A rotating propeller induces a rotating motion in its wake. This reduces the efficiency of the propeller. In order to recover this energy two propellers behind each other are used on concentric shafts, also called as CRP.
The two propellers rotate in opposite directions. The pitch and loading of the propellers is designed such that the resulting rotational energy in the wake is zero. To reduce shaft vibrations the number of blades of both [[propeller|propellers] is different, so that not all blades pass each other simultaneously. The diameter of the front propeller is often slightly larger than that of the rear propeller, to account for the contraction of the propeller wake and to avoid the rear propeller to hit the tip vortex of the front propeller.
The rotational losses in the wake of a moderately loaded propeller are some 6 percent. Half of that rotational energy is generally recovered by the rudder. So the gain of a CRP is not more than some 3%. More important, however, is reduction of frictional losses which is possible by a CRP. This can be understood when it is realized that in the design of a single propeller the optimum efficiency is reached at a certain pitch and rotation rate. An increase in pitch will decrease the rotation rate. The result is an increase of the rotational losses and a decrease the viscous losses. The optimum is found when both effects are equal.
In the case of a CRP, however, it is possible to increase the pitch of the propeller without penalty, because the rotational losses are recovered by the rear propeller. This makes it possible to reduce the rotation rate and thus the frictional losses without increasing the rotational losses. A CRP therefore has a lower rotation rate and the increase of efficiency from a single propeller can be as high as 15%.
A low rotation rate can be difficult for the engine, however. The rear propeller is often directly couple with the main engine and its rotation rate is defined by the rotation rate of the engine. A gear box can be used to reverse the direction of rotation of the other propeller and to reduce its rotation rate.
Planet gears are possible to reduce the rotation rate of both propellers, but until now ship owners have to been reluctant to use these gears for reason for reliability: when such a gear fails the ship is without propulsion. A major problem is also the bearing between the inner and outer shaft.
- The propeller-induced heeling moment is compensated (negligible for larger ships).
- More power can be transmitted for a given propeller radius.
- The propeller efficiency is usually increased.
- The mechanical installation of coaxial contra-rotating shafts is complicated, expensive and requires more maintenance.
- The hydrodynamic gains are partially compensated by mechanical losses in shafting.
Contra-rotating propellers are used on torpedos due to the natural torque compensation. It is also used in some motor boats. The cost of boring out the outer shafts and problems of mounting the inner shaft bearings are not worth pursuing in case of normal ships.
- Marine Propellers and Propulsion by J S Carlton. Publisher: Butterworth Heinemann
- Propulsors by Prof. G.Kuiper. Publiser: MARIN
- Ship Design for Efficiency and Economy by H.Schneekluth and V.Bertram. Publisher: Butterworth Heinemann