Honda Marine - Technical Talk

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 plaining
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 through their service network. To find your nearest service outlet use the 'Locate a Dealer' button in the store menu, or e-mail marine.help@honda-eu.com .

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 navigation…)
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. However, in reality the blade is moving through water which acts as a lubricant and allows the blade to slip.

Diameter
The diameter of the propeller chosen for an outboard engine will fall within a fairly small band. Depending on the size of the engine the propeller drive shaft will be a specified distance from the anti-cavitation plate. So a propeller can be selected with a slightly different diameter for the same amount of pitch, depending on application.
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.

Both diameter and pitch must be taken into account when selecting a propeller.

Setting up the propeller
Put the boat in the water with its normal load and, assuming the engine is run in, take it 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.

As a general rule, a difference of 1 inch of pitch can alter the engine speed by as much as 300 rpm. A propeller with a larger pitch will reduce the top rpm, wheras 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.