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LETTER | 178
RRS Ernest Shackleton –
a breakthrough in icebreaker
hull protection
On site seal repairs around the world
keep ships out of drydock
Hydrex White Paper No. 6

Contents
Page 3
RRS Ernest Shackleton –
a breakthrough in icebreaker
hull protection
Page 8
On site seal repairs around the
world keep ships out of drydock
Page 13
Hydrex White Paper No. 6
Introduction
ISO 9001
certified
Underwater services and
technology approved by:
Supreme Rudder
Protection
Ecospeed gives a very thorough
and lasting defense
against cavitation and corrosion
damage for a ship hull’s entire
service life.
The coating equally provides the
rudder with an impenetrable protective
layer while its flexibility
enables absorption of the forces
that are produced by cavitation.
This prevents the damage normally
caused by this phenomenon.
Without proper protection against
cavitation and the resulting erosion
and corrosion damage, the
financial consequences can be
severe.
By removing the existing paint
layers and applying Ecospeed
on the rudder we can break the
never ending cycle. of painting,
suffering damage, having to perform
extensive repairs in drydock
followed by a full repainting,
again and again.
With an Ecospeed application no
full repaint will be needed during
drydocking. Ecospeed is guaranteed
for ten years. At the most,
minor touch-ups will be required.
Belgian headquarters
Phone: + 32 3 213 5318
Fax: + 32 3 213 5321
info@ecospeed.be
2
US Office
Phone: + 1 727 443 3900
Fax: + 1 727 443 3990
info@ecospeed.us
www.ecospeed.be

RRS Ernest Shackleton –
a breakthrough in icebreaker
hull protection
When British Antarctic Survey’s
RRS (Royal Research Ship)
Ernest Shackleton was drydocked
recently in Denmark, the superintendent,
engineers and paint specialists
there to check the condition of the hull
paint were amazed. After two seasons
of battering its way through ice up to
2.5 meters thick with a high content
of gravel and volcanic lava adding
to its abrasiveness, the hull coating
was virtually intact and undamaged.
This was in strong contrast to the
Shackleton’s previous drydocking,
when almost the entire hull, bearing
a conventional ice-going underwater
hull coating, was practically stripped
to bare, unprotected steel.
The difference lay in the fact that
when the Shackleton left drydock in
2009, the hull was newly coated with
Ecospeed, a glassflake vinyl ester
resin underwater hull coating proven
to have extraordinary anti-corrosion
protective strength and flexibility.
Even though Ecospeed is not intended
specifically for ice-going ships and
icebreakers, it consistently outperforms
the specialized ice-going ship bottom
paints.
The success of the new underwater hull
coating on the Shackleton, whose hull
can genuinely be said to undergo the
harshest of conditions of just about any
vessel afloat, is a story well worth telling
in detail. As in many cases, a picture is
worth the proverbial thousand words.
In this case, two sets of pictures tell
the story more dramatically than any
description. Shackleton was initially
coated with a conventional ice class
paint in 1995, from build. BAS took
over the operation of the Shackleton
in 1999 and since then has only ever
RRS Ernest Shackleton in Antarctic ice up to 2.5 m thick.
repaired the hull coating using the
recommended repair product. The
pictures below show the condition of
Shackleton’s hull after 13 years without
“Ecospeed...consistently outperforms the specialized icegoing
ship bottom paints.”
a suitable ice class paint system and the
comparison of Ecospeed after two hard
years working in ice (next page).
It is worth noting in the photos on the
right above that neither the rudder
nor the hull above the water line were
coated with Ecospeed in 2009. This
Three photos of the hull of RRS Ernest Shackleton showing the condition of the paint (conventional ice paint and corresponding
repair coating) before Ecospeed was applied.
3

Three comparison photos of similar areas of the hull after the switch to Ecospeed underwater hull coating system and two
seasons of sailing in the ice. Note that the boot top and rudder, seen to have suffered damage in the photos on the right, were
not coated with Ecospeed in 2009 along with the rest of the underwater hull. This has now been remedied in 2011.
has now been rectified in the 2011
drydocking so that the rudder and the
hull above the water line will have the
same protection as the rest of the ship
bottom.
Some background on the
Royal Research Ship Ernest
Shackleton
Based in Cambridge, England, British
Antarctic Survey (BAS) is one of
the world’s leading environmental
research centers, responsible for the
UK’s national scientific activities in
Antarctica.
The RRS (Royal Research Ship) Ernest
Shackleton was built by Kverner Klevin
Leirvik A/S, Norway and launched
originally as the MV Polar Queen by
Rieber Shipping of Bergen in 1995.
She was deployed in the Antarctic by
various national programs before being
RRS Ernest Shackleton in Antarctic ice up to 2.5 m thick.
acquired by British Antarctic Survey
in August 1999. The vessel was then
renamed RRS Ernest Shackleton after
Sir Ernest Shackleton, perhaps the
most famous Polar explorer of all time.
RRS Ernest Shackleton, ice strengthened
and capable of a wide range
of logistic tasks as well as having a
scientific capability, is primarily a
logistics ship, used for the resupply
of the BAS’s stations, with occasional
scientific and specialist tasking.
In September/October of each year the
Shackleton sails from the UK to the
Antarctic and returns the following
May/June. After annual refit/drydock,
RRS Ernest Shackleton is chartered into
4
commercial survey work. At the end of
this northern summer work she loads
cargo and scientific equipment in the
Humber for return to the Antarctic. The
trade pattern of the Ernest Shackleton
includes breaking ice from 1- 2.5m
thick. According to the crew, she has
“really been bashing heavy ice” during
her recent voyages.
Ecospeed performance
Stephen Lee is the Senior Marine
Engineer for British Antarctic Survey,
the BAS’s equivalent of a Technical
“The biggest thing was the surprise at seeing the areas
where you’d expect it to have taken a lot of damage...”
Superintendent. Stephen and the
Antarctic Marine Engineering (AME)
department were instrumental in the
initial research which led to replacing
the underwater hull coating in 2009.
He recalls the reaction of those
present when the Ernest Shackleton
was first pulled out of the water at
Frederikshaven drydock in early 2011.
“The biggest thing was the surprise at
seeing the areas where you’d expect
it to have taken a lot of damage...
when she first came out of the water
and onto the blocks it was a complete
shock to all those present. All of us
there commented on the condition of
the hull and in particular that there
was negligible damage at the bows,
merely some scratch marks. None of us
there would have predicted this. I then
jokingly asked the question, ‘Are you
sure you’ve taken this ship to the ice?’”

Examples of the minor scrapes and chips which were the totality of the damage to the Ecospeed coated hull after two seasons in
the ice.
ice belt where mechanical damage
normally occurs. The rudder was also
coated with Ecospeed during the 2011
drydocking so that it too could benefit
from the same impenetrable protection
as the hull.
total removal of all the paint which the
crew of the Shackleton had become
used to before the application of
Ecospeed, the damage was negligible
and easily repaired. Only very minor
touch-ups were required in drydock.
Stephen Lee, Senior Marine Engineer
for BAS.
According to Stephen Lee, the crew of
the Shackleton reported that they had
been pushing into 2 - 2.5 meter thick
ice, “...and it’s just not touched it – just
not touched it at all.”
“We were really impressed with the performance
of Ecospeed,” says Stephen,
who was the person mainly responsible
for switching to Ecospeed in 2009. It
was seeing the results after two seasons
in the ice that led him to go up another
level and have Ecospeed applied above
the waterline so that it covers all the
Naturally the condition of the paint
was carefully inspected. “The paint
inspector, Howard Jess, took dry film
thickness measurements around the hull
and found it was basically something
around 970-1000 microns on average so
it’s hardly lost any thickness,” Stephen
explains. The original application of
Ecospeed was 1000 microns DFT on
the entire underwater hull.
Some minor mechanical damage
had occurred to the coating but this
amounted to some chips and scrapes
totaling less than 0.1% of the total
surface area. Compared to the virtual
Stephen Lee stated that BAS also took
advantage of the fact that the ship was
in drydock to apply Ecospeed to the
rudder and the boot top area which had
“...Are you sure you’ve taken this ship to the ice?”
not been included when the hull was
coated in 2009.
Maintenance on the Ecospeed has
been remarkably low. “The only
maintenance undertaken has been one
whole underwater scrub in August
2010 in Frederikshaven during our
last maintenance period, and then she
went down south,” says BAS’s Senior
marine Engineer.
5

Newly Ecospeed coated boot top area above the water line
and rudder as the Ernest Shackleton is readied for departure
from drydock.
Finding the right hull coating
system
Stephen Lee recalls his reaction on
seeing the Shackleton in drydock for
the first time, not long after he joined
BAS. “In 2008 when she came out of
the water it was amazing to see the
state of the hull. There was next to no
paint protection left on the vessel. The
decision was made then that we should
start actively searching for an ice class
paint that we could put onto the hull to
give us that protection.” This was the
search that led to the application of
Ecospeed in 2009.
“We looked at all the alternatives
including Ecospeed,” recalls Stephen.
“There were a lot of comparisons
between all of the products. Because
of the nature of our business and where
we operate we also required a paint
system that would have significant
environmental benefits as well as
conforming to the polar code and latest
classification societies regulations. We
required a paint system which was cost
effective in purchase, application and
maintenance. We wanted a simplified
paint system that no matter where you
went in the world a paint contractor
would be able to apply it without
having to rent in expensive equipment
or shielding to ensure application
could continue. We also wanted to be
able to conduct minor repairs either
by the yard paint contractor or our
own crews. Ecospeed gives use this
capability. Application of some of
the more traditional icebreaker paint
requires twin feed paint system which
requires a great deal more care during
the application process as well as
ensuring all the environmentals are
correct which can include tenting up
space heaters around the area that is
going to be painted. Comparing the
other paints with Ecospeed they’re
very comparable as far as purchase
price and performance generally in
the broadest of terms, but the main,
huge difference is the actual cost and
complexity of application of the paint.
The preparation is the same, 2.5 SA
over the hull, but the actual application,
not having to get the environmentals
“...the main, huge difference is the actual cost and
complexity of application of the paint.”
right, not having to tent up the area,
if it’s slightly cool not requiring space
heaters, if the area is gingered slightly
which may or may not require a sweep
blast before you can put the primer on
– there’s a huge amount of preparation
and logistics that have to go into
getting the initial coat of traditional ice-
going paint onto the hull, whereas with
Ecospeed its minimal as long as you
have a good paint inspector, and only
minimum environmentals are needed.”
Cathodic protection areas,
Impressed Current Cathodic
Protection (ICCP)
Ecospeed was applied with the standard
thickness of 1000 microns over the
hull. The only area where an increase
of thickness up to 2000 microns was
applied was around the cathodic protection
areas.
Stephen Lee explains, “One of the
inquiries I made before we put the
Ecospeed on the first time in 2009
was to see how compatible it was with
the ICCP that the vessel had because
the ship was suffering from the ICCP
System not functioning properly.
The main problem was not having
an adequate paint system. Cathodic
protection works best when the hull is
undamaged and has a complete paint
system covering the hull, unlike in
Shackleton’s case. Cathodic protection
as most people understand works in
conjunction with the paint. ICCP has
never given complete hull protection
but with a good paint system it will.
Effectively a common ratio I use is
80% protection from paint & 20%
protection from ICCP. A failure in
one or the other will quickly see a
6

deterioration in the paint system. With
Shackleton’s hull the ICCP continued to
be ineffective simply because there was
no complete paint system and therefore
the ICCP system produced eddy
currents around the hull, which resulted
in the ICCP continually tripping,
rendering the system inoperative most
of the time. Once we knew what type
of paint system we were looking for,
we checked into its compatibility with
the cathodic system and realized that
the actual paint system would act as
a dielectric, so we’ve gone to 2000
microns around the anodes.”
Paint inspector’s perspective
Howard Jess was the paint inspector for
the initial Ecospeed application in 2009
and Stephen Lee was so impressed with
“his diligence, his commitment and his
professionalism” that he specifically
requested Howard’s attendance at the
yard for the additional application
of Ecospeed to the boot top area and
rudder in 2011.
Currently working as an independent
paint inspector specializing in the
marine sector, Howard Jess is a NACE
Level 2 Marine Paint Inspector, developer
of commercially adopted innovative
coating technology who has
published a number of technical papers
and spoken at many international
events. Howard studied chemistry at
the Glasgow College of Technology
and has nearly 30 years of experience
in the paint industry. He has overseen
a number of Ecospeed applications,
including the original coating of the
Shackleton in 2009, major cruise ship
and RO-PAX ferry newbuilds in 2009
and 2010, and a number of other applications,
small and large.
“I was very impressed with the condition
of the coating on the Shackleton after
Howard Jess at a paint inspection job.
two seasons in the ice,” says Howard.
“Apparently she had been trapped in
the ice on several occasions and the
procedure is to reverse and then crash
forward at full speed. Yet the coating
remained intact – pretty impressive.
I would have expected to see damage
down through the coating exposing the
hull. However the bow looked as if it
had just been painted. Crew members
who had seen the ship out of the water
on numerous occasions said that they
had never seen the hull looking so good
after two seasons in the ice.”
Experienced in paint inspection jobs
with a number of different types and
brands of marine paint, Howard notes a
difference with Ecospeed applications:
“If you are ever involved with any
marine coating job where there are
“I’m not aware of any other company that gives a 10 year
warranty on their hull coating.”
a number of paint suppliers you will
see that inspectors from other paint
companies will maybe come in once
or twice a week to check things,
sometimes daily but only for a coffee
and a chat. Ecospeed inspectors are
there whenever paint is applied. They
also control when the paint can and
cannot be applied. This level of control
inevitably results in better system
performance. There has been a lot of
development work into anti-fouling
coatings in recent years. Ice coatings
tend to have been left behind due to the
small market sector so the majors tend
to push their new super-duper hightech
coatings which somehow never
seem to live up to expectations. I’m
not aware of any other company that
gives a 10 year warranty on their hull
coating.”
Asked to compare the various ice-going
hull coatings available, Howard Jess
commented as follows: “Taking the
technicalities out of the equation, in my
view the greatest advantage in using
Ecospeed is cost. Two applications
means only two times painters’ costs
when using Ecospeed. When using
other ‘conventional’ systems we could
be talking about painters costs times
seven. Do the maths! Add to that,
reduced repair work meaning less
time in dry dock, less time off-hire
and increased fuel efficiency and the
product should just about sell itself.”
Howard also has some advice for
shipowners applying Ecospeed to their
ice-going vessels: “For some reason
the current mind-set is to stop at the
waterline. Given that ice tends to ride
over itself and up the hull it would
seem sensible to extend the coating
to 2–3 meters above the waterline.” A
piece of wisdom obviously shared by
Stephen Lee, evidenced by the fact that
while the Shackleton was in drydock
recently, the level of Ecospeed coating
was raised from the water line to well
into the boot top area for protection.
Conclusion
The RRS Ernest Shackleton is living
evidence that when it comes to protecting
the underwater hull in the very
harshest of conditions including 2.5 m
ice mixed with gravel such as only the
Antarctic can provide, Ecospeed offers
easy-to-apply, long lasting, complete
protection, and does so in a highly
economic way.
7

On site seal repairs around the
world keep ships out of drydock
Hydrex has carried out repairs and
replacements on all types of seals
in-situ, and in most cases underwater,
for a number of years now. This helps
owners to extend their vessel’s drydock
interval and eliminates the loss of
time and production brought about
by drydocking. Using the Hydrex
flexible mobdocks, fast response to
any emergency call is guaranteed to
locations around the world from the
various Hydrex offices.
Since Hydrex divers first carried out an
underwater face seal replacement 15
years ago, we have constantly worked
to advance the techniques used for all
kinds of seal repairs. This led to the
development of our unique flexible
mobdock (mobile mini drydock) technique
which allows Hydrex teams to
create a dry underwater working environment
around a seal assembly. The
technique made it possible to replace
stern tube seals in their entirety underwater.
Hydrex diver/technician teams work around the clock if needed to carry out the
repair as fast as possible.
The technique has won one of the
prestigious Lloyd’s List Awards on two
occasions. Hydrex has now received
tools. These centers were designed
specifically to increase speed of service.
This made it possible for Hydrex to
“On site seal repairs helps owners to extend their vessel’s
drydock interval and eliminates the loss of time and
production brought about by drydocking.”
full class acceptance from several
major classification societies to perform
underwater stern tube seal repairs.
Five years ago Hydrex started working
together with manufacturer-independent
seal specialists AEGIR-Marine,
providing Hydrex with the capability
to repair or replace all the major seal
types. Since then Hydrex has continued
to develop the mobdock technique to
adapt it to all possible situations.
All Hydrex divers have experience with
working inside the flexible mobdock.
The following case studies give an
account of some of the more important
recent seal repairs performed by
Hydrex.
Underwater stern tube seal
repairs in France
The Hydrex flexible mobdock technique
was used to reposition the aft
stern tube seal assembly of a 210-meter
container ship in Le Havre. A Hydrex
diver/technician team performed this
operation to stop an oil blockage that
occurred just after the vessel came out
of drydock.
Every Hydrex office has a fast response
center equipped with all the latest
facilities, lightweight equipment and
8
All welding work is carried out by
experienced welders.
mobilize a diver/technician team to Le
Havre from the headquarters in Antwerp
immediately.
After the rope guard was removed the
diver/technicians discovered that the
oil flow through the stern tube seal
assembly was blocked because part
of the assembly had been positioned
180 degrees the wrong way. After the
flexible mobdock was installed, the
assembly was opened and all parts were
closely examined and cleaned. This
inspection revealed that all seals were

While the team prepared a first insert
plate on shore, the inspection of the
seal assembly revealed that the seals
were worn and needed replacement.
Next they installed the first insert and
secured it while the second plate was
prepared. Simultaneously another part
of the team opened the stern tube seal
assembly and it became clear that they
needed to renew the running area of
the seals as well. The team did this by
All Hydrex offices have state-of-the-art equipment at their disposal.
in good condition but that the bonding
was faulty. The seals were rebonded
and the seal assembly refitted correctly.
The superintendent of the container
ship in Le Havre was very satisfied
with the service offered by Hydrex. He
said, “Even after the typical late call
on a Friday afternoon and even later
vessel while it was berthed in Ghent,
Belgium. Following this inspection
the team replaced the worn seals and
installed a spacer ring, thus creating a
new running area for the seals.
Prior to the operation the vessel was
trimmed as much as possible. The
Hydrex team then built a scaffolding
“Using the Hydrex flexible mobdocks, fast response to any
emergency call is guaranteed to locations around the world
from the various Hydrex offices.”
that evening, Hydrex still managed to
assemble and organize a team of divers
that arrived the next day in Le Havre.
The job was completed after four days,
including full re-welding of the rope
guard and shifting of the vessel to
another berth. In between, new divers
and technicians arrived, all of whom
seemed very professional and skilled in
their work. We had also used Hydrex
on this ship before it went into drydock
for a propeller polishing in Dunkerque
and the response at that time was also
very fast and the job professionally
executed.”
Rudder and stern tube seal
repair on tanker in Belgium
A Hydrex diver/technician team performed
a crack repair on the pintle area
of the rudder of a 181-meter tanker
and carried out a detailed inspection
of the stern tube seal assembly of the
around the rudder pintle and the stern
tube seal assembly. Next they removed
the rope guard and the damaged areas
Hydrex divers are trained to carry
out seal repairs above and below the
waterline.
of the outer plating of the rudder. This
allowed them to perform an inspection
of the stern tube seal assembly and start
the repairs to the rudder.
The Hydrex flexible mobdock allows us
to create a dry underwater environment.
installing a new spacer ring on the stern
tube flange after which they replaced
and bonded the three seals.
Hydrex performed all operations under
DNV requirements which were verified
by an attending surveyor. The diver/
technician team rotated in shifts to finish
both repairs in the shortest possible
time and avoid any unnecessary delays
for the vessel.
Fast response prevents time
loss for general cargo vessel in
the U.S.A.
When oil was leaking from the stern
tube seals of a general cargo vessel,
Hydrex mobilized a certified diver/
technician team to the vessel’s location
in Mobile, Alabama, to perform underwater
stern tube seal repairs before the
9

and a dry underwater environment was
created, the damaged seals could be
replaced.
In order to provide the customer with
the fastest possible response, flexibility
was essential throughout the entire
operation. Hydrex was able to perform
the repairs in a very tight timeframe
and made sure that the new charterer
could sail the vessel free of oil leaks.
Stormy weather conditions did not
stop stern tube seal repair in Manila.
Hydrex diver inside flexible mobdock
communicating with team leader.
ship was transferred to a new chartering
party.
Hydrex had already performed a similar
operation on one of the customer’s other
vessels so he was aware of Hydrex’s
well-trained diving teams and ability
to handle this kind of situation without
loss of quality or time for the customer.
Because the U.S. Coast Guard has
very strict policies concerning environmental
risks, they would not
allow the vessel to sail to a different
location before the oil leak had been
permanently fixed.
“Hydrex has now received full class acceptance from several
major classification societies to perform underwater stern
tube seal repairs.”
Typhoon does not stop
stern tube seal repair in the
Philippines
When an oil leak prevented a 225-meter
bulker from continuing its sailing
schedule, a Hydrex diver/technician
team mobilized to Manila together
with one of the company’s flexible
mobdocks to perform emergency
underwater repairs at anchorage.
A typhoon was crossing over the
Philippines at the time the team
arrived. The storm grew to a climax
improved slightly and full safety could
be guaranteed for the divers.
Still under terrible sea conditions,
the rope guard was removed and an
inspection revealed that a fishing net
had been caught in the assembly and
was tangled up around the entirety
of the seals. The flexible mobdock
was then installed. This created a dry
working environment for the divers at
a depth of twelve meters in which they
could replace the damaged seals.
Damaged stern tube seal.
A team immediately left from the
Hydrex office in Clearwater, Florida,
together with the needed equipment,
and set up a diving station at the
berthing location of the ship. After the
Hydrex flexible mobdock was installed
around the stern tube seal assembly
Grinding work on stern tube seal assembly prior to reinstallation of rope guard.
just after preparations had been made
for the repair. Unfortunately this delayed
the underwater operation by a
day, beginning when the weather had
Even though they were forced to halt
the repair briefly during the peak of the
typhoon, the team worked through the
rest of the storm to make sure that the
10

delay for the customer was kept to an
absolute minimum. Very strict safety
measures were taken during the entire
operation, as is the case with every job
Hydrex performs.
In-situ propeller blade seal
replacement in Togo
Oil was leaking from one of the blades
of the propeller of a 210-meter RORO
vessel and the vessel could not use its
propeller anymore. Hydrex therefore
mobilized a team to the ship’s location
in Port of Lome, Togo to perform
emergency repairs.
The operation was carried out while
the vessel was at anchor and trimmed
so that the affected blade seal surfaced.
After all the necessary equipment had
arrived on site together with a workboat
and a pontoon, the repair started with
the installation of chain blocks to
enable the team to lift the blade from
which the oil was leaking.
Underwater stern tube seal
repair on vessel in Nigeria
Recently a Hydrex diver/technician
team performed an in-situ underwater
stern tube seal repair on the mechanical
seal of a 150-meter general cargo vessel
in Lagos. A rope was caught in the seal
assembly causing an oil leak.
The owner asked Hydrex to assist
his vessel in Lagos because a similar
operation had been performed on one of
Pontoon next to trimmed propeller of Ro-Ro vessel in Togo.
wet. This allowed the underwater team
to arrive at the location at the same time
as the vessel and only days after the
enquiry was made.
The repair started with the removal
of the rope that had caused the oil
leak. With the aid of special tools the
spring unit was compressed and a
small opening was created between
the different parts of the assembly. The
team could then remove the remains
“Very strict safety measures were taken during the entire
operation, as is the case with every job Hydrex performs.”
Welding work prior to installation of
lifting chains in Togo.
Seven of the eight blade bolts were
easily removed, but the last bolt was
firmly stuck and could only be removed
by cutting it with the aid of special
equipment. Subsequently the propeller
blade was lifted and the damaged seal
was replaced. After the new seal was
bonded, the blade was repositioned and
the remaining seven bolts were secured
again.
his other vessels and he knew Hydrex
could carry out the repair in-situ within
a very short time frame.
Finally a successful oil pressure test and
an underwater inspection of the entire
propeller were performed, concluding
the repair. With the oil leak repaired the
vessel could use its propeller again and
was able to leave Port of Lome.
Diver working on a seal assembly.
As the ship was equipped with a
mechanical seal assembly there was no
need to mobilize a flexible mobdock
as the repair could be performed in the
11
Dry underwater stern tube seal repair
inside flexible mobdock.

of the rope and clean the area. Next
the spring unit was repositioned. The
repair was completed in less than a day.
Much to the satisfaction of the ship
owner Hydrex was able to perform
the repair in a very tight timeframe
and keep the loss of time to the bare
minimum.
Underwater stern tube seal
repair in Panama
When a 295-meter container ship
developed an oil leak from its stern
tube seal assembly, caused by an
entangled fishing net, Hydrex mobilized
a diver/technician team to Panama
where underwater repairs were carried
out using the Hydrex unique flexible
mobdock technique.
Diver working on a stern tube seal
assembly.
Working closely together with a local
support base, three seals were replaced
in one smooth operation while the
vessel was anchored at the entrance to
the Panama Canal. Corrosion on the
running area of the seals prevented the
new stern tube seals from completely
closing off the inside of the ship, so
the decision was made to remove the
spacer ring. This adjustment brought
the seals beyond the corroded area.
Hydrex special lightweight equipment
allows for an almost immediate mobilization
to the location of a vessel.
We combine this capability with a
worldwide network of offices and
service stations established over the last
37 years so that we can offer the best
and fastest service to our customers.
In its quest to provide cost
effective services to customers,
Hydrex developed procedures to
address different kinds of damage
to propellers. This research led
to the design of the Hydrex cold
straightening machines first used in
2002.
By taking advantage of this technique
damaged blades can be straightened
underwater, allowing the ship
to return to commercial operations
without the need to drydock. Blades
can be brought back close to their
original form, restoring the propeller’s
optimum efficiency.
The cold straightening machines
have been in use for quite some
time now but the Hydrex research
department has been looking into
Removal of the fishing net on container ship in Panama.
This gives them the opportunity to have
damaged seals replaced without having
to change the sailing schedule of the
vessel or to take it into drydock, saving
valuable time and money.
Cold straightening of
severely bent
propeller blades
ways to expand the technique even
further to improve our services. A
new version of the straightening
machine was recently put into practice.
It is compatible with the existing
models and is used to restore more
severely bent propeller blades to
their original condition.
12

Hydrex White Paper
No. 6 Introduction
Rudder Cavitation Damage Solved
How to put a permanent end to costly, repeated rudder repair and replacement
As any shipowner knows, a ship’s
rudder is particularly prone
to damage caused by erosion and
corrosion. The problem features
more prominently in high speed container
carriers and other fast ships,
which are more seriously affected
than slower vessels. However, it is a
potential problem and hazard for all
ships and boats.
This problem results in frequent costly
repairs to or replacement of this vital
part of the ship’s underwater equipment.
So far, the bulk of efforts to relieve this
problem have not been fully effective.
Why the rudder?
A ship’s rudder, placed directly behind
the propeller to give the ship maximum
maneuverability, is particularly prone
to erosion followed by corrosion.
The erosion in this case is caused by
hydrodynamic cavitation.
Hydrodynamic cavitation is a phenomenon
that accompanies turbulent fluids.
The turbulence in the fluid, in this case
caused by the ship’s motion through
the water and in particular by the action
of the ship’s propeller, results in areas
of greatly reduced fluid pressure. (The
physical laws involved are clear and
well documented, but not relevant to
this White Paper which is intended for
shipowners/operators, not scientists.)
Due to the low pressure, the water vaporizes.
This causes small vapor-filled
cavities or bubbles in the fluid up to
about 3 mm in diameter. The cavities
travel through the water and the pressure
around them increases, causing them to
collapse suddenly. It is the collapse of
the cavities which is accompanied by
very high pressure pulses, speeds and
temperatures in the water, that cause
the damage. The implosion of the cavities
is accompanied by a complex set
13
of physical processes. The cavities
can travel some distance before they
implode, so those caused by the ship’s
propeller can be transported back to the
surface of the rudder before imploding
and delivering their force there.

The forces involved are very large. It
is as if the surface affected has been
subjected to repeated, heavy blows from
a hammer, as well as high temperatures.
This causes what is known as cavitation
erosion as the surface material, first
paint and then steel, begins to flake
away. This process can be greatly
magnified by the presence of gravel or
other hard particles in the water.
One need only examine a ship’s rudder
that has been subjected to cavitation
damage to see that, whether one
understands or believes the theory, in
practice very real damage is caused
by this phenomenon. Rudders become
deeply pitted, paint coatings and hard
steel simply disappear. Whole plates
can fall off and the rudder practically
disintegrate altogether, all as a result of
this cavitation damage.
Cavitation is caused by the flows caused
by the motion of the propeller, the
cavities being transported rapidly back
to implode on the rudder surface. But
the cavitation can also be caused by the
turbulence around the rudder itself, and
the collapse of the cavities can occur
almost immediately after the cavity
is created. So the rudder is subjected
to cavitation damage from two main
sources: the turbulence caused by the
propeller and that caused by the water
flowing over the rudder itself.
Cavitation damage is not limited to
the ship’s rudder. The propeller is
also subject to the phenomenon as is
the vessel’s hull and other parts of the
underwater vessel where the water
flows are particularly swift or turbulent.
But the rudder is particularly prone to
this phenomenon due to its position and
structure.
The process is gradual, but not necessarily
slow. This process can occur
in a remarkably short period of time.
Sometimes six months is all it takes for
serious rudder damage to be present.
The first step is that the cavitation
causes the paint coating on the steel to
erode, exposing bare steel. The erosion
of the steel is then accompanied by the
electro-chemical process of corrosion
because the steel is no longer protected.
The effect is multiplied as the cavitation
continues and the erosion it causes
is added to by the natural corrosion
of bare steel exposed to water and
the electro-chemical process and the
oxidation which this brings about.
Attempted solutions
Rudder cavitation damage is a well
known and extremely well documented
phenomenon. There is a vast amount
of literature on the subject. High speed
video has been used to capture the
cycle in action. Computer programs
have been developed to model the
damage and predict where the rudder
will be most affected, depending on the
construction and shape of the rudder.
Many scientists have investigated the
phenomenon and papers on the subject
abound.
There have been many attempts to
prevent the damage caused by cavitation.
In the main these attempts fall
into the following categories:
1. Change the position of the rudder
so that it is not behind the propeller.
This reduces cavitation on the
rudder, but is impractical since
the ship loses its maneuverability.
The ideal placement of the rudder
is directly behind and in the wake
of the propeller. The more rapidly
moving water makes the rudder
more effective. In other words,
positioning the rudder so that it
carries out its function in the best
possible way renders it most liable
to cavitation damage.
2. Redesign the rudder so that it is less
affected by the flows and turbulence.
Some inventors have developed a
twisted rudder which is marketed
and in use. The twist is an attempt
14
to reduce the turbulence caused by
the flow of the water caused by the
action of the propeller by changing
its angle of attack on the rudder. This
has met with some success but has
not eliminated the problem.
3. Strengthen the surface of the
rudder to increase its resistance to
cavitation erosion, often with some
other metal. This has only partially
relieved the problem, and can in
fact be counterproductive if the
combination of metals increases the
electro-chemical/corrosion factor.
The difference in potential between
metals can cause very rapid
corrosion to occur. Historically, the
most dramatic example of this was
perhaps the attempt to put copper
sheathing on steel hulls to protect
them from fouling. The interaction
of the copper and the steel resulted
in very rapid corrosion of the steel.
4. Use cathodic protection systems to
reduce the electro-chemical/corrosive
effects. Since the corrosion only
sets in after the protective coating
has been eroded by cavitation, this is
a bit like putting a lock on the barn
door after the horse has been stolen.
It may reduce the corrosion, but it
does not address the primary cause,
which is the erosion damage caused
by the cavitation.
5. Develop better coatings and rudder
protection. It is in this area that the
solution presented in this White
Paper lies. There have been many
attempts to devise a better protection
system for the hull. Most of these
have been ineffective.
Current practices
The most common practice is to use a
conventional type of rudder and coat
it with a typical antifouling scheme
consisting of primer, epoxy coats,
midcoat and biocidal AF paint; the
rudder area is often also surrounded
by a number of sacrificial anodes for
cathodic protection. Depending on the
design of the rudder, the usual cruising

speed of the vessel and the presence
or absence of abrasive particles in the
water, cavitation erosion sets in rapidly
or not so rapidly; the paint is eroded
away leaving bare steel; the steel is
subjected to the combined damaging
effects of cavitation erosion plus corrosion;
the rudder becomes pitted
and damaged, usually in a specific
pattern; inspection reveals the damage,
hopefully before it is too late, and the
rudder must be repaired or replaced
before it disappears completely.
The repair usually consists of welding
to restore and build up the surface
where the metal has eroded or corroded
away, followed by repainting. Plates
may need to be entirely replaced.
This usually takes the form of lengthy
and expensive hot work performed in
drydock. Alternatively, it can involve
expensive, drawn out underwater repairs
to the rudder to keep it functioning
until the next opportunity to drydock
the ship.
The vessel sails and the repaired rudder
is subjected to further cavitation.
Weaker now, the damage occurs more
rapidly. Before too long the rudder
must be replaced entirely.
This all adds up to a continuing economic
nightmare for the shipowner/
operator. Drydocking is expensive in
many ways, not the least of which is
the off-hire time it entails.
Successful approach
One particular coating, a specially
formulated glassflake vinylester surface
treated composite (STC) has been found
to be extremely effective in completely
preventing rudder cavitation erosion
from occurring in the first place, thus
breaking this vicious circle. This was
an entirely practical solution, almost
stumbled upon by the manufacturer,
since the coating system was designed
for protection of the underwater ship
hull and fouling control, not developed
specifically for rudder protection.
Observation of the coating system in
action demonstrated that hull areas
which are normally prone to cavitation
erosion were successfully protected
with this STC. There was no cavitation
erosion where it normally would be
expected to occur. This then led to its
experimental application to rudders. So
far in the eight years that this system has
been in use on many different rudders
(hundreds of rudder applications have
been carried out in that time) not one
has suffered any cavitation erosion.
The rudders so treated have not even
needed to be repainted, let alone
repaired or replaced.
The STC not only offers protection
against rudder cavitation damage, it
has also been used to repair rudder
damage where it has occurred due to
an ineffective paint scheme. In cases
where the steel was pitted but not
completely worn away, the STC was
used to build up and repair the pitting,
before being applied to entire rudder
to protect it against future damage.
This has also proved to be 100%
effective. Due to its high glass content,
this coating is extremely tough and
resilient and has the added advantage
of being an electrical insulator which
successfully prevents electro-chemical
corrosion from taking place.
While the experiment with the STC has
not yet been attempted on every type
of vessel’s rudder, and it remains to
be tried on some of the higher speed
ships, results to date show a potential
final solution to all rudder cavitation
problems.
This White Paper covers the problem
of rudder cavitation damage in detail,
and discusses the results and potential
of using this type of coating system to
completely prevent such damage from
occurring, thus ending the endless and
expensive cycle of rudder damage,
repair and replacement.
15
In-situ bow
thruster
operations
The Hydrex lightweight flexible
mobdocks are designed to be
easily transported around the world
and are used to close off the thruster
tunnel on both sides, allowing divers
to perform repairs and other operations
in a dry environment around
the bow thruster unit.
This technique enables them to
reinstall the propeller blades of an
overhauled thruster inside the thruster
tunnel after the unit has been
secured or replace the blades or seals
and perform repair work on a specific
part without removing the unit.
Since the development of this flexible
mobdock technique, numerous thruster
repairs have been carried out by
Hydrex diver/technicians around the
world.
There is no need to send the vessel
to drydock as all operations can
be carried out in port or while the
vessel is stationary at sea. Normal
commercial activities can therefore
continue without disruption.
Phone: + 32 3 213 5300 (24/7)
Fax: + 32 3 213 5321
hydrex@hydrex.be
www.hydrex.be

The professional team to get you back
in business from first call to finish
Hydrex offers turnkey underwater
repair solutions to
shipowners wherever and whenever
they are needed. Hydrex’s multidisciplinary
team will help you find
the best solution for any problem
encountered with your ship below
the water line. We will immediately
mobilize our diver/technicians to
carry out necessary repair work
without the need to drydock.
Hydrex has a long track record of
performing complex permanent underwater
repairs to thrusters, propellers,
rudders, stern tube seals and damaged
or corroded hulls. By creating
drydock-like conditions around the
affected area, our diver/technicians can
carry out these operations in port or at
anchor.
All the projects we undertake are
engineered and carried out in close
cooperation with the customer and
any third party suppliers, relieving
the customer of all the hassle of
coordination, planning and supervision.
Headquartered in the Belgian port of
Antwerp, we have offices in Tampa
(U.S.A), Algeciras (Spain), Mumbai
and Visakhapatnam (India), and Port
Gentil (Gabon).
All Hydrex offices have fully
operational fast response centers where
an extensive range of state-of-the-art
equipment is available at all times.
Headquarters Hydrex N.V. - Antwerp
Phone: + 32 3 213 5300 (24/7)
E-mail: hydrex@hydrex.be
Hydrex Spain - Algeciras
Phone: + 34 (956) 675 049 (24/7)
E-mail: info@hydrex.es
Hydrex LLC - Tampa, U.S.A.
Phone: + 1 727 443 3900 (24/7)
E-mail: info@hydrex.us
Hydrex West Africa - Port Gentil, Gabon
Phone: + 241 04 16 49 48 (24/7)
E-mail: westafrica@hydrex.be
www.hydrex.be
Hydrex India - Mumbai
Phone: + 91 222 2046 988 (24/7)
E-mail: mumbai@hydrex.be
Hydrex India - Vishakhapatnam
Phone: + 91 891 2711 863 (24/7)
E-mail: vishakhapatnam@hydrex.be