Loss of power


We regularly receive reports that engines are failing to reach their maximum rated speed with the Kiwiprop propeller we have supplied.


The dimension calculation is carried out from an extensive database covering some 2000 installations and as long as we receive the correct engine and gearbox characteristics, we are certain of the validity of our diameter calculations and not of our propellers.


Obviously the inclination of the shaft, the clearances and the displacement have a small influence on the maximum possible engine speed, but in 90% of all installations this data does not require any modification of the pitch we have provided. The two most likely causes of an engine’s inability to reach maximum speed are:


This problem appears especially with older engines. The manufacturers all insist on the maximum level of back pressure beyond which an engine cannot develop its rated power. This player is especially important on the smallest engines. Because of its small diameter, any restriction of the exhaust line has disproportionate effects.


Many owners of older engines cannot determine the cause of their loss of power. When disassembling the exhaust elbow, it is common to find that corrosion caused by seawater or even heated fresh water has created a rust accumulation on the internal walls of the collector. A reduction of just 10 to 15% in diameter can cut the power output in half.


Generally the surface is also very rough, which in itself is a factor of loss of power by increasing back pressure. This is the first check that must be made in an installation that fails to achieve the power that many identical engines have developed with the same propeller.


This problem appears on both relatively new and older engines. Any situation that allows warm seawater or seawater vapor to reach an open exhaust valve that remains warm after the engine is shut down is a likely source of significant corrosion within minutes.


While the manifold cools after the engine is stopped, the seal created by the water present in the water lock can create a vacuum thus sucking in water that can settle on the exhaust valves.

This phenomenon can be seen before any disassembly of the engine, by measuring the compression ratio in each cylinder. White exhaust gases also show how too little compression can lead to incomplete fuel combustion.


This problem most often occurs on the cylinder closest to the exhaust. Many motors are installed inclined. In this case it is therefore the cylinder most at the rear that is damaged first in case of water inlet on the exhaust valves.


Intake valves can also be prone to corrosion, but since their temperature is much lower, the damage is much less even if it also causes a loss of compression.

Any decrease in valve tightness generates a loss of compression and therefore power.