The invention relates to a trawler door of the kind described in the preface of claim 1. The trawler door is attached to trawler wires to obtain spreading of these by trawling. The trawler doors must have stability both at high and low speed, and also give the highest possible ratio between spreadforce and pullforce.

Trawler doors with two or more blades that are attached side by side are known, e.g. from British patent- application No. 523.452. Here, hcvever, the blades form a curved wall with slits between each pair of adjacent' blades. In this way the total surface area can be somewhat increased with a given blade area. A very favourable spreadforce/ pullforce ratio, however, will not be obtained with such an arrangement.

The object of the invention is therefore to create a trawler door where less pullforce at a certain spreading is needed and which also has stability and accept¬ able manufacturing costs. According to the invention this can be obtained by working out the trawler door corresponding to claim 1.

It has been demonstrated that such a door gives better spreading than known trawler doors and therefore gives possibility for boats normally having an engine which is too small, to accomplish an effective trawling. As an alternative the trawler door according to the invention can be made smaller and still keep the spreading properties which exists by bigger trawler doors.

Different advantageous features of the invention are given in the subclaims.

The invention will be described by referring to the drawings, in which

fig. 1 is a side elevation of a first embodiment of a trawler door corresponding to the invention, fig. 2 is an end elevation of the trawler door in fig. 1, fig. 3 is a plan view from the upper side of the trawler door in fig. 1 and 2, while fig. 4 and 5 are plan views from the upper side of two trawler doors which are modified embodiments compared to that shown i fig. 1-3.

The trawler door referred to in fig. 1-3 has as main components four wingshaped blades 11-14 that are attached in a row between an upper and a lower beam, resp. 15-16. To the lower beam 16 a tkeel formed as a runner 17 is attached and arried by a plate ISfThe runner 17 is curved up onto the lower beam 16 at the front edge 19. To fasten the trawler wire a bracket 20 is attached approximately in the center of the blades 11-14 in height direction, while for fastening of the trawler a trawling connection 21 is decreed which projects at the opposite side of the trawler door compared to the bracket 20. The trawling connection 21 is formed as a paw placed at the back end of the trawler door, seen in the pulling direction.

The bracket 20 is in the example extending over the whole door length, to- strengthen the construction and to distribute the forces that are to be transferred. A little in front of the center -it is widened with a wing 22, which forms attaching point for the trawler wire.

The blade 11 at the front edge of the trawler door has in the example form of a streamline with a front edge rounded off and a rear end getting narrower. The two blades 12 and 13 in the middle have about resiprocal paral- lelity and are also parallel with the adjacent side of the first blade 11. The blade 14 in the back is bent and placed in a way such that the rear part is extending about parallel with the main axis of the trawler door. All the blades 11-14 are crosswise curved and placed a little over¬ lapping with the concave side turned on to the attaching bracket 20. The front edge of the blades 11-14 forms an angle of about 30-40° with the main plane of the trawl _<

door.

An alternative embodiment as shown in fig. 4 where all the blades 11'-14' are formed from curved plates, and where a tube formed stay 23 resp. 24 is attached at every end of the trawler door between the upper and the lower beam. The other components are unaltered and the blades are also placed in about the same way as in the fore¬ going example, except that the rear blade 14 at its rear end is laying onto the rear stay 24 at the side where the water is streaming.

Another embodiment with only three blades 11", 12" and 13" is shown i fig. 5, where the front blade 11" and the rear blade 13" have a streamlined profile and the blade in the middle has form as a plate. The blades in front of the rear blade in the examples have an approximately even curving crosswise.

The main plane of each of these blades forms about the same angle with the main plane of the trawler door. As shown in the embodiment in fig. 5 this angle is in the example 15°, while it according to fig. 4 is 12 . This angle can vary a little from embodiment to embodiment

The angle between the door plane and the stream¬ ing direction of the water, which mostly falls together with the direction of the trawler wire, can be in the area around 35°. It has been shown that the blades ought to be placed in a way to obtain a blade angle or foil angle with the streaming direction of the water of 10-30 . This means that the blades should be placed in a plane which makes from about 12 to about 25° with the main piane of the trawler door.

The overlap between the single neighbouring blades is not essential, but the blades are placed in such a way that openings or channels through which the water streams are formed- side by side between two and two neighbouring .blades. It is not quite known what is the reason of the great spread effect which is obtained by means of trawler doors according to the invention, but it may £Qme from the effect obtained by forming more lamina _^ ia^-^-p

streams through the trawler door based on the embodiment of the . channels through which the water can stream.

Practical ^" fests ^" have shown that trawler doors embodied m correspondance to the invention need a dragging force which is less than, the half of the spreadforce at normal dragging speed.

OMPI