ABSTRACT

The expected additional resistance due to the propeller race when towing large structures is discussed. Model tests and full scale tests, measuring the velocity distribution behind working propellers are performed. The axial force due to the propeller race on 3 different structures inserted in the propeller race is I1ieas-lired. The results show that in extreme cases the force on the towed structure may be greater than the propeller thrust, and this results, in a motion opposite of the intended. Within the accuracy of the measurements no scale effects on the velocity distribution of the propeller race could be detected during the tests. To study the effect of the propeller race on towed structures model tests, even with small propellers, will be a useful tool.

INTRODUCTION

It is well known that the propeller race behind a tug causes an increased resistance of a towed structure. This fact has, however, been of minor importance when towing lines have been long and the breadth and draft of the towed-structure small compared to the length of the towing line. Offshore activities, including towing of large structures, necessitate a more careful look upon the forces that may be expected on a towed structure due to the propeller race.

The present paper presents some experimental results obtained from an introductory study on the action of a tug's propeller race on a towed structure. The intention of the paper is to demonstrate the magnitude of the forces and to study the scale effects on the velocity distribution in the propeller race when applying very small model propellers. A physical explanation of the additional forces on the towed structure due to the propeller race is found by considering the conservation of momentum in the propeller race. I.e. the forces on the structure can be related to the change of flow direction of the propeller race, see Fig. 1.

THEORETICAL CONSIDERATIONS.
Conservation of momentum

The flux of momentum may be expressed as:

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Assuming stationary conditions we can express the flux of momentum through a surface A as:

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where n is the unit normal to the surface and p the density of the fluid.

Assuming a propeller working at zero velocity of advance, i.e. bollard pull, we define the mean velocity of the water through propeller disc, UO by considering the propeller thrust:

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where Mox is the axial component of the flux of momentum. Uo is used as a reference velocity for the axial velocities in the flow field. Consider a propeller working in an infinite fluid where there are no other forces acting. The flux of momentum will then be constant through every y-z plane behind the working propeller. If we insert a body in the propeller race, the force in x-direction on the towed body may be found by considering the flux of momentum through a plane ahead of the body and a plane aft of the body. The difference in flux of momentum through the two y-z planes is an expression of the total force on the body in x-direction (see Fig. 1):

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