Propeller arrangement for a marine propulsion unit

A double propeller arrangement for a marine propulsion unit, for example of the type which includes a so-called stern-drive. Each propeller has a hub consisting of an inner sleeve with splines for attachment to a propeller shaft, an outer sleeve with propeller blades, and an elastic bushing between the sleeves. Particular to the propeller arrangement is that the inner sleeve of respective propeller hubs is provided with an axial passage for the discharge of exhaust gases from an internal combustion engine connected to the propeller drive.

The present invention relates to a propeller arrangement for a marine 
propulsion unit, comprising first and second concentrically supported 
propeller shafts for rotation in opposite directions, each shaft carrying 
a propeller hub having a first sleeve member secured to the shaft for 
rotation therewith, a second sleeve member provided with propeller blades, 
and a bushing of elastic material disposed between said sleeve members. 
Hitherto, propeller arrangements of this type when used in conjunction with 
marine propulsion units having so-called stern-drive, for example of the 
Aquamatic.COPYRGT. type, have had their exhaust passages from the engine 
discharge via a cavitation plate positioned above the propellers. 
Discharge of exhaust gases via the cavitation plate is also used in single 
propeller drive systems, though with such drive systems it is also common 
to direct the exhaust gases through exhaust passages in the propeller hub. 
In such a case, the construction of the hub, as well as its sealing to the 
drive unit, is somewhat more complicated, though this is offset by the 
advantage that the exhaust gases from the hub fill out the central 
cavitation void behind the hub centre which is created during high-speed 
cruising. This allows the stern cone on the propeller hub to be dispensed 
with. In addition, a somewhat higher propeller efficiency can be attained, 
at least in the higher speed range. 
However, no-one has yet applied the method of exhausting gases through the 
propeller hub known for single propellers to a double propeller drive 
system, since it has generally been considered that any possible 
advantages would be outweighed by the disadvantages associated with 
increased costs due to the more complicated construction of the double 
propeller drive, etc. 
It is therefore a general object of the present invention to provide a 
propeller arrangement of the type mentioned in the introduction, which 
provides an increase in efficiency at least in the higher speed range when 
compared to conventional double propeller systems, though which is not 
prohibitively expensive. 
This object is achieved in accordance with the present invention by means 
of the propeller hubs being provided with at least one axial passage for 
the discharge of exhaust gases from an internal combustion engine forming 
a part of the propulsion unit. 
It has been shown that exhaust gas discharge via the propeller hubs in a 
double propeller system provides an increase in the total efficiency of 
the propeller arrangement which is greater than what could be expected in 
view of that which is achieved with single propeller systems, and that 
this result can be achieved using a relatively uncomplicated construction. 
In known single propellers with exhaust gas discharge through the hub, the 
exhaust passage is arranged between the outer hub sleeve which carries the 
propeller blades and the inner sleeve member. Such an arrangement can lead 
to problems with the front propeller due to the relatively large diameter 
of the tubular shaft. Depending on the material of the hub, there is a 
risk that the hub sleeve casting be too thin when employing presently 
specified sizes of exhaust passages and radius of the propeller blade 
intersection surface. 
In a preferred embodiment of the invention, the exhaust passage is arranged 
in an inner sleeve member. The inner sleeve member includes an inner 
sleeve that is shaped with inwardly directed grooves and splines for 
engagement with corresponding grooves and splines on the propeller shaft, 
as well as an outer sleeve which is concentric with, and spaced from, the 
inner sleeve, the spacing forming an exhaust passage which is divided into 
several axial channels by wall elements which unite the outer and inner 
sleeves. 
By arranging the exhaust passage in the sleeve member radially inwardly 
with respect to the rubber bushing, the outer sleeve which carries the 
propeller blades can be sized and cast with sufficient wall thickness. At 
the same time, the rubber bushing retains its large diameter and optimal 
adhesion surface area for attachment to the surfaces of the hub sleeves.

In FIG. 1, reference numeral 1 denotes a lower wall region of a stern-drive 
unit, for example of the Aquamatic.COPYRGT. type, which is connected to a 
not-shown internal combustion engine. A first tubular propeller shaft 2 
and a second propeller shaft 3 are concentrically rotatably supported in 
the drive unit and connected to a drive shaft in a known manner via a 
not-shown conical gear arrangement so as to be driven in opposite 
directions. Each propeller shaft 2 and 3 carries a propeller, generally 
denoted by 4 and 5 respectively, which comprises blades 6 and 7 resp. and 
a hub 8 and 9 resp. 
The propeller hub 8 presents an inner sleeve member 10 which consists of an 
inner sleeve 11 and an outer sleeve 14. The inner sleeve 11 has internal 
splines 12 for engagement with corresponding external splines 13 on the 
tubular shaft 2 and is connected to the outer sleeve 14 via axial wall 
elements or webs 15. The sleeves 11, 14 and the webs 15 together delimit 
three exhaust channels 16 which communicate with an exhaust pipe 17b in 
the drive unit via an opening 17a in the wall region 1 concentric with the 
propeller shafts 2, 3. 
The described sleeve member 11 can be produced by extrusion in long lengths 
which are then cut to the actual hub length. A rubber bushing 18 is cured 
or adhered to the outer surface of the outer sleeve 14 and a pair of 
spacer rings 19 of a relatively hard plastic material are affixed to the 
sleeve 14, with one ring 19 to each side of the bushing 18. The position 
of the spacer rings 19 is determined by shallow circumferential grooves 20 
in which the rings sit. The outer diameter of the distance rings 19 is 
equal to the inner diameter of the outer sleeve member 21 to which the 
propeller blades 6 are affixed. As is apparent from FIG. 4, in its 
non-influenced state, the outer diameter of the bushing 18 is greater than 
the diameter of the spacer rings 19. During assembly of the hub sleeves 10 
and 21, the bushing 18 is compressed so that it thereby expands axially, 
as illustrated in FIG. 1. The bushing 18 is fixedly adhered or cured to 
the outer sleeve 21 and serves as a torsion member, whilst the distance 
rings 19 take up radial loads and prevent the outer sleeve 21 with the 
blades 6 from "wobbling". 
The propeller hub 9 is constructed in the same manner as the hub 8 and 
corresponding components have been denoted by the same reference numeral, 
though with the addition of a prime. A few minor differences can be 
identified; for example, the sleeve member 10 is shorter than the sleeve 
member 10' and the inner sleeve 11 does of course have a greater inner 
diameter than the sleeve 11' to allow it to be slid onto the tubular shaft 
2. 
The inner sleeves 11, 11' have conical end surfaces 30 and 31 (see 
particularly FIG. 4) and are clamped between conical surfaces on 
respective shafts 2, 3 and lock nuts 33, 34, of which the lock nut 33 is 
secured by a bolt 34 which is screwed into a threaded bore in the end of 
the shaft 3. 
As previously mentioned, the described embodiment lends itself to be 
produced from extruded aluminium. Thanks to the rubber bushing 18 and 
spacer ring arrangement, the outer sleeve 21 with the propeller blades is 
electrically insulated, which implies that materials other than aluminium 
can be used in the outer sleeve 21, for example stainless steel, without 
the risk of galvanic corrosion.