Guidelines for the Construction and Rigging of Common Steel Vee Doors
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Guidelines for the Construction and Rigging of Common Vee Doors
Guidelines for the Construction and
Rigging of Common
Steel Vee Doors
-1-Guidelines for the Construction and Rigging of Common Vee Doors
Common Vee Doors
Many fishermen are misled by the efficiency of the more modern door designs, if they had a better
understanding of the potential of a correctly rigged vee door then the difference in performance between
the more modern designs their present door would rapidly diminish.
These guidelines will attempt to provide a better understanding of vee doors and point towards more
efficient ways of improving their performance and rigging.
1. Otterboard Design
The common steel vee otterboard or trawl door is very simple in construction and versatile in
performance. It will perform reliably in a multitude of conditions and can perform efficiently if it is rigged
correctly for the prevailing ground and sea conditions.
Generally vee doors are made entirely of mild steel although plastic versions are made. Occasionally
hardened steel shoes are added to improve wear resistance but these are considered an extra.
Warp towing points can be hinged bracket or towing chain depending on the manufacturer. The
differences between towing with a chain or a bracket are explained later.
The main structure is bent along the horizontal plane to form a vee angle of approximately 25° to 30°
dependent on manufacture. This inherent design produces a rugged, very strong and stable
configuration and helps the otterboard ride over stony ground. It also permits easy recovery if the doors
collapse when a turn is completed.
Both port and starboard otterboards are designed to be symmetric around the middle vertical axis and
both otterboards are therefore identical, if the towing arm is dis-regarded. This reduces cost and also
enables the fisherman to interchange the otterboards when the shoes are part worn.
The generic design calls for large radius top corners. This is a useful feature and prevents damage on
wooden vessels when hauling. The bottom corners also have a generous if smaller radius, this assists
them to climb over seabed obstructions.
Overall, the vee otterboard is simple in design and construction; this promotes economic production.
-2-Guidelines for the Construction and Rigging of Common Vee Doors
2. Otterboard Characteristics
Aspect Ratio
The aspect ratio describes the relationship between the height and the length of the door. If a door had
a height of 0.5m and a length of 1m then it would have an aspect ratio of 0.5; this is achieved by
dividing the height by the length. Vee doors are usually built with an aspect ratio in the region of 0.65.
This means that the height of the door is just over half its length.
Consider two near identical Vee doors, one with an aspect ratio of 0.65 and one with an aspect ratio
of 1.0; the one with the lower value will have better low speed stability than the other.
Angle of attack
This is the angle between the shoe of the door
and the direction along which it is been
towed.
Heel angle
The heel angle refers to its natural behaviour
to lean inwards or outwards while towing.
Pitch
An otterboard is described as pitching
when it is lifting up at the front or
rear of the door while towing.
-3-Guidelines for the Construction and Rigging of Common Vee Doors
3. Otterboard Rigging
3.1 Warp
Towing chain: Otterboards with this method of
attachment are easy to store due to the absence of
brackets, are economical to produce, simple to
maintain and have a multitude of adjustments
available. The chain acts as a very simple form of
hinged bracket. Problems may occur if the chain
stretches due to poor quality or simply wear, both
will affect the angle of attack, heel or pitch.
Evidence has suggested that shooting in heavy sea conditions may require a change of technique to
prevent the tension being released from the chain when hit by or dropping off a wave. This may be
particularly true if only a single or twin backstrop system is used. Chain brackets fitted with triple
backstrops tend to perform better in heavy weather.
Horizontal hinged bracket: Using an
otterboard fitted with this type of attachment
produces excellent rough ground
performance and is favoured because of the
hinges' ability to allow the otterboard to
pivot over large boulders. It can be stored
easily on the vessel with the bracket folding
across the face of the otterboard.
The disadvantages include an obvious additional construction cost due to the requirement for careful
setting up of the pivoting arm during manufacture. There is also a maintenance element; the hinge bolts
need to be checked regularly for wear and stiffness in order to prevent deterioration of performance.
Also if the arm is damaged or bent the hinge joints will stiffen and shooting may be more difficult; it has
been observed that doors with this problem will collapse onto their backs when touching down on the
sea-bed.
3.2 Backstrops
Most otterboards are rigged with one, two, three or four chain backstrops. Numerous adjustments are
available with chain systems and they are versatile, easy to adjust and to service.
-4-Guidelines for the Construction and Rigging of Common Vee Doors
1,2,3 and 4 Chain Backstrop Systems
It is recommended that all backstrops regardless of gear size are made from quality chain and all
backstrops are cut from the same length, this will avoid un-even wear or stretching. If wires are used
they will eventually kink or may have been made with un-even lengths, small differences will lead to
errors in the heel angle.
If rope is used due to the differing snatch loads applied during towing the ropes will stretch differently
and the heel will be effected.
Only when using short lengths and towing directly from the top and bottom of the door to the headline
and footrope of the net is it advised to use rope.
-5-Guidelines for the Construction and Rigging of Common Vee Doors
4. Angle of Attack Adjustments
4.1 Warp
It is important when setting angle of attack of the door to consider the prevailing weather, sea and
ground condition. Reliability is paramount and efficiency may have to be sacrificed in order to achieve
this.
The angle can be measured using the combination guage (see below). If the angle of attack is too high
then door spread will not be maximized and also more fuel than necessary will be burned.
If reference is made to the graph of the spreading forces acting on the common vee door (Page 6) it can
be seen that the best angle regarding door spread is between 28o-30o degrees. Angles as low as 33
degrees can be achieved on good level grounds and sea conditions; in poor weather or stony ground,
to achieve reliability, the angle of attack may have to be increased to 35 degrees or more; experience
will dictate what angle can be reliably maintained.
-6-Guidelines for the Construction and Rigging of Common Vee Doors
Generally, adjusting the warp position is considered a coarse adjustment of the angle of attack. To
increase the angle of attack the towing point needs to be moved aft or away from the face of the
otterboard. Angles of attack above 40° are considered inefficient and produce high fuel consumption
and low spreading performance. If a reduction in angle is required then the towing point will have to
be moved forward or closer to the face of the otterboard. Care must be taken to avoid angles of attack
below 30°; poor shooting behaviour and instability often occurs with many designs at these low angles.
The adjustments for the different types of warp attachments are discussed below and in addition these
are illustrated by diagrams. Included on each diagram is an arrow which indicates the adjustment
necessary to give an increase or decrease in the angle of attack.
Chain towing point adjustment: The warp can be shackled to the towing chain by either one or two
chain links.
The angle of attack can be adjusted by either inserting/removing links from the towing chain or by
moving the warp shackle along the towing chain.
-7-Guidelines for the Construction and Rigging of Common Vee Doors
Horizontal hinged bracket towing point adjustment: The hinged bracket arrangement is a little
restrictive and usually only three options are available to change the angle of attack. This provides a
quick and simple means of changing the angle of attack. However, if lower angles are required then an
additional bracket can be welded on to the warp arm or a modified arm with more holes designed. (see
Seafish drawing and section on construction).
Vee otterboards work well if the upper face is
near vertical; the otterboard will then skim
over the ground and produce good spreading
characteristics, however, if the otterboard heels
over further onto its back a depressing force
will be created and the otterboard will dig into
the seabed. Its efficiency will be reduced if worked in this manner.
Although the efficiency of both vee and flat otterboards are similar in practice, when changing from flat
to vee otterboards fishermen tend to chose one size larger. This can be attributed to the additional
spreading force generated by the outward heel and digging action of flat otterboards on normal or mixed
grounds. This would not be applicable on very soft grounds as the towing point would be raised for a
flat otterboard to prevent outward heel and thus prevent the otterboard from burying.
4.2 Backstrops
Single Backstrops
Vee otterboards are regularly worked with a
single chain backstrop on rough ground. This
helps the otterboard pivot over large rocks and
pinnacles. There are no set rules on the length of
the single backstrop, this depends entirely on the
working method of the individual. The only
consideration is the weight of the chain; if this is a
very heavy short link chain it may have an effect
on the attitude of the otterboard.
Twin Backstrops
Twin chain backstrops tend to be used more on
cleaner grounds, in shallow water or where
precise control of the heel angle is required.
It is recommended that the length of each leg of
twin backstrop chains is between 1.0 and 1.5
times the length of the otterboard.
-8-Guidelines for the Construction and Rigging of Common Vee Doors
Design Problems
Care must be taken with the design and positioning of the door backstrop attachment brackets. Many
designs allow both the single and double backstrop chains to foul the adjacent vertical stiffeners. This
fouling will be evident by abrasion or damage on the stiffeners and this will prevent fine adjustment at
efficient angles of attack.
For vee otterboards with equal upper and lower plates, equal backstrop lengths are normally used
unless working in shallow water or at low towing speeds when a single shackle can be inserted into the
lower backstrop leg.
If the backstrops are much shorter than the length of the otterboard then the sensitive adjustment using
shackles will be lost. Fine adjustment of the angle of attack is normally carried out using the backstrop
chains.
With both single or double backstrop and the angle of attack is changed by moving the chains to a
different hole.
Three and four backstrops chains
In this arrangement the forward long chain is
normally equal in length to the distance from its
forward attachment point to the aft end of the
otterboard. Shortening this chain will increase
the angle of attack. The two short aft chains
can be adjusted to help control otterboard heel.
Vee otterboards with the upper plate larger
than the lower will have the upper backstrop
longer than the lower in a triple system.
Care must be taken when adjusting the three
and four chain systems to ensure all chains
remain in tension, although this is very difficult
with four chain systems. If only an angle of
attack change is required then both aft chains
must be adjusted by the same amount. If only
one chain is adjusted then the heel angle may
inadvertently be altered.
Use of 3 and 4 chain systems can increase the
speed with which an otterboard will stand up
and spread following collapse onto its face. When tension comes on the sweeps/bridles the upper
backstrops come tight but the lower ones remain slack. This gives a pull on the upper half of the
otterboard raising it off the sea bed. Changing from a 1 or 2 backstrop system to a 3 or 4 backstrop
system will initially result in an increase in angle of attack. Three and four chain systems are adjusted
as shown below.
-9-Guidelines for the Construction and Rigging of Common Vee Doors
5. Adjusting the Heel and Pitch
The common steel vee door is usually constructed with the towing point on the horizontal centre line.
At angles of attack greater than 30° if the backstrops are adjusted in length, only the heel angle will
be affected; no significant change in pitch angle has be found to occur.
If the bottom chain is lengthened then the otterboard will heel outwards and potentially a greater
downward force is exerted; if the top chain is lengthened then the otterboard will heel inwards and the
downward force is reduced. Extensions needed to adjust the heel angle are small and should be
carefully considered. If extensions greater than 5% of the length of one of the backstrop chains are
inserted these will cause radical changes in the heel angle and are therefore not recommended. The
effects of backstrop adjustment on heel are shown below.
- 10 -Guidelines for the Construction and Rigging of Common Vee Doors
Adjusting Angle of Heel for 3 Chain Backstrop Systems
Looking at the position of the polish on the shoe will normally indicate the heel and pitch angle the
otterboard was working at during the previous tow. Care must be taken to consider the seabed
conditions as undulations or towing with one otterboard along a bank may well temporarily change its
normal working attitude and hence the polish.
Heel and pitch angles are interrelated and occasionally adjusting one will affect the other.
- 11 -Guidelines for the Construction and Rigging of Common Vee Doors
Heel and pitch angles are affected by any or a combination of the following:-
• Height of warp attachment point
• Warp out
• Otterboard weight
• Ground conditions
• Rigging arrangements
• Towing speed
Often, for the same rigging arrangements, a change of fishing ground from soft mud in one area to hard
sand and stones in another will induce different friction characteristics and produce a different pattern
of polish on the shoe. Minor adjustments may be required to correct for any ground ‘effect’.
Heel angle adjustment: The heel angle can be adjusted in five ways:-
• Moving the towing point up or down
• Changing the warp/depth ratio
• Increasing or reducing the weight
• Adjustment of the backstrop chains
• Changing towing speed
An outward heel can be reduced by adjusting one of the following:-
• Raising the towing point
• Adjusting the backstrop chains
• Increasing the warp out
• Reducing the speed
• Increasing weight
An inward heel can be reduced by adjusting one of the following:-
• Lowering the towing point
• Adjusting the backstrop chains
• Reducing the warp out
• Increasing the towing speed
• Reducing weight
• Increasing the upper plate height
- 12 -Guidelines for the Construction and Rigging of Common Vee Doors
Pitch angle adjustment: The pitch angles of most otterboards cannot be controlled by the adjustment
of backstrop chains. This applies particularly to otterboards with straight shoes worked at normal
angles of attack and in firm contact with the sea bed.
There are other influences on the pitch of an otterboard, these are as follows:-
• Towing point height
• Warp out
• Otterboard weight
• Speed
If the otterboard is heeling in and also pitching up at the forward end then:-
• Lower towing point
• Reduce the warp out
• Reduce otterboard weight
• Increase speed
To reduce pitching down at the forward end of the otterboard when it is heeling out:-
• Raise the towing point height slightly
• Increase warp out
• Increase the otterboard's weight
• Reduce speed
- 13 -Guidelines for the Construction and Rigging of Common Vee Doors
6. Otterboard Shoes
The shoe of an otterboard is usually straight. The forward part of the shoe is curved up in order to ease
the passage over irregular grounds. Straight shoes give a good contact with the ground. However, oval
types of otterboard are fitted with curved shoes which are usually made in sections. This allows worn
sections to be replaced individually.
The digging effect of the otterboard depends on the width of the shoe and the rigging arrangement. On
muddy grounds it is recommended that wide shoes are used or the rigging is modified so that the digging
effect is reduced.
The shoe's wear rate depends upon the following factors:-
• Weight of the otterboard
• Width of the shoe
• Resistance to abrasion of the steel material
• Ground conditions
• Rigging arrangement on the otterboard
• Towing point height
All will contribute to how fast the steel shoe will wear away and hence the length of time between re-
shoeing.
If excessive shoe wear is experienced on good ground then the otterboard may be too heavy for the
warp/depth ratio or the shoe material could be soft mild steel.
If the otterboard is heeling out in excess 10°, then this will drive the otterboard into the seabed
generating heavy wear and increased fuel consumption. Adjusting the towing point slightly upwards will
help. If this is not possible then the length of the lower backstrop chain can be shortened to reduce the
heel angle.
If ground conditions are hard then the following is suggested:
• Replace the shoe with a tough abrasive resistant material (e.g. a redundant length of railway
line) although this can be difficult and expensive if welded in place. It is very important after
re-shoeing to check that the warp towing point is at the correct height.
• Increasing the width of the shoe will spread the load and should reduce wear.
- 14 -Guidelines for the Construction and Rigging of Common Vee Doors
7. How to Reduce the Spread of your Trawl Doors
Sometimes there is a requirement to reduce the spreading power of trawl doors, this can be achieved
in the following manner:-
• Reduce the warp out
• Increase the angle of attack (excluding Bison design)
• Reduce towing speed
• Reduce bridle length
• Increase length of upper twin backstrop by one shackle
• Cut out a section of the trawl door
Reducing the warp out is the simplest and quickest method and has the capability of drastically
reducing the door spread. The doors will pitch up on to their nose and heel out and they maybe
unstable, but they will be easily pulled over rocky ground.
Increasing the angle of attack can be carried out by moving the warp shackle away from the face
of the door. This has the capability of reducing the door spread by approximately 10% but will increase
the fuel consumption; the door should still be stable.
Reducing the towing speed can be used but the ground coverage will suffer and potentially fish
caught.
Shortening the length of bridle will also reduce the door spread but again the catching area will
diminish.
If working a twin backstrop an additional shackle in the upper leg will allow the door to heel more
this should reduce door spread but stability will be effected and difficult shooting could result.
A drastic solution would be to cut out sections of the door, this could be in the form of a sliding or
replaceable bolt on panel. Careful thought should be made of the panel design or the structural integrity
of the door could suffer.
It must be considered that if the door spread is dramatically reduced then the
gear and net geometry will change which will alter the catching potential of
the trawl.
- 15 -Guidelines for the Construction and Rigging of Common Vee Doors
8. Vee Door Construction
Background Information
As explained earlier in the text the swinging arm or chain will normally be mounted on the centre-line
of the completed door, this will allow good all round performance. Any extra shoes added after
manufacture may up-set the positioning of the towing point raising it above the centre-line. This may
well cause excessive heeling in when towing at or just below normal towing speeds.
Manufactures of vee doors vary the weights of the doors considerably. Vee doors should be heavy
enough to run on the bottom at all times without lifting off when turning, if the doors are over-weight for
the fishing conditions they will dig hard and plough the sea-bed causing excessive fuel consumption and
wear on the shoes and with additional handling problems. The required weight of a door will vary
according to the warp to depth ratio, the towing speed and the ground conditions and can only be
accessed by the individual.
8.1 Trawl Door Size
A graph indicating door size against vessel H.P. is included below. This has been derived from
information gathered over the years by Seafish officers; it is a basis to start from when selecting the
door size, but it can only be considered a very rough guide.
Graph not available
The door size should be accessed according to the net size and its construction. Two nets of the same
construction and size but different twine thickness will ideally require different door sizes but this is not
always possible in practice; also the towing speed will have an obvious effect on the door size.
Once the overall length has been selected then the height can be calculated. This is achieved by
multiplying the length of the door by the Aspect Ratio which for a vee door is 0.65. Therefore the
dimensions for a 6ft.- 0in door would be as follows:-
72in x 0.65 = 46.8
- 16 -Guidelines for the Construction and Rigging of Common Vee Doors
Hence a 6ft vee door with an aspect ratio of 0.65 would be 72 x 47in
Using the drawing below it is possible to work out most of the major dimensions for construction
purposes. All the dimensions are expressed in the form of percentages of the length; for example the
corner radii are 15% L this can simply be worked out as follows:-
1% L = 72 ÷ 100 = 0.72 in
10% L = 72 ÷ 10 = 7.2 in
15% L = 0.72 x 5 + 7.2 in Total = 10.8 in
8.2. Centre of Gravity
This can be located by lifting the door up by the towing arm clear of the ground, the centre of gravity
is a point directly below the suspension point. This should be below the horizontal centre line and
approximately on the vertical centre line. If the centre of gravity is above the horizontal centre line then
difficulty with the door shooting may well be experienced. It is worth checking the freedom of the
towing arm as this point, if held against the top surface of the door and given a light push it should drop
under its own weight and hit the lower plate. If it sticks on the decent then it will probably stick when
shooting; this may lead to a door falling onto its back or not standing up correctly. A matching set of
doors should have the same centre of gravity. If the centre of gravity is to high the doors weight should
be re-distributed.
If changing from one make of doors to another consider the difference in the weight in water. Wooden
doors will loose up to 50% of their dry weight in water where a steel vee door will only loose 15%
therefore there will be a considerable difference on the sea-bed.
8.3 Construction Drawings
For additional information three drawings have been included; one 4ft.-6in., one 5ft.6in. and the other
6ft.-6in. Both 4ft.6in. and 6ft.6in. designs were developed by Seafish staff for projects on trawl doors
and both were built and tested successfully. The 5ft.6in. is a drawing of a door used successfully in the
south coast. The Seafish doors have been worked on different fisheries, at different depths, speeds and
warp out variations to establish what variations can be made to a basic design and to compare a
common vee door with the ability to be set up at the correct angle of attack.
A noticeable feature is that there are more than the standard three holes in the warp arm of the Seafish
doors, it was necessary to vary for experimental purposes the warp position and to achieve an efficient
angle of attack; the standard arm of three hole configuration would not allow this angle to be selected.
- 17 -Guidelines for the Construction and Rigging of Common Vee Doors
8.4 Backstrop Attachments
When machining the holes for the single backstrop make sure that they will allow the backstrop chain
to clear the rear stiffener when in use. Also positioning the twin backstrop brackets must allow the
chains to clear the stiffener in all positions.
Single and Twin Backstrop Adjustment
8.5 Shallow Water Design Modifications
When a door is to be used in shallow water and with long warps it is essential that the door is designed
and manufactured as light as possible with a very low centre of gravity. To achieve this the weight needs
to be concentrated low on the door and the upper section very light.
With warp to depth ratios commonly 6:1 in shallow
waters, to cope with the added weight of the warp
horizontally in front of the door it is useful to design
the height of the upper section 15% greater than the
lower section. This will assist in stabilising the door
and the door and the door spread by allowing the
upper plate to adopt a vertical orientation.
- 18 -Guidelines for the Construction and Rigging of Common Vee Doors
Scaling Seafish door sizes:
If a different door size is required but based on the Seafish design it is possible to work out the various
sizes by using a scaling factor. This can be calculated by dividing the required door size by the Seafish
door size.
e.g. Required door size 3ft-6in Seafish door size 4ft-6in
Therefore divide 42in by 54in = 0.77
Multiply all dimentions and sizes by Scaling facture = 0.77
If a larger door size than the Seafish door is required:-
e.g. Required door size 8ft-3in Seafish door size 6ft-6in
Therefore divide 99in by 78 = 1.28
Multiply all dimentions and sizes by Scaling facture = 1.28
Please note that all component sizes must be scaled down as should all thickness and diameters. If steel
is not available in the selected size then the nearest size should be obtained. Remember to concentrate
the weight as low down on the door as possible as this will assist low speed pick-up and stability.
- 19 -Guidelines for the Construction and Rigging of Common Vee Doors
9. Estimating Otterboard Spread at Sea
In order to enable a comparison to be made between the various rigging arrangements it is necessary
to have a method of assessing the otterboard spread. One common method adopted is the warp
divergence technique. This method assumes that the warps do not have any curvature and a very simple
calculation is all that is required to estimate an otterboard spread. To calculate an otterboard's spread
proceed with one of the following two methods.
Towing from a single point: Mark a point on each warp one fathom down from the towing point,
then measure the distance between the two marks on the warps as shown in fig ? Due to the movement
of the warps it may be necessary to take an average over 15 to 20 seconds. This measurement, `the
warp divergence', is then multiplied by the length of warp out in fathoms. The result is the distance
between the otterboards. It may be more convenient to measure over 1 metre and multiply by the warp
length in metres. Accuracy of the divergence measurement will be improved by subtracting any small
separation that exists between the warps in the towing point itself.
- 20 -Guidelines for the Construction and Rigging of Common Vee Doors
Towing from the gallows: Measure and record the distance between the towing blocks, then mark
a point one fathom down on both warps and measure the distance between the warps at this point as
shown below. Subtract the measured distance between the gallows to find the `warp divergence'. The
warp divergence is then multiplied by the warp out. The distance between the gallows is added on to
give the distance between the otterboards.
When comparisons are made between tows any errors generated in the calculation by the curvature in
the warps will be constant and so cancel each other out.
This method has been used in conjunction with electronic systems and has been shown to be reasonably
accurate when comparing different otterboard settings, although varying sea and weather conditions may
add errors into the calculations.
If otterboard spread is determined from the calculation then inevitably it will be on the low side
compared with the true value. This is due to the curvature of the warps. Errors are dependent on warp
length and are usually in the order of 10 to 15%.
- 21 -Guidelines for the Construction and Rigging of Common Vee Doors
10. Common Problems Affecting Otterboards
This section is intended to identify the most commonly occurring faults and describe how to correct
them.
Incorrect or no Polish on Shoe and Poor Shooting Performance
• Towing point incorrectly set for type of seabed
• Too much extension in upper backstrops.
• Warps too short for depth of water.
• Towing speed too high.
• Otterboard too light.
Differing Polish Marks on the Shoes of a Pair of Otterboards
• Unequal backstrop chains.
• Backstrops stretched or twisted.
• Towing points on otterboard out of alignment.
• Redundant shackle left in backstrop leg
• Otterboards not constructed similarly.
• Length, height and towing point variations between otterboards.
• Towing with one otterboard along a bank or in a gully
Both Otterboards Heeling in or Out Excessively
• Otterboards too heavy or too light for depth of water.
• Warp out incorrect.
• Wrong backstrop rigging arrangement for fishing conditions.
• Towing points incorrectly set for conditions experienced.
• Incorrect towing speed for otterboard rigging.
• Seized or tight pivot points on vee otterboards
Worn Shoes
The rate of shoe wear is dependent on several factors. The position of the wear or polish can indicate
potential problems. It is usual to see the shoe polish along three quarters of its length; the forward end
is rarely worn or polished due to an upward supporting force applied by the warp. As the shoes wear
the heel angle of the otterboard will be affected.
Bent or Damaged Brackets
This will affect the angle of attack and heel of the otterboard. The damage must be corrected as soon
as possible.
- 22 -Guidelines for the Construction and Rigging of Common Vee Doors
Loose or Worn Brackets on Flat Otterboards
The heel of the otterboard will be unpredictable. Shooting may be troublesome.
Worn Pins on Hinged Towing Arms
Towing point height slightly affected depending on warp out. The otterboard will heel progressively
inward as more warp is paid out.
Difficult Shooting with Hinged Vee Otterboard, Variable Pitch Experienced
The hinged towing arms should be free to fall under their own weight from the upper face to the
lower face. If they are stiff this may well be the cause of the problem.
Running Backstrops
Using large sliding shackles on continuous chain will prevent positive control of the heel angle. After
shooting it will lock in a random position and predicting the heel angle will be impossible. Evidence
suggests that the shackles do not move during normal fishing operations, but instead they will only
be encouraged to move if the otterboard hits large boulders or pinnacles. A roller block is often
used instead of a shackle to help overcome the problem.
Incorrect Warp to Depth Ratio
If too much warp is used the otterboard will heel in and the warp in front of otterboard will polish.
The result will be poor otterboard spread. If too little warp out is used then the otterboard will
pitch up at the forward end and heel out. Again the otterboards will be under spread.
Heavy Wear on Point of Towing Brackets
Considerable wear on both otterboard towing points indicates that:-
• The otterboard is too heavy for the towing speed
• Upper backstrop too long for fishing conditions
• Warp attachment point set too high
• Too much warp out
Heavy Wear on the Point of Bracket of One Otterboard
Possible faults:-
• Upper backstrop too long
• Warp length too long on affected side
• Warp attachment point higher or damaged
• Vessel turning continuously to one side
Light Polish on Towing Point
This is frequently found and it is caused during turning or when stopped and hauling in. This is not
a problem.
- 23 -Guidelines for the Construction and Rigging of Common Vee Doors
For further information contact :-
Adrian Strickland
Flume Tank Engineer
Marine Technology Group
Sea Fish Industry Authority
St Andrew’s Dock
HULL
HU3 4QE
Tel: (01482) 327837 Fax: (01482) 223310
or Flume Tank Fax No: (01482) 587013
Email a_strickland@seafish.co.uk
The Sea Fish Industry Authority, 1995 Ref: 1995/30/FG - V3
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