Stern Design

We have previously discussed how we have dealt with the design of flow and pressure distribution at the bow. We now look at the critical design of the aft end of the vessel. The flow around the stern is of very great importance as this will influence not only drag, but also the propulsive efficiency of the propeller. In an ideal situation a propeller would prefer to have ‘clean’ water flow towards it without turbulence and at the same speed as the boat is travelling (Speed of advance Va is the speed of the water that approaches the propeller. Unless the propeller is in front of the boat in clean water Va is usually less than V which is the speed of the vessel ). The common hull design used in the canal system has a traditional swim whereby the side shell is pulled into a point on the centreline. This results in rapid deceleration of the fluid and a change in direction causing a pressure drop in front of the propeller and also a reduction in Va. In

the Ideal situation, the flow direction is changed gradually over a long distance (reducing the pressure drop) and the volume of the engine pod reduced to the bare minimum, cleaning up flow into the propeller, resulting in a lower reduction to Va and thereby improving the propeller efficiency.

The diagram illustrates the variation in geometry and the clean flow into  the Floating Ohm propeller compared with the traditional ‘Swim’

The long sweep from the bottom shell into the swim above the propeller in the Floating Ohm design does not detract from the accommodation volume and is approximately 3.5m long and directs cleaner wake into the propeller with a Va closer to the speed of the boat through the water. (Va = Speed of advance)

The traditional swim could be modified by significantly lengthening it, to achieve similar flow but would result in a considerable loss of volume in the accommodation, an approach that has been implemented in the past with negative client feedback.

There are additional benefits to this design approach in that the available useable volume in the machinery space is improved giving better options for laying out equipment and better access for maintenance.