Build on the above equation an analysis was performed to verify the assumption that L〜pe based on obtained results from engine test bed.
The results have been compiled in table 1 (7) in which, load indicator position and calculated value of Δ is given. During the study of this issue it was assumed that for Ne = Nen at n = nn the position of the load indicator corresponds entirely with the mean effective pressure pe i.e. L 〜 pe. From this assumption it becomes obvious that for the nominal load Δ = 0.
Analysing the results in table 1 it can be stated that between the four engines the deviation Δ = f(n) assumes positive and negative values within the limits + 8.4% to - 6.8%. The average for all engines is about + 5%, this can be considered as a rather moderate deviation and measuring error, what in turns allows to consider the load indicators as a tool sufficiently determining the engine operating point in the load diagram. The quoted figures in table 1 are for ships and engines (Sulzer RND type) built in the 1970'S.
A quite interesting and striking results contains table 2 (8). The given in this table data stems from a recent (July 98) sea trial results of a new built ship in one of a well known shipyard. During the sea trial of this ship (a 45 ・ 103 DWT bulk carrier) for the measurement of the engine (a 6RTA 58T) torque three torquemeters were installed while the fourth engine torque value was calculated from the product load indicator x engine revolutions (L・rpm).
The reason for such unusual measurement arrangement was a heated dispute between the shipyard and propeller maker who insisted that the calculations of engine power by using the formula L・rpm is an accurate method of power calculation and comparable with the results obtained from torquemeters readings. The dispute actually started when during sea trials of a previous ship of the same class an unaccepted difference between shipyard torquemeter readings and the L・rpm readings did occur see table 3 (9). As can be seen from table 3 the difference in power calculation by the torquemeter and the L・rpm formula gave quite satisfactory results for the first ship. But as the second ship is concerned the difference during the first sea trial appeared to be quite large and the shipyard didn't accept the method of L・rpm as a credible way to calculate the engine power (on what the propeller maker insisted). So based on the torquemeter results it was concluded that the propeller is too heavy and a cutting of propeller blades was carried out. This ship went then for a second sea trial during which the discrepancy between the torquemeter and L・rpm formula became really horrendous. The propeller turned out to be now too light and it was difficult to achieve the contracted speed with initially set up in the contract engine revolutions.
From this rather unprecedented case it can be concluded that if the calibration of a torquemeter is not correctly done quite a significant error may occur.
4. CONCLUSIONS
It has been distinctly shown that an assessment of a propeller efficiency especial of a new unconventional design like CLT is hedged about with difficulties stemming from:
- shipowners conservatism,
- makers of conventional propellers worries about their lack of know-how in technology design of CLT propellers,
- last but not least the lack of accuracy of propulsion power which may originate in erroneous measurements with error exceeding the level of propeller efficiency gain. This errors may have at least two sources:
a) wrongly tuned torquemeter
b) not aproprialy assessed weather conditions during sea treals.
The described by the author case of not best and accurate measurements are fortunately an exceptional one, but nevertheless it illustrates the traps one can encounter in propeller efficiency measurement procedures.
Table 2 provides and undoubted proof how accurate is the estimation of engine power by using the L・rpm formula as well that there are differences between each of the installed torquemeters but the range of deviations lies within an accepted limits. It can be concluded from table 2 that the formula L・rpm has given in all power ranges higher values then torquemeters recordings but the average deviations were about + 2.5% thus by getting a little higher values the operator of the engine calculating the power output by the L・rpm formula is on the safe side.
Finally it should be remembered that there are certain factors which have however, to be considered when using the formula L・rpm to obtain trustworthy results. These factors to be considered are as follows: - fuel calorific value of presently used fuel oil and density comparable with the one used during the shop trial. If not a correction of the calculated value has to be carried out.
Important parameters such as scavenging air pressure turbocharger speed and exhaust gas pressures have to be compared with the shop trial values. The state of the injection system, injection pumps, nozzles and injection timing must be as during shop trials. The reason for this is that for example a worn linkage would result in a different load indicator reading result. As long as above parameters of the measurement and the shop trial are comparable there is no reason to doubt the power calculation.
But finally it can be said that the majority of CLT propeller now fitted (250) perform the very satisfaction of the shipowners. It is to be believed that a CLT propeller is certainly the ultimate in ship propulsion and we can look forward to seeing all ships in the world fitted with CLT propellers.
