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OTTER BOARDS.ppt
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- 2. • Otter boardsare 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 commontype 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 relatedto 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: Itis 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 flatrectangular otter board, A = h/l • And for oval, A = h2 / s • Where: h = height l = length S = area of otter board
- 9. • Divergence ofthe 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 otterboard 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 determiningthe 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 Otterboard • 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 relationshipbetween 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 boardsare 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 OTTERBOARDS • 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 boardsare 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.
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- 18. Rectangular curved otterboards (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 boardshave 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 mainlimitation 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.