The relationship between the power developed by the main engine - KW and the product of load indicator index (Li) and propeller rpm (N) deduced from said graphic is as follows:
KW = 17.5 (LixN) - 1500 (1)
Yet another factor should also be taken into account, namely the proper interpretation and correction of existing weather conditions. It should be understood that when comparing the performance of two ships during sea trials at the same wind force for example B 5 doesn't mean that the trials were carried out in the same conditions. Important is the sea state itself.
The sea state corresponding to the speed trials carried out with ship "A" was clearly originated by the action of the existing wind. The speed trials started while the sea was completely flat. However the sea state has been evaluated by B 3. Further during these speed trials the wind intensity was increasing progressively so that when the runs corresponding at the higher ship speeds were conducted the wind intensity was B 5.
According to volume III "Hydrodynamic in Ship Design" published by SNAME and written by H. E. Saunders in order that the sea waves reach associated intensity corresponding to B 5 it is needed that the wind was blowing with such intensity during at least 10 hours. So it is evident that the existing sea waves during ship "A" runs corresponding to the maximum power had a wave height lower than the one corresponding to an authentical sea state equal to B 5.
During the shipyard trials carried out with the ship "B" the sea state existing in the area of the intended speed trials oscilated between B 6 and B 8 through two days at the second day evening the wind force started to decrease.
The Shipyard representative defined the wind force at the beginning of the shipyard trials by B 5 in order to take into account that in spite that the wind velocity was about B 4, the existing sea waves were more severe than the ones corresponding to B 4. To this end it must be mentioned that sea waves created after a 2 days lasting wind force B 7 have a duration time until 27 hours. The official speed trials of ship "B" were conducted 4 hours later after the wind decreased to B 4 but unfortunately the shipowners and shipyard did ignore the existence of a left high swell which essentially influenced the speed trials and absorbed power, so obviously the obtained results concerning the non conventional CLT propeller, could not be trustworthy at all. The propeller maker have obtained a reliable weather report from WNI/Oceanroutes from Aberdeen covering among others the weather for the time and area where the sea trials were carried out. As the report states during the ship "B" trials there was a swell having a wave height between 3.5 to 4 m [6]. Thus carring out trials at such state of weather without corrections it isn't possible to asses objectively any savings even if they are present.
In the following text an account is made how measurements by a torquemeter wrongly adjusted can be totally misleading and how reliable is the product of load indicator (Li) and revolution (n) rpm indications to have reliable estimation of propulsive power developed.
If a torquemeter is not available on board then there still exists some way to determine in a quite precise way the power of an engine. Namely the engine load indicator indications multiplied by the engine revolutions are an accurate way to calculate the engine power. This statement needs some further considerations substantiating the above made assumption. To get orientated what convergence do exists between the engine torque Mo (or mean effective pressure Pe) and the engine load indicator "L" in order to be able to ascertain eventual problems during transferring the parameter values expressed as a function of Mo into the engine load indicator indications some following considerations have to be done.
In accordance with the theoretical propeller curve the engine has to develop a power defined by the equation
Ne = C・n3 (2)
where: C - constant, n - engine revolutions
The needed (service) torque is
