Polishing or Cleaning of Propellers

By Keith Henderson at April 24, 2012 00:55
Filed Under: Company News, General, Navy insights

In the April 2012 Journal of Ship Hull Performance from the Hydrex Group, international
specialists in underwater hull treatments, a White Paper called the 'Ship Propeller
Maintenance: Polish or Clean, explains how to save up 15 per cent of propulsion fuel
costs without harm to the environment.


Caption: USS_Arleigh_Burke: average periods between full cleaning of the US Navy
DDG51 Arleigh Burke class destroyers is five years with interim cleaning including
propellers at about six month intervals.
Image Credit: US Navy

It describes how a propeller is most efficient when its surface is smooth. Immersed in water,
within hours it starts to loose its smoothness and become rough. The rougher a propeller is
allowed to become before it is remedied, the more rapid further roughness occurs in an ever
worsening spiral.

There are a number of reasons why or the speed at which a propeller becomes rough and get rougher in service. Split
into groups, there are two man made sources, material of manufacture and improper polishing or
cleaning. Static in water sources are marine fouling, corrosion and calcareous deposit (chalk
layer) as a secondary effect of corrosion protection. In service sources are impingement attack
(small water bourne objects), cavitation erosion and mechanical damage from impact with foreign
objects, flotsam etc.

Despite the reason, the effect must be remedied and there is no remote method of assessing
propeller surface roughness. A manual check must be made either by a diver or in drydock:
obviously the former is faster and cheaper. Failure to check regularly causes loss of speed with an
increase in fuel consumption the consequence

This was accurately quantified in trials by the US Navy on the destroyer USS McCormick which
indicated that about two-thirds of the increase in fuel consumption due to fouling was due to
propellers fouling. After 226 days out of drydock the average fuel consumption required to
maintain a given speed had increased to 115.8 per cent of the consumption with a clean bottom.
After cleaning the propellers (only), the fuel consumption dropped to 105.5 per cent. Thus in
seven months the propellers alone were responsible for a 10 per cent increase in fuel consumption.


Caption: Trials by the US Navy on the destroyer USS McCormick indicated that about two-thirds
of the increase in fuel consumption due to fouling after 226 days was due to propellers fouling.
Image Credit: PD-US Navy

Average periods between full cleaning of the US Navy DDG51 Arleigh Burke class
destroyers is five years with interim cleanings (including propellers) at about six month
intervals, however rates of fouling depend very much where and how the vessel is
being used.

The conclusion of the White Paper is that "a little and often" is preferable to "seldom and severe"
when it come to propeller cleaning. To illustrate the point a case study of a 442 ft (134 m) cruise
ship is given. The propellers were cleaned by one of the ship's crew who was a diver, using a
rotating brush alone - no grinding or polishing disc was required. As the propellers are often
cleaned, the fouling was not very heavy so both propellers could be cleaned in a total of 40
minutes. The resultant fuel savings were calculated at 6 per cent. In the first 30 hour trip, the fuel
savings were estimated to be $2,142. Had the propeller been cleaned by an outside company it
would not have cost more than about $2,000. Since the fuel saving would accumulate until the
next cleaning - in one or two months, the savings are obviously substantial.


Caption: Cleaning a propeller with a brush and abrasives. The ship's two propellers were cleaned
in approximately 40 minutes.
Image Credit: Hydrex

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