawls with 50 meters of sweeps. We expected to be able to use the same doors. In front of the doors was a two-warp split-wire rig - i.e., 90 meters in front of the doors split warps were attached which could go to the centre clump weight, intended to keep the centre sweeps on the seabed. Anyone interested can request a specification from me.
The first two hauls were not particularly successful, but at the third attempt - after some adjustments - the rigging seemed to be squared up and working fine. Unfortunately we were not able to measure the distance between the doors (giving us the angle of attack of the sweeps) during this attempt, so some fine-tuning is still to come.
Hauling two full codends is very encouraging
The skipper was still a bit sceptical, but after having consulted with colleagues on the same fishing ground, he found out that we were out-fishing all of them.
The trials are part of a selectivity study, where a number of codends are tested against each other. The twin rig is by far the best way to make such comparisons. As for the result of this test, we will have to wait a few months until a reasonable number of tows have been completed, and the data has been analysed.
Long-term experience with a twin-trawl rig on small inshore NZ vessels is still to be gained, but the good results last week will certainly speed up the conversion process.
Posted November 21st 2008 Comment
Twin Rigging
A twin rig is two trawls towed by one vessel. The technique has been used for some time for shrimp trawling in the Gulf of Carpentaria in Australia. Trawls, trawl doors and vessels, however, are specific to this fishery and the method has little relevance to a New Zealand fishery.
Twenty-five years ago, another form of twin rigging evolved in the North Atlantic. Here, two normal bottom trawls are towed next to each other, with normal trawl doors and normal sweeps and bridles: everything behind the trawl doors is the same as in single trawling.
It is the way these two nets are towed which makes this method different, in that three warps are used, with normal trawl doors on the outside and a clump weight in the middle.
I have been promoting twin rigging for the fishermen I meet because it is generally recognized as being more efficient than single trawling. I am often met with an immediate comment that the skipper's vessel does not have sufficient towing power. But there should not be such concerns; just like a single trawl the selection of trawl sizes for a specific vessel is a very individual case, and often on a knife's edge. Hence it is always possible to find the correct size of nets for the vessel - as it is for a twin rig.
My prediction is that in a few years most of the inshore bottom trawlers in NZ will operate twin-rigged trawls. It took a similar amount of time following the introduction of twin-rigging for it to become established in Denmark, and later in other countries bordering the North Sea.
In a later contribution to this blog I will focus on how to square up a twin rig. However, fishermen interested in the idea are very welcome to contact me and I will guide them through the obstacles. In SeaFIC's Resource Centre you will also find several reports and articles on twin rigging.
I will also comment on a twin rig towed in two warps. This method will most likely be used by many NZ fishermen as an intermediate solution until they make the decision to install a winch for the centre warp.
Posted November 5th 2008 Comment
Successful workshops
I have just been travelling around the North Island. In Tauranga and Whangarei, I met with fishermen for day-long talks and discussions about trawl technology. In both places, 10 participants showed up in the morning. We covered a lot of topics and saw several small videos of underwater footage of trawls. Among the topics were the use of T90, trawl doors, twin rigging and fish behaviour around trawls. The meetings lasted until late in the afternoon, and we left no stone unturned!
Posted November 5th 2008 Comment
Buoyancy
Trawls need lift - that's a fact. But flow alone is not enough to provide that lift. The lifting force can be created either by static forces such as floats, or by hydrodynamic forces from for instance a kite. In general it is easiest to use floats and to forget all about trying out kites made from canvas, plastic, boards or other flexible or hard materials. They are often a troublemaker on a normal trawl.
In an earlier blog I mentioned that the resistance from the floats in a normal trawl is around three to five percent. Therefore it is very difficult to save fuel by altering the trawl's buoyancy, and kites are definitely not the way to go.
Kites are very difficult to mount because - just like trawl doors - they operate best at a very specific angle of attack. Moreover, they only give a reasonable lift at high towing speeds - that is, more lift for the incurred resistance.
The only place I can think of where kites have some relevance is with big-meshed mid-water trawls where floats can tangle up with the big meshes and do a lot of harm to the net at shooting and hauling.
Posted November 5th 2008 Comment
T90
The T90 codend, lower, has many advantages and a larger volume than the normal
T90 can denote either a Russian war tank, a Canon camera - or it can be used to describe the netting in a section of a trawl that has been turned 90 degrees in relation to normal diamond netting. In New Zealand trawl fisheries it has for some time been common to use a T90 section as a lengthener just in front of the codend, and it has been called a turbo lengthener.
When the netting is turned 90 degrees the meshes stay much more open, and that feature can be used to give more volume to a codend or it can be used to reduce resistance, when applied in the belly of the trawl.
It is only recently that the T90 features were revealed. In an attempt to find a codend with less turbulence than the standard diamond codend, flume tank experiments found that when the net was turned 90 degrees the result was a codend with a vastly increased cross-section area. This in turn resulted in a large reduction in turbulence. A T90 codend can therefore be seen to have little movement in the water, and fish are not washed from side-to-side. This will preserve the quality of the fish better than a net where the codend is waving from side-to-side for hours. The large volume also means that the codend is likely to catch more fish, a fact which has been proven in several fisheries.
It is vital for the success of implementing T90 in your codend that you join it to ordinary netting correctly. A joining ratio of three normal meshes to two T90 meshes is normally found to be fine. In thick twine it could be closer to 2:1 and in thin twine maybe closer to 4:3. If the joining ratio is not correct the net before or aft of the joining round might start pleating up.
Posted September 19th 2008 Comment
- and T45
And what is T45 then? That is a new term for square mesh! And it is a good term: square mesh is ordinary diamond netting turned 45 degrees. T90 has often been mistaken for square mesh, which is very wrong in my mind. Netting turned 45 degrees owes its ability to stay open from the forces going down the netting through a bar. No force is trying to close the mesh. Square mesh has the same qualities as T90 in preserving the quality of fish, catching more and a good selectivity. On the negative side it is only using half the number of bars to take the strain from the catch and it is not flexible when the catch builds up. In knotted netting the meshes will soon be drawn into a rectangular shape. Only knotless netting can expect to remain in a square shape, when turned T45.
Posted September 19th 2008 Comment
What is the right size of doors for my trawl?
These doors look large for the vessel, but are they?
The easy answer to this question is: "That is the size of door giving you the correct spread on your trawl". No more - no less.
In most cases it is therefore a trawl door giving you an angle of attack of the sweeps and bridles of 13-17 degrees, - depending on the fishery. Several sizes of doors can do that: you can have a large door set at a low spread or a small door set at optimal spread. Or you can try to work longer warps to reduce the inward pull and enable your small doors to spread properly, and vice versa. In some cases altering the length of the sweeps might also have an effect on the angle of attack of the sweeps.
In normal fishing it is often possible to make a check of the match between trawl door size and trawl size by altering towing speed: if the distance increases between your trawl doors when you are speeding up, the doors are large; if the distance decreases, the doors are small. Note that I do not claim that it is a mismatch to use small or large doors; you may have your reasons.
Posted September 19th 2008 Comment
Trawl monitoring
It is not easy to know how your net is performing while it's being used. However, there have for some time been instruments available to attach to your net, which can send information to the wheelhouse about the net's geometry. These instruments are well known in New Zealand, but there are only a few in use in the inshore fleet. There may be good reasons for that - for example, they are not cheap. But what if they are worth the money? Fishermen in other parts of the world give the impression that they are.
The most important piece of information about a net's operation is its spread. With this information you have a basic understanding of the well-being of the trawl. It is particularly important in twin rigging set-ups, with two warps.
The energy efficiency programme of SeaFIC - of which I am part - is going participate in a small number of demonstration projects. Therefore I have contacted SCANMAR, a Norwegian manufacturer of a wide range of trawl monitoring instruments, and have arranged for us to have access to a portable system for measuring door spread. It is a specially made portable system which can be taken on board different vessels without further installations. Contact me if you want to know more about this system.
Posted August 28th 2008 Comment
Distribution of drag in a trawl
If you want to invest in fuel savings you need to know where to best spend your money. The following table shows the drag of the different components in a trawl.
| Trawl warps | 5% |
| Doors | 20% |
| Sweeps and bridles | 6% |
| Groundgear | 8% |
| Netting | 58% |
| Floats | 3% |
These figures represents a generic bottom trawl, and there is naturally a lot of variation from trawl to trawl. However, it certainly points out that you should not spend a lot of money on changing the the warps or the sweeps, and that the floats don't need a lot of attention from an energy point of view.
Posted 26th of August 2008, Comment
Underwater video footage
One of the things which always catches a fisherman's attention is underwater videos showing trawl gear in operation and the reactions of the fish in front of the gear, as well as the behaviour of fish as they pass down the net towards the codend. There are a few research institutes with their own camera, but today a camera is often mounted on the net to reveal very specific information for a specific purpose. Such footage is not very useful. Only a few institutes have recognized the wealth of information they could provide to the industry about ground contact, trawl operation, trawl design and a lot more.
One of the most comprehensive collections of edited videos for the industry has originated from the Marine Laboratory in Aberdeen, Scotland. Unfortunately many of them were made more than 25 years ago. Not that the gear and fish performance has changed that much, but the media the videos is on is old fashioned and not many of us can play U-Matic tape any longer. Therefore it is significant that Marine Lab has recognized the World Heritage nature of these videos, and has allocated some time to digitise the tapes to DVD. Currently the price is unknown, but I'll advise as soon as I know more. Watch this space!
Posted August 25th 2008 Comment
Question
Sweep angle - Is that important?
Bottom contact and height are affected by the sweep angle.
One of the most frequent questions I have heard from fishermen is about gear spreading.
First of all let us define sweep angle as the angle of attack the sweeps and bridles have with the direction of towing. It is easy to understand that this angle is crucial for herding fish: A big angle means that the sweep is coming to the fish, very fast, perhaps too fast? A small angle means you are not covering very much ground because of the small distance between the doors. So there is an optimal angle, which is important to maintain and check, over and over again.
But the angle of sweeps and bridles has a number of other important implications. It affects the height and bottom contact of a trawl. A large spread will give a low height and a poor bottom contact around the bosom; a small spread gives more height but could lead to a digging in of the groundgear.
The optimal angle depends on a lot of different things: target species, towing speed, design of your net and so forth. It is normal to see figures of around 13 – 17 degrees. At a later stage in this forum we'll talk about how you can calculate the angle of attack.
Posted 25th of August 2008. Comment
Introduction
The flume tank - discussing the rigging of a twin trawl. The water is circulated and the bottom is a conveyor belt so that the trawl can remain in a fixed position for closer studies
My first step on this project was to travel around both the North and South Island giving workshops in the large fishing centres. Those of you, who heard and saw me, will remember that I pointed you to this site, and advised you to have a look here from time to time.
Well, now is the moment of truth! This is my first contribution to something I hope can be of use to fishers trying to make a living on catching seafood with a trawl.
I have worked with fishing gear technology and fishing efficiency since the 1970’s. In 1981 I held a position at a new private institute which was going to explore fishing gear. The institute was setting up a flume tank for testing fishing gear, mainly towed gear like trawlnets. The flume tank was built as part of the North Sea Centre in Denmark. The centre is situated in Hirtshals, one of the big fishing ports along the west coast of Denmark facing the North Sea.
The tank opened in 1982 and quickly became an important centre for European commercial trawl development. Fishermen and net manufacturers can come to test their good – and bad! – ideas. They can learn more about the gears' sensitivity to alterations in the rigging. More than once I have heard a fisherman saying "I'll go home and change this on my trawl immediately". That’s when you know the flume tank work is valuable and directly applicable. So far more than 800 scale models of trawls from all over the world have been tested, filmed, measured, admired, rejected, or they have formed the training focus for thousands of apprentices, young fishermen, skippers, students, and researchers.
See pictures from the tank at http://www.sintef.dk/.