Thrust loss on azimuthing thrusters due to Coanda effect

Abstract

The main objectives in this master's thesis is to investigate how the Coanda effect influences a thruster jet which further causes a thrust loss.

The tendency of a thruster slipstream to be deflected towards a nearby surface, for most practical situations the hull of a vessel, is called the Coanda effect and is likely to produce a significant thrust loss under certain geometric conditions.

The approach in this master's thesis is to perform an experiment measuring the direct thrust loss related to the deflection of the slipstream. Results from the experiment are presented along with comparisons with similar experimental results and predicted results acquired from mathematical model. The mathematical model has proven accurate in predicting the deflection of the thruster jet.

Results show that both distances from the thruster to the hull have an influence on the results, and in particular the tilting of the thruster nozzle. The greatest thrust loss measured in the experiment is approximately 25 % loss, which during for example critical offshore operations is significant and may lead to dangerous situations. Variations in thrust force, however, do not represent thrust loss variations according to the experimental results within the specified thrust range. Increasing the distance between thruster and hull bottom reduces the thrust loss with approximately 5 %.

By tilting the thruster downwards 7.5o one can reduce the thrust loss due to Coanda effect with approximately 10 - 15 %. Analyses show that tilting the thruster farther than 7.5o does not result in a reduced thrust loss, and is not advised as a counter measure for the Coanda effect.