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1
Vessel Modification
and
Hull Maintenance Considerations –
Options & Pay Back Period or Return On Investments
By
Dag Friis
Christian Knapp
Bob McGrath
Ocean Engineering Research Centre
MUN Engineering
2
Overview: Energy Efficiency Related Hull Form problems
commonly found in Length Restricted vessels
Hull Maintenance Ghost Weights
Hull Surface Fouling
Appendages
Lengthening Bow Half Angle
Transom Immersion
Anti-Roll Systems
Bulbous Bows
3
Vessel/Hull Maintenance:
Ghost Weights: The ever increasing weight!
Excess Cargo, Gear and Miscellaneous Equipment Left On-Board unnecessary weight = Increased Fuel Consumption
Hull Surface Roughness has a significant influence on resistance. Hull surface should be as smooth, even and fair as possible.
ROUGH AND FOULED HULLS
IMPROPERLY
FAIRED
FIBREGLASS
HULL
4
Estimated Hull Surface Roughness Efficiency:
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
12.00%
14.00%
0 2 4 6 8 10 12
Pe
rce
nta
ge
Dif
fere
nce
[%
]
Speed [knts]
Comparison of Percentage Increase in Fuel Consumption due to Hull Fouling over that of a Clean Hull
35 Footer
65 Footer
Effects More Pronounced at Slower Speeds
5
Hull Surface Treatments: Possible Actions:
Clean off Marine Growth on a regular basis
Apply Fairing compound, sand and finish with a smooth coating to a clean hull.
Paint for Surface Protection Especially Steel Against Corrosion
•REFLECTIONS =
PROPER LOOK OF A
CLEAN & FAIR HULL!
•Moulded Fibreglass
Planing Craft – with good
surface finish,
•Could be cleaned and
scratches filled
ROUGH AND FOULED
PlANING HULLS
6
Small Boat Surface Roughness
ROUGH AND FOULED HULLS
•Moulded Fibreglass Planing Craft –
with good surface finish, could be
cleaned and scratches filled
Steel should be painted against Fouling
and Corrosion
Fibreglass Surface Roughness easily
Faired with Sandpaper, Epoxy, Anti-
Fouling Paint and Filler compound
7
Hull Appendages : Sonar Domes:
Design should be faired into hull during installation to reduce drag
Additional Appendages such as Struts, Trunks, Heat Exchanger Pipes, Ice Deflectors, etc All Affect flow along hull as well as inflow
conditions to the propeller, thereby increasing stern pressure, hull drag and reducing propeller efficiency
IMPROPERLY FAIRED
ICE DEFLECTORS
CONSIDER USING A
SEACHEST
Proper Strut or
Cavitation Plate
Position = Immersed to
minimise Cavitation!
8
Effect of Stern Tube and Skeg Shape on Design: Stern Tube, Skeg and Rudder and
Rudder Post Faring Abrupt Changes in Shape Result in
poor Flow to the Propeller
Poor In-Flow Conditions Significantly Reduce Propeller Efficiency
CLEANER PROP IN-
FLOW
ABRUPT PROP IN-FLOW
ASYMMETRIC
PROP IN-FLOW
9
Rudders: Rudders:
The Common flat plate variety produce less Lift Force which is required to steer the
vessel compared with Foil types requiring increased helm motion
Increases the drag force produced when rudder angle is applied relative to the airfoil type slows vessel
Incorrect Design of Rudder supports such as heel, trunk and struts can contribute:
Significantly Increased Drag and Flow Turbulence
Worse propeller outflow conditions and lower propeller efficiency and loss of manoeuvrability
Potential Rudder Cavitation Erosion
Plate Rudder
with Heel Un Faired
Foil Half Spade Rudder
- Faired Trunk & Stock
Rudder Cavitation
Erosion
10
Estimated Hull Appendage Efficiency:
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
12.00%
14.00%
0 2 4 6 8 10 12 14
Pe
rce
nta
ge
Dif
fere
nce
[%
]
Speed [knts]
Percentage Increase in Fuel Consumption for a Non-Faired Hull Appendages Over Faired Appendages
64'11"
34'11"
Effects More Pronounced at Slower Speeds and Smaller Vessel Size
11
Rudder Selection:
Twisted Rudder- Asymmetric
Design Uses Swirl to Generate
Added Lift & Reduce Cavitation
Rudders: Proper Rudder Selection can
Decrease Thrust losses attributed to Propeller Rotational Flow and Cavitation,
Reduce vibration & Noise
Increase Propeller Fuel Efficiency and Vessel Manoeuvrability
Reduce Necessary Helm Motion
Affect Overall Vessel Performance Reduce Fuel Consumption
Flap Rudder with Heel – Increased
Manoeuvrability & Course Keeping
High Lift Shilling Rudder –
70 Deg max. Angle & Low
Drag
12
Vessel Lengthening: The 35’, 45’, and 55’ vessels may be lengthened by 5 ft
The 65’ vessels may be lengthened by 25 ft Note: New Small Fishing Vessel Regulations < 24m (78.74 ft)
Lengthening will generally reduce the amount of power required to cruise at a given speed
Allow for partial re-design of submerged hull Form
Will likely improve directional stability, i.e. the Vessel is more likely to be able to go in a straight line. Staying on course reduces the total sailing distance reduced fuel
consumption & Rudder Drag
13
Lengthening: For Fibreglass or Fibreglass-over-Wood hulls one may
require the whole hull to be glassed over depending on how the lengthening is done.
If the boat is cut at amidships and a piece added in the middle the hull will need to be fully glassed over
If a piece is added at the stern one may not need to glass over the entire hull if one can prove that the connection is strong enough to remain intact even in extreme conditions.
14
Lengthening: For steel and Aluminum hulls lengthening is a much
more straight forward thing regardless of whether the lengthening takes place at amidships or at the stern or the bow. The joining of the new to the old structure is done by welding.
Steel or Aluminum hulls will make it possible to cut off the bow and replace it with one with a more suitable angle of entrance
15
Lengthening: Lengthening should be done using a well qualified
Naval Architectural Engineer that will evaluate what additional vessel modifications may be needed in order to make the boat as safe and economic to operate as possible.
16
Stern Lengthening:
Increase in Working Deck Area
Moving Skeg & Stern Tube Aft = Increase in Propeller Clearance
Increase In Directional Stability & Hydrodynamic Efficiency – Better Course Keeping & Reduction in Service Power
17
Effect Of Transom Immersion & Abrupt Changes in Hull Shape: Larger Low Pressure Area Sucking in Water,
Creating Vortices and a Resultant forward water motion Substantial Energy Loss & Inefficiency
Creates a ‘boxier’ profile No change in waterline Length
Less clearance available for Propeller installation
Generally means a greater interference in flow through Propeller Disc
BOX LIKE =
Always Max
Submerged Area &
Large Pressure
Drop
RISING STERN = Transom
Immersion Varies with
Displacement = More Efficient
A CONTAINER VESSEL ONLY AT
MOST EXTREME LOADING IS
THE TRANSOM SUBMERGED!
18
Estimated Service Power with Transom Immersion for a 35’:
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9
Serv
ice
Po
wer
[kW
]
Speed [knts]
Estimated Service Power (No Genset) Variation with Immersed Transom: As Percentage of the Midship Draught for a 35' (L/B = 2.33)
100% Midship Draught Immersion
90% of Midship draught Immersion
80% of Midship draught Immersion
70% of Midship draught Immersion
60% of Midship draught Immersion
50% of Midship draught Immersion
40% of Midship draught Immersion
30% of Midship Draught Immersion (As-Built)
20% of Midship Draught Immersion
10% of Midship draught Immersion
~52% Reduction
~4
8%
Re
du
ctio
n
19
Bow Lengthening:
Reduction in Half Angle
More Gradual Change in Hull Submerged Area
20
Lengthening: Decreasing Half Angle
-30.00%
-25.00%
-20.00%
-15.00%
-10.00%
-5.00%
0.00%
5.00%
10.00%
15.00%
20.00%
1 2 3 4 5 6 7 8 9 10 11
Fu
el
Vo
lum
e [
l]
Speed [knts]
Percentage Difference in Hourly Fuel Consumption with Variation In Bow Half Angle over As Built Case ( for L/B = 2.89)
110% As-Built Half Angle (60.5 Deg)
105% As-Built Half Angle (57.75 Deg)
95% As-Built Half Angle (52.25 Deg)
90% As-Built Half Angle (49.5 Deg)
85% As-Built Half Angle (46.75 Deg)
80% As-Built Half Angle (44 Deg)
75% As-Built Half Angle (41.25 Deg)
70% As-Built Half Angle (38.5 Deg)
As-Built Half Angle (55 Deg)
21
Case Studies Simulated : 65’@ Cruising Speeds (25% Sea Margin, 100 nautical miles)
65’ Base (10 knots ): 1583 Litres 1923.9 Engine RPM
65’ Optimised Speed (7.5 knots): 520 Litres 1025 Engine RPM
65’ Lengthened to 75’(10 knots): 692 Litres 1340 Engine RPM
35’@ Cruising Speeds (15% Sea Margin, 100 nautical miles)
35’ Base (8 knots ): 250 Litres 1989 Engine RPM
35’ Optimised Speed (6 knots): 75 Litres 940 Engine RPM
35’ Lengthened to 40’(8 knots): ~100 Litres 1439 Engine RPM
22
Anti-Roll-Devices and Energy Efficiency: Paravanes , Active Fin Stabilizers and Batwings:
add significantly to vessel resistance and are more efficient in stabilizing the vessel at cruising than fishing speed
Anti-Roll-Tank very effective at all operating speeds Can be installed away from work and hold areas Must be ‘tuned’ and designed to specific vessel’s
parameters
EFFECT OF ANTI-ROLL TANKS ON SEAKEEPING TESTS
Bulb B (L/B = 3)
Irregular Waves
Bulb B with ART (L/B =3)
Irregular Waves
CAUTION: ANTI-ROLL TANKS MUST BE PROPERLY DESIGNED AND FITTED FOR EACH INDIVIDUAL VESSEL
24
Anti-Roll-Tanks: Properly Designed and Tuned Anti-Roll-Tanks will reduce roll motions in normal
conditions by the order of
55% 60% is likely a reasonable expectation.
This will allow one to fish in worse conditions and likely reduce fishing time even in good conditions.
Roll for Model with Bulb B with and without an ART
-40
-30
-20
-10
0
10
20
30
40
0 5 10 15 20
Time (s)
Roll
(Deg
)
ART
No ART
25
Bulbous Bows: Reduces resistance by creating a wave that is
sufficiently out of phase with the wave generated by the bow that it reduces the resulting combined wave
Reduces pitch motion and phasing relative to waves if properly designed and significantly reduces the added resistance in waves
WARNING: Bulbs can amplify pitch motion and increase the added resistance in waves if not properly designed OBTAIN PROFESSIONAL DESIGN SERVICES
Properly designed bulbous bows have proven to reduce resistance by 15% to 40% depending on steaming speed and overall hull proportions etc
EFFECT OF BOW TYPE ON SEAKEEPING TESTS
Standard Bow (L/B = 3)
Irregular Waves Bulb A (L/B =3)
Irregular Waves
Effect of Bow Type on Pitch Motion
0
2
4
6
8
10
12
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
Pit
ch A
ng
le (
De
gre
es)
Wave Frequency [rad/s]
Comparison of Pitch Motion for Conventional and Bulbous Bows at Design Draught (L/B=3) in 2m Significant Waves
Bulb A
Standard Bow
28
Bow Type Fuel Rate :
0
100
200
300
400
500
600
700
0 2 4 6 8 10 12 14
Fu
el
Ra
te [
l/h
]
Speed [knts]
Comparison of Estimated Hourly Fuel Consumption for a Design Draught for L/B = 3 and 25% Sea Margin
Standard Bow 0 Deg Trim
Standard Bow 3 Deg Trim
Bulb A 0 Deg Trim
Bulb A 3 Deg Trim
~40
% R
ed
uc
tio
n
~66% Reduction
29
Advantages ofBulbous Bows:
Bulbous Bows are effective in reducing resistance at cruising speeds :
Roughly 4.5 7.5 knots for a 35’ vessel (Estimated)
35’ with bulbs exist, however, we have not currently tested them.
Likely the payback period will be longer for smaller vessels
Roughly 5.0 8.5 knots for a 45’ vessel
Roughly 5.5 9.5 knots for a 55’ vessel
Roughly 6 10 knots for a 65’ vessel
Economics Depend on Vessel Size, Construction Material, Operational Life, Steaming Distance and Frequency of Trips to determine IRR and Payback period
30
ECONOMIC COSTS: ART’s :
~ $3000 For Professional Design + Construction and Trials ($25,000+ total estimate)
Paravanes : ~10% Increase in resistance equivalent to the loss of 1 knot at steaming
speeds (for Paravanes = 0.3-0.4 m2) Require a few knots ahead Speed to gain dampening effect
Alternate Rudders & Additions: Can Deliver 3-6% increase in Fuel Efficiency Increased Manoeuvrability and Course Keeping Increased Lift and Lower Drag Less Cavitation Erosion, Fewer Vibrations and Reduced Noise
Bulbous Bows: Steel: ~$50,000 $120,000+ Depending on Vessel Length and Type
(Cylindrical or Faired: Less for smaller vessels) Fibreglass: Significantly less as it is easier to Retro-fit
31
Estimated % Power Variation 65’:
-40.00%
-20.00%
0.00%
20.00%
40.00%
60.00%
80.00%
100.00%
4 6 8 10 10.5
Pe
rce
nta
ge
Dif
fere
nce
[%
]
Speed [knts]
Estimated Effect of Hull Fouling, Appendage , Immersed Transom and Half Angle on Service Power (L/B ~ 2.89)
Appendage Drag
Fouling Drag
33% Increase in Transom Draught
66.7% Increase in Transom Draught
100% Increase in Transom Draught
33% Reduction in Transom Draught
66.7% Reduction in Transom Draught
10% Increase in Half Angle
10% Reduction in Half Angle
25% Reductio in Half Angle
32
A CONCLUDING SLIDE
Remember: “COST vs. BENEFIT!”