The propeller delivers the thrust to push the hull of the boat through the water. The size, shape and number of blades on the propeller will have to be selected to suit the size of engine and the shape and weight of the hull if the boat is to reach the design speed.
Choosing the right propeller will give:
- A comfortable ride
- High performance
- Higher speeds
- Economic fuel consumption
- Optimum engine rpm
- Better acceleration
If the wrong propeller is fitted you get:
- A bumpy ride
- Lazy performance
- Inferior speeds
- Problems planning
- Higher fuel consumption
- Poor engine rpm
- Restrained acceleration
Generally, on smaller horsepower outboards the standard propeller that is supplied with the engine is suitable for most applications. If a different propeller is needed Honda have a range of propellers available - e-mail or phone us if you need assistance.
On larger horsepower engines the propeller will need to be matched directly with your boat and application. Our largest engines are supplied with a choice of aluminium propellers.
In case of unforeseen circumstances it makes sound marine sense to carry a spare propeller at all times.
Points to take into consideration when choosing alternative propellers include:
- Type of boating you do (fishing, water-skiing, cruising, inland navigationà)
- The size and weight of your boat
- Your Honda engine horsepower
- The performance you expect to achieve
Pitch and diameter
Getting the correct pitch and diameter of your prop will depend on the boat and engine combination.
Pitch-Imagine a corkscrew turning and driving itself into the cork of a bottle. Each full rotation will drive it down the distance that it takes the tip of a section of the blade to complete a rotation of 360 degrees. Take any propeller and stand it on a flat surface. Walk it, by hand through 360 degrees following the path of one blade tip and measure the distance it walks along the flat surface. That is the pitch. This information will be stamped on the propeller along with the diameter.
In theory, one revolution of the propeller will move the boat through the water the distance of the pitch.. Once you understand the basic principle of how a propeller blade works itÆs easy to see the differing pitches. However, in reality the blade suffers from slip as it moves through water which acts as a lubricant. Some slip is essential , excess slip is called cavitation which is explained below. Diameter-The diameter of the propeller chosen for an outboard engine will fall within a fairly small power band. Depending on the size of the engine the propeller drive shaft a specified distance from the anti-cavitation plate will be specified. A propeller can be selected with a slightly different diameter for the same amount of pitch, depending on application. Diameter and pitch must be taken into account when selecting a propeller . A slightly larger diameter will increase the thrust from a propeller of similar pitch. The blades will be larger and therefore have more contact with the water . A slightly larger diameter will increase the thrust from a propeller of similar pitch. The blades will be larger and therefore have more contact with the water.
Setting up the propeller
With the boat in the water carrying a normal load and, assuming the engine is run in, take the outboard motor up to maximum speed. The tachometer reading should be at or near to the Honda maximum recommended rpm. If the engine is revving above this level a propeller with an increased pitch or diameter is needed. This will only apply to boats that have been out of the water for a reasonable period of time that is dry and have a clean bottom. Any fouling of a boat will affect the performance. We have encountered boats that loose in excess of 70% of their performance that have a dirty bottom. In our experience the main cause of poor performance has been from fouling and NOT an incorrectly selected propeller. As a general rule, a difference of 1 inch of pitch can alter the engine revs by as much as 300 rpm. A propeller with a larger pitch will reduce the rpm, whereas the finer a pitch will increase the rpm. Cavitation is an indicator that a propeller is overloaded. Propeller Cavitation is when a vacuum on the reverse side of a propeller is so great it pulls air from the surface. This air meets the following blade with devastating results for the boats performance. Cavitation can often be felt through the hull of the boat as a vibration. If Cavitation is left for a long period of time not only will the engine be damaged, the propeller can be destroyed. It cannot be stressed enough that careful consideration to propeller size and pitch must be taken into account as excessive revs can cause damage to a marine outboard engine. Damage can also be sustained when a propeller of too large a pitch is used. This would be just like driving your car up hill in the wrong gear continually. Excess Fuel will be used which can end up diluting the engine lubricating oil.
A propeller with a larger pitch will reduce the top rpm, whereas the lower a pitch will increase the rpm.
Continuous excessive revs can cause serious damage to an engine. Serious damage can also be caused when a propeller of too large a pitch is used, especially when the engine is under load.
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