DRIVE MEANS IN A BOAT

Abstract: Drive assembly in a boat, comprising a propeller drive (5) which is arranged on the outside of the boat hull (4) and has twin counter-rotating propellers (18, 19). The drive is drivably coupled to a drive unit (1) arranged on the inside of the hull. The drive assembly is characterized in that the housing of the drive is connected rigidly to the hull.

Description: DRIVE MEANS IN A BOAT The present invention relates to a drive assembly in a boat, comprising a propeller drive which is arranged on the outside of a boat hull and has an at least essentially vertical drive shaft which, via an angle gear enclosed in an underwater housing, drives in a counter-rotating manner a pair of at least essentially horizontal propeller shafts each with their own propeller, and a drive unit which is arranged on the inside of the hull and to which the vertical drive shaft is drivably connected. It is a known fact that, with an outboard drive with twin counter-rotating propellers coupled to an inboard engine, it is possible to achieve considerably higher overall efficiency than with an inboard engine coupled to a straight shaft with a single propeller in fast motor boats . Until now, outboard drives in fast motor boats have, with few exceptions, been of the type which is suspended steerably as well as trimmably and tippably in a shield in the flat of the stern of the boat. The advantage of being able to trim the drive at different angles in relation to the flat of the stern of the boat is that the drive angle can be adapted to the position of the boat in the water, which depends on loading, speed and weather conditions, so that optimum propulsion can be achieved under different operating conditions. The advantages of being able to trim the drive are most apparent in smaller and medium-sized fast-moving boats up to about 40 feet. The larger and heavier the boat is, the less its position in the water is affected by said factors and the smaller the need to be able to trim the drive. At the same time, the cost of the drive increases considerably, the greater the power that it is to transmit. For these reasons, inter alia, outboard drives are seldom used in boats in the size class over 40 feet, but in this case the engines drive straight propeller shafts with a single propeller via inboard-mounted reversing gears. It is known per se to equip larger vessels with straight, concentric and counter-rotating shafts each with their own propeller. Concentric shafts which are long in relation to their diameter have to be produced with very close tolerances and are therefore very expensive. Furthermore, mounting is complicated. Twin counter-rotating propellers have therefore not been adopted for use in large fast motor boats in spite of the fact that the overall propeller efficiency would be higher than in single-propeller installations. The object of the present invention is to provide a drive assembly of the type referred to in the introduction, which is primarily but not exclusively intended for larger fast motor boats in order to make possible higher overall propeller efficiency than with an installation with straight propeller shafts. According to the invention, this is achieved by virtue of the fact that the propeller drive is connected rigidly to the boat hull. A rigidly mounted drive assembly of this type, which does not have arrangements for trimming and tipping, can have its dimensions increased so as to transmit higher torques and power levels than steerable and tippable outboard drives, without the cost of the drive assembly becoming unreasonably high in relation to the gain in efficiency. The cost-saving achieved as a result of the absence of steering and tipping shafts, bearings, trimming and tipping cylinders, drive shaft junctions etc. more than compensates for the increase in cost for the larger dimensioning of other drive components. The drive assembly can therefore advantageously replace drive assemblies with straight shafts in heavier fast-moving motor boats so as to increase the speed of the boat at a given throttle level or reduce its fuel consumption at a given speed. In a development of the drive assembly according to the invention, the underwater housing of the propeller drive is connected to the boat hull and the drive unit via members which, in the event of a given load acting on the housing in the direction of the propeller shaft, bring about controlled separation of the housing from the drive unit and the hull. Violent grounding of a boat with straight propeller shafts frequently has serious consequences. Shafts and bearing brackets are destroyed and, not infrequently, the engines are pulled loose from their attachments resulting in damage to the engine seatings. In the worst case, the propellers may be pushed up against the bottom of the boat and tear holes resulting in leakage which in the most serious case may lead to the boat sinking. In the developed embodiment of the drive assembly according to the invention, the effects of grounding with impact against the drive are limited to damage to the drive alone. The invention is described in greater detail with reference to exemplary embodiments shown in the appended drawing, in which Figs 1 and 2 show diagrammatic side views of two embodiments of a drive assembly according to the invention. In the figures, 1 generally designates a drive unit consisting of an engine with a reversing-gear mechanism which is fixed on a seating 2 which rests against an inner surface 3 on the bottom 4 of a boat. An underwater housing 5 is fixed against a plate 7 which is pressed against an outer surface 8 on the bottom 4. In front of the plate 7, a streamlined body 7a adjoining the plate 7 is fixed against the bottom 4. A sloping plane 7c is fixed in a gap between the rear edge of the plate 7 and a plate 7b. The seating 2 and the plate 7 are interconnected and clamped against the respective surfaces 3 and 8 by means of screws 9 which extend with play through openings 10 in the bottom 4. A vertical drive shaft 11 extends through an opening 12 in the bottom 4 into the housing 5 and, via a bevel gear 13 and two bevel gears 14 and 15, drives a pair of propeller shafts 16 and 17, of which the shaft 17 is a hollow shaft through which the shaft 16 extends. The shaft 16 bears a propeller 18 and the shaft 17 a propeller 19. Cutting elements 20, which have cutting edges 21 facing towards the shanks of the screws 9, are fixed in the openings 10 in the bottom 4 of the boat. If, in the event of grounding, the housing 5 is exposed to an impact with a force F, which overcomes by a predetermined margin the frictional force between the seating 2 and the surface 3 and also between the plate 7 and the surface 8, the entire drive assembly 1 and 5 including the seating 2 and the plate 7 will be displaced and the cutting edges 21 of the cutting elements 20 will penetrate the screws 9 and shear these off, so that the housing 5 with the plate 7 is separated from the bottom of the boat. Sealing members (not shown) , which close the openings 10 and 12 and prevent water penetration when the plate 7 is removed, can be arranged in the openings 10 and 12. The force F required in order to detach the housing 5 is defined on the one hand by the frictional force between the seating 2 and the plate 7 respectively and the bottom 4, which in turn depends on the clamping force of the screws 9, and on the other hand the additional force necessary in order to shear off the screws. This force is defined by the dimensions and material of the screws and can also be influenced by virtue of the screws being designed with break indications. The drive shaft 11 is divided into two parts 11a, lib which are interconnected by means of a splined joint lie, so that the lower part lib can easily be detached from the upper part 11a when the separation takes place. The sloping plane 7c imparts to the plate 7 with the housing 5 a downwardly and rearwardly inclined movement in order to ensure that the shaft lib is drawn out of the sleeve lie. Comparative tests carried out using two identical 36 foot planing motor boats with twin engines, one of which had drive assemblies with straight propeller shafts with one propeller on each shaft and the other of which had drive assemblies according to the invention with twin counter-rotating propellers, revealed significant improvements for the latter. The top speed increased by 10% at the same time as the fuel consumption at top speed was reduced by 10%. At a speed of 25 knots, the fuel consumption was as much as 15% lower in the boat with the drive assemblies according to the invention. Fig. 2 shows a drive assembly which differs from the drive assembly in Fig. 1 only in that it has pulling instead of pushing propellers. In particular with regard to drives with pushing propeller (s) , it can be an advantage to connect the drive rigidly directly to the flat of the stern of the boat instead of to its bottom.

Claims: Claims 1. Drive assembly in a boat, comprising a propeller drive (5) which is arranged rigidly on the outside of a boat hull (8) and has an at least essentially vertical drive shaft (11) which, via an angle gear (13, 14, 15) enclosed in an underwater housing, drives in a counter-rotating manner a pair of at least essentially horizontal propeller shafts (16, 17) each with their own propeller (18, 19) , and a drive unit (1) which is arranged on the inside of the hull and to which the vertical drive shaft is drivably connected, characterized in that both propellers (18, 19) are arranged on the same side of the vertical drive shaft (11) . 2. Drive assembly according to Claim 1, characterized in that the fore propeller (19) has a greater diameter than the aft propeller (18) . 3. Drive assembly according to Claim 1 or 2, characterized in that the propellers (18, 19) are arranged on that side of the housing facing astern. 4. Drive assembly according to Claim 1 or 2, characterized in that the propellers (18, 19) are arranged on that side of the housing facing ahead. 5. Drive assembly according to any one of Claims 1-4, characterized in that the underwater housing of the propeller drive (5) is connected to the boat hull (4) and the drive unit (1) via members (9) which, in the event of a given load acting on the housing in the direction of the propeller shaft, bring about controlled separation of the housing from the drive unit and the hull .

Inventors: HEDLUND BENNY, HEDLUND, BENNY
Applicants: HEDLUND BENNY, VOLVO PENTA AB, AB VOLVO PENTA, HEDLUND, BENNY
Application year: 1998
Main classification: B63H5
IPC codes: B63H5, B63H21