Descaling an Engine
Over time raw water cooled engines can develop lime scale in the waterways. This leads to partial blockages and the engine running hot.
There is a lot of discussion on this on the PBO forum. Various agents to remove limescale are discussed: Rydlyme, Fernox-DS3, Hydrochloric acid, Citric Acid.
Various comments by chemists warned against using very harsh acids and questioned the efficacy of mild acids.
Rydlyme is quite expensive so was discounted. I plumbed for Fernox-DS3 in the end as it is readily available in plumbers merchants, sulphamic acid based, and widely used to descale domestic central heating installations.
This is the bucket arrangement to circulate the fluid arround the engine. The other end of the hose attaches to the intake of the water pump.
There is no easy way of knowing what's going on inside the engine. So I decided to take off the thermostat housing and see what sort of state that was in.
I mixed up a 10% solution of fernox which is the highest recommend concentration on the instructions. So 500g fernox to 5 litre of hot water. I ran this through the engine for about half an hour. The engine ran quite warm and it was a cool day so often difficult to see what was going on in the bucket.
After the half hour was up I inspected the thermostat housing again. It was quite obvious that there was now bright metal visible. I also noticed that inside the bottom of the housing the gunk coating the surface was no longer hard but a more viscous coating.
So reasonably encouraged I mixed up another batch of fernox and ran for another half hour. The colour of the solution is pale yellow which is supposed to turn to green when it is spent. Mine just turns black within a few minutes. At the end of this cycle I put some of the solution on a barnacle chipped of the rudder. I could see no visible signs of a chemical reaction, so probably the solution was spent at this point.