Sec. 67.16 



UNDERWATER-HULL DESIGN 



521 



67.15 Notes on Three- and Five-Screw Instal- 

 lations. Although triple screws are, at the time 

 of writing (1955), not favored for modern ship 

 designs, there have been many successful installa- 

 tions in the past. Notable among these were the 

 U. S. cruisers Columbia and Minneapolis of the 

 1890's, many early turbine-driven vessels of the 

 1900's, the passenger vessels Great Northern and 

 Northern Pacific of 1914, and the many cruisers 

 and large German capital ships of the World 

 War I and World War II periods. An excellent 

 photograph of the Cunard liner Carmania, 

 fitted with triple 3-bladed propellers, is shown on 

 Plate 161, SNAME, 1905. A photograph of the 

 Columbia in dock, showing the three 3-bladed 

 built-up propellers, is published in Cassier's 

 Magazine [Feb 1896, p. 325]. The triple-screw 

 arrangements on the Argentine battleships Moreno 

 and Rivadavia are illustrated in Schiffbau [11 

 Oct 1911, p. 19]. There was a proposed triple- 

 screw World War II German destroyer of the 

 Z-51 class, with internal-combustion engine drive 

 [ASNE, Feb 1948, pi. opp. p. 30]. There were four 

 engines of 10,000 horses each coupled to the 

 center shaft and one each of 10,000 horses to the 

 two wing shafts. The center propeller was of 

 course much larger than the two others. The 

 vessel was to carry a large centerline spade rudder 

 and smaller offset twin rudders. 



The triple-screw stern, despite its use on many 

 medium and large vessels in the past, presents 

 difficulties with propelling machinery and pro- 

 peller design. The hull shape in way of the center 

 skeg or center portion ahead of a middle screw 

 is vastly different from the shape ahead of the 

 wing screws, even when the latter are carried by 

 bossings. The friction wakes, the angularity of 

 the flows, and the uniformity of water speeds 

 over the discs will not be the same for the center 

 and the wing propellers. Although not strictly 

 necessary, it is almost never possible to make the 

 center propeller absorb the same power at the 

 same rate of rotation as the wing propellers. 



Many designers have been disillusioned by 

 attractive proposals to use the center propeller 

 solely for intermediate-speed and cruising pur- 

 poses, only to find in service that: 



(1) There were practical difficulties in uncoupling 

 the wing shafts and permitting their propellers to 

 free-wheel while cruising 



(2) There were many problems in designing a 

 center propelling plant which would operate 



economically and efficiently at both cruising 

 speed and at full speed. 



While attractive from the point of reducing 

 the number of propelling plants and propulsion 

 devices on a high-power ship, triple screws may 

 have to be associated with something rather 

 new and different in the way of ship sterns to 

 realize their full possibilities. 



It is not beyond the bounds of probability that 

 with further increases in ship power it may be 

 found advisable to fit five screw propellers on a 

 ship. A stern design for such a vessel appears to 

 involve many problems of both the triple-screw 

 and the quadruple-screw stern. Because of this 

 and other reasons it has not, so far as known, 

 been attempted on more than model scale. 



67.16 The Arch Type of Single-Screw Stern. 

 With the increased reUabiUty, decreased weight 

 and space, and reduced fuel consumption of the 

 larger sizes of modern ship propeUing plants it 

 becomes increasingly desirable to take advantage 

 of the inherent simpUcity and lower first cost of 

 single-screw propelling machinery, to say nothing 

 of its higher propulsive coefficient. Propeller 

 design has progressed to the point where powers 

 much higher than those dehvered to single wheels 

 in the past can be absorbed at reasonably low 

 rotational speeds. If the rate of rotation is 

 sufficiently increased, if the propeller is made 

 large enough, and if it is kept adequately sub- 

 merged, the shaft power of 50,000 horses men- 

 tioned elsewhere may be achieved in the not 

 distant future. 



Much higher propulsive coefficients can be and 

 have been obtained with single-screw than with 

 twin- or multiple-screw propulsion but these are 

 predicated upon advantageous flow conditions at 

 the propeller disc. Good flow is obtained with 

 relatively long and thin vertical skegs ahead of 

 the propeller, giving reasonably high hull effi- 

 ciencies [riH = (1 — 0/(1 — ■"')] at the propeUer 

 position without excessive circumferential varia- 

 tion and without objectionable separation or 

 eddying. 



As the size of vessels in this category increases, 

 so does the beam-draft ratio, because the draft 

 is more severely Hmited by depths in the water- 

 ways of the world than the beam is limited by 

 berthing and docking facihties. The progressive 

 increase in beam, over the years, due largely to 

 damage-control requirements, has meant in- 

 creased difficulty in fining the waterUnes forward 



