enabling permanent repairsof rudders without dry-docking the ship. Permanentrepairs were hitherto notpossible and ships had todrydock in case a majordefect was found. The newlydesigned equipment is light-weight and can be mobili-zed very rapidly in ourspecial flight containers.Therefore this new service isnow available worldwide.
Major defects on rudders very often cause unscheduleddrydocking of ships. The new method designed by our technical departmentallows engineers, weldersand inspectors to performtheir tasks in dry conditions.Class approved permanentrepairs in-situ, without mo-ving the ship, are now possi-ble and commercial opera-tions can continue. Steelrepairs and replacements can be performed and pintle
and bushing defects can besolved without the loss oftime and money associatedwith drydocking.
The equipment can be mobi-lized within hours to any portin the world and is availablefor rapid mobilization fromthe Hydrex headquarters inAntwerp.
UNDERWATER TECHNOLOGY
Permanent rudder repairs now possible without drydocking
3
Hydrex started activities in 1974to help ship owners solve the
problems they might encounterunderwater with their vessel andthis purpose still holds firm today.We made ‘helping you’ our respon-sibility and it is work we continue toenjoy.
The first article in this magazine is a good example of the complete package of services we offer to ourcustomers. In this article you can readof an operation that was carried out bya Hydrex diver-technician team on atanker at anchor in Port Gentil.
While one unit of the team was perfor-ming a class approved insert repair on the side shell plating of the vessel,the second unit was simultaneouslyperforming a full presale inspectionand cleaning of the underwater hull ofthe vessel, combined with a propellerpolishing. The combination of thesetwo jobs enabled the new owner tosail his vessel with a permanentlyrepaired hull and at the same timehave the ship’s performance restoredas close to its optimum condition aspossible.
Best regards
Boud van Rompay
Editorial
KEEPING SHIPS IN BUSINESS
To receive a free copy, fax to:Hydrex N.V. +32 3 213 53 21or e-mail to [email protected]
Insert repair, underwater hullcleaning and presale inspectionof tanker in Port Gentil 4
Cutting away damaged area on side shellplating of tanker
Table of contents
Slip and propellerefficiency 6-7
ISO 9001 certified
The value of regular under-water propeller polishing 5
insert which was subsequently positio-ned and secured with a deep pene-tration weld according to the classapproved Hydrex procedures.
When the insert plate was fully welded,ultrasonic testing was carried out withpositive results. The full penetrationweld was also inspected from the out-side, found in a good condition andauthorized by the class. This allowedthe new owner to sail his vessel with apermanently repaired hull.
Recently Hydrex was asked toperform an insert repair com-
bined with a presale inspection, an underwater hull cleaning and propeller polishing on a 230 metretanker while the vessel was atanchor in Cap Lopes Bay, PortGentil. A diver-technician team wastherefore mobilized from the localHydrex office.
The operation was performed with twowork barges, one on which all theequipment needed for the insert repairwas loaded and one on which a monito-ring station was installed which wasused for the underwater hull inspectionand the underwater hull cleaning work.This enabled the team to split up andperform the two parts of the operationsimultaneously.
After doing a full underwater inspec-tion, the hull cleaning team removed all marine fouling from the underwaterhull of the vessel and carried out a propeller polishing with the Hydrex in-house developed range of under-water cleaning units. This restored thevessel’s performance to a level as closeto optimum as possible.
Meanwhile the repair operation startedwith the installation of scaffolding onboth sides of the hull to give the teamaccess to the affected area on the side
shell plating of the vessel. Next both anonboard and an outboard inspection ofthe damage was performed. This revea-led that an area of 2255 mm x 1760 mmneeded to be cut away and replacedwith a new insert plate.
While the new insert was fabricated tothe exact measurements, the affectedshell plating was cut away togetherwith several frames that were alsodamaged. New frames were then instal-led and the edges of the hole were prepared for the installation of the
Insert repair, underwater hull cleaning andpresale inspection of tanker in Port Gentil
Hydrex equipment loaded onto work barge New frames after installation
Preparing the edges of frames prior to installation of new ones New insert prior to full penetration weld
4
condition and can keep a vessel’s fuelconsumption as low as possible, lea-ding to significant financial savings.
An observed reduction in shipperformance is readily associa-
ted with the condition of the ship’shull but the effect of the propellersurface condition is often overloo-ked. Nevertheless, the effect can be significant. In economic terms, a high return for a really low invest-ment can be obtained by propellermaintenance.
When considering the propeller sur-face condition a distinction has to bemade between fouling and surfacedeterioration. The effects of propellerfouling in terms of a power penaltyare much greater than those of surfacedeterioration. Research has shownthat propeller fouling can quicklyreduce the delivered horsepower by20%. While rough propellers can be asdestructive of fuel economy as roughhulls, the remedy is much cheaper.
Propeller roughness is most easilymeasured underwater by divers usinga comparator gauge, whereby the pro-peller surface is compared to surfacefinishes of known roughness. The pre-dominant effect of an increase inroughness of the propeller blades is an increase in the propeller torque,thereby lowering the propulsive efficiency. By using a propeller per-formance analysis tool the losses inpropulsive efficiency were calculated.
This showed that significant lossesresulting from blade roughening canbe regained by cleaning and polishingof the blades.
Fuel savings of up to 5% can be gai-ned if a polishing is performed on apropeller after 1 to 2 years in service.Exact figures depend on a number offactors such as operating conditionsand propeller design, but it is advisedto perform propeller polishing at regular intervals to avoid losing propulsion efficiency. This keeps the propeller in optimum operational
The value of regular underwaterpropeller polishing
Propeller blades after polishing Hydrex diver during propeller blade polishing
Propeller blades prior to polishing
Hydrex diver polishing propeller blades
5
6
Most shipowners provide theirengineers with standard log
sheets for the daily entry of perfor-mance data. These log sheets almostinvariably contain a column for therecording of 'slip' and the super-intendent consults these columns toobtain information on the perfor-mance of the ship, engine and pro-peller, and possibly also to comparethe performance with that of otherships in the fleet.
What is the real significance of this slip figures, and what relationship exists between slip, propeller effi-ciency and ship performance? Towhat degree does the condition of the propeller affect its slip, and howdo repairs impact on this?
Propeller efficiency
The function of the propeller is toabsorb the horsepower delivered bythe engine at the correct rate of revo-lutions, to ensure optimum enginerunning conditions, and must itselfhave the highest possible efficiency in
order to produce maximum thrust andhence maximum speed.
The power is absorbed as torque at thetail-shaft, i.e. delivered horse-powerd.h.p. at N revolutions per minute.Thrust horsepower, t.h.p. is given bythrust T x speed of advance Va, andpropeller efficiency is given by t.h.p.
d.h.p.
Slip
Perhaps the simplest conception ofthis is to consider the screw to workin a solid block, when, in one com-plete revolution it would advance a distance equal to its pitch. Due tothe fluidity of the medium in which it actually works, however, the screwadvances forward a distance some-what less than the pitch, and the difference between these is called the 'slip'. This can be shown dia-grammatically in the form shown in Figure 1.
A better, and more scientific explana-tion is to consider that the propellerblade works as an aerofoil, and as a
result gives its best lift/drag characte-ristics at low angles of incidence. It iscustomary to present aerofoil data ona base of angle of incidence as repre-senting the operating conditions, andthis can be shown to be the same asthe slip function commonly used forthis purpose by marine engineers.
Fig.1. Diagrammatic representation of slip
Fig.2. Typical examples ofthe difference between effective and face pitch on various blade sectionshapes.
Slip and propeller efficiency
7
Referring to diagram 1, it will be seen that if the velocity at right angles tothe direction of advance is plotted as27πRN, and the ship speed V is plot-ted in the direction of advance, theresultant apparent water flow will bealong the line OV. The attitude of theblade is, of course, defined by thepitch and revolutions, and thus for agiven propeller the angle of incidenceis defined by the slip shown.
Slip is usually expressed as a percen-tage of the pitch, thus, if the propellerturns at N r.p.m. and the forwardspeed is V in knots, then slip = NP-V x 6080
60 x 100 = 1-101.3V x 100NV NV
The ship's engineer sometimes refersto NP, the product of revolutions andpitch, as the 'engine speed'. It is use-ful to note from the above, for futurereference, that slip is a function of
V/N i.e. the relationship between shipspeed and propeller revolutions, andthat therefore some of the parametersoften used in presenting propellerinformation are functions of slip. Forconvenience' therefore, it is usual for e.g. Advance coefficient J =V
NDDiameter constant = ND
VEffective Pitch
Another anomaly concerns the pitchused in the slip calculations. Thepitch that determines the powerabsorption characteristics of the pro-peller is the mean effective pitch,sometimes called the virtual or no-liftpitch, which is always greater thanthe face pitch when the blade has fini-te thickness. The difference betweenthe effective and face pitch can varyfrom propeller to propeller because ofthe different types of blade sectionprofiles used. Some typical examples
of the difference typical examples areshown diagrammatically in Figure 2.
The effective pitch of the three sec-tions shown is the same in each casebut the face pitch decreases progressi-vely from (a) to (c). The effectivepitch can only be found by calculationor experiment and cannot be measu-red from the propeller or readilyshown on the propeller drawing. For convenience, therefore, it is usual forthe engineer to use the face pitch inhis slip calculations, this being thepitch that can be measured on theactual propeller.
From the above it will readily be seen that it can be misleading and is indeed incorrect, to compare values of slipfrom propellers of different designfamilies. Likewise, one should guardagainst claims that one propeller issuperior to another merely on thegrounds that it gives less slip.
In its quest to provide cost effec-tive services to customers,
Hydrex developed procedures to address different kinds ofdamage to propellers. This re-search led to the design of theHydrex cold straightening machi-nes first used in 2002.
By taking advantage of this tech-nique damaged blades can bestraightened underwater, allowingthe ship to return to commercialoperations without the need to drydock. Blades can be broughtback close to their original form,restoring the propeller’s optimumefficiency.
The cold straightening machineshave been in use for quite sometime now but the Hydrex researchdepartment has been looking intoways to expand the technique evenfurther to improve our services. A new version of the straightening
machine was recently put intopractice. It is compatible with the existing models and is used to restore more severely bent propeller blades to their originalcondition.
Cold straightening of severely bent propeller blades
KEEPING SHIPS IN BUSINESS
8
Keeping ships in business
Hydrex underwater tech-nology and services pro-
vide high quality solutionsto the repair and replace-ment problems encounteredby ships and offshore ves-sels.
We deliver a complete line of
services that may reduce oravoid off-hire time entirely.From major projects to simpleinspections, Hydrex has theworldwide facilities and capa-bility to meet your demands.
Drydocking is not necessaryso time, trouble and expense
are saved by doing work in-situ. Hydrex services coverhighly technical major repairsor replacements of a ship’sexternal underwater equip-ment such as thrusters, pro-pellers, rudders, stern tubeseals and damaged or corro-ded hulls.
![INSERT TAB HERE Table of Contents [Sections B – S only]](https://static.documents.pub/doc/80x56/6328751b5c2c3bbfa804aa29/insert-tab-here-table-of-contents-sections-b-s-only.jpg)
