For optimum performance and fuel efficiency under engine, it’s important to have the right propeller on your boat. Tony Davies reports
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?
Propeller specifications explained: Propeller theory
All propellers can be described in basic terms using a set of two numbers: diameter x pitch. Hence a prop with an 18in diameter and 12in pitch is described as 18 x 12. The diameter of the propeller is the circle which the tip of a single blade describes in a complete rotation. It is found by measuring from the centre of the propeller boss to the tip of one blade and then doubling the result.
Pitch is the forward distance that a propeller would theoretically travel in a single rotation if there were no slip present – imagine a screw being driven into a piece of wood. The angle at which the propeller blades are set governs the distance travelled. A fine pitch (meaning little angle on the blades) gives a lesser distance than a coarse pitch (greater blade angle).
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.
What is Hull Resistance?
Resistance is the hull’s natural tendency to drag through the water. Bare hull resistance is produced by skin friction of the hull’s surface, waves created by the hull running through the water and the shape of the hull below the water. To the bare hull, the drag of appendages must be added such as rudders, shaft support brackets, shafts, depth sounder transducers, stabiliser fins, etc. However, the design of the lift surfaces of the hull also have a very significant effect. Designers work to ensure that the lifting characteristics give the hull the least resistance at normal operating speed.
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 slow-revving 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.
Does my boat have the 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.
What is the benefit of new propeller blade design?
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.
The heavy sectioned blade is no longer the preserve of the plodding working boat, it can also be used to advantage on high-speed vessels where performance and economy are of equal importance.
Take a look at the original propeller on my own boat (below). 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.
Theoretically, fewer blades provide a more efficient propeller. In contrast, more blades make for smoother running and less vibration. With modern propeller design the type of propeller used for any particular purpose is no longer easy to designate as there are often elements of all types incorporated into the design.
Originally published in Practical Boat Owner magazine May 2019