116 JOINT DISCUSSION ON TWO PRECEDING PAPERS. 
matter was covered to some extent in the original paper and the discussion, it is one of such 
serious practical importance as to seem to warrant further remark. 
Even the largest of the three model propellers was too small te give a good efficiency in 
any position. I give in the table below its estimated actual slip and the corresponding effi- 
ciency of propulsion together with the wakes, etc., already given. 
Data ESTIMATED FOR FULL-SIZED SHIP WITH PROPELLER AS PER MopeE . 571. 
Propeller tip below surface...... 134” 134” 134” 13.8” 13.8” 13.8” 
Propeller center abaft stern post. . 11.58” 6.66” 2.15/ 11.58” 6.66/ 2.15/ 
Ship speed, knots ............. 11.57 11.57 11.57 11.57 11.57 11.57 
Wel Guatlossaoescoodsacdaac 1.74 2.43 2.55 3.82 4.40 4,97 
Speed of advance ............. 9.83 9.14 9.02 7.75 Uo 6.60 
Wake) fractionyaa-fi-)iret)- ici 15 21 22 33 38 43 
Thrust deduction.............. 19 26 Oxi 21 30 
Hullivefiiciencymiy-rieere yr -96 94 94 1.17 
-A\p parentys|ipmerieietieretritexie -09 08 -09 .01 
Atctualislippercen reteset 23 0 .29 233 
Efficiency of propulsion ........ 64 : .58 -70 
E.H.P. E.H.P. 
I.H.P. IP. 
The efficiency of propulsion is not where P is propeller power. 
It is seen that, though there is a gain of something like 20 per cent of hull efficiency in 
the higher position, the gain of propulsive efficiency is 4 to 10 per cent only, and similar 
tables for the smaller propellers would show an actual loss in some cases. The reason seems 
fairly obvious. In the lower position the speed of advance of the propeller through the 
water in which it acts is 9.02 to 9.83 knots. In the upper position it is 6.60 to 7.75 knots. 
Now the propeller, if it were sentient, would know nothing about hull efficiency. It would 
know only that it was absorbing a certain power at a certain revolution and advancing 
through the water in which it operates at a certain average speed. This speed of advance is 
a vital factor in the case, and the propeller efficiency depends largely upon how well its design 
suits the speed of advance. 
In the case above, propeller No. 571 from open-water experiments has a maximum effi- 
ciency of 65 per cent at a slip of 17 per cent figured on nominal or face pitch. In the low 
position, however, it is not working anywhere near its maximum efficiency, the slip averaging 
some 27 per cent corresponding to an efficiency of 62% per cent. In the high position, in 
order that the propeller with a speed of advance of 6.60.to 7.75 knots may drive the ship at 
11.57 knots, the slip has to go up to an average of 39 per cent with a reduction of pro- 
peller efficiency to 56 per cent or some 10 per cent less than the efficiency in the low position. 
No wonder, then, that the gain of hull efficiency of some 20 per cent is associated with a gain 
in propulsive efficiency of only 10 per cent at most. 
The question at once arises: Is this condition unavoidable? The answer is certainly 
not, provided the design is properly made in the first place. In the model experiments 
