Foldable propeller

Technical field

The present invention relates to a foldable propeller for a boat, comprising a hub for fitting to a driveshaft of the boat and at least two propeller blades pivotably arranged about a spindle of the hub between a substantially folded first position and a substantially deployed second position according to the preamble of the subsequent claim 1.

State of the art

Use of a so-called foldable propeller on sailboats is well-known. Such a propeller is usually adapted to being usable together with an engine to propel the boat forwards or rearwards.

During sailing, a propeller gives rise to a certain drag resistance when it is not in use. For this reason, propellers are made foldable, i.e. with propeller blades fastened to a hub pivotably so that they can be folded together (by the boat's movement through the water) in the direction defined by the propeller's driveshaft, to a position in which they extend in the longitudinal direction of the boat. When the propeller is to be used, the blades are deployed by centrifugal force due to rotation of the driveshaft. In their folded state, propeller blades are normally designed to constitute a streamlined body, thereby reducing drag resistance.

A number of foldable propellers are previously known. For example, WO 93/01972 refers to a propeller comprising at least three blades which are arranged to pivot between a deployed position and a folded position.

Owing to their streamlined shape, propellers with blades which can be folded together usually result in little drag resistance, but the propulsive action of these known propellers is relatively poor when travelling forwards and, above all, when travelling rearwards. A particular problem of previously known foldable propellers concerns propulsion power when travelling rearwards. High propulsion power rearwards when the boat is moving forwards, e.g. when decelerating close to a landing, is usually achieved by increasing the weight at the blade tips and thereby increasing the centrifugal force which causes the blades to pivot about their pivot spindle. The angle of opening of the blades is thus increased. However, increasing the weight at the blade tips entails problems either in the form of thick blade portions with bad cavitation characteristics, or in the form of long blade portions which tend to reduce propeller efficiency, particularly at increased boat velocity, when the propeller is acting in the forward direction.

A further problem of known foldable propellers is the relatively large forces which occur when propeller blades deployed by the propeller being caused to rotate reach their limit-position stop, resulting in undesirable noise and a "power shock" which is transmitted via the driveshaft and the engine to the hull.

A general problem of propellers is achieving both high propulsive force and high propeller efficiency when travelling forwards at any desired velocity. The general solution to this problem is large propeller diameter combined with low driveshaft speed. In addition, the propeller's radial load distribution needs to be optimum and the blade area large enough to prevent cavitation. Last but not least, the blades need to have thin bent sections of the aerofoil type. Providing foldable propellers with thin blades is a particular problem in that doing so often conflicts with the need for the blades to be heavy enough to ensure that they will always deploy.

Summary of the invention

The object of the present invention is to provide a foldable propeller which solves the problems mentioned above. This object is achieved with a foldable propeller according to the present invention, whose characteristics are defined in the subsequent claim 1.

The invention thus relates to a foldable propeller for a boat, comprising a hub for fitting to a driveshaft of the boat and at least two propeller blades pivotably arranged about a spindle of the hub between a substantially folded first position and a substantially deployed second position. The propeller is distinguished particularly by:

the fact that the propeller blades take the form of a first and a second blade element, each of which elements comprises blade segments on opposite sides of said spindle, and by

the fact that a blade segment from one blade element and a blade segment from the other blade element together constitute a propeller blade in the deployed position. The result is a propeller which has relatively low drag resistance while at the same time meeting the requirements for an optimum blade profile, and hence good efficiency, since the propeller blades do not have to be provided with particularly thick/heavy blade portions to ensure that the propeller blades will deploy. In addition, the fact that the blade elements pivot about said spindle makes possible a relatively controlled/balanced deployment of the propeller, thereby preventing both the undesirable noise and the "power shocks" of known solutions.

According to a preferred embodiment of the invention, the blade segments of each blade element are provided with different blade shapes on opposite sides of said spindle. More precisely, the blade shape of one blade segment, the main blade segment, is designed to cause greater drag resistance than the second blade segment, the secondary blade segment. The result is that the propeller folds together when it is not rotating and the boat is moving through the water, e.g. during sailing.

According to a further preferred embodiment of the invention, said greater drag resistance of the main blade segment is achieved by the latter' s blade area being so disposed as to cause more resistance than the blade area of the secondary blade segment. More precisely, the main blade segment is provided with a larger blade width at a radius (r) from said spindle than the secondary blade segment at the same radius (r).

According to yet another preferred embodiment of the invention, the hub is provided with a recess for accommodating two of the secondary blade segments of the blade elements when the propeller is in its folded position. The propeller thus meets the requirement of relatively low drag resistance when it is not being used for powering the boat, i.e. when it is not rotating.

According to a preferred embodiment of the invention, said recess is provided by the hub being fork-shaped with two parallel fork limbs extending in the longitudinal direction of the hub, whereby said recess takes the form of an aperture running through between the fork limbs.

According to a further preferred embodiment of the invention, a limit-position stop against which the respective blade element abuts when the propeller is in its deployed position is provided on the inside of each of the fork limbs, i.e. in said recess. This enables accurate mutual positioning of the blade elements, thereby ensuring that an optimum propeller blade profile is always achieved.

Further advantages and objects of the invention may be appreciated on the basis of the attached claims and the description set out below.

Description of the drawings

The invention is described below on the basis of examples of preferred embodiments and the attached drawings, in which

Figure 1 shows a sideview of the propeller in its deployed position according to the present invention,

Figure 2 shows the propeller as seen from directly behind in its deployed position with the propeller rotating and operating in the forward direction,

Figure 3 shows the propeller as seen from directly behind in its deployed position with the propeller rotating and operating in the reverse direction,

Figure 4 shows the propeller in a position halfway between its folded and deployed positions, and

Figure 5 shows a sideview of the propeller in ks folded position. Preferred embodiments

In Figure 1 , ref. 1 denotes generally a propeller with a hub 2 for fitting to a driveshaft 3 of a boat (not shown). In a preferred embodiment, the propeller 1 in its deployed position has two substantially identical propeller blades 4, 5. The propeller blades 4, 5 take the form of a first blade element 6 and a second blade element 7, each of which elements comprises, on opposite sides of a spindle 8, blade segments 9, 10, 11, 12 whereby a blade segment 9, 10 from one blade element 6 and a blade segment 11, 12 from a second blade element 7 together constitute a propeller blade 4, 5 in the deployed position. Each blade element 6, 7 is made of, for example, bronze, aluminium bronze (containing 8-10% aluminium), steel or a plastic fibre composite. The hub 2 may be made of similar material. In addition, each of the blade elements 6, 7 is fastened, pivotably about said spindle 8, to the hub 2, said spindle 8 extending through cooperating holes 13 in the hub 2

An important characteristic of the invention is that the blade segments 9, 10, 11, 12 of a blade element 6, 7 are provided with different blade shapes on opposite sides of the spindle 8. More precisely, the blade shape of one blade segment 10, 11, the main blade segment, is designed to cause greater drag resistance than the second blade element 9, 12, the secondary blade segment. The result, when the propeller is not rotating and the boat is moving through the water, is a greater turning moment of the main blade segment 10, 11 about the spindle 8 than of the secondary blade segment 9, 12, thereby leading to the main blade segment 10, 11 being folded rearwards, i.e. folding together of the propeller 1, e.g. during sailing. More precisely, the blade area of the main blade segment 10, 11 is larger than the blade area of the secondary blade segment 9, 12 since the main blade segment 10, 11 is provided with a larger blade width at a radius (r) from said spindle than the secondary blade segment at the same radius (r). According to a preferred embodiment, the blade width of the secondary blade segment 9, 12 is about 0.3 to 0.4 times the blade width of the main blade segment 10, 11 at a radius (r), and a blade width 0.35 times the blade width of the main blade segment 10, 11 is particularly preferred. According to a preferred embodiment, the area of the main blade segment 10, 11 is 70-130% larger than the blade area of the secondary blade segment 9, 12, and a 90-110% relationship is particularly advantageous. It should also be noted that the two blade elements 6, 7 are identical, making it possible to simplify manufacturing and hence be able to ensure relatively low production costs.

It is possible to define a front edge 14, 17 and a rear edge 15, 16 for each of the propeller blades 4, 5, when the propeller 1 is driven forwards F, i.e. clockwise in Figure 2. In reverse running, the front edge 14, 17 becomes the "rear edge" and the rear edge 15, 16 becomes the "front edge". It should be noted that according to the invention the propeller 1 may also be designed to rotate anticlockwise when it is driven in the forward direction F.

In the ensuing, the situation during operation of the propeller 1 according to the invention is described with reference to Figures 2 and 3. During propulsion in the forward direction F as depicted in Figure 2, the pressure centre 18, 19 is situated at a distance from said forward edge 14, 17 which corresponds to about one-third of the total deployed width of the propeller blade 4, 5, i.e. on the main blade segment 10, 11, and the blade element 6, 7 is thereby caused to pivot about the spindle 8 until it abuts against the limit-position stop 22 (see Figure 4). The propeller thus maintains its deployed position, i.e. with the blade elements 6, 7 perpendicular to the longitudinal axis of the hub 2. During reverse propulsion R as depicted in Figure 3, the pressure centre 20, 21 shifts to a distance which corresponds to about one-third of the total deployed width of the propeller blade from the new front edge 15, 16. The fact that the secondary blade segment 9, 12 is so designed that the pressure centre 20, 21 is situated on it results in the turning moment on the blade element 6, 7 about the spindle 8 being greater from the secondary blade segment 9, 12 than from the main blade segment 10, 11, thereby, in a manner similar to that described above, causing the blade element 6, 7 to abut against the limit-position stop 22.

When the propeller 1 is not being used for propulsion, e.g. during sailing, then, as described above, the drag resistance of the main blade segment is greater, causing the latter to move rearward while the less resistant secondary blade segment moves forward, as depicted in Figure 4, into a recess 23 provided for the purpose in the hub 2. The fact that the hub 2 is fork-shaped with two parallel fork limbs 24, 25 extending in the longitudinal direction of the hub results in said recess 23 taking the form of an aperture running through between the fork limbs 24, 25. Limit-position stops 22 (only one of which is visible in Figure 4) against which the blade elements abut when the propeller 1 is powering the boat are provided in the recess 23, on the inside of each of the fork limbs 24, 25.

Figure 5 depicts the propeller 1 in its folded position, i.e. with the blade elements 6, 7 substantially parallel with the longitudinal direction of the hub 2 and thereby constituting a streamlined body with low drag resistance.

The invention is not limited to the examples of embodiments described above and depicted in the drawings but may be varied within the scopes of the attached claims. For example, instead as described above, the propeller may be provided with four propeller blades.