Before we dive into the history of propellers and their many inventors this post will explain the two types of vessels commonly fitted with propellers, displacement vessels (tankers, cargo, cruise ships) and planing craft (power boats, RIBs, racing boats). Both types use similar propellers but have very different requirements on their design, performance and other aspects such as cavitation requirements. But key to this blog series, is the demand on the power both types have. The hydrodynamics dictate very different engine sizes and power demands for both types of vessel, so let’s have a look at these two vessel types.
If you think of a classical ship type, chances are you will be thinking of a big vessel such as a ferry or a cargo ship. These are common and very well understood by Naval Architects. Pointy end on the front, propeller on the back – in it’s simplest form. The key to this type of vessel is that the water is pushed around the vessel as it moves forward; that is, it displaces the water to achieve speed through the water. But because it pushes water out of the way it creates waves, and these waves become the limiting factor in the powering and speed of the vessel. Naval Architects spend a great deal of time optimizing displacement type vessels for their mission, to use less fuel or go faster. Remember most ships are optimized for a mission profile, that is optimum speed, so a cruise ship will be designed for, let us say 21 knots, and be expected to achieve this speed throughout it’s life, from handover to decommissioning. It is also not like building a car, seldom do you get chance to build a prototype, ships are almost always one offs, designed and predicted from model tests, CFD and other standard series methods. In other words you have to get it right first time!
Looking at the little slideshow on this post lets address a couple of things. Slide 2 and 3 show the Power required for a displacement vessel, say the tanker shown, over a range of speeds. As the speed goes up, you need more power, initially this requirement is a shallow curve, but it soon becomes a steep curve as the wave making of the vessel dominates the power required. So displacement craft have a relatively smooth curve (there are humps at certain points) but it is in general a smooth curve that becomes dominated by the waves the vessel makes.
Now planing craft do things different. Slides 4 and 5 show the same resistance curve for a planing craft. Actually the vessel shown is the ‘daddy’ of planing craft called Turbinia, fitted with the first steam turbine in 1895 but I digress!
So the funny hump in the curve is the hydrodynamics of the vessel helping as the vessel increases in speed. It works like this, the planing craft increases in speed and to a point behaves like a displacement regime making waves and everything. The increase in speed causes the running trim of the vessel to pitch up by the bow exposing the bottom of the hull to the incoming flow and generate lift. There is a lot going on that I am skipping over but the lift helps the vessel out of the water a little and this causes the dip in the power curve. The vessel is being dynamically supported at that speed and as power increases it will drop further to an optimum and then increase as normal. The wave making is not the dominant part but the proportion of the hull in the water making the Frictional Resistance. This is why in a seaway some vessels put a ton of power down and cannot get over the first hump and plane – the wave resistance is taking too much energy. A smoother sea state and the vessel jumps onto the plane no problem.
So how does this all relate? Well I will be discussing the history of the displacement vessel propeller. These are lightly to moderately loaded propellers with conventional blade shapes and not too many whistles and bells, although this is changing. For the planing craft the propellers get interesting, whether they are outboard specific, surface piercing (part of the propeller intentionally in the air at all times) or super-cavitating (intentionally cavitating the propeller there are so many unique variants.
So about those paddlewheels…