When repair won?t do

When a vessel has been in operation for more than 10 years, several onboard systems may be well into their useful lives

.

The same applies to the propeller, which may have been repaired, cleaned and polished several times during this period

.

A modernisation could be in order, or even replacement

.

It is difficult to say how bad a propeller has to be before it needs to be replaced and this differs from case to case

.

When the propeller is damaged in such a way that the classification society does not approve it anymore, or approves it for a limited time, it is clear that replacement is in order

.

Fixed pitch propellers (FPPs) on vessels more than 10 years old are usually replaced for four main reasons: the propeller may have too much damage and cannot be welded; upgrading of the propeller design to modern standards may give better efficiency, resulting in fuel savings and/or higher speed, or it may reduce propeller-induced pressure pulses resulting in increased crew comfort and lower dynamic excitation of the ship?s structure; the mission

profile

of the vessel may have changed; or, the propeller may have been of poor initial design

.

If a new propeller is required because of damage, it can be redesigned using the latest technology

.

This latest technology is a combination of increased computer power, which enables more detailed calculations, especially when talking about computational fluid dynamics (CFD) simulations and more research

.

Tip-rake development is one of the recent research projects into hydrodynamics

.

If a propeller is heavy running, a pitch reduction will make the propeller run lighter

.

A new and better propeller design can easily compensate for the slight decrease in efficiency caused by reducing the pitch

.

If a vessel?s

profile

changes, the existing propeller has to be replaced because only one optimum design condition can be defined for a propeller

.

In other conditions the propeller will operate at sub-optimal efficiency

.

It is not uncommon that the propeller was of a poor design in the first place when it is taken into account that vessels could be anything from five to 20 years old with propeller designs representing technologies of that period when prediction and design methods were less sophisticated than today?s

.

When it is time to replace or modernise a vessel, Wärtsilä advises taking a critical look at the propeller

.

Does the propeller still fulfil its requirements? Is it damaged too much? Has the mission

profile

changed?

Improving propeller design

New techniques allow propellers to be designed with higher efficiencies than in the past, resulting in lower fuel consumption without increased noise and vibrations

.

Of course, a propeller can be further optimised by for instance changing the diameter, RPM, number of blades, etc

.

But to keep it simple this article assumes that the geometry of the replacement propeller should lie within the geometry of the present propeller

.

It is also assumed that the boundary conditions do not change and that nothing is added to the propulsion system

.

There are four options that increase efficiency through a new propeller design: reduce the blade area; change the blade

contour

; modify the radial pitch distribution; or apply the Lips

tip-rake concept

.

Reducing blade area

Reducing the blade area ratio to the maximum raises efficiency by more than 3% and this can be achieved without an unacceptable increase in pressure pulses or the risk of erosion

.

Changing blade

contour

When reducing the blade area we assume that this is equally scaled over all radii

.

If it is not possible or desirable to reduce the blade area, choosing another blade

contour

can increase the efficiency of the propeller instead

.

In the outer radii where the speed is highest, the viscous friction losses are highest as well

.

Of course the generated lift is more significant at the outer radii and it could be necessary to apply more chord length owing to the less favourable cavitation characteristics

.

Modifying radial pitch distribution

In the past it was usual to apply some kind of constant radial pitch distribution, comparable to the pitch distribution of the well-known Wageningen B-series propellers

.

It is known that this kind of pitch distribution is theoretically favourable with respect to efficiency

.

The outer radii in particular have a positive impact on efficiency

.

High pitch values at the outer radii, however, impose a negative effect on cavitation behaviour and consequently lead to higher pressure pulse levels

.

Such high tip loadings also cause intensive tip-vortex cavitation, which may damage the rudder

.

Where the inner radii are concerned, high pitch values do not contribute so much to high efficiency values and this also has a negative effect with respect to hub vortices, which could lead to cavitation erosion on the rudder

.

To summarise, the loading of the propeller blade should be shifted away from the tip and the root

.

To achieve equal pressure pulses as a result of the constant and variable pitch distribution, one can choose a lower blade area for the propeller with variable pitch distribution

.

As mentioned above, this will lead to better efficiency

.

In other words, when the pitch distribution of a propeller is similar to the constant pitch distribution, it is worthwhile investigating what the exact consequences of changing the pitch distribution will be

.

Applying the Lips

tip-rake concept

In cases where it is no longer possible to reduce the blade area, change the blade

contour

, or change the radial pitch distribution, the Lips tip-rake can be applied to achieve better efficiency resulting in fuel savings and/or higher ship speed

.

Normally, there are no restrictions that prevent the application of tip-rake

.

Tip-rake does not lead to higher pressure pulse levels

.

It has also been shown that propellers with tip-rake are quieter and therefore more comfortable for the crew

.

A Lips tip-rake propeller differs from a “normal” propeller only in the radial rake distribution

.

Figure 1 shows the clearance curve and rake distribution (axial shift with respect to the generator line) for both a normal and a Lips tip-rake propeller

.

As Figure 1 shows, the rake is not different for all radii

.

The tip-rake distribution arises from 0

.

9R towards the tip and the blade has been bent towards the pressure side (face side) of the blade

.

Relative efficiency improvements of 2-3 % compared to normal propellers have been measured so far

.

The Lips

tip-rake concept

resembles the principle of tip-plate propellers

.

In a tip-plate propeller, however, a tip fin is fitted at right angles to the propeller blade, which means adding blade area and increasing frictional drag

.

This is not the case for the Lips

tip-rake concept

, where the tip fin is integrated with the propeller blade

.

The working principle of the Lips

tip-rake concept

is based on a virtual increase in propeller diameter

.

It is well known that a larger propeller diameter yields a higher efficiency

.

However, it is not always possible to increase the diameter because of restrictions in propeller-hull clearances

.

Applying tip-rake has a similar effect to increasing the diameter, especially in the area of highest speeds and therefore highest lift

.

At full scale it is hardly possible to measure an efficiency improvement of 2% directly, but shipowners have reported a noticeable reduction in fuel consumption

.

Several have also remarked that the tip-rake propeller is much quieter than a conventional propeller and this has also been demonstrated by vibration measurements

.

The production of a tip-rake propeller results in slightly more grinding costs due to the more complicated shape and therefore they cost slightly more than a comparable propeller without tip-rake

.

Wärtsilä points out that the propellers are no more prone to damage than existing propeller designs as due to the bent shape of the tip, the strength of a tip-rake propeller is even better

.

Repair of tip-rake propellers is for us not much more difficult than for normal propellers

.

Our repair engineers have the knowledge and specialise in repairing this kind of propeller

.

Change of mission

profile

It also becomes necessary to replace an existing propeller when the mission

profile

of a vessel changes

.

For example, a fixed pitch propeller (on a ferry) originally designed to operate on two engines cannot be used on one of the two engines only

.

Figure 2 shows that the propeller curve of a fixed pitch propeller, originally designed for a twin-engine mode, lies above the “single-engine curve”

.

If the schedule of the ferry is changed, making it unnecessary to run the propeller on two engines, a new service curve must be defined

.

There are other situations in which the mission

profile

of a vessel could change

.

A vessel removed from sailing in ice conditions will no longer need ice classification, for example

.

Again, a vessel?s service speed could be raised (or lowered) owing to a new schedule, or because the vessel sails more often on another draught and/or trim, or because the vessel has a different sea margin (e

.

g

.

no longer sailing in the Atlantic, only the Adriatic)

.

n

Marcel van Haaren is Project Engineer Hydrodynamics for the Fixed PitchPropellers product group, Wärtsilä Propulsion Netherlands