NOTES FROM THE MODEL BASIN. 87 
DISCUSSION. 
THE PRESIDENT :—This paper which has just been presented entitled “Notes from the 
Model Basin,” is now open for discussion. Is there any member who wishes to discuss it? 
Proressor C. H. Peasopy, Member of Council:—I wish to point out one item, not in 
the way of criticism, because I think Naval Constructor McEntee is perfectly conversant 
with this matter, namely, that these experiments show in a marked manner the disadvan- 
tage of roughness, but clearly do not allow us to estimate the effect of roughness, as it does 
not follow the law of mechanical similitude. 
This work is extremely interesting, relatively, but in my opinion we do not need to 
look for such a great loss of efficiency as these curves would indicate. They do show the 
great advantage of having propellers properly finished. However, there is another reason 
why propellers should be finished, and that is, cast propellers seldom have their intended 
pitch, if, in fact, they have any pitch at all. 
Mr. Witi1am GaTEwoop, Member:—Mr. McEntee states that if compared on the 
basis of the same thrust the prejudicial effect of the rough surface of the propellers would 
be even more pronounced. I think it would be very interesting if Mr. McEntee would 
prepare a curve on the basis of thrust and show that up. On the face of his curves, they 
appear to indicate that a smooth propeller has a lower efficiency at small slips than a rough 
propeller ; but when you plot on the basis of thrust you will find that the smooth propeller has 
increased efficiency throughout the whole range of the experiments. 
I think it would also be well if Constructor McEntee will give us a little information in 
connection with the air resistance on the ship’s model, on what basis of comparison he ar- 
rived at the probable value of the horse-power required to overcome the wind resistance 
for the full-sized vessel. 
NavaL Constructor McENTEE (Communicated) :—The point made by Professor Pea- 
body that the results of these experiments cannot be extended directly to the full-sized pro- 
peller because of lack of the same relative roughness on the cast model propeller and the 
cast full-sized propeller, is well taken. In order that the comparison could be made direct, 
it would probably be necessary that the linear dimensions of projections forming rough 
places on the full-sized propeller should be in proportion to the size of the propeller. This 
with a casting, of course, is not at all likely, because using sand of the same coarseness in 
moulding, the full-sized propeller would be relatively less rough than the model propeller. 
The variation from the true pitch likely to be found in a cast propeller would probably 
affect both the model and full-sized propeller in about the same degree. 
With regard to the possible roughness of a propeller, I think that an examination of 
the photograph of model No. 285 (Fig. 5, Plate 66), cork painted, will give an idea of the 
scale of the roughness, and that it could easily represent an actual full-sized propeller which, 
by fouling, would have much the same appearance if photographed and reduced to the same 
size as that given in the paper. In other words, I think that on an actual propeller it would 
be possible, by a heavy accumulation of sea growth, to reduce the efficiency by one-half. 
Referring to Mr. Gatewood’s discussion, it will be noted that the curves of efficiency for 
the cast propellers show a higher efficiency at very low or negative slips where the amount 
of power and thrust developed are very small. As this is a point at which a properly de- 
