otterboard for Trawler

1. OTTER BOARDS
By
Dr.Sudhansu Shekhar Mahanand

2. • Otter boards are devices used for keeping the
horizontal opening of the trawl net.
• Prior to the introduction of otter boards, the
mouth opening was maintained by means of
beams (wood or steel).
• An otter board was first used in Scottish waters
in 1894. Earlier, otter boards were directly
attached to the wings of trawl net.
• In 1920, Vigneron Dahl introduced the system
by which sweep lines and bridles were used to
connect the otter board to wings of trawl net.

3. • The common type of otterboard consists of a flat or
curved surface, a bracket or chain, back strop rings,
and a heavy iron shoe.
• The otterboard contribute to about 25% of the total
drag of the trawl system and thus responsible for
about 16% total fuel consumption in trawling
operations.
• The flat or curved surface provide necessary spread
force by diverting the flow of water, the bracket or
chain is used for attaching the trawl warp, the back
strop forms the connecting point for the legs or
bridles and the heavy shoe gives stability and helps
to keep the board close to the ground.

6. The terms related to the otter board
assembly are:
• Angle of attack: It is the angle at which the
otter board is presented to the flow of water
and it is the angle between shoe of the otter
board and direction of tow.
• Critical angle of attack: It is the critical angle
at which the sheer to drag ratio is maximum.
• Heel: The inclination of the plane of the otter
board from the vertical is termed as Heel. It is
termed either inward or outward according to
whether the inclination is towards or away
from the trawl warp.

7. • Tilt: It is the angle, which the otter board
makes with the horizontal. This is termed
as ‘positive’ or ‘negative’ according to
the movement of the board whether nose
up or nose down.
• Aspect Ratio: It is the ratio which
defines the relationship between the
height and length of otter board.

8. • For flat rectangular otter board, A = h/l
• And for oval, A = h2 / s
• Where:
h = height
l = length
S = area of otter board

9. • Divergence of the warp is the angle
between the warps
• Declination of warp – The angle between
the warp and the horizontal
• Both the divergence and declination vary
with the length and flow of water against
the warps.

10. Area of otter board is determined by using
two equation
• 1) S = 0.105 P + 4
Where ‘S’ = is the area of the otter board in
sq.ft.
‘P’ = Installed power of the engine.
• 2) S = 0.995 P
Where ‘S’ = is the area of the otter board in
sq.m.
‘P’ = Installed power of the engine.

11. • After determining the area, the length and
height of the otter board are determined
by using the simple equation h/l = A (‘h’
and ‘l’ are height and length respectively
of otter board and ‘A’ is the aspect ratio)
• For small trawlers ‘A’ is taken as ½ which
gives l = 2h from this ‘l’ and ‘h’ are
determined.

12. Weight of Otter board
• The weight of otter board and its distribution
along its length and breadth has an important
bearing on the stability of the otter board and
frictional resistance.
• The weight of the board must be determined
according to the dimension of the net but its
optimization has been achieved in the design of
the net. The weight of the board can be taken to
be proportionate to the engine power. For
engines below 100 HP, which are normally
engaged in coastal fishing,

13. • The relationship between weight of the board
and power of engine is given by the formula
W = 2.7 P
Where: ‘W’ is the weight of otter board in
pounds and ‘P’ is the HP of the engine.
• As the vertical component of the warp pull
exerts a great influence on the behaviour of the
otter board, vis a vis its weight, it is necessary
to increase the weight, if lesser scope ratio
(length of warp / depth of water) is adopted.

14. Material
• Otter boards are made of steel, wood or a
combination of both. In the case of coastal
fishing boats wood is used, as a rule for
construction of otter boards with iron
brackets, shoes and rings.
• All steel otter boards are not recommended
for small boats because of the high
fabrication charges and cost of material.

15. TYPES OF OTTER BOARDS
• Rectangular Flat Otter boards
• This is the least expensive form of otter board
and is still widely used for bottom trawling.
This can be fabricated either with wood or
steel, the smaller ones are commonly built out
of wood. The common timbers used are
venteak, aini, or other hard wood, which can
withstand sea water.
• The board is assembled by joining planks and
fixing them together with long bolts or mild
steel straps. A wide shoe is provided at bottom
to prevent the board from digging into mud and
is rounded off at the leading edge so that it can
ride over obstructions.

16. • These boards are not very efficient hydro
dynamically but are comparatively
inexpensive and easy to handle.
• They are however, not suitable for rough
grounds as they cannot easily tide over
obstructions. The Sheer to drag ratio of
flat rectangular otter board is 1.14. Aspect
ratio of this board is 0.55 and common
angle of attack is 40o.

17. Rectangular Otter Board

18. Rectangular curved otter boards
(Horizontally curved)
• Curved rectangular otter boards; both
horizontally and vertically are used
respectively for bottom and midwater /
pelagic trawling.
• These boards are scientifically designed
and are hydro dynamically more efficient
than flat rectangular due to stream lined
water flow of water. However, these
designs are not widely accepted and their
use is very much limited in our area.

19. Rectangular Vertically Curved
• These boards are very efficient hydro
dynamically and have been found to be
extremely good for one boat mid water
trawling. They have high aspect ratio of
2:1.
• Normally they are of steel construction
however wood is used for making these
boards for small vessels.

21. Oval Otter boards (Flat with slots)
• These boards, which have an oval shape, are
provided with a vertical slot at the joining of
the two sections of the otter board. These
bords can be made of wood, with steel framing
or complete covering with steel sheet.
• A thick shoe plate is provided quite often in
more than one piece, for easy replacement.
The keel is usually given a concrete filling to
increase stability.

22. • These boards have slightly better
hydrodynamic properties than flat rectangular
boards. The rounded lower edge though
adversely affects the spreading performance,
improve overall performance on uneven or
hard ground because it reduces ground friction
and mechanical stress.

23. • The main limitation of oval flat otter board is
its lower spreading force in plain ground as
compared to the curved rectangular board of
the same area. On such grounds, the oval board
will also have less spreading force than the
conventional rectangular flat board because of
less ploughing and for the same reason that
oval board has less bottom resistance so that
its efficiency is higher. These boards are
unsuitable for mid water trawling.