We regularly receive reports of engines that are not able to achieve close to their rated max rpm with the Kiwiprop we have supplied.
Sizing is developed from an extensive database covering some 2000 installations and given we receive the correct engine and reduction gear specifications – we are confident of our ability to size the correct propeller as to diameter and pitch.
Obviously shaft angle / clearances / displacement have small impacts on maximum achievable engine rpm – but over 90 % of all installations do not alter the pitch we have supplied.
The two most likely causes of an engine not achieving near it's rated max rpm are as follows:
This applies in particular the older the engine.
Manufacturers are strict about the level of back pressure that cannot be exceeded for an engine to develop it's rated power. This is very important on smaller engines with smaller exhaust pipes where any restriction has disproportionate effects.
Typically on older engines we see examples where the owner can't figure out why the power loss – yet when opening the exhaust bend finds corrosion from hot salt and even fresh water has caused a build up of corrosion on the internal walls of the manifold.
Reductions of just 10 – 15 % in diameter can halve power output.
The surface is usually also very rough – which in itself contributes to power loss from additional back pressure.
This should be the first area for analysis for any installation that is unable to achieve what many similar engines with the same propeller have been able to achieve.
This can apply equally to relatively new as well as older engines.
Any situation that allows hot salt water or salt water fumes to reach an open exhaust valve that is hot when the motor is shut down is likely to cause severe corrosion literally in a matter of minutes.
As the manifold cools when the engine is stopped – the seal from the water lock can create a vacuum and suck water back nearer or in contact with the exhaust valves.
The following article illustrates the problem well.
This can be tested for before pulling the motor apart by measuring the compression on each cylinder. White exhaust smoke is also a give away at starting as the low compression causes incomplete combustion.
If this has occurred it is most likely to be the on the cylinder nearest the exhaust. Typically the engine is installed on an angle – ensuring it's the aft most cylinder that will be damaged first in the event of water penetrating to the exhaust valves.
Inlet valves of course can suffer corrosion but being so much colder – they typically suffer less damage – but can still cause compression losses.
Any loss of valve sealing will generate compression losses and this significant power losses.
The following pages gives a detailed analysis from an owner of what can happen when hot salt water gets to an exhaust valve and what the solutions are – maintenance and design wise to prevent this from ever happening.
The following was prepared by Mr Gordon Croudace as a result of his own experience with power losses and provides an excellent background to the whole issue.