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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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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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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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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 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