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Hi Sailnet,

I've recently acquired a 30 year old Yanmar 3GMD which runs at 67C (152.6F) at the crankcase when revved to 1800rpm after a few minutes. This is a little hot for my liking as I'd like to try and get it to 2000rpm if possible - beating into 25knt wind in a very narrow and shallow channel will be impossible otherwise.

When I replaced all the zincs recently (there are 3) I noticed a massive amount of white scale which I'm assuming is zinc oxide inside the engine block, so I'm wanting to descale. I bought Rydlyme as after some research it seems the most gentle.

My concern is some of this white zinc oxide is visible on the outside of the engine. Does this mean there is a small leak in the raw water system which if descaled will turn the engine into a leaky bucket?

If there are leaks, is it possible to seal, say if part there's a crack or corrosion in the engine block itself?


Thanks a lot for your help,
Brandon
 

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I would guess you're looking at salt water scale, not zinc oxide.

Rydlyme is great stuff. You need to fashion a way to fully recirculate the product for a period of time. That may require pulling the thermostat and/or impeller. It depends on where you plan to enter/exit.

It was so effective on mine several years back that the scale clogged the pump. A filter of some kind would be a good idea, even if just a strainer than return fluid dumped through.

The exterior corrosion appears to be from leakage, so I'd try to seal those back up again. I can't tell from the pics, but if the paint had been compromised in those locations, they may simply be exterior corrosion.
 
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