Solving the Shearwater Cooling Problem

Just about a year ago, I had the Shearwater’s bottom painted. This was one of only a few times I haven’t done this task myself.

Almost immediately after the paint job, the engine began to overheat. While several people had suggested the overheating may have been caused by the bottom job, I didn’t see the obvious.

Eventually I went under the boat with a mask and discovered the painter had covered nearly the entire thru hull strainer with enough paint to severely restrict the water flow, causing both an overheat condition and eventually causing the impeller to fail by slipping on its shaft. I pried off as much paint as I could and removed several fins. The overheat situation improved but wasn’t perfect.

During the process of finding the overheat, we tried to remove the thermostat. The easiest way to test a thermostat is to remove it – if the problem goes away, you found the cause.

Originally, I went to the parts manual and service manual to “find” the thermostat.  There it was, exactly as I expected it, on Page 24 of the parts manual (part: 165001-35710) with the water pump parts. I removed the assembly and there was no thermostat inside.

So, I did what every retired engineer did, I asked for help. Fortunately, help came from another ranger owner and from the Yamaha Distributor in South Florida. who pointed me to Page 27, where the print called for a second thermostat.  Apparently Yanmar fits some engines with a page 24 thermostat and some with a page 27 thermostat. (see part number 15: 120651-44520).

Keep looking, it was not there either.  A third thermostat is show on figure 11, as part of the turbocharger assembly. Finally found the engine’s thermostat, as installed on the Shearwater.  See part 19, 10650-44570 in the figure below.

This time, there was actually a thermostat.  We removed it, it was jammed, and we decided not to replace it until the engine was proven to run below normal temperatures at high rpm. The last two thermostat “removals” were from places too tight for my aging fingers so while the shop was hunting for the thermostat, they sent the heat exchanger off for cleaning and pressure testing. No problems were found.

After spending a lot of money and time, and not solving the problem, I had the Shearwater pulled out of the water by the shop, and discovered:

Removed Thru Hull
Thru Hull with Excessive Marine Growth

In addition to the clogged fins, the entire internal flow path was clogged with marine growth.

The shop removed the thru hull and replaced it with a larger thru hull mushroom. We speculated that since the boat lives in a slip in a drainage canal from the Okeechobee Waterway, there is a foul combination of salt and freshwater organisms that caused the excessive growth. This growth and the paint caused the reduced water flow.

The thru hull was diameter was raised to a 1.5″ bronze fitting, about twice the water carrying capacity. The original factory fitting has proven a problem on several R25 that are ran at higher speed in South Florida waters where marine growth is a challenge.

New 1.5″ bronze thru hull.

While the clogged thru hull was being replaced, the shop also installed a new 1.5″ bronze seacock and strainer. They removed the elbow that was previously in the raw water path and found it partially clogged with dead marine growth. The new straight line strainer, seacock, and thru hull can all be cleaned as necessary by inserting a 1/4″ threaded rod in the open sea strainer and “filing” out the marine growth. The top of the sea strainer is located at the water level so it is easy to clean as necessary.

Straight thru water screen with no elbow and easy cleanout

Located on the left side of the image is the new flow sensor that, hopefully, will detect a clogged sea strainer before the engine or muffler are damaged.  I have also installed an exhaust overheat sensor that should detect exhaust gas overheating because of low water flow.

Finally, it was nearly impossible for me to remove and replace the heat exchanger cap since it is located under the door sill. As you can see in the photo below, I have trimmed as much fiberglass as I felt could be safely removed so it is possible, but difficult, to remove the cap.

Heat Exchanger fill cap under door sill

You can see the cap stored temporarily in the oil fill area.

We fixed the engine coolant fill access problem by replacing the overflow bottle with a pressurized stainless steel overflow container that is operated at engine coolant pressure. This container retains and replaces the small amount of coolant that is expelled as the engine heats, returning it – and any makeup coolant needed for leaks – to the top of the engine.  I removed the pressure seal ring inside the heat exchanger cap and installed a new pressure seal ring on the new container.

The slick looking polished 2-liter jug is bolted just aft of the domestic hot water heater and is easily accessed without moving anything, injuring myself, or potentially spilling or dripping coolant anyplace inside the cabin where the sole would be damaged.