A RAW SEAWATER cooled diesel engine
will quickly foul internally if the thermostat is not designed to open at about 135 degrees F. Typical thermostats on Yanmars are designed to open at 180 degress F. (82 deg. C)
What happens is when seawater is brought to about 145 degrees F (63°C)the dissolved Calcium Carbonates in the seawater will become insoluble & 'drop out' (precipitate) of the seawater and will form a thick scale on just about all internal surfaces of the cooling
passages. This is a gradual process. This scale is a retardant to proper heat transfer and must be removed so that your engine
returns back to proper heat transfer conditions. The fouling will be in both the engine internal 'water jacket' AND inside the water
jacket of the exhaust
Such fouling is removed by 'pickling' the raw water
circuit with either muriatic/hydrochloric acid or (much much better) a commercial
organic boiler descaling compound. The acid when finished dissolving the scale will begin to attack the base metal of the engine; commercial
boiler descalers are organic acids and will not dissolve the engine's base metal. I notice that you are in S. France
, so you will have to search that area for a supplier of an organic 'boiler descaling' compound. In the USA such compounds are known as "Rydlyme" or "Marsolve", etc.
If you cant find a commercial organic acid boiler descaler use/obtain the muriatic(hydrochloric) acid (typically 35% concentration) dilute that by 2:1 or 3:1 (by adding acid TO the water
and NEVER water to acid to dilute - important if you dont want to get scalded and attacked by splattering acid), remove the cooling
water intake hose and suck the mixture into the engine, let soak for about an hour or so, then reattach the hose and flush with sea water. After any 'pickling' it is very important to bring the engine up to full operating temperature as soon as possible and then run for several hours if possible so that you restore the 'protective' black rust coating (ferrous oxide) on the internal cast iron. If you dont bring the engine up to and let 'heat soak' at normal engine temperature, you will begin to form destructive red rust (ferric oxide) and that will start to 'push the casting' apart if there are any defects from the original metal casting process.... forming rust *pressure* that will begin to push the 'stratifications' of the original cast iron apart if these pathways are 'communicating' with the interal surfaces. Inotherwords .... run the HELL out of engine after you pickle it.
Try to locate a thermostat that fully opens at 135 degrees F (57 deg C.) which although not perfect will greatly retard the rate of internal scale formation. After pickling, be aware that the exhuast manifold can possibly issue large 'plates of rust' from the internal surfaces at a much later time that can possibly block the outlet and block the water flow to the whole circuit .... these 'platelettes' usually can be broken loose by pushing a stiff wire into the manifold from the water outlet - this doesnt happen all the time, just a 'cautionary note' if the engine suddenlly stops flowing cooling water in the future.
To help decide when to pickle the engine in the future, record
the thermal operating conditions before you pickle and after you pickle. At a reference rpm
(2000 for example), record
the inlet water temp and the outlet water temp and note the difference between the two at the constant rpm
(after running at that rpm for some time) - this is the base 'thermal terminal difference' of the engine. In future when the engine is appartently 'misbehaving' and if the terminal difference is SMALLER than as after you pickled it, then you need to pickle the engine again to restore the proper operating 'terminal difference'. Scale is somewhat like an 'insulator' and will retard the caloric 'flow' from the engine to the cooling water and a fouled engine will have a LESS terminal difference than when clean / unfouled.
First check the amount of water coming out the engine's exhaust
(look at your manual - will specify a specific volumetric rate at a specified rpm) ... you measure with a bucket and stopwatach at the exhaust. If the flow is withing 'spec', .... then "Pickle" the engine's water circuit, run the Hell out of it after pickling and get a 135 degree thermostat.
If the volumetric flow is below spec .... you simply have a blockage somewhere: broken impelller vane lodged in the piping, "zebra type" mussels growing in the intake, kinked hose, large slab of platelette rust blocking the exhaust manifold, etc. Hope this helps.