Basic physics law, in order for an object to move, it needs to overcome its resistance. In the case a ship it is divided in several components, as explained in the previous post. In that article, we’ve focused on the impact of the frictional component of the overall wetted surface and then on the wave and viscous resistance influenced by the bow shape. Yet the stern is also influencing the viscous resistance, considering that the wake is generated as the flow is breaking away from the submerged body (i.e. the hull). A smooth shape is limiting the wake and consequently reducing resistance, but as the stern is the area where the propellers and rudders are housed, the needs of these appendices take priority and finally define stern’s form.
For example the propeller has to be protected from the hull, because breaking a blade results in thrust losses and simultaneously can source excessive vibrations. Therefore the keel protrudes and protects the blades from the sea bottom. Moreover there needs to be sufficient gap between the top edge of the propeller and the bottom of the hull in order to avoid once again vibrations, as well as noise. Simply the propeller is laterally adding to the viscous resistance and directly to the resistance from appendages, for self explanatory reasons.
Furthermore it is contributing significantly to the efficiency losses from the shaft arrangement. It is estimated that the delivered horsepower to the propeller is 95-98% of the break horsepower (horsepower from the engine), but when it comes to the propeller efficiency, it is between 65-75%. In other words one third of the delivered horsepower is lost from the propeller alone. By now it is obvious that the propeller, while it is the main driver for the vast majority of ships, it can simultaneously be the primary headache.
Definitely the industry has researched to engineer better alternatives, like the ducted propellers and more notably the Kort nozzle, that have been developed to regulate the flow and achieve higher efficiencies. Practise has shown that it can be a very effective arrangement for lower speeds, but it is losing momentum when the speed goes over 10knots. Looking into the yachting industry a speed lower than 10knots would be very low.
Another option is the controllable pitch propeller. In CPP the blades can be adjusted to accommodated the exact load and revolutions required, therefore the engine can maintain high rpm in severe weather conditions, while the propeller is operating in its optimum revolutions. Manoeuvring is also more responsive as the rotational direction of the propeller is independent of that of the engine. On the downside it has about 1-2% lower efficiency from the fixed pitch propeller, which combined with their higher price and maintenance cost are deterrent factors. Overall though one can say that CPP is using all the power that it is given, making best use of the available resources.
Modern design and specifically computational fluid dynamics (CFD) can be a benefactor in maximising the propeller’s efficiency. In order to appreciate the advantages of mathematical models we need to juxtapose them against the traditional approach for selecting the propeller. The calculation of hull resistance has been the first step, taking into account the bare hull. Next the propeller performance in open water (no hull in front) is guesstimated with the input of, for example, the B-series models, later corrected by coefficients, giving the final characteristics. It has been proven that this procedure is far from perfect. CFD on the other hand provides the opportunity to examine hull resistance and propeller efficiency in the same step, evaluate the actual wake/ inflow of the propeller and return to make hull shape changes, if needed, in parallel to defining propeller’s data. Indeed the polynomials incorporated in this process are still under development, but at least this method has growth potential and worths investing further into.
Summing the overview of the propeller, considering that it is the preferred propulsion method of the industry, we can note the following.
- Controllable pitch propellers are utilising the power of the engine more effectively and in respect of the environmental considerations are advantageous;
- Further tests and development of CFD of the stern in combination with the propeller need to be done, in order to have the best possible shape for the selected propeller characteristics;