Marine Propulsion System

TECHNICAL FIELD: The invention relates to a marine propulsion system comprising contra-rotating propellers driven by a planetary gear system in the propeller hub, wherein the drive shaft to the hub operates at high revolutions, geared up 1.5 to 3 times the engine speed. The gearing up takes place after the engine via a unit comprising a reverse gear and step up gear.

The preferred embodiment comprises a steerable outboard propeller leg with the drive train arranged such that when the boat is moored the propeller leg can be rotated clear of the water.

BACKGROUND OF THE INVENTION:

There is a general trend to demand easily manoeuvrable efficient marine propulsion systems which can convert the power of an engine into thrust with the lowest possible losses. A number of obstacles lie in the way for achieving high efficiency and, to maximize the thrust, it is necess¬ ary to study each partial loss.

The propeller is of course a very important element and the most efficient of the known systems is the contra rotating propeller which can achieve an efficiency of somewhat over 80%. Propulsion systems with contra-rotating propellers are known from for example SE 433 599 and SE 451 572,

US 1,381,939, US 1,088,080, US 2,672,115, IT 36986421, and FR 121.325.

Nevertheless, such a propeller achieves no more than about 84% of the theoretical maximum possible efficiency (an

impulse disc) compared to 78% for a single propeller. By further lowering the rotational speed of the propeller, the efficiency for a contra rotating propeller can be further increased to a little over 88% of the maximum. The reason why this is not done in practise is that a propulsion system which works at low speed (and therefore high torque) has a tendency to be large and bulky. Thus, it becomes not only expensive but also requires large dimensions, some¬ thing which increases the size of all components, even those in the water (shaft, housing), which increases the flow resistance.

A further factor of importance for the total efficiency is the dimensions of the propeller. Many manufacturers have one or maximum two propeller series and it is questionable whether the propellers which are available for a certain engine are as efficient as they could be due to the fact that the rotational speed and the propeller's dimensions are not well matched.

The submerged body (rudder, propeller shaft, outboard housing) consume in an optimal performance system 10 to 15% of the propeller's thrust. A slender submerged housing (small frontal area), well shaped, can reduce this loss to 6 to 8% of the thrust.

A common problem is plant growth on the propeller and drive system. Even very little growth dramatically increases the friction between water and propeller and submerged housing. Investigations show that the friction quickly doubles due to growth of micro-organisms and algae. The propeller system can thus lose a further 10% of its thrust as well as increased resistance for the submerged housing.

Outboard drives are often equipped with cooling water intakes to take in water to the cooling system of the

engine. An unsuitably shaped inlet can unfortunately imply extra flow losses due in part to turbulence which is created, as well as indirectly due to the fact that cavitation bubbles and turbulence can reduce the propeller efficiency and increase the resistance of the submerged housing.

The size of the engine is of course of decisive importance for the energy consumption of the boat. An efficient propulsion system often results in a drastic decrease of the necessary power demands since there is a close rela¬ tionship between the power of the engine, total displace¬ ment and thus also the boat resistance.

Experience shows that an increase of the propeller effi¬ ciency by 10% (for example from 0.70 to 0.77) for a planing boat having a diesel engine reduces the power demands by about 20%. Particularly for large heavy boats, the engine power is primarily determined by the demand for reasonably quick acceleration (for example maximum 20 seconds from travelling slowly to planing), with the desired top speed being only of secondary importance.

During acceleration, the propeller is heavy to rotate and the engine can therefore not rev up to attain its maximum power. It is obvious that a 2-speed gear box would make it possible for the engine to rev up. This would make it possible to equip the boat with a less powerful engine which would automatically imply lower fuel consumption.

As has been previously mentioned, it is known to use contra-rotating propellers.

In outboard drives, it is known (GB 2 094 894) to employ a conical gear according to Fig. 25 where the vertical shaft

59 drives a gear wheel pair, i.e. the forward gear wheel 60

(connected to the rear propeller 62) and the rearward gear wheel 61 connected to the front propeller 63. In order to attain a slender submerged housing having low resistance, such an embodiment must operate at high revolutions with small propellers (in relation to the power). Despite this, the efficiency is relatively high, particularly at higher speeds, 35 to 40 knots.

Another known embodiment is an epicyclic planetary gear system according to Fig. 26 in which an inboard engine 64 drives a contra rotating propeller 71 and 72 via an inboard planetary gear system. The engine 63 drives the shaft 65 to the sun gear 66 which drives the planet gears 67 which are supported by the planet carrier 68 which drives the rear propeller 72. The crown gear 69, which is driven by the planet gears 67, drives the front propeller 71.

The latter mentioned solution suffers from the problem that the revolution reduction is too great; the propellers rotate too slowly to be able to work efficiently. According to US 4,963,108, an extra gear has therefore been intro¬ duced with gear wheels arranged around the periphery of the driven gear. These gear wheels drive "planet gears" which drive an outer pinion and an inner pinion. It is to be noted that the "planet gears" are carried in a stationary housing and that they thus do not migrate around the drive wheel during their rotation. The proposed solution implies a large torque up-gearing and large dimensions of the shaft and tubular shaft which connect the transmission to the propellers. An analysis shows that such a shaft system weighs 3 to 4 times more per meter than that proposed by the present invention.

According to the invention (see Fig. 15) the sun gear 1 of the planet gear is driven via a shaft 10 which has been geared up to 1.5 to 3 times higher rotational speed than

that of the engine. The planet gears 2 are not restrained in a stationary housing but instead migrate around the periphery of the sun gear 1 and, via the planet carrier 3, drive the rear propeller 4.

US 4,642,059 discloses a solution to the rotational speed problem mentioned above by exchanging the cylindrical planet gear by an inboard mounted conical gear of the same type as in Fig. 25.

A further solution with planet gears in the hub is US 4,604,032. This invention describes a system with 3 or more rotors. The number of rotors is determined by the basic gearing of the planetary gear system (N = the ratio of the crown gear diameter to the sun gear diameter) which determines the balance, i.e. the torques which apply for the crown gear and planet gear. According to the invention, the system can have N reverse rotating rotors (connected to the crown gear) and N+l forward rotating rotors (connected to the planet carrier) relative the sun gear. If the input torque is Q, the difference in the braking torque of the propellers is (N+1)*Q - N*Q = Q. The basic gearing thus corresponds to N. When N=l , a conical gear is attained.

DynaProp is based on a different principle. It employs two propellers. The rear propeller has the same or possibly somewhat greater diameter than the front propeller and can thus make use of the contraction of the flow downstream of the first propeller, see Fig. 16, where the front propeller 6 and the inner portion of the rear propeller 7 lie within the same flow 7 and are thus both true contra rotating propellers. The outer tip of the rear propeller 4 lies however in its own flow 8 which results in a blade tip which works as a single propeller. This additional small propeller provides necessary torque balance.

A further problem which the invention deals with is the method for reversing. Established practise is that the propeller system is driven via a transmission which can provide forward and reverse rotation which is achieved by means of a reverse gear in the system. The problem is that the large marine propellers (often of bronze) are heavy and have a large moment of inertia (which is further increased by the water mass which attaches itself to the propeller blades). During reversing, when the rotational direction is to be changed, shock impulses will therefore arise which increase the loading on shafts and gear wheels. In order to reduce these loads, engagement of reverse gear does not always take place as quickly as is desired, something which somewhat reduces the manoeuvrability, particularly for smaller boats.

A further known problem with reversing is that the boat travels far too quickly when the engine is idling. A pleasure boat can have an idling speed of about 5 knots and a large ship up to 10 to 12 knots. One known solution for this is trolling which implies that, at low speeds, clutch is made to slip.

Another tried and tested alternative to reverse gearing of a conventional propeller is adjustable propeller blades.

This system provides a gentle and quick switching from forward to reverse, as well as the possibility to finely adjust the manoeuvring forces all the way down until the speed of the boat is zero. In twin installations with variable blades, the manoeuvring ability can be increased to provide full control of the manoeuvring of the vessel

(forwards-backwards, side displacement, rotation) by being able to exactly control for each propeller both the propulsive force and its direction. For example, by making one propeller provide a forward thrust and the other

propeller a reverse thrust, a stationary vessel can be made to rotate about its own axis.

In addition, adjustable propellers provide the possibility to vary the engine rotational speed (and thereby the engine power) if for example the resistance of the boat has increased due to weather and wind.

The object of the invention is thus to provide a simple and cheap system for adjustable propeller blades in a contra- rotating propeller system. Such systems are known for vessels (DDR 523 815) and for air-craft (US 5,152,668, GB 2 231 623, GB 2 180 892 and GB 173 863A) .

SUMMARY OF THE INVENTION:

The intention of the invention is to provide an efficient propulsion system which avoids the disadvantages which have been mentioned above.

The objects of the invention are achieved by means of a propulsive transmission system which consists of: a hub with contra rotating propellers the propellers connected to a planetary gear system in the hub the planetary gear system driven by a horizontal or slightly angled shaft to the sun gear the sun gear operating at a rotational speed consider¬ ably higher than that of the engine by gearing up the speed of the engine by 1.5 to 3 times in a step up gear parallel gear wheels can be arranged (with clutches) in parallel with the step up gear to temporarily create another gearing ("gear box") the hub can be mounted in a steerable outboard housing which may also be provided with trimming means for an inboard motor having a steerable housing, the power is

transmitted via a cardan joint to thereby permit trimming and steering

A particularly interesting embodiment is a cylindrical planetary gear system having large reduction (5 to 7 from sun gear to propeller) which permits a high rotational speed for the drive line (6,000 to 8,000 rpm for engines 150 to 450 hp). The low drive line torque allows for small dimensions and thus a slender submerged housing with low resistance despite the fact that the propellers operate at an ideally low speed (and efficiency).

BRIEF DESCRIPTION OF THE DRAWINGS:

The invention is described by the following drawings:

Fig. 1 shows how the drive system in a form a inboard/outboard (I/O) drive is mounted to the transom of a boat

Fig. 2 shows a raised I/O drive

Fig. 3 shows an embodiment for inboard engines with a planetary gear system in a hub

Fig. 4 shows an I/O propeller system with a planetary gear system in the submerged housing

Fig. 5 shows a step-up gear for an engine with built-in gear box and forward/reverse unit

Fig. 6 shows flow past a propeller

Fig. 7 shows how the flow changes velocity when passing through the propeller

Fig. 8 shows the lift force and resistance for a lightly loaded propeller

Fig. 9 shows how the blade friction affects the effi- ciency

Fig. 10 shows a wing profile of a non-cavitating propeller

Fig. 11 shows the wing profile for a partially cavitating propeller

Fig. 12 shows the blade friction for both a non-cavitating and a semi-cavitating propeller

Fig. 13 shows how the blade speed (0.7R) affects the fric¬ tion efficiency for a typical I/O propeller

Fig. 14 shows the relationship between lift coefficient and drag coefficient (angle of attack or camber is varied) for a wing profile

Fig. 15 shows how the propeller is connected to the plan¬ etary gear system

Fig. 16 shows the flow past contra rotating propellers and how the stream from the front propeller contracts and how the outer peripheral flow passes the blade tip of the rear propeller

Fig. 17 shows a yoke-supported drive of a standard type

Fig. 18 shows the principle construction for a complete system designed according to the invention

Fig. 19 shows a rotatable submerged housing formed in accordance with the invention

Fig. 20 shows how the velocity field behind the submerged housing creates a transverse force on the front propeller

Fig. 21 shows force pulses acting on the side of the front propeller

Fig. 22 shows those forces and torques which balance the submerged housing

Fig. 23 shows how an outboard drive is acted upon by flow during manoeuvring and those forces which thus act on the housing and propeller

Fig. 24 shows how the lateral centre for the submerged housing can be approximately calculated

Fig. 25 shows a conventional outboard drive with contra rotating propellers

Fig. 26 shows a known system for vessels with contra rotating propellers

Fig. 27 is a view of an outboard drive with an adjustable propeller and a planetary gear system according to the invention

Fig. 28 shows the planetary gear system with hub brake incorporated in the drive

Fig. 29 shows a rear propeller adjustable via a hydraulic piston and a front non-adjustable propeller

Fig. 30 shows the supply of hydraulic fluid via the drive shaft to the rear propeller and a measuring

arrangement for the position of the adjustable blade

Fig. 31 shows a principal embodiment for an inboard drive

Fig. 32 shows the linear relationship between the rotational speed for the two propellers

PREFERRED EMBODIMENT The most favourite embodiment requires an insight into how the propeller losses can be affected by the change in rotational speed and which drag reduction is attainable for the propeller shaft or the submerged housing.

The losses for a propeller can be divided into a number of groups:

1. the friction between propeller blade and water in the form of viscous friction and turbulent fric¬ tion in which small vortices induced by the pres- sure difference from the suction to the pressure side are included

2. axial displacement energy (corresponding to the reaction to the thrust)

3. rotation of the flow downstream of the propeller (swirl).

The friction of the propeller (drag D) reduces the thrust (the component in the direction of the velocity of the boat Va) of the propeller and increases the torque which is required to rotate the propeller (the component in the direction of the rotational velocity U) , see Fig. 8. Inter¬ estingly, the friction has a more damaging effect when the propeller has small blade angles (beta in Fig. 8), in other words for a low pitch. How large this effect is is shown in Fig. 9 in the form of friction efficiency which shows how great a proportion of the supplied engine power is con-

verted into thrust and displacement energy in the propeller stream.

For example, the blade drag 7.5% (=the drag/blade lift) the efficiency for friction = 80% for the blade angle 20° (pitch P/D = 1.14) and 86% for the blade angle 40° (pitch P/D = 2.64).

Inboard propellers often have a low pitch (P/D = 0.6 to 0.8 and blade angles 11° to 14°) and are thus even more disad- vantaged by the effect of friction.

It is a fact that the effect of friction becomes greater at higher blade velocities of the propeller. This has to do with the cavitation sensitivity of the blade. The higher the blade velocity, the thinner (and broader) the blade has to be made to be able to operate without cavitating. It would be ideal if the blade could operate with a somewhat higher lift coefficient since a more advantageous relation- ship between lift and drag is thereby attained. This can be illustrated in a polar diagram according to Fig. 14. The majority of propellers operates with lift coefficients about 0.08 to 0.14 (the point Normal in Fig. 14) whilst it would be optimal if the construction could provide a lift coefficient of about 0.2 or just above (the point Optimum in Fig. 14). In most cases, this is impossible because of cavitation.

Why are propellers now made with "small" blade angles?? Well, quite simply because low blade velocities imply low revolutions and thus high torque and large drive shafts, which cost money. This increase in the size of the drive train results in a further large loss in a form of a weight penalty. Furthermore, it should be pointed out that single propellers exhibit a large loss in rotational energy (swirl) at low revolutions, a loss which contra-rotating

propellers need not be encumbered with and it is to this embodiment that the invention relates.

With conventional flow theory, one can analyze how the blade drag of the propeller appears when taking into account that the propeller blade is to operate without cavitation. For the sake of completeness, a semi-cavitating section has also been studied, a profile form which is used for high-speed boats with high power density for the propeller, see Fig. 10 and 11.

Fig. 12 shows the calculated relative drag (drag/lift) as a function of the blade velocity (for an I/O drive with typical propeller diameter about 0.4 to 0.5 meters and made of light metal).

The majority of I/O propellers have a typical blade velocity (at 0.7R) of about 65 to 75 knots. It is apparent that it would be interesting to construct the propeller for blade velocities down to 55 to 60 knots. This is indicated by the friction efficiency in Fig. 13. A reduction of the blade velocity from 70 knots to 60 knots would provide 8% more thrust.

We have now arrived at the principle expounded by the present invention. In order to attain a compact and slender drive line, we reduce the number of revolutions only where it is needed, i.e. in the hub by the propeller in the water. Furthermore, after the engine, we introduce a step- up gear which increases the number of revolutions 1.5 to 3 times. The invention will be applied to V-engines operating at 4,600 rpm and diesel engines down to 2,500 rpm. We have discovered that drive line revolutions about 6,500 to 8,000 rpm (for engine power 200 to 400 hp) are interesting, though high peripheral velocities for the gear wheels must be avoided in order not to generate pumping losses. Note

that the established method in the marine branch has always been to reduce the number of revolutions after the engine, often by a factor of 1.5 to 3, sometimes 4 to 6.

An interesting parallel to the proposed marine drive train is trucks. A revolution reduction means is provided in the wheel hub and the remaining drive line components (from the engine to the wheel) are permitted to operate at high revolutions to thereby reduce weight and production costs.

Compared to a drive system for single propellers, the torque in the drive line will be 3 to 4 times lower, which implies that the shaft dimension can be reduced by 35%. This gives a considerable reduction in the frontal area of the submerged housing and thereby its drag in the water. Compared to a conventional submerged housing for outboard drives, this new body will be slimmer (reduced width, reduced thickness). An optimizing analysis shows that it is beneficial to slightly increase the propeller diameter (which gives an improvement in the thrust) and thus extend the submerged housing somewhat.

How great can the hub reduction be? The proposed cylindri¬ cal planetary gear system according to Fig. 4 can provide very large gearing variations, up to 10.

The power is supplied to the sun gear 1 (rotated by the input torque +Q„) which drives the planet gears 2 journalled to the planet gear carrier 3 which is connected to the rear propeller 4. The rear propeller is braked by the water with a torque -Q ₂ . The planet gear carrier rotates in the same direction as the sun gear, but the planet gears encounter resistance from the crown wheel 5 which, in the opposite direction, drives the front propel- ler 6 which is braked "in the other direction" by the water, torque = +Q,.

Since the planetary gear system is in equilibrium, the sum (with signs) of all the torques = 0, i.e. Q ₀ + Q, + -Q ₂ = 0.

Depending on the planetary gear system's basic gearing (=Z,/Z ₀ , i.e. the relationship between the number of teeth on Z, on the crown gear and the number of teeth Z ₀ on the sun gear), different relationships between the torques can be created, for example:

D Q,:Q ₂ = 1:2 provides a reduction of 3 for Z,/Z ₀ = 1

□ QitCh ^β 2:3 provides a reduction of 5 for Z,/Z ₀ = 2

□ Q,:Q ₂ - 5:7 provides a reduction of 6 for Z,/Z ₀ = 2.5 π Q,:Q ₂ = 3:4 provides a reduction of 7 for Z,/Z ₀ = 3

Since there is a linear relationship between the number of revolutions, the propellers 1 and 2 do not necessarily have to have exactly the same rotational speed. It can be advantageous to construct them for both different speeds and different power. Nevertheless, the planetary gear system always seeks equilibrium, i.e. the propellers automatically adapt their rotational speed to achieve torque equilibrium.

In can be interesting to unload the front propeller since cavitation bubbles from the front propeller can inflict mechanical damage on the rear propeller.

In order to now be able to achieve the highest efficiency of the contra-rotating propeller, we ought to take into account how it operates. A contra-rotating propeller has high efficiency i.a. because the flow which leaves the rear propeller does not rotate (because the rotation flow received from the first propeller is counter-acted by the rear propeller rotating in the opposite direction). Can this now be achieved if the two propellers operate with different power?

Fig. 6 shows how the flow passes the propeller and is accelerated. The flow q kilograms water per second passes through the propeller. The velocity of the water increases from V upstream of the propeller to V+dV downstream of the propeller. According to Newton's law (force=change in momentum) the propeller's thrust T is

T = q*dv

Exactly the same law applies tangentially however, for the propeller, see Fig. 8 which shows the propeller blade seen from above.

In Fig. 8 the propeller blade rotates with the velocity U (= tangential velocity at a certain "typical" radius about 70%). The flow q passes through the propeller. We see that in the axial direction the velocity increases from V to V+dV, the change of velocity dV corresponding to the thrust T. In the same manner, the propeller has changed the tangential velocity from zero to dU, i.e. according to the same law the tangential force F on the blade is

F = q*dU

If we now have two (contra rotating) propellers in series, substantially the same flow will pass both propellers. If the two operate with the same torque, the tangential force for the two propellers is the same and thus they subject the water to equal rotation dU but in opposite directions, i.e. the sum is zero, the exiting flow does not rotate and the rotation loss is zero.

It is to be noted that the above theory is not dependent on the rotational speed of the propellers and that the propellers, in order to achieve the best efficiency, should operate with the same torque. However, different rotational

speeds imply different power. This provides a possibility of allowing the front and rear propellers to be of differ¬ ent types, for example cavitating and non-cavitating, whereby each propeller operates at its optimal rotational speed. This can be particularly interesting for high-speed boats and inboard boats; the front propeller can operate at lower power and thereby made extra cavitation-safe. For inboard drives, the front propeller could receive for example 1/3 of the power, have "thick" blades with rounded nose and thus be less sensitive to the angled velocity field which propellers are subjected to with inclined propeller shafts. Said power transfer from the front propeller to the rear propeller provides lower levels of vibration and thus reduced transmitted noise in the boat. The front propeller provides the rear propeller with an even velocity flow and the rear propeller can also operate more favourably.

As has previously been mentioned, a characteristic of the planetary gear system is that it provides different torque to the rear and front propeller, for example in the relation 5:7 for a rotation reduction of 6 times. In accordance with the above reasoning concerning the kinetic rotational energy (swirl), this results in a certain worsening of the efficiency since, with differing torques for the two propellers, a certain rotation in the flow behind the propeller remains. This is now addressed according to the preferred embodiment of the invention by not making the rear propeller smaller than the front propeller as is usually done, but instead perhaps even making it somewhat bigger. The principle is explained in Fig. 16. The front propeller 6 accelerates the flow which is contracted to a central propeller stream 7 which leaves the front propeller 6. The rear propeller 4 now operates with the central stream 7 as well as the outer ring-shaped flow 8 which passes the blade tips of the rear propeller 4.

Analysis now shows that in this manner the rear propeller can be allowed to operate at 5 to 10% higher power without worsening the total efficiency (which with unchanged total power with this mode of operation implies an improvement of the total efficiency by 1 to 2 percent). The proposal implies that the entire central portion of the rear propeller works contra-rotatingly and that its blade tips in addition work as a single propeller.

In order to avoid cavitation bubbles from a cooling water intake in the central submerged housing and to balance the underpressure of the suction effect in a suitable manner when water is drawn into the cooling water pump, and the overpressure of the "ram effect" from impinging flow, the housing according to Fig. 18 is provided with a precisely manufactured nose cone 55 having such shape that the outer flow only creates a weak overpressure at the ring-shaped cooling water intake in order to balance the suction effect of the cooling water pump. In the I/O drive embodiment shown in Fig. 1, plant growth can be prevented on the underwater components (45, 4, 6) by swinging the propeller leg (45, 46, 44) clear of the water according to Fig. 2.

Finally, we can now discuss the adjustable variant of the propeller system. For a contra-rotating propeller of normal type (Fig. 25), both of the propellers must be adjustable, something which mechanically is very complicated. According to the invention, the torque balance of the planetary gear system is used which implies that the propellers automati- cally adapt themselves to the rotational speed which provides a constant relation shape between the torques of the propellers. The propeller blades are therefore made adjustable for one of the two propellers. One embodiment is shown in Fig. 27.

Operation is the following:

If for example the blade angle for the one propeller is reduced and the pitch angle is reduced (from normally 20° to 35°) it rotates more freely and its rotational speed has a tendency to increase in an attempt to maintain torque balance in the planetary gear system. At the same time, the rotational speed for the second propeller starts to fall because, in relation to the other propeller, it is heavily loaded (its blade angle has not changed) and because it is connected to the other propeller via the planetary gear system. The connection provides a linear relationship between the propeller rotational speeds (see Fig. 32) and an increase in rotational speed for one propeller always results in a rotational speed reduction for the other propeller according to the following.

The linear relationship is:

the crown gear's rotational speed (minus rotation) is (-n _k ) = [1 - n _p *(N+l)]/N where N is the diameter ratio crown gear/sun gear and the planet gear carrier's rotational speed = n _p .

DynaProp would seem to have a suitable basic gear reduction of about N = 2 to 3 (which allows for suitable proportions and room for bearings). For example, N = 2.5 is (-n _k ) = [1 - n _p *3.5]/2.5. The rotational speed (relative to the sun gear) is thus:

Equal revolutions: (-n _k ) = n _p = 1/6 = 0.17. Locked crown gear: (n _k = 0) gives n _p = 2/7 = 0.29. Locked planet gear (n _p = 0) gives n _k = 2/5 = 0.4.

The equilibrium of the planetary gear system ensures that the torque Q, of the front propeller always is the propor-

tion a = N/(N+1) of the torque Q ₂ for the rear propeller. For N = 2.5 in the example, a = 5/7 = 71% (and thus Q, = 0.71 Q ₂ ). A reduction of Q ₂ (rotating the blades) results in a corresponding reduction of Q, (the rotational speed reduces) .

The conclusion is that the torque absorption for the two propellers can be regulated by adjusting one propeller. In our example, the rotational speed for the rear propeller will thus be able to oscillate in the interval 0% to 40% of the rotational speed of the sun gear and, for the front propeller, between 0% and 29%. In this manner, the system can be adapted to different loading (weather and wind as well as the load on board increases the drag of the vessel) .

The system also operates for reversing. Assume that the boat engine is a diesel (the accelerator application provides a constant engine speed) . We adjust the blade angle for one propeller so that it approaches zero. Both propellers will thus run more freely (until they almost no longer absorb any torque), the rotational speed for the propeller with fixed blades becomes lower and lower. Once this condition is reached, a hub brake (the multiple plate clutch according to Fig. 28) locks the propeller (with fixed blades) to the gear housing. The hub brake can be a hydraulic multiple plate clutch, a disc brake or a common drum brake. In order to take effect, it is not necessary that the system totally locks the front propeller; instead it can be allowed to slip somewhat provided it provides a braking torque. In fact, it is possible to finally adjust the reverse thrust by braking to varying degrees; if the brake is released a little, the reverse thrust becomes less. If the front propeller is locked, only the propeller with adjustable blades rotates and its increased rotational

speed (about 70 to 80%) provides a noticeable reverse thrust.

A mechanically simple solution has the hydraulic piston mounted in the rear propeller according to Fig. 27.

DESCRIPTION:

Fig. 4 (A and B) shows the embodiment for an I/O drive. Via an inclined conical gear 11, the inclined "vertical" drive shaft 9 drives the horizontal propeller shaft which mechanically can be a direct extension of the sun gear 1. The planet gears 2 drive an inwardly cylindrical gear in the crown gear 5. The drive train is mounted in the submerged housing 13. The sun gear spindle 10 is journalled in the forward portion of the housing in bearing 18 and indirectly by the planet gear carrier, bearing 22. In this embodiment, the sun gear spindle does not accommodate any thrust from the propeller. The thrust from the planet gear carrier 3 is taken up via the body in bearing 19 whilst the thrust on the crown gear is transmitted to the body via bearing 23.

Fig. 3 (A and B) shows the invention applied to an inboard drive. According to the proposed embodiment, the propeller thrust is transmitted directly to the inner propeller shaft 15 via the attachment 16 to the external propeller shaft 14. The entire thrust is transmitted to bearing 17 which acts on the attachment at 16. The system is preloaded via bearings 17 and 19. The thrust on the front propeller (crown gear 5) passes via bearing 18.

The step-up gear which is necessary for the system is shown in Fig. 5. The shown embodiment also comprises a two-speed gearbox (which however can be omitted). Via the flywheel 24, the engine drives the input shaft 25 which, via the clutch 26, unites the shaft 25 to the drive gear 27 which

transmits the power to the upper shaft 28 which carries gear pinion 29.

When it is desired to change gear, the clutch 26 is released, whereby shaft 25 begins to speed up until the freewheel 30 connects lower gear pinion 31 to the input shaft so that the power can instead be transmitted to upper gear pinion 32 which is also connected to upper shaft 28. The power to the outboard drive is transmitted to the drive flange 33. During forward motion, the shaft in the vicinity of the drive flange 33 is connected with clutch 34. During reversing, clutch 34 is released, clutch 35 is engaged, whereby the conical gear 36 drives the opposing conical gear 37 (providing reversed rotational direction) via the two conical planet gears 38.

The embodiment with I/O drive can either be executed in conventional manner with a support yoke according to Fig. 17 in which the drive train 39 is of the high speed type and there being a cylindrical planetary gear system in the hub 40.

The cylinder gear arrangement 27 to 29 adapts the rotational speed of the engine to the optimal propeller rotational speed. For a given, correctly constructed propeller series, it is namely so that the propeller operates at a rotational speed n which is cubically related to the engine power P and can be expressed as (0 = con¬ struction point for the propeller power, 1 other engine power):

If for example the rotational speed of the drive line is to be 7,000 rpm at 350 hp, the drive line for the same propel-

ler(series) for an engine of 250 hp will operate at rotational speed:

7,000 (250/350)" ³ which gives 6.260 rpm.

In order to make this adaptation as simple as possible, the preferred embodiment has a series of available gears 27-29 which are staggered by 10% in gearing difference, i.e. 1, 1.1, l.l ² = 1.21, l.l ³ = 1.33, etc.

The embodiment with the gearbox also makes use of this, so that in principle the same gear pairs 27-29 are used for the gearbox 31-32 (see Fig. 5). Suitably, a smallest difference of 1.1 (1 stage), maximum 1.3 (3 stages) and most typically 1.21 (2 stages) is used. This implies that we can now adapt the system for the majority of engines on the market.

An alternative to a reverse gear is adjustable blades for the propeller. In this manner, the load on the propeller can be adapted so that the engine operates at a suitable rotational speed. This is achieved according to the invention by preferably turning the rear propeller blades as shown in Figs. 27 to 31.

Fig. 27 is a general view. The front propeller 6 which has fixed blades is driven by the crown gear 5. The rear propeller 4 which has adjustable blades is driven by the planet gear carrier 3. A hydraulic piston 74 is located in the hydraulic cylinder 80 and, via the connecter rod 75 acts on a crank pin 77 on the base plate 84 of the blade. Displacement of the piston thereby results in an angular change of the propeller blade. This is also apparent from Fig. 29 which shows the rear oil chamber 78 and the front oil chamber 79 to which oil is supplied (or removed) via the oil conduit 81 and the concentric passage 85 which is

formed between the oil conduit and the central channels in the connecting rod 75 and the planet gear carrier spindle 3 (as well as the sun gear spindle 15 in Fig. 27).

In order to adjust the position of the hydraulic piston, oil is supplied (removed) according to Fig. 30 via oil hoses 82 and 83 attached to respective oil nipples 86 and 87 affixed in the outer oil sleeve 88. The oil passes through the oil sleeve 88 and into (out of) respective conduits, the outer conduit 89 connected to the concentric "outer" passage 85 and the inner conduit 90 which communi¬ cates with the oil conduit 81.

In order to adjust the position of the blade angle to a desired value, the angle of the blade is read by determin¬ ing the axial position of the hydraulic piston. Since the oil conduit 81 is located within the hydraulic cylinder 74 (see Fig. 29), the oil conduit 81 will also be axially displaced together with the piston and the position can be measured using a sensor mounted directly in the hub according to Fig. 30. It is also feasible to employ a mechanical link system to transmit the displacement to a sensor located for example in the upper housing of the drive (to thereby facilitate replacement). In the present proposal according to Fig. 30, the oil conduit 81 co- rotating with the propeller is connected via bearing 92 to a coarsely threaded screw tap 96, which when subjected to the axial displacement, rotates a journalled 94 disc 93 provided with a number of metal screws 97. During rotation of the disc 93, when screw 97 passes the sensor 95, a pulse is generated which is transmitted via a cable 98. The number of pulses counted in a micro-control system deter¬ mines the actual position.

When the boat is to be reversed, the propeller blades 4 (see Fig. 27) are rotated towards zero. Thus, the front

propeller rotates more slowly in its attempt to torque balance the rear propeller via the planetary gear system, which rear propeller now rotates more freely (and more quickly) . Should the propellers rotate too easily, it is not possible to obtain reverse thrust (compare with a car without a limited slip differential, if one wheel spins on ice, the other wheel cannot be driven). According to the invention (see Fig. 28), the ability to reverse is there¬ fore obtained by locking the front propeller 6 when it is rotating sufficiently slowly to the gear housing 13 via a hydraulicly operated multiple plate clutch 99 which has alternate plates connected to the hub (the crown gear spindle 5) and remaining plates connected to an outer sleeve 100 affixed to the gear housing 13.

The described system with rotatable blade can also be mounted to a displacement boat with straight shaft accord¬ ing to Fig. 31 in which the engine 103 via a clutch 104 drives a shaft which increases the engine rotational speed and transmits this to the sun gear spindle 15 which, via the planetary gear system, drives the two propellers 4 and 6.

The preferred embodiment of the outboard drive propeller leg has a Z-shaped drive line which is shown in Fig. 18. The cardan joint 40 is driven by the drive flange 33 from the reversing gear and drives via shafts via two conical inclined gears, upper 42 and lower 11, the sun gear spindle 10.

This inclined arrangement implies that the swinging centre (at the joint 40) is located at such a distance from the transom that the drive (without contacting the transom) can be swung clear of the water by angling the special cardan joint 120°.

Swinging the drive clear of the water effectively prevents plant growth and thereby a corresponding performance loss. However, for "normal" boats (non-racing) it is undesirable have the drive a long way back because, on the one hand, a too long extension can result in damage to drive during manoeuvring in harbour and, on the other hand, because the flow to the drive in the vicinity of the transom is well defined (in a position a distance behind the boat the water surface can "swell up" over the drive and during certain operational positions can create a large extra drag). Accordingly, the Z-shape of the drive train implies that we "return" the submerged body so that it lies close to the transom. A drive according to this embodiment is shown in Fig. 1 where 43 is the rotational centre, 44 the upper housing and 45 the lower housing. The shown embodiment has the lower housing rotatably mounted to the upper housing. The swung-up position is shown in Fig. 2.

The preferred embodiment according to the above comprises a rotatable submerged housing which, with the present propeller system, must be designed in a particular manner to be able to be steered with high steering torque.

The submerged body 45 which is rotatable for steering is, according to Fig. 19, journalled at an angle in the upper housing 44 via the attachment cone 46. The input shaft 9 (torque Q) has its rotational speed reduced by U times via the conical lower gear 11, the horizontal output shaft 10 of which drives the propellers via the sun gear 1. The propellers are braked by the water with the torque U*Q (neglecting the small losses in the gears).

In order to balance the housing, the rotational torque about the steering axis should be zero. The geometry according to Fig. 19 dictates that Q = UQcos α, i.e. the gear reduction shall be chosen as U = 1/cos α, which is a

previously known condition for balancing the housing. Drives designed according to this patented specification have been shown however not be balanced, either when travelling straight ahead or when manoeuvring. The reason for this is that the propeller is always influenced by a transverse force which is caused by the water velocity just behind the underwater housing being less than the surround¬ ings (the velocity of the boat) see Fig. 20.

When the propeller blade 6 passes through the velocity field behind the submerged housing 45, the flow force 53 on the blade increases during a very short period when the wake passes. The larger blade force 54 during the passage results in both a force pulse 52 forwardly (in the direc- tion of the pressure force) as well as a transversely directed force pulse 51 which also affects the steering torque. The transverse force thus consists of a number of short force pulses which arise each time a propeller blade passes through the wake behind the submerged housing, see Fig. 21. The velocity just behind a submerged housing is 10 to 20% less than the surroundings (the velocity of the boat) and gives an average transverse force 51 of a few % of the thrust of the propeller. Only the front propeller is affected by the velocity field; the rear propeller is subjected to an equalized velocity field from the front propeller.

The average transverse force ΔF (51) has a lever distance H about the steering axis (see Fig. 22) and thus generates an additional steering torque ΔQ = ΔF*H which is to be added to the other torques Q and UQ. It is to be noted that the force ΔF has a direction (+) according to the above for a right-hand rotating propeller (which applies for the proposed embodiment) and opposite direction (-) for a left- hand rotating propeller (can be applicable for other embodiments) .

In order to determine the balance requirement, well-known propeller equations can be used:

thrust force T = k, p n ² D ⁴ torque M = k _q p n ² D ⁵ (the torque of the front pro¬ peller is M - U,UQ)

(where k are dimensionless functions, p density, n number revolutions and D diameter)

The average transverse force ΔF can now be related to the thrust T: ΔF = eT which gives the steering torque ΔQ of the transverse force in relation to the propeller torque M (=U,UQ) as ΔQ/M = e k, H/k _q D = C _r .

The torque Q _s about the steering axis is (+right, -left):

Q _s = +Q+(±ΔQ)-UQcos α

which, for the balance requirement Q _s = 0 gives

1/U = cos a -(±U, C _r )

As an example let us take the steering axis being inclined at 60° and the reduction in revolutions should be 2 (=l/cos 60°) . We estimate for that which is applicable for a planetary gear system with Q,:Q ₂ = 2.5:3.5 (thus U, - 2.5) in the embodiment according to Fig. 21 (right-hand rotation +):

1/U = 0.5 + 0.7*(0.02 to 0.03)*2.5*(4 to 5) = 0.5 + 0.14 to 0.26

which implies that the housing is balanced for a gear reduction U = 1.56 to 1.31; in other words a lower reduc-

tion <2 since the right-hand rotating propeller provides an "opposed" torque.

We have now analyzed the case when travelling straight ahead. During manoeuvring, both the propeller and the housing will be subjected to an oblique flow with an angle of attack which constitutes the difference between the rudder angle and the yaw angle of the boat. Exactly how large these angles are is complicated to determine and varies from boat to boat and with the sailing conditions and tightness when turning. Nevertheless, what generally applies is that both forces are proportional to the angle of attack and a balance requirement can therefore be drawn up which implies that, with oblique flow, for example during cornering, the side forces acting on the submerged housing (S _n ) and the propeller (S _p ) balance each other about the steering axis. Their lever distances (see Fig. 23) are H _h and H _p and thus the balance requirement is S„*H _h = S _p *H _p .

The lever distance H _p for the propellers is to their centre (approximately the centre of gravity for their lateral surface) whilst the lever distance for the submerged housing is taken to be the lateral centre of the housing which is approximately determined by the centre of gravity 57 for "half" the housing 56, see Fig. 24.

The balance requirement must be determined from case to case with analysis and practical tests since both the propeller type and the speed range in question have great effect on the result.

The balance requirement can be expressed mathematically: Hh/Hp = e p ή tg β

where p is a relative power density typical for the drive, p = P/flTpV ³ ) in the order of 1/2 to 5 and

A is the side area V velocity m/s p the density of water e propeller function ("sensitivity factor" for oblique flow) η propeller efficiency

P engine power β blade angle propeller

Analysis shows that H _h /H _p is of the order 0.1 to 0.3 with the lower value for balance at high speeds (35-45 knots) and the higher value for balance at low speeds (20 to 25 knots). Generally, the pressure centre H„ for the housing should thus lie forward of the steering axis at the distance 10 to 20% of the lever distance H _p from the steer¬ ing axis to the propeller. For the drive which is the subject of the patent application, the distance H _p is approximately the diameter of the propeller.