engine overheating at 20 rpm+

What engines? What year? Raw water cooled or coolant cooled with heat exchangers?

Is plenty of water coming out of the exhaust? Your risers should be warm to hot, but not too hot to put your hand on.

If low water flow in exhaust, check the intakes under the boat. If those are clear I'd check/replace the impellers, and backflush the raw water side of the cooling system looking for debris. You could have rust in your exhaust manifold blocking cooling water.

More thoughts when I know what engines and how they are cooled... How mechanical are you?

Last time I had a gasser and had that problem it was the exhaust manifolds rusting up and clogging.
Mine were Chryslers, 360's I think, and the risers were done.
Same symptoms, I could run at cruise with no problems and overheat as soon as I backed down to idle.

I'm going to guess by your location these have been running in salt water. That engine runs raw (salt) water through the exhaust manifold, it is normal to replace the exhaust manifolds and risers every few years when run in salt water. It will take moderate mechanical skill and tools, probably should look for a mechanic.

You really need to be able to check for water flow from your exhaust, every time you start engines. This is not an option, unless you have money to burn on new engines every so often. I have sensors and alarms for water flow, but still check the exhaust. Check the oil and other fluids, check bilge, start engines, check gauges, check water flow, then go out.

Typical troubleshooting on the raw water side would be:
- cleaning the sea strainer (which you've done),
- changing the impeller (fishing out any broken vanes or other pieces while you're at it),
- backflushing the intake thru-hull (in case something is clogging that)...
- and then if necessary cleaning the heat exchanger (often that means removal and either mechanical and/or chemical cleaning)

There's a way to flush the raw water side of the heat exchanger in situ, using products like Rydlyme or Barnacle Buster... and that could address other segments of the raw water cooling system, if you have any (oil coolers, fuel coolers, aftercoolers, whatever)... in case there's a clog somewhere else along the line.

Probably useful to check the freshwater side: Thermostat working? Coolant levels OK?

And then with gas engines, as folks have said, it's typical to replace the risers and manifolds periodically. Gas owners in our club often recommend every 5 years in salt water...

-Chris

speaking re impeller changes on generators and mains. what is the consenses re changing impellers when running is fresh water only. we operate the boat about 4to 5 months a year with around 75 to 100 hrs. the yard wants to see them changed every couple of years. i was doing that but noted not much diffwrence between the new impellers and the ones we replaced.

That is cheap insurance if you've only got to do it every couple years.