Perkins 4-108 0verheating

Hello Pirate and Stirfry -- Our heat exchanger was remanufactured by the previous owner in 2003, and includes a pencil zinc, which is located on the bottom port side of the heat exchanger. I don't know what size the bolt is -- but it is a brass or bronze bolt head that goes into the main rounded body part of the heat exchanger (not the end cap bolts). The original heat exchanger had corroded away, so presumably the new one had a zinc included to reduce that corrosion problem. So other boats of this age (1978) might also have a similar situation with a remanufactured heat exchanger and pencil zinc.

We don't seem to have a big electrolysis problem in our marina (not like the one the boat lived in for many years). I check the pencil zinc about twice a year, and replace as needed. The first time I took the zinc bolt out, it was completely gone. In the past, I replaced it when it was almost all gone; I will probably replace more often now that I know it will corrode and clog the raw water outlet. As for the shaft zinc, I replace it about once every 12-14 months, and usually about 25% of the zinc is still there. So your problem might be your marina, but also might be what Pirate indicated -- something to do with if "the engine is properly bonded to the boat's main anode", which I will check on my boat as well (Thanks Pirate).

Dave

 

Tachometer accuracy

I know the thread is way old but it pops up on Google, so here's another thing to consider: tachometer accuracy. We had the same problem as Jim, prop overpitched, but it was masked by an inaccurate tach: boat new to us, plus new batteries and regulator left the tach reading about 30% too high. So we could reach cruising speed (though never as high as I would have liked) at what appeared to be a reasonable engine speed, but were in fact lugging the engine. Got an Aetna tach, repitched the Maxprop, problem gone.