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Review of Engine Shafting, Propulsion and Transmission Systems

Key Considerations for Industry

ByDag Friis

Bob McGrathChristian Knapp

Ocean Engineering Research CentreMUN Engineering

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Scope: Components of the Propulsion System

Where engine power goes

Propeller types

Propulsive efficiency

Cavitation

Selection Guidelines

What can I do with my as-installed system?

Testimonials and Simulations

Conclusions

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Propulsion and Shafting System: Is a massive “family” that includes:

Main Engine

Gearbox

Shafting

Shaft couplings

Journal and stern tube bearings

Propeller

Must be designed to work in harmony

Changes or problems with one component effect the entire system

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System Power Evaluation:

Indicated Power Delivered PowerShaft PowerBrake Power

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Where Engine Power Goes:Gear Losses

4%Shaft

Losses3%

Prop Losses (thrust

deduction)25%

PTO (if applicable)

20%

Remaining48%

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Propeller Types:

Fixed Pitch

Least Expensive in initial cost

Efficient for wide range of operations

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Controllable Pitch Propeller: Controllable Pitch

Great for multi-mode operations.

Engine RPM remains constant while pitch is varied for different loading conditions, or both simultaneously

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Nozzles:KORT RICE

Depends on application and how much clearance you have if

using a nozzle makes sense

• Built Low Speed Efficiency

• Loses operational efficiency

when majority of time spent

steaming

• Built for Steaming Efficiency

• Multiple options by going

with either speed or towing

nozzle

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Mewis Duct:

• Designed for vessels with poor inflow due to hull form

• Stabilizes water inflow to propeller. Uniform load distribution.

• Rotated fins pre-swirl the flow, generates higher load on propeller and

more thrust

• Guaranteed efficiency gains (when designed and optimized for vessel

and coupled with rudder technology)

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Achieving Good Propulsive Efficiency:

The Power characteristics of the engine have to be matched to the best possible propeller characteristics for this application.

The main propeller characteristics are:

Diameter Pitch RPM Number of Blades Blade Area Ratio

Achieving Good Propulsive Efficiency:

The greater the propeller diameter the more efficient the propeller, i.e. choose the largest propeller that can be reasonably accommodated in the available propeller aperture.

Propeller clearances (inches)

Prop diameter (inches)

60 72 100

minimum maximum minimum maximum minimum maximum

a 4.8 12.0 5.8 14.4 8.0 20.0

b 4.8 15.0 5.8 18.0 8.0 25.0

c 9.0 18.0 10.8 21.6 15.0 30.0

d 1.8 3.6 2.2 4.3 3.0 6.0

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Cavitation:

Cavitation occurs when the pressure in an area of the propeller falls below the vapour pressure.

This results in bubbles or Cavities of steam forming

The problem is that when the steam cavities collapse on the surface of the propeller it leads to erosion of the blade material

Collapse also generates noise

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Cavitation:TIP CAVITATION

SHEET CAVITATION

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Choice of Blade Area Ratio:

The smaller the blade area ratio the higher the open water propeller efficiency

The choice is made on the basis of choosing the smallest ratio that will give satisfactory propeller performance from a Cavitation point of view.

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Hull and Propulsion System Interaction:

Interaction Between Hull and Propeller

Flow speed through the propeller (Wake fraction )

Effectiveness of the thrust developed by propeller (Thrust Deduction Fraction)

Hull Geometry and Characteristics

The higher the L/B ratio the better the flow of water to the propeller

Results in a more gradual change in direction of water flow

Lowers likelihood of flow separation and eddy making

Increases flow velocity through propeller

Results in more uniform flow velocity through propeller

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Selection of Propeller Characteristics:

In order to be efficient, the propeller characteristics have to be selected based on:

Maximum Allowable Propeller Diameter

Flow conditions at the propeller (hull form)

Cavitation

Operational Scenario

Operating RPM (gear ratio)

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Selection of Propeller Characteristics:

Propeller type has to be selected based on operating regime:

Fixed Pitch is best suited to a single speed operation Fixed Gear Fishery, i.e. Propeller Designed for Cruising Conditions

Controllable Pitch when towing fishing gear Nozzle can be detrimental for boats that spend a major portion of

their time steaming to and from the grounds due to the increased drag at cruising speed

Nozzle Propeller when towing fishing gear, and affordable This is only likely to be the best alternative if the vessel spends most

of its time towing gear Usually fitted with controllable pitch to optimize performance at

both operating conditions

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What can I do with my as-installed system?

Have Clearance? Increase your diameter /decrease rpm(mind tip cavitation)

No Clearance? Alter pitch and gear ratio (mind cavitation)

Clearance AND Pitch restricted? Alter gearing ratio (mind cavitation and prop loading)

Reduce unnecessary hotel loads (extra deep freezes, clothes dryers, T.V.’s, cabin lights, etc)

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Testimonials:

86” diameter propeller achieving best fuel econ. and highest speed at lowest rpm

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5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400

GPH

RPM

64'11" RPM VS Fuel Economy

60" Diameter

66" Diameter

86" Diameter

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Testimonials:

28” diameter propeller achieving highest speed at identical RPM

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1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30

Speed (kts)

Prop Diameter (in)

Identical 34'11" Vessels, Speed vs Prop Diameter at 660 RPM

22" Diameter

26" Diameter

28" Diameter

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Simulations:35’ fixed gear vessel:

• Altered as-built prop from 25” to 30” diameter

• Achieved 12% fuel savings per hour

65’ mobile gear vessel:

• Constrained in diameter due to as-built specs

• Achieved 2% fuel savings per hour by altering pitch

• Greater savings achievable by altering of gear ratio

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Conclusions: Have your propeller checked by a qualified professional for suitability of Diameter,

Pitch, RPM, and Blade Area Ratio and resulting efficiency for your operation

If you are towing fishing gear a significant part of the time, a controllable pitch and possibly a nozzle propeller may be the best choice

If you are not towing gear a well designed fixed pitch propeller is your best option

Check that changing propeller and/or gear ratio makes economic sense for the remaining vessel life.

Time and money spent in R&D can save and even make you money in the long term, but the analysis has to be done.

Remember your decisions should make business sense.

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QUESTIONS?

VS

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Propeller Cavitation Design Chart:

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Considerations for Outboards:

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Propeller Design Chart:

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Symptoms and Causes:

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Nozzle Propeller: If flow separation occurs around the

nozzle one will get a significant increase in drag, i.e. reducing the efficiency of the nozzle-propeller

Nozzle-Propeller diameter will be less than for regular propeller, therefore resulting a reduction in efficiency

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Propeller Types:

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Propeller Design Parameters:

The Optimal Open Water Efficiency:

Rises with increase of Propeller Diameter

Rises with increase of Propeller Speed of Advance This is governed by hull characteristics and its effect on

slowing of the water flow through the propeller disk (wake fraction)

Decreases as the Blade Area Ratio Increases Governed by cavitation avoidance considerations

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Achieving Good Propulsive Efficiency:

The greater the propeller diameter the more efficient the propeller, i.e. choose the largest propeller that can be reasonably accommodated in the available propeller aperture.

This is done by allowing for reasonable propeller clearances in order to reduce likelihood of pressure pulse vibrations being induced in the local hull structure.

Propeller clearances (inches)

Prop diameter (inches)

60 72 100

minimum maximum minimum maximum minimum maximum

a 4.8 12.0 5.8 14.4 8.0 20.0

b 4.8 15.0 5.8 18.0 8.0 25.0

c 9.0 18.0 10.8 21.6 15.0 30.0

d 1.8 3.6 2.2 4.3 3.0 6.0