What does a propeller do?
A propeller not only pushes the boat along but also has a direct bearing on engine RPM. As most engines develop their maximum power at full rated RPM it is important to select a propeller that allows the engine to reach its full RPM. An example of this would be if your engine should have an RPM of 2500 but you can only get 2300 your propeller is too course in pitch. This can be altered by having the pitch reduced by one inch which will give you the needed 200 RPM to bring the engine to it's full rev's and efficiency and so reduce fuel costs. This RPM alteration is presuming a gear reduction of approximately 2:1. Pitch alteration is one of the many services that we can offer at a very reasonable cost, and you will probably save this money in reduced fuel costs and unnecessary wear to your engine. You should prop your boat so that you can achieve the maximum RPM from your engine. Once you have done this you will get the maximum horsepower from your engine and thus the best possible performance from your boat. Therefore Max. RPM = Max. Power = Max. Speed = Economy at cruise.
What is cavitation?
Cavitation can give the feeling of slip whilst the propeller is in use. It is actually bubbles in a partial vacuum caused by use of a damaged propeller, excessive propeller speed, loading or an incorrectly sized propeller being used. When a propeller moves through the water, the water is forced against the suction side of the propeller. The faster the blades cut through the water the more vacuum bubbles implode against the propeller. This implosion can happen with such force that the bubbles actually suck the metal right off the surface causing erosion or a "cavitation burn". Although it is not possible to repair cavitation burns it is possible to reduce cavitation and therefore reduce the possibility of further damage. This is done either by cupping the trailing edge of the propeller or ensuring that the propeller has the appropriate exhaust ring fitted if applicable, we can provide both of these services for you. You may also have the propeller blades thinned down as this reduces the high pressure build up on the blades.
What is ventilation?
Ventilation is the effect of external air or exhaust gases being made available to the propeller. The propeller blades during their operation have a pressure and suction side. When air is induced into the propellers working area, the suction or back side of the blade will accept any migrant air, thus causing loss of thrust, an increase in motor rpm, and poor performance.
What are the most common causes of cavitation and ventilation?
The most common reasons for cavitation and ventilation are damaged and out of balance propellers If the bottom of the boat or the lower unit is fouled this will have an effect not only towards cavitation and ventilation but to the overall performance of the boat. The loss of a converging or exhaust ring from a thru-hub exhaust propeller will mean that the exhaust gases going through the propeller could either escape between the propeller and the gear casing if the converging ring is missing or not be properly deflected if the exhaust ring is missing. Too high a transom setting, a severe turn when in use, wrong motor angle and in general incorrect boat "set up" will all have an effect on the performance of your boat.
What is cupping?
Cupping is a slight up turn on the trailing edge of the propeller. Cupping will reduce cavitation , increase acceleration, reduce propeller vibration, and give better propeller "grip". Cupping is also the equivalent of increasing the pitch of the propeller by approximately one inch and this should therefore be taken into consideration. If you would like your propeller cupped this is one of the many services that we can provide.
What does the propeller "bush" do and can it fail?
Propeller bushes can be made from rubber or composite plastic and some propellers have torsion rods in their centers all give protection to the lower unit. All act like a shock absorber and decreases the stresses to which the lower unit and engine gears are subjected to in the cases of sudden engagement of drive or when the propeller hits an obstruction. Propeller bushes are usually reliable. If your bush is damaged you will get little drive from it probably only walking pace. However if your boat is going faster than this and you are experiencing "slip" this is probably not a faulty bush but an effect of cavitation or ventilation. If you do have a faulty propeller bush this can usually be replaced by a competent engineer.
Frequently Asked Questions
What do the measurements on my propeller mean?
On your propeller there will be some numbers stamped such as 14" x 12". The first number relates to the diameter, in our example 14". The diameter is determined by doubling the distance between the blade tip and the center of the hub. Diameter can also be looked at as the distance across a circle that the propeller would make when rotating. The second number 12" in our example, refers to the pitch. Pitch is the blade angle and also the 12" pitch indicates that in each propeller revolution the boat theoretically would advance 12" through the water. However as water is not a solid substance there is some slip approximately 10-30% which means that the actual advance is some what less. It may sometimes be difficult to find the size marked on a propeller and also some propellers may have their measurements given in the form of a code so if you are unsure what size your propeller is we can advise you.
A~ the propellers leading edge
B~ the trailing edge
C~ converging ring
D~ the exhaust ring
E~ the shaft
F~ the propeller nut & fittings
G~ the "bush"
How do I know if my propeller is left or right hand?
Propellers are either left or right handed. Left handed propellers rotate anti-clockwise as viewed from behind the boat looking towards the stern. Right hand propellers turn clockwise.
Propeller Repairs & Sales
240 Merton Road, London, SW18 5JQ, England
TEL 020 8874 7059
The UK's leading propeller specialists trading since 1958