Propulsion system

Details Propulsion system Propulsion system

2000-08-04

2001-12-25

Basinger, Sherman (Department: 3617)

Marine propulsion

Screw propeller

Having means to control flow around propeller

C440S049000, C440S050000

Reexamination Certificate

active

06332818

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a propulsion system for high speed marine craft. In particular, the invention concerns an improved surface drive propulsion system having high operational efficiency.
Known propulsion systems for high speed craft display considerable limitations in performance at low speeds, at high speeds, or throughout the desired speed range. The simplest form of propulsion for marine craft, the submerged propeller, has many limitations and tends to have low efficiency characteristics at high speeds. An improved drive system, the ‘Z’ (or stern) drive, introduced in the 1960's, provides improved efficiency at higher speeds for smaller craft. However, at very high speeds problems are experienced with this type of propeller and often a surface-piercing propeller must be fitted instead.
For high craft speeds surface-piercing propellers fitted either to a specialised surface drive system, or to a Z-drive, give the highest efficiencies. However, conventional surface-piercing propellers are extremely power-absorbing at low speeds. One reason for this is that because these propellers are designed to be run semi-immersed their diameter is large compared to a conventional propeller. Thus until the craft has achieved planing speed the propeller is normally excessively immersed such that the flow and torque requirement are excessively high. A second factor, which is less well understood, is that at low speeds and high power the blades are running at a high lift coefficient, the vapour cavity behind the blade is wide and the distance between the external surface of one blade cavity and the propulsive surface of the succeeding blade is small. Thus the blade is effectively pushing against a vapour bubble with an evident loss of thrust. These two factors in particular cause craft fitted with surface drives to have considerable difficulty getting onto the plane which means they have to be fitted with excessively powerful engines. As a result of the limitations imposed by these drives their usage remains restricted and their cost is high. Also, such propellers are normally mounted well behind the hull which renders them vulnerable to damage when manoevreing or at berth. In most cases, the propeller cannot be raised sufficiently to enable the craft to be beached.
In recent years, jet pump drives have also become increasingly employed for two classes of craft: small performance boats and personal water-craft (jet bikes etc.), and larger luxury yachts and performance work-boats. However, jet drives suffer from a number of distinct disadvantages: in practice the efficiency is usually less than 60% and is often less than 50%. Jet drives are also relatively complex and tend to be expensive; installation is also more onerous than for other drives.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a propulsion system which substantially avoids or minimises one or more of the foregoing disadvantages.
Accordingly, we provide a propulsion system for a water craft, the system comprising: at least one shaft which extends rearwardly from the transom of the hull of the water craft; a surface-piercing propeller mounted on said at least one shaft, proximal to a trailing edge of the hull, so that said propeller operates in a transom cavity created behind the transom of the hull in use of the craft; and drive means for driving said at least one shaft; wherein said propeller comprises a hub and a plurality of blades angularly spaced apart, preferably substantially equidistantly, therearound, the ratio (S/C) of the spacing (S) of the blades to the blade chord (C) being at least 2.0 along substantially the whole length of the blades.
An advantage of the propulsion system of the invention is that the high ratio of blade spacing to blade chord (at least 2.0 along substantially the whole length of each blade) enables much greater efficiency to be achieved than with prior known propulsion systems of the surface-piercing propeller type in which the ratio of blade spacing to blade chord has generally been in the region of unity or, more often, less than unity.
One reason for the increased efficiency which is achieved is that the distance between the trailing surface of one blade and the propulsive surface of the succeeding blade is relatively large and therefore, in use, the pressure field of the latter (following) blade is well behind the blade cavity created behind the trailing edge of the former (preceding) blade. The increased efficiency due to this feature is mainly seen at low craft speeds. Another advantage of the relatively small blade chord is that the transition periods during which each blade enters the water and leaves the water form a relatively small part of the propeller cycle as compared to the prior known surface drive systems where the blades are of relatively large chord. The blades operate at considerably reduced efficiency during these transition periods.
It will be understood that the term “transom cavity” in relation to the hull of the craft refers to the air pocket created immediately behind the trailing edge of the hull, after start up of the craft and at low and high craft speeds. By positioning the propeller close to the trailing edge of the hull, so as to operate in said transom cavity created thereby, we avoid the excessive power losses attributable to churning effects in conditions where the blades are operating in areas of high water swirl velocity (as in conventional propulsion systems).
In relation to each blade, it will be appreciated that the term “blade cavity” refers to the (underwater) vapour space created behind the trailing edge of each blade of the propeller in use thereof.
The ratio (S/C) of the blade spacing (S) of adjacent blades to the blade chord (C) is desirably in the range of from two to five or more, preferably in the range of 2.3 to 4.0, along ubstantially the whole length of the blades. Although the ratio S/C could be chosen to be higher than five, it will be appreciated that there will be an upper limit at which the blades are too spindly to be sufficiently effective in practice. We believe this is likely to be the case in most situations for a ratio of S/C which is above 10. A high ratio will, however, be desirable where one wishes to convert a low engine power into high propeller speed.
The ratio (S/C) of the blade spacing (S) to the blade chord (C) is preferably at least 2.0 along at least 90% of the length of the blades, desirably along at least 95% or more of the length of the blades.
Preferably, the hub of the propeller is relatively large. Desirably, the ratio (H
d
/P
d
) of hub diameter (H
d
) to propeller diameter (P
d
) is at least 0.35, preferably 0.4 or more. An advantage of such a large hub diameter is that at low speeds (where the propeller tends to be in a lower position in the water than at high, planing, speeds) a large percentage of the area swept by the blades in each rotation of the propeller is in air. This is particularly beneficial at low craft speeds where a high percentage of swept air is required to obtain high thrust at such low speeds. In the known prior art propulsion systems, at low speeds a large percentage of the swept area is underwater, (operation of the blades thus requiring greater power absorption from the engine), leading to significantly reduced thrust at these low speeds, as compared with the system of the present invention.
Additionally, a large hub diameter to propeller diameter ratio means that there tends to be much less variation in water flow velocity along the length of the propeller blades than in conventional propellers and as a consequence the variation in performance in off-design conditions is less marked: generally, the shorter the blades relative to the hub diameter the less the radially acting flow forces generated in the water and, in turn, the greater the efficiency of the propulsion system. Also the shorter the blades, the less is the torsional bending and deflection of the blades during use. In many case the blades may be of constant

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