Short Sea Oil Product Tanker

Resistance performance testing at SSPA 2000-12-06 and 12-07

- Preliminary report comparing initial design assumptions to measurements

Report no: 0001 Resistance testing
The resistance testing was made for the Ecoship coastal tanker for four different draughts. Design draught of 7.3 m and for two higher draughts of 7.7 m and 8.0 m respectively as well as for a ballast condition. (See table in appendix A.)

The tests predicting the full scale bare hull resistance have been compared to the figures earlier calculated from regression analysis program SHIPRED. The measurements show that the bare hull resistance in the lower speed region up to design speed (15 kn) will be about 4% to 9% lower than what was estimated for the design draught. At 16 kn on the other hand the resistance is some 2% to 8% higher than calculated for all draughts. For scantling draught (8.0 m) the resistance is almost 8% higher than calculated for 16 kn and also at 15 kn the resistance is a little higher than estimated in SHIPRED. 17 kn was also tested for design draught and here the resistance was again more than 6% lower than what was calculated. (See diagrams below.) Note that all figures include corrections for appendix drag such as bow thruster, bilge keels, centre keel and rudders.

SSPA comparison shows that the bare hull resistance values for the Ecoship bare hull resistance is among the very best ever tested at SSPA facilities. The comparison was made for hulls of similar dimensions and design speeds.

Fig. Bare hull resistances in MW. Calculated with SHIPRED and corrected for bilge keel and rudder.
(Rudder -0.4% and bilge keels +0.8%)

Fig. SSPA Predicted bare hull resistance from measurements including bow thruster. 
(Bow thruster +2%)


Fig. SSPA resistance in % of SHIPRED predictions. 
(100% means equal resitance.)

Paint testing (Flow field study)
The low bare hull resistance achieved indicates that the alignment of the chines of the developable surfaces in the flow direction has been successful. Looking at the paint test this can also be verified. It is only in a small portion of the aft body where cross flow over the sharp chines does occur. It should also be noted that where this happens it is over very blunt angles. 

What can also bee seen from this study is that the connection between the bulb and hull can be improved. Making the intersection between the bulb and hull aft of FP smother will probably result in further gains in hull performance.

Even though the hull analysed in SHIPFLOW in the thesis work conducted by Björn Nyberg and Oskar Wahl earlier this year was similar but not the very same as the one tested at SSPA the flow pattern resemblance is striking. SHIPFLOW predicted the cross flow in the aft shoulder region correctly and indicated also correctly that the bilge location was very well aligned with the flow.

Preliminary prediction of propulsive power
The predicted full scale propulsion power for speeds between 10 and 17 knots has been estimated from data given by ABB Azipod oy in Helsinki, Finland. This prediction has also made ground for the RPP called "BKTJ_SSPAslap2.xls".

The following efficiencies have been used for the calculations:

Pod electrical engine efficiency = 97.3%
Generator efficiency = 95.4%
Electrical freq. Converters = 98.5%
Propeller open water efficiency = 63 to 65% depending on speed (see App E).

The total efficiency was calculated as the product of all efficiencies and the result has been plotted for speeds in the range of 10 to 17 knots below. (See also MATLAB code in App E.)

From the total efficiencies derived the full scale propulsive power has been calculated. The results are presented in the diagram below.