TORM, the Danish ship-owners, experienced problems with vibrations in the stern of six new tankers. By a simple, rapid and inexpensive operation in the form of two fins welded on to each side of the hull, FORCE Technology succeeded in reducing the vibrations considerably.

Where do the vibrations come from?

On the basis of noise and vibration measuring, it was ascertained that the vibrations matched the propeller blade frequency. Thus, the solution to the problem was most probably to be found here, and therefore TORM contacted FORCE Technology’s Division for Maritime Industry.

Wind simulating water

FORCE Technology has great experience in solving this type of problems. Vibrations of this kind are often due to a bad flow to the propeller typically caused by a very voluminous stern where inflow of water to the top of the propeller can be very limited, and therefore the propeller ’operates” in a very varied speed field. On account of the great speed variations in the propeller disk, vibrations may arise because the individual propeller blades are exposed to different loads.

In order to examine the inflow of water at the stern FORCE Technology built a model of the hull, about 0.7 m long, which was then tested in one of their wind tunnels. The wind tunnel tests were based on the fact that the flow over the hull is the same whether using wind or water.

In the wind tunnel FORCE Technology’s specialists made tests by spraying a mixture of oil and graphite over the hull and then subsequently see how the flow spread the liquid substance. The specialists could ascertain the flow to the propeller over the hull to be unfavourable which meant that the propeller did not receive an optimal inflow of water and thereby was the cause of vibrations.

Rapid and easy solution

As it is not possible to change the hull shape of a ship, the problem can be solved by changing the inflow to the propeller. In order to improve the water inflow to particularly the upper part of the propeller, FORCE Technology therefore suggested mounting a fin about 2 m deep on each side of the hull under the waterline. The positioning and angle of the fins were determined very exactly on the basis of the wind tunnel tests to ensure an optimal inflow to the propeller in order to make the speed variation in the propeller disk as uniform as possible.

It took FORCE Technology about three weeks to complete the whole process from model making via tests to the final proposal for design of fin. Subsequently drawings were made for the approval of the classification society, and the fins were constructed and mounted on the ship in connection with a planned docking.

Vibrations reduced

After TORM having tested the fins on two of their six ships, the feed-back from the crews has been very positive. According to measuring made, vibrations have been reduced to about 10-20 % of the original level.

FORCE Technology’s Division for Maritime Industry has contributed to the project with their expertise during the whole process from solution proposal with shaping and positioning of fins to supply of drawings for the approval of the classification society and performance of the actual tests.