EXPERTISE ON MEWIS DUCT EFFICIENCY GAIN FOR m/t „TAMBOURIN“
m/t „TAMBOURIN“, built 2004 at shipyard „Brodotrogir“ has been retrofitted during 2009 with so called MEWIS DUCT, invented and sell by Becker Marine Systems(BMS).
Shipowner ordered from „Yacht Design“ an expertise on efficiency gain after duct was installed.
All necessary inputs were available, such as model test reports, sea trial reports for all 6 sister-ships, report on speed/ power relation and HSVA evaluation of both speed trials etc.
Expertise has proven that 5-6% gain-as claimed by BMS- can be possible improvement on very bad aft hull form but not on well designed one-as it was the case with m/t „Tambourin“.
YD revealed many irregularities in reports and concluded that gain can be in the range 1-2% which could, however, be effect of newly applied low friction silicone antifauling rather than duct effect.
m/t „TAMBOURIN“
SEA TRIALS WITH MEWIS DUCT
EVALUATION OF SEA TRIALS RESULTS
According to available data sea trials of m/t „Tambourin“ with Mewis duct installed in dry dock are carried out instantaneously after leaving the shipyard.
Figures given in three xls charts are studied and following is noted:
- There is discrepancy between draughts reported by Mr.Zimmermann of
BMS(10.7m) and Captain (11.0m)
- There is discrepancy in readings “speed over ground” recorded by Mr.
Zimmermann and Captain. Mr Zimmermann gives speeds rounded to
tenth of knots while Captain displays speeds in 1/100 fractions of the
knot .It is not clear why Mr. Zimmerman’s mean speeds are , in
average, abt. 0.15 kn higher than Captain’s mean speeds.
- Recorded sea water density is very low, i.e. 1006 kg/m3. Ship’s
resistance is proportional with density of media she sails in.
- m/t “Tambourin” conducted delivery sea trials under adverse weather
conditions (sea state 3-4, rel.wind speed up to 43m/sec) resulting in
maximum speed being abt. 0.4 knots inferior to average achievement
of other five ships in series.
In other words, comparison of Mewis duct trials results with
“Tambourin” official (delivery) sea trials results would not give
realistic measure of improvement in efficiency.
Therefore, m/t “Tapatio” (Nb.304) is selected for comparison as
she has conducted speed trials at similar or somewhat worse weather
conditions than it happen to be during duct testing on m“Tambourin”
(relative wind speeds 12-28 m/sec and sea state 1-2).
Note that m/t “Tapatio” was the worst one –speed 14.85 kn only-
among the remaining five ships.
- Zimmermann’s speed/power data(Curve 2) are plotted in enclosed diagram adjacent to “Tapatio” (Curve 1) measured mile official curve (BI Report No.5698-B). Captain’s speed readings (Curve 3)are also plotted on the same engine outputs and mean power/speed curve (4) is established.
Speed gain can be read out as 0.22 kn corresponding to 395 kW reduction
in power at 90%MCR (6912kW) i.e. abt 5.7%.
- However, this percentage need to be diminished due to draft and density effects:
Draft: providing that draft was 10.7m (instead of 11.0m as it was on
comparative ship) we can use official speed gain (tank tests
correlation based) due to decrease of displacement.
From diagrams on pages 14 and 15 of m/t “Tapatio” Measured
Mile Report No.5698-B we find speed gain from 12.2m draft
(57158t ) to 11.0 m draft (51089t) amounting to 0.37 kn.
Extrapolating this further down to 10.7m draft there should be
further gain of 0.09 kn.
Accordingly, Mewis duct contribution is to be diminished for
this amount so it remains 0.22 -0.09 = 0.13kn
Translated into percentages this can be abt. 2.3%.
(in this consideration 0.2m aft trim is disregarded-it is difficult to
evaluate what could be gain or drop in speed arising from this
small trim)
Density: As resistance/power is proportional with the density of the
fluid we need to multiply power recorded at 10.7m draft with
ratio of 1028.7 over 1006 and this gives abt. 2.2% increase.
(1028.7kg/m3 is density recorded on m/t “Tapatio” official trials)
Totally, on account of these two corrections , we diminish duct
contribution from 5.7% to abt 1.2 %.
These corrections are based on model tests and physical laws and they cannot be matter of dispute.
However, there are two more subjects of concern:
- Effect of silicone based antifouling and
- Too high specific fuel consumption.
Ad.A: Producers of silicone based fouling release coatings claim a
reduction in friction coefficient of abt 5% compared to tin free
self-polishing coatings.
Considering that friction resistance constitute abt. 55 % of total
resistance (for subject vessels at 15kn) we arrive to possible power reduction of abt. 2.7 %.
This subject need to be further investigated through evaluation of shell roughness and specific and new coating features.
One HEMPEL report dealing with this subject is enclosed.
Ad.B: Specific fuel consumption –as measured-in the range 210-211
g/kWh (for calorific value 42.7 MJ/kg) exceeds for abt 5.7%
consumption measured during official sea trials(?)(which was in
the range 198-200 g/kWh)
There could be two or three possible reasons:
B.1 Engines are in bad condition (claim for MAK) or
B.2 Recorded M.Es outputs are not true ones, i.e. they are shown
lower than they really are –resulting in specific consumption
being too high.(Torsiometer accuracy ?)
B.3 Recorded flow includes return fuel (surplus fuel) which is
returned to tank instead of being returned to inlet pipe –
between engine and flow-meter.
As conclusion, we wish to point out that all concerns listed in “Testing program for evaluation of efficiency of Mewis duct fitted on m/t “Tambourin” has appeared in reality obstructing us in creating clear, true and definite value of efficiency gain due to Mewis duct.