One of the problems of navigating a sailboat that draws 6’3” inches in the shallow Intracoastal waterways in Florida is the inevitability of the occasional grounding. This does not normally cause damage for a long keel boat like Britannia, because the bottom is mostly soft black mud, but depending on how hard she goes on, it can be a quite a trial to refloat. With any attempt to refloat, the propeller, that is whirling madly only a few feet from the muddy bottom is certain to disturb lots of silt and sludge that can be drawn into an engine water cooling intake and cause havoc.
Most freshwater cooled marine diesel engines operate on similar cooling principles. Simply put, cold seawater is drawn in by the engine pump and this cools the hot fresh water in the engine, through a heat exchanger. The residual warmed seawater is then pumped out of the back of the boat usually through the exhaust.
The large mesh filters fitted to most boats’ seawater intakes will capture large lumps of debris like sea grass and even tiny fish, but minute particles of sand and sludge can still pass through, clogging first the neoprene impeller pump then working their way into the engine's heat exchanger(s) and cooling passageways. The first sign of this is often a rise in the engine's temperature along with a reduction in revolutions and performance. I had noticed all of these things during a few day-sails and decided to investigate
I started at the engine main filter and found the inside gauze was completely blocked with black mud. Next the plate on the front of the Jabsco seawater pump was removed and to my utter amazement the impeller vanes had completely vanished leaving only the hub on the shaft! This must have happened when the primary filter become completely clogged, causing the impeller to run dry and shred the vanes. There is only one place the broken bits can then go - straight into the bowels of the engine.
The first place the water went after the pump was into the oil heat exchanger located right at the back of the engine in a most awkward to reach part of the motor. I removed the inlet water hose and poked a finger inside feeling just soft rubber and dirt! This meant removing the double barreled oil cooler that was no easy task with a bulkhead in the way. I finally managed to get it off the engine and using long nosed pliers I pulled as many bits of impeller out as I could, then back flushed the twin pipes with a water hose.
The next path for the raw seawater was to the larger engine heat exchanger that was also partially blocked with tiny bits of the impeller and lots of sludge. This was when I concluded that there was nothing else for it, but to completely dismantle and clean all the seawater passageways in this big engine.
DISMANTLING.
All the parts of the cooling system had to come off, to be examined and cleaned. But access was impeded by two floor beams that had been positioned right over the top of the engine, and it was impossible to get a wrench on some of the nuts. The beams had to come out, so I used my circular saw to make 45 degree cuts on each end of the beams and remove them. Later, after the engine was repaired I glued a sliver of wood on the beam ends to make up for the thickness of the saw cut, then fitted guides on each side of the joint to prevent the beam slipping off and they now just rests on the slanted ends.
Even after removing the beams, to be able to get to the heat exchanger I first had to remove the intake manifold and then the heavy cast iron exhaust manifold, to be able to even reach the nuts securing the heat exchanger… It is at times like this when I would have liked to get my bruised knuckles around the necks of the people who designed this engine!
Three hours later I had the heat exchanger in my hands but it was not easy to clean the large device, with its honeycomb of tiny tubes inside carrying the engines fresh water to where it is cooled by the flow of seawater, that is, when there is any.
I counted about 100 1/8 inch diameter honeycombed pipes inside the body, 
but only about 30 capable of passing water. The rest were blocked with sludge. No wonder poor old “Perky” didn't have any strength, he needed multiple bypass surgery
I bought a 24” inch long 1/8” inch drill bit, then carefully rotated it by hand down every tube I could get to and extracted reams of dirt from each. Some of the tubes were so solidly ingrained that I had to drill them clean using an electric drill. This is called “rodding” the tubes, and needs to be done very carefully indeed, because if the tubes become punctured with the drill, freshwater and seawater will mix and the expensive heat exchanger becomes useless. I then immersed the whole thing in a bath of Rydlyme Marine dissolving fluid. This is a descaling liquid used by diesel engineers to clean inaccessible parts of engines. Then I pressure washed the pipes until clean water flowed through both the seawater and freshwater tubes.
It was clear that debris had permeated through the whole seawater system, no doubt as far back as the exhaust elbow. I think this must have been building up for a long time on our trips in the ICW, but it is a wonder to me how the engine actually kept running with such blockages in essential pipes
Another item to inspect was the thermostat, but it wasn't where the engine manual said it was. This was when I then found out that my so-called “British” engine was actually a “North American model,” and the thermostat was located under the large header tank at the front of the engine. When this tank was removed the thermostat was there, covered with great black blob of muck.
By now things were completely out of hand and I had bits of the motor all over my garage. I thought; “In for a penny, in for a pound,” and decided to remove the main freshwater pump to be able to examine the inside of the block, but to do this I had to remove the thermostat housing bolted on top of it.
All these heavy cast iron pieces were fastened with rusty nuts and studs into the block that had probably not been unscrewed in 45 years. Some were so welded up with rust I needed a long socket handle that I whacked with a hammer to break them loose. These I threw away and bought new fasteners. I also had to cut many of the hoses and lever them off the pipes. I had no Perkins part numbers for these so I took the old hoses to my local auto parts store where I rummaged through the many pipes in their storeroom for similar shaped pipes. New gaskets from the Perkins dealer cost $130.
I discovered that some of the hose clips had actually snapped, leaving the pipes holding on by corrosion and simple friction. The most serious of these were two 4 inch diameter clips securing the exhaust elbow to the large exhaust muffler. These were both broken and the joint was literally held on with corrosion. If it had parted hot exhaust would have entered the boat and easily caused a fire.
The correct type of hose clips for pressurized engine pipes are the sort that have indentations where the worm drive engages in the continuous stainless band. The clamps with open slots are thinner and weaker and liable to fail over time. I 
replaced twenty one of these weaker clips.
The engine block freshwater drain taps were completely seized-up and impossible to open. They were unscrewed from the block, dismantled and cleaned, then the holes in the block reamed out with a drill bit releasing a torrent of filthy brown water. Clearly, the block also needed cleaning out so I tipped all the remaining Rydlyme Marine fluid into it and left it overnight in hopes of dissolving much of the ingrained grime and silt.
By now the engine was completely naked, with hardly anything on the sides or ends of the block. After cleaning everything with degreasing fluid I spray painted the block and all the parts with blue engine paint, ready for re-assembly. I wanted my hard work to at least look good.
I would mention here that all this was in the height of the summer in Florida, with daily temperatures in the 90s Fahrenheit and sometimes over 100F. All I had for protection against the blistering heat was the air conditioning that managed to keep the inside temperature at around 80F, otherwise I would still be at it.
REASSEMBLY.
After draining the dissolving fluid from the engine block, which from the looks of it had done an excellent job, I began the reassembly in the reverse order of dismantling - more or less. I couldn't remember which pipes went where and I felt like one of “All the King's men, who couldn't put Humpty Dumpty together again.” It was a good thing I had taken photographs as I went along.
On the port engine side, first the heat exchanger was relocated along with its four connecting hoses, one of which was a long copper pipe coiled around the back of the engine to the other side. Then came the bulky and heavy exhaust manifold that had to be bolted to its equally massive exhaust elbow with new hose clips to secure it to the muffler. Then I bolted the inlet manifold to complete that side of the motor.
On the starboard side the all important raw water pump was re-bolted in place with yet another new impeller and glycerin grease just for good measure. I decided to fit a second water filter directly after the impeller pump to catch any the bits from any future impeller failure and anything else that managed to pass through the water pump.
The difficult to reach oil cooler was also replaced and its pipes connected to the engine and gearbox.
After double checking all the clamps I then primed all the pipes with water from the seacock through both heat exchangers and exhaust. This would ensure that a circulation would occur the moment the engine fired up, instead of the impeller running dry even for a few seconds.
The freshwater pump was reconnected to the front of the engine with the thermostat housing and a new thermostat. I then filled it with fresh water, ensuring that water filled the cylinder head. I checked for leaks in all the pipes and gaskets and thankfully there were only a few hose clips to tighten. Then the header tank was refitted and filled, the alternator reconnected and the belt tensioned. The rebuild was now complete - well, nearly.
I had drained all the oil out of the engine sump, by hanging upside down into the bottom of the engine to unscrew the drain plug. I replaced it with a 90 degree elbow and a reinforced pipe up to a hand vacuum pump so changing the oil will now be much easier. I filled the engine with new oil along with a new fuel filter. The transmission was also drained and filled with new oil.
It was now nearing the moment of truth, to see whether the engine would start, but much more importantly remain at its nominal operating temperature of 180F. I opened the seacock and gingerly pressed the start button and the engine fired, and all systems were suddenly, go!
Within seconds 60 psi was showing on the oil pressure gauge. An absolute torrent of water was also gushing out of the exhaust pipe, more than I have ever seen on any single engine boat I've ever been on. But then, so it should be because the whole water system had been completely dismantled and meticulously cleaned, probably for the first time in nearly 40 years
It still took a few anxious minutes for the water temperature gauge to begin to move. It slowly rose to 180F then stopped as the new thermostat opened, allowing for full circulation of water throughout the engine. Whew, what a relief! I engaged forward gear and slowly increased the engine speed to 2000 rpm, and with mooring lines bar tight the temperature remained steady.
Taking Britannia out for a longer trial run was next, and with the boat plowing along at 2,250 rpm, (maximum break horsepower), the engine temperature remained constant. When it had cooled down I drained the freshwater out of the block and refilled it with a 50/50 mixture of Ethylene Glycol, (antifreeze) and distilled water. Apart from freeze protection, an antifreeze mixture boils at a higher temperature of 230F than water alone.
This was a long and very difficult revitalization of a 40 year old engine. It is a fine testament to these heavy old motors that it ran at all. However, I have to say, I wish both the engine and boat builders had given more thought to their customers who had to work on the boats. Many items could have been positioned for much easier access
Britannia had been immobile since early May 2022 and it was now mid August. Being retired, (that is, from an income earning job, not a boat), I was able to work on her at my leisure, but delays in delivery of parts, repairs, weather, etc., all took their toll on my time. My actual work log showed 130 hours for this rebuild.
You can be sure I will be very careful in future, to (a), avoid groundings and, (b) if I have one, to immediately check the water inlet filters. I certainly don't want a repeat of this hard labor or the costs.
RESTORE YOUR ENGINE COOLING SYSTEM