Let us first dwell for a moment on the Distance sensors: When Scanmar, more than 25 years ago, chose to develop these; it was mainly for the fishermen to know if the doors lie down.  Research reports we have studied showed that in certain areas errors could occur in 30% of the hauls. Simultaneously the varying door distance would immediately reveal that something was wrong. But for many the greatest reward was perhaps in connection with the rigging of new gear, after reparations, during shooting etc. In other words; an array of other useful “applications” that were not thought of the first time around, but came “on their own” after some time.

• Weld the holder to the upper door panel or central rib, as parallel as possible with the door’s vertical axis and its horizontal lower edge respectively.
• Place the instrument on the ground, or on a vertical surface. Press the measurement button.
• Insert the instrument into the holder and press the measurement button again.
• Enter the figure shown on the screen into the bridge instrument.

Now you will always see on the screen how much the door angles differ in the horizontal and vertical plane, and you can adjust the attachment to get it exactly as you want.

Door angles (bottom doors)

Here we are talking about three angles:

• Attack angle: Angle between trawl shoe and towing course
• Roll angle: The door’s roll angle inwards (+) or outwards (-)
• Pitch angle: The shoe points upwards (+) or downwards (-) 

Scanmar uses the doors’ vertical axis (the line from the door’s top to bottom) as an indication of the vertical position and roll angle. This is important for correct and a collective statement for all angles on all the doors, and not to mention to understand the forces that affect the door.  

The fishermen have “always” been most concerned with the Attack angle, and in “the old days”, in the lack of anything better, it was probably sensible. Today the situation is different.

Attack angle  
There is a range of different doors on the market, and the fishermen make their choice from their experiences, the door’s construction etc. We will not get into that, but look at what makes a door, any door, as efficient as possible.  

There are big differences between the different doors when it comes to attack angle for largest possible square force; it can vary from a little under 30° to a little over 40°. The most interesting thing here is that most doors seem to have an area of about 6-8° around the optimal point where they have about the same square force in the entire area. What is interesting with this is that one can fish at different depths (change the warp lengths) without it having any significant meaning for the square force. Thus; if the door is rigged correctly for the area being fished in, we can forget about the attack angle.

Roll angle  

The port door loses bottom contact, the trawl collapses. (1+2) Alot of pitch and roll makes the door unstable and loses bottom contact. (3) the trawldoors lay down and lose contact.

The roll angle is the factor that most of your attention should be directed to. We all have an understanding of the trawl door’s crucial meaning for the trawl to fish efficiently, but do we share the same thought of what actually makes a door efficient?

Studies of trawl door efficiency have been conducted, where the square forces and towing resistance are measured in flume tanks; the measurements are taken while the doors are vertical and with different attack angles. Although this can be interesting information, it may perhaps also contribute to taking the focus away from what is important:  

What is important is how much the square forces reduce when the door is not vertical; how much do you have to increase the towing speed in order to achieve the same square force, and how much will the fuel consumption increase as a result of the increased towing speed?

We will look a bit closer at these questions:  

If we disregard the square forces that emerge from the door’s friction against the seabed (relatively insignificant for modern doors), which increases somewhat when the door tilts a little outwards, and reduces when the door tilts inwards, we have the following conditions:  

A small roll angle means little; up to 15° means no more than 3-4%. If the angle increases more, up towards 30°, the force reduces with 13-14%, and if we get closer to 45° it will be 30%. From these numbers it seems that it means little if the door’s vertical position changes a bit, but over 15° it will have significant consequences for both square forces and fuel consumption.  

Efficiency curves (square forces in relation to towing resistance) for trawl doors have been created, but since they are of little importance in comparison to the significance of the roll angle of the doors and the towing resistance form the trawl, we can disregard it.   

We can then make the following, very simplified overview for how much it “costs” to increase the speed in order to obtain the same square forces on the trawl doors:  

The coefficients for the trawl doors’ square force and towing resistance is measured in a flume tank when the doors are vertical, and since they are close to each other in value and the majority of the towing resistance (80%) comes from the trawl and gear, we can ignore them in a very simplified calculation:  

Given that the towing resistance increases 5 times the square force multiplied with the increase in towing speed; meaning that if the towing speed doubles from 1 to 2 knots, the towing resistance quadruples. For trawl doors of 4 square meters this means that the towing resistance increases from 400 kg to 1600 kg.

If we imagine the same basis, but the trawl doors tilt approximately 30° inwards, the door area that is responsible for the square force (together with the towing resistance) is reduced by about 0.6 square meters. This means that the towing resistance has to be increased substantially in order to have the same square force, which again leads to a significant increase in towing resistance and fuel consumption.  

If the towing speed usually is at 3.5 – 4 knots, the effects will be very significant.

One should focus on the meaning of roll angles between 15° and 30° and you can then say that the lost square force lies between 5 and 15%, and make a rough estimate of how much the towing speed and fuel consumption increase. (NB! Remember to use the correct door size and towing speed in your calculations). 

What also is important to many, is to make sure to rig a bit differently depending on depth conditions/warp lengths.  

Another thing; many do perhaps have trawl doors that are not quite customed to the trawl, in which case adjustment of angles, also the attack angles, can be of great significance.

NB! Be aware that when the towing speed increases the doors can straighten up, and a tall trawl will lose its height, so that the towing resistance does not increase as much as the calculations would imply.  

Although what has been mentioned above are some of the greatest advantages of using a DoorAngle sensor, there is an array of other beneficial elements as well:

• In the shooting phase the Door angles instantly show if something is about to go wrong, and you can interrupt the shooting and wait for the angles to show that the door is in the right position again.
• They are almost indispensible in the landing phase; you immediately see when the doors hit the seabed, and can begin to tow before the doors lie down.
• You immediately discover if a door loses contact with the seabed, or is about to, so that necessary steps can be made.
• Errors and damage that occur while fishing are instantly reviled by a change in the door angle.
• The door angles can be adjusted so that they react quickly in pelagic towing near the seabed or surface. 
• It is simple to make changes in the towing speed, course or warp lengths if the doors are in different depths.

We have also received feedback on several other advantages, perhaps especially when fishing semi pelagic in a lot and varying current, but it would take too long to touch on all the elements here.