blocked riser?

[LARRY VALENTINE]

Anyone with any experience with a riser blockage in a DD 6V92TA, 53 ft Hatt Motoryacht, 1985
vintage? The engine overheats when in gear for about 15 minutes, then drops to normal temp when
put in neutral and kept running. The riser is cast iron. When observed by removing the pipe from
the heat exchanger where it enters the riser it appears that small peripheral holes in the riser seem
partially blocked by rust and calcium or salt deposits. Where do you get a new riser? Any way to
clean the holes? There is only about a three inch access to the riser interior where the raw water
is injected into the riser if one were to try drilling the holes.

The system leading to the heat exchanger has been explored with new impeller and is open to
the exchanger with a replaced heat exchanger which was nearly blocked with calcium deposits.
Any problems downstream of where the raw water meets the exhaust in the riser would require
a boat lift because that remains below the waterline.. The boat was last bottom painted a month
ago when the barnacles and oysters were removed,the through hull lever opened and closed and
a new bronze strainer holder inserted. I cruised back to my house afterwards and because the
engine overheated I found my impeller was shredded. It was then replaced with rubber fragments
removed. { air lock}

 

[MikeP]

I'm not clear from the description but if you are describing the "showerHead," which has peripheral holes around its circumference and introduces the cooling raw water into the exhaust stream. If the holes are partially clogged, that could produce exactly the symptoms you describe. The water flow at higher RPM would be too obstructed to properly cool the HE/engine while at low speed it would be fine.

The showerhead can be removed and the holes cleared. But it should be removed as opposed to trying to clear it from the outside while in-place.

 

[SEVEN]

Mike are those able to be boiled out? I do not have the issue but would like the info for future just in cases.

 

[SeaEric]

A fowled heat exchanger can produce that same symptom.

 

[oldawg]

A fowled heat exchanger can produce that same symptom.

Like one with a chicken stuck in it?

 

[MikeP]

Yes, a fowled HE would cause the overheating the same way but wow, what a mess...chicken parts everywhere.

I pulled our shower heads not long after we bought the boat and found several of the holes plugged with what looked like small rocks. The "rocks" were loose. IOW, they would fall away from the holes when the engine was not running but plug the holes when water pressure forced them into the holes again when the engine was running.

They may have been formed from mineral deposits and, if so, boiling would have dissolved them but at the time I thought they might have been zinc chunks. In any case I just broke them up with long screwdrivers, etc

 

[LARRY VALENTINE]

I'm not clear from the description but if you are describing the "showerHead," which has peripheral holes around its circumference and introduces the cooling raw water into the exhaust stream. If the holes are partially clogged, that could produce exactly the symptoms you describe. The water flow at higher RPM would be too obstructed to properly cool the HE/engine while at low speed it would be fine.

The showerhead can be removed and the holes cleared. But it should be removed as opposed to trying to clear it from the outside while in-place.

Mike , thanks I assume the heat jacket removal allows access to the "showerhead' because the
only access I now see is through the raw water injector 3 inch pipe which allows only a glimps
of some of the blocked holes in the showerhead and will not allow opening the holes. Do you
have to haul the boat to get to the showerhead?. We did extract about a 3 inch zinc pencil " floating"
in the showerhead but that did not solve my problem of overheating. I have also tried multiple
gallons of CLR, Metal Rescue and now phosphoric acid with no change in the raw water flow. My
mechanic will return in a week to recheck the entire raw water system leading to the Showerhead.
He says the worse case scenerio is a SSrebuilt system in the range of 10K.. When we first
0pened the showerhead access a garden hose at at full flow, it spilled back. Now it will allow full
flow without kicking any water back....but it still overheats at any RPM when in forward gear.
going to a higher RPM in neutral brings the temp back down. I cannot imagine the fresh water
thermosets causing this problem.

 

[johnjen50]

Ok, I'm confused. You said the impeller was shredded because the engine overheated. If your impeller was really shredded, that's because it was not getting enough water. Of course then the engine would overheat, but I think you are looking at the wrong thing. If the impeller wasn't getting enough water and shredded I would look at the thru hull inlet to see if the bottom painters painted over the holes. If the impeller was getting water it would not have self destructed.

 

[johnjen50]

Re reading your post I'm thinking that the yard that did the work messed something up. Was the boat running fine prior to the bottom paint, and barnacle removal???

 

[oscarvan]

Where are you getting the "overheat" diagnosis? From the gauge? Is it really? IR gun, in line gauges..... data is everything. Get the data (right) and narrow it down. Process of elimination. Think.

 

[LARRY VALENTINE]

Ok, I'm confused. You said the impeller was shredded because the engine overheated. If your impeller was really shredded, that's because it was not getting enough water. Of course then the engine would overheat, but I think you are looking at the wrong thing. If the impeller wasn't getting enough water and shredded I would look at the thru hull inlet to see if the bottom painters painted over the holes. If the impeller was getting water it would not have self destructed.

There was an airlock after leaving the haul out at the marina. That shredded the impeller.

 

[LARRY VALENTINE]

A fowled heat exchanger can produce that same symptom.

The heat exchanger was replaced by one that had been professionally cleaned and looked like brand new.

 

[LARRY VALENTINE]

Re reading your post I'm thinking that the yard that did the work messed something up. Was the boat running fine prior to the bottom paint, and barnacle removal???

Yes There was a crack in the plastic jacket that holds the SS strainer. There was no plastic container available so I elected to have the whole mechanism replaced except for the thru hull valve. It was bronze and cost about $500.
We have opened the strainer retrieving port with the thru hull lever open and water flow seemed adequate. A new impeller was then installed.

 

[LARRY VALENTINE]

Where are you getting the "overheat" diagnosis? From the gauge? Is it really? IR gun, in line gauges..... data is everything. Get the data (right) and narrow it down. Process of elimination. Think.

Both guages on the flybridge as well as thepilot house show 195 before the alarm goes off. Heat gun showes 190 degreed on fresh water side of exchanger with 100 degrees at raw water inport to exchanger , 175 degrees leaving
the exchanger[raw water].