vessel modification and hull maintenance considerations ... modifications and... · vessel...

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

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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

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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

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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






30% of Midship Draught Immersion (As-Built)

20% 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)





80% As-Built Half Angle (44 Deg)



As-Built Half Angle (55 Deg)

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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

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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

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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

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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

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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

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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

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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

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A CONCLUDING SLIDE

Remember: “COST vs. BENEFIT!”

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