Choosing a propeller
For optimum performance and fuel efficiency under engine, it’s important to have the right propeller on your boat. Tony Davies reports
For optimum performance and fuel efficiency under engine
Understanding propeller design can help solve performance problems that occur when a boat is being driven under power. The wrong prop may be wrong in a number of different ways including diameter, pitch, blade shape or blade area. Your prop may suffer from cavitation, ventilation or slippage – your boat may be burning too much fuel or lacking acceleration or top speed.
But how can you tell if you have a propeller problem and how do you know
which of these issues might be affecting your propulsion?
A propeller is usually designed with two to five blades (though sometimes more). In the first instance, the number of blades is decided by the weight of the vessel: the greater the weight, the greater the blade area required to push it through the water with a minimum of slip and cavitation. To specify the correct type and size of propeller, a standard set of boat, engine and gearbox measurements is required.
Designers will use the boat’s length, beam, draught and displacement in
combination with the boat’s underwater drag characteristics to calculate hull resistance (see panel for more detail).
Once this resistance is known, engine and gearbox specifications can be combined with maximum propeller diameter (clearance beneath the boat is typically 12% of the prop’s own diameter) to determine the prop's minimum requirements. The vessel’s ‘duty cycle’, which is simply a judgement as to whether a boat is to be used for heavy or light commercial or pleasure purposes, then also influences a designer’s final specification.
Propeller diameter is first decided by the type of boat. A large-diameter slowrevving propeller is the most efficient type but this won’t provide the necessary acceleration to get a sports cruiser onto the plane. Acceleration is achieved by increasing the rotation speed, which usually means a smaller diameter propeller. The fact that many modern sports cruisers have limited propeller clearance space is another reason for fitting smaller diameter propellers.
To accommodate the loss of thrust from the smaller diameter, the pitch is increased to give more thrust from the propeller’s movement through the water rather than from the blade area. It is in fact a combination of the three elements: diameter, pitch and rpm that provides the thrust at the given speed of the boat. The higher the speed of the boat, the higher the pitch should be. Also the higher the pitch, the greater the efficiency of the propeller. Higher pitch can be used by selecting higher gear ratios, but this is not really viable for most outdrive powered craft due to the small choice of ratios available and in this case it is usually necessary to work with the supplied ratio.
Cavitation and ventilation
Cavitation is often confused with ventilation as the symptoms are similar: increased shaft revs without any
increase in boat speed – or sometimes even a sudden reduction in speed.
Ventilation is caused when air is drawn down into the propeller from the surface. This can be due to the propeller being too near the surface (a design fault) and spinning too fast. The air entering the propeller causes the blades to lose contact with the water and allow it to lose ‘bite’.
Cavitation is totally different and is often caused when a heavy, fast vessel has propellers with too little diameter and blade area. The weight of the boat puts too much load on the propeller blades increasing the pressure on the face side while decreasing the pressure on the backside of the propeller.
At very low pressure water starts to vaporize, which causes a vacuum to form on the surface of the blades in the form of tiny bubbles that implode upon themselves. These implosions can damage the surface of the blades with pitting. Over a period of time, these tiny pits, or craters, can seriously weaken the blades and the bubbles can additionally cause erosion on the rudder surface.
The answer to cavitation is to use blades of larger diameter or greater area to increase thrust. This will allow the revs to be reduced by altering the gearbox ratio. However, this is often not possible due to lack of clearance beneath the hull – in which case, the props just have to be replaced more often.
Wrong size prop?
If the boat is over-propped the engine will not reach maximum rpm and the boat will underperform. Equally, if a boat is under-propped and the engine easily reaches or exceeds maximum rpm, the propeller needs more ‘bite’ to get the boat to maximum performance level.
With outdrives, adjusting the pitch of the prop is often the only option, as the maximum diameter propeller is often already installed. Increasing the pitch will decrease the revs and vice-versa. If the engine revs seem OK with the current prop, but you want better acceleration, it is often possible to reduce the diameter and increase the pitch by roughly the same amount to give added acceleration without losing the top-end speed.
Blade shape has moved on in the last 15 years or so, thanks partly to computer aided design and a better understanding of what advantages a particular blade shape can offer.
New blades for old
The heavy sectioned blade is no longer the preserve of the plodding working boat, it can also be used to advantage on high
‘propeller diameter, pitch and rpm are the three elements that provide the thrust at any given speed’
speed vessels where performance and economy are of equal importance.
Take a look at the original propeller on my own boat. Not much more than a three-bladed slab of bronze that did its job of pushing the boat through the water.
Now move on ten years and look at the sophisticated four-blade foil design that runs much more quietly, with less vibration and gives increased fuel economy at semi-displacement cruising speed.
Brave new blade design
With modern propeller design and construction methods the three main criteria (diameter, pitch and rpm) are no longer the only governing factors.
Increasingly, shape is just as important. If the diameter needs to be smaller to provide better acceleration then the blade area can be increased by adding more blades. For optimum performance, fuel economy, fast acceleration and smooth vibration-free running, it is the shape and cross section of the blades that make the crucial difference.
For the everyday boat owner blade shape is governed by the boat’s dimensions, its engine specifications and the owner’s choice of propeller manufacturer. High-tech computer designed and machined propellers only become viable for yachts over 24m (superyachts) where the shape of the blades is tailored to the requirements of the vessel taking into account resistance as well as all the normal information required to build a propeller.