Properties of Bronze for Marine PropellersCOMPARATIVE SERVICE DATA. The superiority of nickel aluminum bronze has been convincingly demonstrated by an in-service test.

One propeller of each material was installed on a 6,000 ton twin screw carrier, which subjects its wheels to unusually severe service by operating on the Orinoco River in Venezuela.

The sand bars and sand in suspension in this river are responsible for damage to the ship’s propellers, needing frequent reconditioning or replacement.

The nickel aluminum bronze (starboard) propeller has suffered little mechanical damage or erosion, while the manganese bronze (port) propeller was rather severely damaged and required frequent repair.

The U.S. Navy is using nickel aluminum bronze material for propellers for ice-breaking service, and after a full season of operation excellent results were noted.

Propeller efficiency. Less obvious perhaps than the data presented in Table 1, but more important from a service standpoint, is propeller efficiency. The design engineers are most impressed by the ability of nickel aluminum bronze to retain its original smooth machined surface over a long period of time, thereby retaining its high efficiency factor. Numerically the improvement in efficiency would lie in the order of 1.5-3.0 per cent, with resultant fuel savings. The exact efficiency increase would depend upon the propeller size, design factors and length of service.

Design Benefit. As nickel aluminum bronze is in itself approximately 10 per cent lighter in weight than manganese bronze, and can be designed to thinner sections because of its higher  strength, other advantages become apparent. For example, loading stresses on the tailshaft and bearings are reduce, thus permitting smaller shafts.

Resistance to Notch Sensitivity. The ability of nickel aluminum bronze to resist failure under impact when notched, contributes greatly to its value as a propeller material.

Maintenance .  Maintenance of nickel aluminum bronze propellers is greatly reduced compared to that of manganese bze it has superior resistance to bending, breaking and wearing, including cavitation are directly associated with the material properties.

Reparability. Nickel aluminum bronze is readily reparable with the inert gas process, or by direct electric rod welding. Little or no pre-heat is required, and unlike to copper-zinc brasses, it is not subject to stress corrosion cracking and therefore does not necessarily require a stress relief treatment.

Propeller cost. Reduced weight of nickel aluminum bronze in conjunction with increased strength of the material allows designing the propeller approximately 15 % less in weight than a comparable manganese bze propeller. Although the former is more costly by the price per pound, the long term cost reduction is appreciable.

 TABLE 1 - TEST RESULTS OF NICKEL ALUMINUM AND MANGANESE BRONZE

Item Nickel Aluminum Bronze Manganese Bronze

Chemical composition %

Copper 78 - 81 55 – 60Zinc In “all others” ReminderNickel 4.5 - 5.5 -Iron 3.5 –5.5 0.9 – 2.0Manganese 0.5 – 1.0 0.3 – 0.9Aluminum 9.0 – 10.3 0.7 – 1.0Lead 0.01 max 0.4 maxTin In “all others” 1.5 maxAll Others 0.5 max -

Mechanical Properties (Normal Range)

Yield 35 – 43,000 psi 27 – 33,000 psiTensile 80 - 95,000 psi 60 - 72,000 psi% Elong in 2 in 15 – 30 20 – 35Proof stress 28,000 psi 14 – 16,000 psiBrinell hardness 152 – 190 112 – 130Fatigue – air 21 – 25,000 psi 9 – 14,000 psiFatigue – salt water 18 – 22,000 psi 9 – 12,500 psiDensity – lb/cu in 0.273 0.297

 Spin Test

Weight loss – grams 2.48 9.21Loss – Mg/in2/day

           Mg/dm2/day

6.5

99.0

24.0

376.0In penetration/yr 0.019 0.063At tip – in 0.005 0.012

 Propellers of nickel aluminum bronze Propellers of manganese bronze

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Tolerances for the manufacture of propellersIn the area of the propulsion many applications exist, that they go from crafts of fishing, of pleasure, of load and of speed between many other's.

It's for that reason that a classification of propeller's exists that determine the tolerances that it should have a propeller, according to the necessities of a craft.

The ISO 484/ 2-1981 Norm establishes the tolerances for the production of propeller's in all their geometric dimension. And divide them in the following classes

This norm contemplates all the dimensions of the propeller's like they are: Pitch, Diameter, Chord Length, Rake, Thickness and separation between blades

The norm requests that they are revised the dimensions of certain radios, this according to the type of propeller that is manufactured.

Class RatioS & I Close to the hub 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95

II Close to the hub 0.5, 0.7, 0.9III Close to the hub 0.5, 0.7, 0.9

The tolerances in pitch vary between the several classes how it show the following chart.

PITCH S I II IIILocal pitch ± 1.5% ± 2% ± 3% ---Prom. pitch for ratio ± 1% ± 1.5% ± 2% ± 5%Prom. pitch for blade ± 0.75% ± 1% ± 1.5% ± 4%Prom. pitch for propeller ± 0.5% ± 0.75% ± 1% ± 3%

The tolerances for the angular deviation between two serial blades are given according to the following chart.

Class Angular Tol.

  S y I ± 1ºII y III ± 2º

TOLERANCES ON TICKNESS

Clase S I II IIIMax. Tol. with a

minimum of+2%

2mm

+ 2.5%

2.5mm

+4%

4mm

+6%

6mm

Min. Tol. with a minimum of

-1%

-1mm

-1.5%

-1.5mm

-2%

-2mm

-4%

-4mm

TOLERANCES ON RAKE

ClassS I II III

Tolerance ± 0.5% ± 1% ±

1.5% ± 3%

NOTE: Rake is expressed like a percentage of the diameter of the propeller.

TOLERANCES ON THE LENGTH OF THE BLADE SECTIONS

ClassS I II III

Tolerance with a minimum of

± 1.5%

7mm

± 2%

10mm

± 3%

13mm

± 5%

15mm

TOLERANCES IN DIAMETER

ClassS I II III

Tolerance ± 0.2% ± 0.3% ± 0.4% ±0.5%All the previous tolerances define the geometry of a propeller in accordance with the ISO 484/ 2-1981 Norm.

The same as this norm for the geometry of the propeller exists, the norm is also used SAE-J755 like reference in order to scheme the hub of a propeller.

This contains specified the dimensions of the most common standard bores, the same as the dimensions of the keyway.

These tolerances are those that they are considered in the production of any RICE propeller.Number of BladesThe choice of the number of blades is one of the first decisions to be made in the design of a screw propeller.

Marine screw propellers usually have 3, 4 or 5 blades, of which four blades are the most common.

Two-bladed propellers are used on sailing ships with auxiliary power, as they offer the lower resistance when in the sailing condition.

The problem with two-bladed propellers for most vessels is that such propellers require very large diameters to get the blade area required for effective thrust.

Three-bladed propellers have generally proven to be the best compromise between blade area and efficiency.

Four or five-bladed propellers and even more blades are useful for two reasons. First, their extra blades create more total blade area with the same or less diameter. 4 blades propellers, however, would seldom be as efficient as the three-bladed because the closer blades create additional turbulence, literally scrambling up each other's water flow.

Another reason to use more than three blades is to reduce vibration. If a propeller is in the habit of producing annoying, rhythmic thumping and humming, a propeller with more blades will often solve the problem. Every time the blades of the propeller pass under the hull or by the strut, they cause a change in pressure that causes a push. If the push is strong enough, it generates a bang. Lots of rapid bangs equal vibration.

Conclusion: the less number of blades the more efficiency, the higher number of blades the smoothest and uniform performance. This must be always taken in consideration when selecting the proper Diameter, Pitch, Blade Area and Shape.

Ship PropulsionThe primary function of any marine engineering plant is to convert the chemical energy of fuel into useful work and to use that work in the propulsion of the ship. A propulsion unit consists of the machinery, equipment and controls that are mechanically, electrically, or hydraulically connected to a propulsion shaft. After reading this chapter, you will have a basic understanding of how a ship's propulsion unit works. You will learn about the three main types of propulsion units used in the Navy. You will also learn how power is transmitted from the propulsion unit to the ship's propeller through the use of gears, shafts, and clutches.

PRINCIPLES OF SHIP PROPULSION

A ship moves through the water through propelling devices , such as paddle wheels or propellers. These devices impart velocity to a column of water and move s it in the opposite direction in which it is desired to move the ship. A force, called reactive force because it reacts to the force of the column of water, is developed against the velocity-imparting device. This force, also called thrust, is transmitted to the ship and causes the ship to move through the water.

The screw-type propeller is the propulsion device used in almost all naval ships . The thrust developed on the propeller is transmitted to the ship's structure by the main shaft through the thrust bearing (fig.). The main shaft extends from the main reduction gear shaft of the reduction gear to the propeller. It is supported and held in alignment by the spring bearings, the stern tube bearings, and the strut bearing. The thrust, acting on the propulsion shaft as a result of the pushing effect of the propeller, is transmitted to the ship's structure by the main thrust bearing. In most ships, the main thrust bearing is located at the forward end of the main shaft within the main reduction gear casing. In some very large ships, however, the main shaft thrust bearing is located farther aft in a machinery space or a shaft alley.

The main reduction gear connects the prime mover (engine) to the shaft. The function of the main reduction gear is to reduce the high rotational speeds of the engine and allow the propeller to operate at lower rotation speeds. In this way, both the engine and the propeller shaft rotate at their most efficient speeds.

Propulsion Systems

There are several different kind of propulsion systems as much as applications, a lot of research has been made to try to get the most efficient system in order to lower operation costs (fuel consumption) without sacrificing performance and life of the vessel. Attempts of radical designs have been made without results, but others have succeeded and are becoming popular as the marine field takes the chance to test one of them, anyway, some of the classical designs have slightly changed too and here are a few of the most common: Fixed- pitch propellers Variable- pitch propellers Controllable Pitch Propellers (C.P.P.) Ducted Propeller Systems Z-Drives Water Jets

These are just some examples of them for the reader to know the advantages of each equipment

All of them have some common applications, these are some of their characteristics

The most common due to its relatively low cost is the fixed pitch propeller , these wheels are also known as “constant face pitch”, this means that the pitch on all the surface of the blade (unlike the blade angles) does not change, they are used in most commercial vessels such as tugs, draggers, fishing vessels, trawlers.

Variable pitch propellers .- As mentioned, most of the wheels have a constant pitch, but there are some special applications (large vessels or high speed boats) where the necessity of taking out the highest efficiency is a must. On these wheels, pitch may vary on each radius (depending on design), but it is most common to find the ones where pitch is usually reduced near the tip in order to reduce the blade pressure and the possibility of cavitation.

Controllable pitch propellers .- These propellers allow the operator to adjust the pitch at will, depending on the operation, this is due to an hydraulic mechanism or a mechanical link that allows the blades to turn on

Open wheel (skewed design)

Variable pitch propeller

their own axis. They offer a big advantage in operation cost, but they are considerably more expensive than the solid ones.

Ducted propellers .- They are surrounded by a hydrodynamic profile shaped shroud, the advantages on these are the increase of thrust (around 40%). There are many profiles that reduce the speed, but there are new improved designs that actually increase it when compared to standard open wheels, there are nozzles designed to operate for maximum performance on the ahead condition and some

for both (ahead and astern thrust). The application for these is limited for low-speed boats (operation below

14 knots) such as trawlers, tugs, draggers.

Z-Drives .- This is the most advanced option when maneuverability is really valuable to the vessel since these systems turn 360 ° and thrust can be directed at any direction. They come with or without nozzles, applies for commercial vessels operating under 14 knots.

Water Jets .- These systems work as a suction pump, they are useful for high speed vessels such as patrol or crew boats, some systems run up to 50 knots.

 

What is Propeller Pitch?By Edgar Reyes.

A propeller can be defined as follows: A mechanical device formed by two or more blades that spin around a shaft and produces a propelling force in boats (or airplanes).

There are several technical terms to define the propeller's characteristics such as: diameter, pitch, disc area relation, hub, bore etc. All these characteristics are calculated to design the optimal propeller accordingly to specific needs of the customer and the boat characteristics.

In this issue we are going to define what is the propeller pitch and the importance at the time to select it.

Pitch: Is the displacement a propeller makes in a complete spin of 360° degrees. This means that if we have a propeller of 40” pitch it will advance 40 inches for every

Controllable pitch propeller inside a nozzle

Z-drive unit

Water jet unit

complete spin as long as this is made in a solid surface; in a liquid enviroment, the propeller will obviously slide with less displacement.

The pitch concept is not exclusive for propellers, other mechanical devices like screws also use it. For instance, a screw with 10 mm of pitch will advance 10 mm for every complete turn when hit by the screwdriver. In fact, the "screw propeller" concept is literally making reference to that the propeller works exactly like a screw.

It is very important that both, pitch and diameter, are properly calculated. If for any given HP the pitch is too big, the propeller becomes heavy and demands more power than the engine can reach and viceversa, if the pitch is too small then we have a light propeller that wouldn't absorb the engine's full power.

So, what would be the appropriate pitch? Certain parameters need to be checked like power, rpms, gear reduction, size of vessel, vessel application (i.e. a trawler or a tugboat needs power while a yacht requires velocity). With this information, Rice's engineering department can help you select the appropriate pitch for your vessel.

Tapers - charts and how to measureYou can measure the taper bore of a propeller with digital vernier calipers.

Measure the small end to the taper inside diameter of propeller - nut end I.D. ( inside diameter)Measure the large end to the taper inside diameter of propeller - front end I.D. ( inside diameter)Measure the overall length of the boss (hub) of propeller Measure the keyway width.

Check how the old prop fits the taper shaft and the amount of draw - i.e. the overhang distance at the nut end so the prop tightens on the taper properly.

Propeller tapers are often not to any standard which makes this game a challenge. There are 4 common tapers used - 1 in 10 , 1 in 12 , 1 in 16 and 1 in 20 and then there are odd back yard ones.

Also check and confirm PROPELLER ROTATION - L/H or R/H - A Right hand propeller rotates clockwise when viewed from astern facing forward. A Left hand propeller rotates counter clockwise when viewed astern facing forward.On a twin screw vessel the left hand prop is normally fitted to the port side.

S.A.E. Specification J755The most widely followed dimensions for tapers is the internationally accepted standard S.A.E. taper dimensions which is 1 in 16 up to and including 5 1/2" diameter. Above this diameter the taper is 1 in 12. This standard is incorporated in the American Boat

and Yacht Council (A.B.Y.C.) rules which are commonly used in Australia. All dimensions for S.A.E. taper standards are based on the SMALL end of the taper. The reason for this is so propellers with different hub lengths (i.e. different diameters at the large end of the taper bore) will fit the same shaft diameter and have the nut face in the same position on the shaft.

1 in 16 taper is 3/4" inch to the foot and the angle center line is 1 degree 47' minutes 23" seconds.1 in 12 taper is 1" inch to the foot and the angle center line is 2 degrees 23' minutes 9" seconds.

S.A.E. Specification J755 (unit: inches)

Shaft Diameter

Shaft Machining PropellerSmall End Taper Small End Keyway

Diam. of Taper Length Diam. Of Taper SizeA B C D

0.750" 0.625 2.000 0.609 0.18750.875" 0.727 2.375 0.711 0.25001.000" 0.828 2.750 0.813 0.25001.125" 0.930 3.125 0.914 0.25001.250" 1.031 3.500 1.016 0.31251.375" 1.133 3.875 1.117 0.31251.500" 1.234 4.250 1.219 0.37501.750" 1.438 5.000 1.422 0.43752.000" 1.641 5.750 1.625 0.50002.250" 1.844 6.500 1.828 * 0.56252.500" 2.047 7.250 2.031 0.62502.750" 2.258 7.875 2.234 0.62503.000" 2.461 8.625 2.438 0.75003.250" 2.664 9.375 2.641 0.75003.500" 2.867 10.125 2.844 0.87503.750" 3.070 10.875 3.047 0.87504.000" 3.273 11.625 3.250 1.00004.500" 3.828 10.750 3.797 1.12505.000" 4.250 12.000 4.219 1.25005.500" 4.672 13.250 4.641 1.2500

NB - *0.500 keyway commonly used in Australia for 2.250" SAE

Propeller Hub Taper 1:10   ISO 4566:1992(E) (unit: mm)

Shaft Dia. Big Dia.D

Small Dia.D1

LengthL

Keyway

b h20 20 15 50 6 325 25 19 60 6 330 30 22 80 8 435 35 26 90 10 440 40 30 100 12 445 45 34 110 14 5.550 50 38 120 14 5.555 55 42 130 14 5.560 60 46 140 16 5.565 65 50 150 16 5.570 70 54 160 18 5.575 74.5 57.5 170 18 680 79.5 61.5 180 20 685 84.5 65.5 190 20 790 89.5 69.5 200 22 795 94.5 73.5 210 25 7100 99 77 220 25 7110 109 85 240 28 7.5120 119 93 260 32 8.5130 129 101 280 36 9.5140 139 109 300 36 9.5150 149 117 320 36 9.5160 159 125 340 40 10.5