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Thu 24 Jul 2008 04:00 AM

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How does a propeller work?

Your prop is as essential as the engine that drives it; neither moves without the other.

Your prop is as essential as the engine that drives it; neither moves without the other.

Like the tyres on your car, the propeller on your boat converts precious power into forward motion - but do you know how to look for wear? How it affects performance? How to maintain it? Or indeed how it works?

The truth is your prop is as essential a part of your boat as the engine that drives it - if your prop is is mismatched or damaged in any way, it will severely reduce your boat's performance. There is no point in having a tiptop engine if you run it with the wrong propeller.

More blades equals more torque, but also an increases in drag.

A propeller pushes the boat along by ‘grabbing' water and ‘pulling' itself through the water by virtue of its spinning blades. When the prop rotates in the water, it forces water behind it and carries itself (and everything attached to it - i.e. your boat) forward.

It works the same way as a threaded screw pulls itself into a piece of wood - this is where the term 'screw' comes from. The speed a prop does this depends upon two factors - its diameter and pitch.

You'll notice all props are reffered to with two sizes, e.g. 13" x 19". The first dimension, in our example 13", refers to diameter and the second 19" is the pitch. Diameter is simply the distance across a circle swept by the extreme tips of the propeller blades.

Pitch is the theoretical distance travelled for one revolution of the propeller, in this case 19". However as water is not a solid substance, this figure is less, due to slip. Blades have a leading edge which cuts the water first, and trailing edge from which the water flows away.

Many propellers have a slight upturn on the trailing edge, this is known as cupping. Cupping has the effect of reducing cavitation. Cavitation feels the same as the clutch slipping on your car, and is commonly confused with bush failure as the effects can be very similar - i.e. the engine's revs suddenly rises but you lose speed.

Cavitation is actually caused when the surface of the blades become covered in tiny vacuum bubbles. A smaller degree of cavitation will cause vibration, but if left in this condition for long periods it will cause 'cavitation burns'.This occurs when the vacuum bubbles implode with enough force to start sucking metal off the blade surface - obviously not a good thing! Severe cavitation can cause the propeller to 'break away' completely; the prop loses traction with the water and spins faster while producing less forward motion.

The reason for cavitation is usually either damage, heavily fouled blades, or overloaded blades due to incorrect propeller type/size.

The pitch of your propeller is also critical to the performance of your boat. As most engines develop their maximum power at their full rated RPM, it is therefore important to select a propeller that allows the engine to reach its potential.

Having a selection of props is the best way forward.

For example, if your engine should have an RPM of 2500, but you can only reach 2300, your propeller is too course in pitch. This can be altered by having the pitch reduced by one inch, this not only provides the required 200 RPM, but also improves the engines efficiency and economy.

The next consideration is the number of blades a prop has; surprisingly, the most efficient number of blades is one; it has nothing to disturb the water flow ahead of it. In practice however this is not possible as a single blade would be impossible to balance.

The next most efficient is a two blade, but again this is impractical for most applications; it would require a huge diameter to provide enough blade area to generate sufficient thrust. For this reason most propellers have three blades.

Some propellers have four or more blades in order to either reduce vibration and provide smoother ride, or increase acceleration for the same pitch. These types of props are favoured by cruisers and wakeboard/waterskiers.

All propellers are a compromise and the choice of a four or five bladed propeller will reduce top end speed as the drag caused by the propeller increases. The drag factor is one of the reasons why surface piercing propellers are very efficient.

They are designed to run with the water line at hub level. This keeps the majority of the blade area clear of the water; reducing drag and keeping propulsion at maximum. However, while surface piercers may be great for pursuit craft or race boats, they are not suited to many applications as they have poor astern characteristics.A propellers blade shape will also affect its performance. Most outboard and sterndrive propellers are either ‘constantly pitched' across the entire high-pressure face, or ‘progressively pitched'.

On a progressive pitched propeller the leading edge will be a lower pitch, so the water is picked up under less pressure. The water then flows along the blade surface increasing in pitch to the trailing edge where it is forced into a tighter thrust funnel -providing higher efficiency.

These progressive pitch props are less prone to cavitation and will give increased acceleration and general performance.

Cupping also reduces cavitation, increases acceleration, reduces propeller vibration, and gives better prop ‘grip'. A cupped leading edge yeilds the equivalent of increasing the pitch of the prop by approximately one inch - this can be added to your current prop buy a specialist.

Some propeller have blade rake; the blades lean or slope, either forward or aft as viewed from the side. Blades that slope aft have positive rake: this provides the effect of increased top end speed and less acceleration, they are generally designed to increase bow lift.

Blade area also has an effect on performance, propellers with relatively high blade areas will have reduced blade loading and therefore suffer less from cavitation. Generally pointed blades, such as cleaver types, will give stern lift. Rounded high rake blade shapes generally give bow lift.

Surprisingly the material the propeller is made from also affects performance. Most outboard and sterndrive propellers are made from aluminium, but fitting a stainless steel propeller will increase performance.

The reason for this is simple: stainless steel is much stronger than aluminium, enabling the propeller to be manufactured with a thinner blade section.

If the leading edge is thinner, less air bubbles will be formed on impact, and thus cavitation is reduced. A thinner blade is much more efficient, so racers take this right to the limit, to the extent you can actually use a racing prop to have a shave!Thin blades however are more prone to failure under stress, so for the enthusiastic boat owner, simply changing from an aluminium propeller to a stainless steel, progressive pitched propeller with cupping will give significant improvements in performance.

For outboard and sterndrive propellers, diameter is related to pitch, Generally the lower the pitch the larger the diameter and vice versa. The reason for this is generally boats that require low pitches are slower and heavier and they need propellers with a larger diameter to give improved thrust.

Faster lighter boats need less thrust to achieve planning speeds and therefore less diameter but larger pitches to give optimum performance.

When you decide on a propeller size you need to carefully consider how you intend to use your boat. Are you looking to tow skiers, or do you want maximum speed? The same propeller cannot deliver high speed and maximum acceleration - it is always a compromise!

One way to save on compromose is to own a selection of props, so you have the ability to change propeller according to boat loading and conditions. As it is always advisable to carry a spare propeller, the latter is the best option.

Once you have a propeller it can always be ‘fine tuned' to your boat set up. It is possible to alter the pitch on an existing propeller up to a maximum of one inch, or a variation in between, to gain or loose extra revs.

Finally, you must keep your propeller in good condition. If it suffers any damage, however minimal it may seem, it will severely affect performance.

You should always have your propeller professionally repaired and serviced, avoid attempting DIY jobs as it will not only cost more in the long run, but it may even render the propeller beyond repair.

A good way of looking at prop bill is; any repair costs to your prop will easily be saved by reducing fuel consumption and keeping the propeller as efficient as possible. You'll save money and improving your boat's performance!