practical experience gained on the application of the mewis duct

8/10/2019 Practical Experience Gained on the Application of the Mewis Duct

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Practical experience gained on the

application of Mewis Duct

By P. Leontis


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- Ship gain increases with speed.

- TT measurements below 10 knots full scale not reliable due to model flow condition.

- Gain normally decreases as draught increases.

- MD economy may be expressed as -PD, -(N = RPM) or +V.

Therefore ,MD does not overload the Main Engine if it reduces RPM.

Any such observations are exceptions, not the rule.

Charter Party work on a tonnes per day concept based on HFO rates regardless ship and

engine speed limits.

As speed decreases, so does fuel consumption, but course-stability deteriorates

adding rudder effort resulting to longer path track through water.

It may also create other problems to the Main Engine due to off-design operation.

MD other effects: 1) Reduces vibration and pitching motion

2) Improves course-stability /maneuverability with low speed3) Equates both turning circles (nearly)

4) Maintains power savings in rough weather

5) Affects EEDI where TT Scantling draught data is necessary


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92.5K Bulk Carrier

Mewis

Duct saving


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Tanktesting

IMO/MSC Chapter 3.1.4 Circ T101053 (16.12.2002) states that

model tests must be considered the most reliable prediction

method.Guided by minimum allowable model propeller size D>210mm, the

scale factor () is decided (best 28


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What does the experiment do?

It records power delivered to the shaft just prior to the stern tube by

means of a carefully pre-calibrated dynamometer.

How can the results be interpreted?

There are three parameters involved: Ship speed, propeller rpm and

delivered power.

At MCR kW/rpm a 0.2-0.4 kn gain can be attained.

At steady ship speed (without the 0.2-0.4 kn gain) an 1-3 rpm gain(reduction) is normally obtained.

Alternatively a statistical average of 4-9% gain in PDis achieved which

may be used either as economy or as extra speed.

There have been cases with smaller and higher gains in Dtoo.

Ballast condition normally yields higher power savings, as ship not

designed to perform in this condition.

Each towing tank facility must be a member of ITTC in order to fulfill the

EEDI requirements. The correction method must be carried out according

to ITTC 1999.


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REPORTED WAKE CONDITION


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Weather performance for a 50K Bulker

Power saving for scantling and heavy ballast draughts

over a range of speeds


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Sea Trials

Proof is considered to be provided by the tanktest

(IMO/MSC Chapter 3.1.4 Circ T101053 (16.12.2002))if further proof is sought, then sea-trials can be arranged

by a mutually accepted third party that conducts trials

and does the necessary corrections where needed, for a

BMS stated figure.


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practical experience gained on the application of the mewis duct

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