I installed a flush port on my C26. This was used anytime I was not planning on running the engine for a few days and the boat was in salt water. I had a ball valve attached to the sea strainer so I could attach a freshwater flush hose to it. This was my procedure.
I used a 5 gallon bucket I kept on board for a wash bucket and also used it for a flush bucket.
I used a clear reinforced 3/4" ID hose 10' long that had garden hose fittings attached at each end
The hose was attached to a garden hose fitting that was connected to one end of the ball valve at the sea strainer.
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I placed the 5 gallon bucket on the swim platform and filled it full of water and placed the other end of the 3/4" hose into the 5 gallon bucket
I started the engine with the raw water thru hull valve open and ran the engine for a few minutes with seawater
I turned the shore water spigot back on and the water would over flow out of the bucket on the swim platform
I opened the fresh water flush ball valve and when I saw the fresh water entering the sea strainer I closed the thru hull seacock valve. The engine was now pulling from the bucket and pumping freshwater thru the system at about 5 gallons a minute
The shore water spigot used with a 5/8" hose will not keep up for a long time I would let it run for 2 or three minutes and then the bucket would start to run dry. At that point I shut the engine down. I knew I had flushed all the salt water out of the raw water cooling and replaced it with fresh water.
I closed the flush valve and left the through hull valve closed too. I would attach the "KEY FOB" chain and float to the through hull valve handle so I would remember to open the valve before starting the engine.
This procedure took about 10 minutes from start to finish.
I agree that using a hose direct to the system could over pressure the pump seal and also possibly enter water into the exhaust system if you are not careful. The procedure I used would prevent both from happening.