Nowaekt and Sharma 
are overestimated in these cases where the Hough and Ordway appro- 
ximation is used. 
The authors have calculated the potential (or displacement) 
and wave wake fractions assuming ideal flow conditions. In real flow 
these fractions are, as pointed out by Dickmann, influenced by the 
viscous wake in a complicated way. This influence can, however, be 
calculated if experimental values of velocity and static pressure at 
the propeller disk are available, see [2] . Are there any plans to 
follow up the investigation with pitot tube measurings ? 
The oscillations in the measured thrust deduction with Froude 
number were found to be much stronger than in the calculated. The 
reason for this is said to be that either the viscous thrust deduction 
oscillates appreciably or that the assumption of the hull sources being 
independent of Froude number is invalid. In my opinion a much more 
likely explanation can be the measuring errors. Everyone acquainted 
with towing tank experiments knows how difficult it is to determine 
the thrust deduction with good accuracy. No direct information on 
these errors is givenin the paper, but the measuring points in Fig.4 
indicate that resistance is determined with an accuracy of -2%. 
Assuming the same accuracy for the thrust measurements the error 
in thrust deduction will be At = t 0.04 - a value which is of the same 
order as the oscillations in t shown in Fig. 30. 
The influence of the free water surface on the propulsive 
coefficients has been determined for the ideal flow case. As pointed 
out by the authors the viscous flow around the ship can make the stern 
less effective in wavemaking. This means that the presented calcu- 
lation procedure instead of giving an exact measure gives almost 
equally important qualitative results and an upper limit of the wave 
influence. 
REFERENCES 
1 DYNE, G., ''A Method for the Design of Ducted Propellers 
in a Uniform Flow'', Publ. N° 62 of the Swedish State Ship- 
building Experimental Tank, 1967. 
2 DYNE, G., "On the Scale Effect on Thrust Deduction", 
RINA 1972. 
1952 
