Impelling evidence

The original impeller at circa 20 hours runtime. One of those vanes is partially torn at its base.

Sometimes on a boat you can't determine what a big problem is, but you can identify a number of small problems that may be performing a decent impression of the big one. The big problem in this case was a lack of cooling water throughput when the engine was switched out at launch. Preoccupied as I was with clearing the slings and getting to our dock on a day that's never described as "relaxing", I didn't notice the absence of water sluicing out with the exhaust. But I did notice that the engine temperature gauge was at about 70C after a rather short run.

Investigation via eyeball confirmed I wasn't getting water through. Generally, I would make the following assumptions:

1. Some sort of blockage in the seacock or standpipe, up to and including "forgot to open engine water cooling seacock". (This was ruled out quickly).
2. Some sort of blockage in the hose to the Perko water strainer I've described before.
3. Some sort of airlock, probably at the "top" of the cooling circuit (which would be the Perko) keeping water out of the "down" side.
4. A failure of the impeller to impel or of its related camshaft.
5. All of the above.

I've learned that boat maintenance has a forensics flavour (if not often a smell) in that chains of causality must be pursued if the correct fix is to be determined. Such was the case here.

1.  Using a bottle washer on a stick, I plunged the standpipe. Some, but not much, vegetation stuck to the bristles. 
2. I removed the hose going to the Perko and gave it a good blow. Attached as it was to a column of water in the standpipe, it amusingly spat lake water back in my face. Apprised of various laws familiar to Boyle and Torricelli, I learned the hose, at least, wasn't particularly mucky or occluded.
3. I opened up the Perko strainer and examined the filter element. Quite a few small weeds, broken shelly bits and some sand were visible; I disposed of them. Probably from last fall's grounding, I thought.
4. I puled the impeller. Huh. Signs of wear, but not failure.

Best to do this right. I got out the spare.

Although one should avoid running dry, this version will do better.

The impeller pump's cover plate was a little worn, but not scarred or pitted. I suspect some sand got past the Perko and that, along with the vigorous throttle action associated with the grounding, had caused the wear.

Yeah, it's time for Speedseal to get my order.

I cleaned up the inside of the cover plate with Scotchbrite pads. It didn't needed sanding or anything particularly intensive.

Good to go with the right gasket.

Further viewing of the old impeller showed it was probably fit to save as an emergency spare. The tear wasn't complete, although it argued for the same "between pump and block" filter basket as I had on Valiente.

An argument for removing the impeller over the winter, I would think, but that complicates the winterizing a bit.

Although the old gasket came out undamaged, I decided to use the new one that came with the run-dry impeller, saving Old G for future service. So I laid a thin coating of grease over it and packed it away. The new one got the same treatment. It helps to get it to stick to the plate as the cover is being restored.

A last confirmation that the new impeller was the right one (yes, prudent seamanship has elements of paranoia) and back it all went.

The red/silver stick to the right is a large dental-type mirror. This gets used a fair bit aboard.

Not pictured is the "cable-tie trick", whereby a cable tie is used to compress the impeller vanes to get it into the pump housing, after which it is slipped off. Another Internet forum trick pays off.

Restored! And yes, not dropping any of the six slot screws (why?) into the bilges was in fact an achievement.

Once reassembled and the Perko primed (by pouring water into it until the level passed both inlet and outlet apertures and then screwing down the gasketed lid to prevent air getting in), the engine gratifyingly produced gushes of raw water out the exhaust. All was cool and collected.

Did I just lose prime? Was it the damaged vane? Did the vegetable and mollusk muck play a role? I can't say if there was one cause or several in concert. But I feel better that the problem of "no throughput" led me to a methodical process, because having plunged, blown and mucked out, I feel I'll likely get better, longer service from the new impeller.