Design hints
An improvement in the hydrodynamic performance must be demonstrated to justify the application of Kort nozzles. In a seaway the efficiency of a propeller with nozzle is less reduced than for a non-ducted propeller due to the more axial inflow. The nozzle efficiency increases in a seaway due to the increased thrust-loading coefficient. In total, the nozzle thus decreases the efficiency losses.
When considering if it is worthwhile to install a nozzle, nozzle construction and initial costs play a major role. For performance improvements greater than 7% and propulsive outputs greater than 1000 kW, nozzle acquisition costs are thought to be already lower than the improved propulsive output when considering costs of shaft, exhaust-gas device, etc.
If the installation of Kort nozzles has been decided, nozzle form and arrangement type must be established. For this purpose, the following aspects have to be individually determined:
1. Fixed nozzle or steerable nozzle.
2. Mounting of nozzle by supports or nozzle ring penetration of ship hull.
3. Propeller diameter and nozzle internal diameter.
4. Nozzle profile shape:
(a) Faired or developable simple-form profile.
(b) Nozzle aft end sharp or heavily rounded.
(c) Concentric nozzle or Y-nozzle.
5. Profile length.
6. Nozzle dihedral angle.
7. Special devices for deflection of inflowing objects.
8. Cavitation and air entrainment hazards.
9. Nozzle axis direction.
10. Standard or Kaplan propeller.
These aspects and alternatives are discussed below; see also Philipp et al. (1993).
(1) Fixed nozzle or steerable nozzle
Steerable nozzles produce virtually the same rudder effect as a downstream rudder of equal lateral projected area. Since the centre of pressure is located at around one-quarter profile length, with the axis of rotation being arranged at around half profile length to avoid propeller impact against the nozzle internal wall, steerable nozzles are overbalanced. Thus, at small deflection a moment arises acting to increase the deflection.
A rudder-like control surface is therefore frequently suspended behind the propeller on the steerable nozzle to 'balance' the entire system. Thus, at small rudder angle, a net restoring moment occurs. Rudder effect is also increased. A further effect is a partial straightening of the propeller slipstream and an associated enhancement of the quasi-propulsive coefficient. In respect of power saving, steerable nozzles offer advantages and disadvantages:
+ The propeller blade tip circle positioned near the after perpendicular is located further aft than in conventional arrangements. Thus either the horizontal clearance between propeller and stern frame is greater than normal (lower thrust-deduction factor) or the waterlines forward of the propeller have a finer run. Separation resistance may be reduced.
— The clearance between propeller blade tip and nozzle internal wall must be kept 50% larger than for fixed nozzles to avoid blade tip impact. Thus, to rotate the nozzle, a greater lateral distance is required and, due to bearing play, greater vertical distance is also needed. Efficiency drops with gap size.
— Steerable nozzle and propeller diameters, depending on the configuration, are smaller than for fixed nozzles. Steerable nozzles are mostly used on small ships.
(2) Mounting of nozzle by supports or nozzle ring penetration of ship hull There are various ways to mount Kort nozzles on the hull:
• Steerable nozzles require a cantilever in the plane of the propeller tip.
• There are various options for fixed nozzles: strut construction between nozzle and hull, either by several shaped struts or a flat strut between nozzle and hull.
• Nozzle penetrates hull.
Hull-penetrating nozzles allow the maximum propeller diameter with highest propeller efficiency, but at the price of a 'lost upper sector'. In this sector the nozzle effect is reduced, but not completely lost. The combined propeller efficiency and nozzle efficiency is often optimized when a penetrating nozzle is chosen. The penetrating nozzle also captures more wake and thus improves the hull efficiency. The penetration of the nozzle should be limited such that the inner contour of the nozzle still accelerates the flow, thus reducing the load at the propeller tip (Fig. 4.9). The wedge-shaped gap between counter and outer nozzle contour should be filled by a connecting piece for strength and hydrodynamic reasons. This connecting piece should either taper out or form a connection to the rudder stock.
Steerable nozzles are usually mounted on the rudder stock. For shallow ship sterns and tunnel sterns v.d.Stein has found that it is often better to integrate the nozzle in a rotating plate (Fig. 4.10).
Other structural measures aiding incident flow homogenization are 'skirts' or other control surfaces. Application of skewback propellers may also be appropriate in this context.
- Figure 4.9 Kort nozzle penetrating the hull with a connecting piece for static and hydrodynamic reasons
Figure 4.10 Kort nozzle integrated in a rotating plate, offering all the advantages of a hull-penetrating nozzle
Figure 4.10 Kort nozzle integrated in a rotating plate, offering all the advantages of a hull-penetrating nozzle
(3) Propeller diameter and nozzle internal diameter
Large propeller and nozzle diameters are normally sought. A large propeller diameter restricts other efficiency-enhancing options, e.g.:
1. Nozzle length for pre-selected profile form.
2. Nozzle dihedral angle.
Both factors are still to be discussed. The gap—the difference between nozzle internal radius and propeller radius—should not exceed 0.75% of the radius.
(4) Nozzle profile shape
(a) Faired profiles—simple forms. For the nozzle profile shape, either faired profiles, e.g. NACA 4415, or simple forms as recommended by Shushkin are used (Fig. 4.8). The simple forms consist of round steel or pipes which at their ends have fully developable surfaces which are essentially conical and cylindrical pieces.
Unlike faired profiles with comparable characteristics, the developable forms are subject to efficiency losses of only 1-2%. Developable forms are frequently used in German inland vessels.
(b) Nozzle after end, sharp or rounded. As with propeller profiles, the nozzle after end is more heavily rounded if greater value is placed on stopping behaviour. By rounding the nozzle profile end, ahead efficiency falls somewhat. Depending on inflow conditions (e.g. outlet-opening ratio), a sharp nozzle after end may also exhibit good stopping and astern operating performance. If the nozzle profile is more heavily rounded aft, ahead operating efficiency may be enhanced through a flow separation corner. Such flow separation corners may also be arranged on the forward ends to improve astern operating performance.
(c) Concentric form—oval inlet cross-section. The theory of Amtsberg and the systematic experiments of van Manen investigated Kort nozzles in axial flow. This provides a good basis and reflected also practice up to the 1960s. Kort nozzles were pre-dominantly used in tugs which back then had very low CB and predominantly axial propeller inflow. The situation in ocean-going ships today is different and the assumption of axial inflow is questionable. The side flanks of the nozzle may be opened and the nozzle axis oriented aft upwards to adjust for the different inflow direction.
A Kort nozzle thus adjusted for the inflow direction reduces power requirements considerably, but increases the costs of model testing and actually building the nozzle.
With simple-form nozzles, the opening is easily widened through the provision of a centro-symmetrical nozzle and subsequent installation of filling pieces. This 'Y-form' may also compensate an excessively small dihedral angle arising on height restriction grounds (Fig. 4.11).
For faired nozzles, an oval inlet can be designed at reasonable expense. 'Reasonable expense' means here that the nozzle is built in concentric form and then split, rather than two concentric nozzle parts and then assembling with intermediate pieces. The angle of the end of the inner part of the nozzle should be 2-3° towards the longitudinal axis. The propeller should always
Horizontal section
Vertical section
Horizontal section
Vertical section
Figure 4.11 Y-nozzle. Simple-form nozzle with lateral widening have sufficient clearance (~2% of the propeller radius). The feasibility of installation of the combined nozzle must be checked.
(5) Nozzle axis direction
Nozzles are normally coaxially aligned with the propeller shaft. However, since the propeller incident flow is not quite coaxial, power requirement with the nozzle is frequently improved through matching of the nozzle axis to the inflow direction. For a twin-screw seagoing tug, for example, an aft-converging nozzle axis run with an angle of around 5° to the centreplane has proved particularly advantageous, despite aft divergence of the propeller shafts. For single-screw ships, an axis raked upwards going aft (Fig. 4.12), offers two advantages. Better adaptation to the flow is obtained, and, for a mounting penetrating the ship hull, better matching of the upper nozzle profile direction to the stern counter run can be obtained on the internal line of the nozzle. For cargo ships, optimum rake angles run from 5° to 7°. For nozzles with axes pointing aft upwards the design guidelines listed for Y nozzles apply.
Figure 4.12 Single-screw ship with aft raked-up nozzle axis (6) Profile length
Optimum nozzle profile length increases with thrust-loading coefficient. Nozzles are built with a length-internal diameter ratio of 0.4-0.8. The trend has
Figure 4.12 Single-screw ship with aft raked-up nozzle axis (6) Profile length
Optimum nozzle profile length increases with thrust-loading coefficient. Nozzles are built with a length-internal diameter ratio of 0.4-0.8. The trend has
been towards smaller lengths. At smaller lengths, a larger propeller diameter may be accomplished within a pre-determined vertical space. Profile length and cross-section shape are limited by strength and stiffness requirements. The profile length may be hydrodynamically optimized by Amtsberg's calculation procedure.
(7) Nozzle dihedral angle
Nozzle dihedral angle is the angle of the 'zero lift direction' or other profile reference line to the nozzle longitudinal axis. The dihedral angle may be optimized according to Amtsberg. At pre-selected nozzle total height, an increased dihedral angle means a restriction of propeller diameter or a more substantial distortion in the profile form in the lower part of the nozzle. Consideration must be given to this fact during selection of dihedral angle. Dihedral angle must also be considered in conjunction with profile form. If, to vary dihedral angle, the nozzle profile were only rotated, the outlet section would then be severely narrowed at large dihedral angles. At very small dihedral angles, there is the risk that the flow diffuser angle will become too large behind and the flow will become separated and eddying. Curved profiles, which avoid these difficulties, have so far been little studied and would also be too expensive to manufacture. The outlet angle to the longitudinal axis should be around 2° for Shushkin profiles and should not exceed 4° for faired profiles. If the dihedral angle is modified, the profile form must be matched to achieve a suitable outlet angle.
(8) Special devices for the deflection of objects flowing into the nozzle
On many cargo ships built without nozzles, such devices would have hydro-dynamic and initial cost advantages. They are not used because operational disruptions are feared through jamming of the propeller in the nozzle, with particular apprehension about fouling by pieces of wood, ice, and stones drawn upwards from the bottom. Of the various ways to protect nozzles against inflowing objects, the preferred choice in practice is use of several annular grooves in the nozzle internal wall (Fig. 4.13). The boundary layer is thus
thickened, with the result that inflowing objects are drawn inwards, leaving the gap between propeller blade tips and nozzle internal wall free.
(9) Cavitation and air entrainment
Since nozzles generate a strong depression field, cavitation and air entrainment can easily occur. Cavitation chiefly occurs at the nozzle internal wall in the proximity of the propeller. To avoid erosion damage, the internal wall is generally made of high-grade steel. Two measures are generally used to prevent air entrainment:
1. The nozzle is located as deep as possible. This requirement conflicts with the requirement for a larger diameter.
2. Arrangement of lateral skirts or a tunnel.
(10) Standard or Kaplan propeller
Kaplan propellers achieve better efficiencies in nozzles than propellers with elliptical contour lines. Kaplan propellers should not be run in steerable nozzles, since even greater gap widths are necessary. For ships operating in shallow waters, Kaplan propellers are more liable to be damaged by shingle than standard propellers. Therefore intermediate forms (Fig. 4.14) or standard propellers are used in these cases.
Figure 4.14 Blade tips of standard propeller, Kaplan propeller, and intermediate forms
Often errors are made in designing the Kort nozzle itself or its arrangement which can be easily avoided:
(a) Often the pressure side (exterior) of the nozzle is built as a cone which directly ends in a circle. The small curvature at the end is thus directly connected to an infinite radius of curvature of the straight section. The flow tends to separate due to this abrupt transition, at least at model scale. In full scale, flow separation is far less pronounced or absent. For model tests, it is thus advisable—or even necessary—to have a gradual change of curvature (Fig. 4.15). Comparative model tests show differences in efficiency of 6%.
(b) Accommodating the nozzle under the counter such that it penetrates the ship hull allows the maximum possible propeller radius and exploits the wake as far as possible. Furthermore, the attachment of the nozzle
- curvature at nozzle entrance (bottom)
is very stable without using brackets which would increase resistance. The arrangement should ensure flow acceleration at the entrance in the upper region to avoid cavitation.
The nozzle contour declines downstream and the counter rises downstream. Therefore an intermediate section is necessary for strength reasons. This intermediate connection should not converge to a point, rather than a transom. Often, a hydrodynamically good solution is to fair the intermediate connection to the rudder contour (see also Fig. 4.26).
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