Can you describe your fresh water flush setup/procedure? How long does it take to perform?
Re: Rydlyme -- my understanding is that unlike generic scale solvents (CLR, muratic acid, etc.), Rydlyme is specifically engineered for this type of use, and will not attack metals.
To fresh water flush here is what I do:
1. After the engine raw water seacock, I installed a "T" fitting with a shut off ball valve. From this install a nipple and a hose long enough to reach your cockpit. Take a bucket and install a hose nipple at the bottom of bucket.
2. When I return to the dock, I keep the engine running and then run hose to the cockpit and attach to the bucket hose nipple. Fill the bucket with fresh water. now open the ball valve located at the "T" at the raw water seacock. Make sure the hose an bucket are full of water (no air pockets). My hose runs nearly stright up from seacock to the bucket so all air is purged when bucket is full of water.
3. Shut raw water seacock so that now all cooling water is coming from your bucket full of dock water. Run the engine for 10 minutes while topping off your bucket with dock water- do not allow bucket to go dry. For my 3gmd at ideal it runs maybe 2 gpm. You are now doing two things, flushing the engine and letting it cool down (manual says to cool engine for 10 mins prior to shut down). Just before shutting engine down I do a 30 second full throttle to blow out carbon per Yanmar instructions.
4. While I am keeping bucket topped off, I am also rinsing the boat deck and rigging down to save time.
It is important to flush as above and let the engines raw water pump to flush the engine. Never hook a hose directly to engine or the raw water suction side of pump. The hose pressure could damage engine. My dock hose has 100 psi - way too much for engine- might even blow some engine gaskets or freeze plugs out.
I have heard tha Ryde Lyme is ok, still a little afraid to use it though. If it ain't broke don't fix it may be true here.