Engine freshwater flush?

[tobnpr]

The raw water strainers for my engines are equipped with a gate valve connection for a garden hose, presumably for a freshwater engine flush. I've never used them; two questions...

I recently used Rydlyme to clean out the raw water sides, how many of you flush with freshwater? Given that the raw water side components are designed for salt exposure, is flushing necessary more than occasionally for removal of salt/scale buildup?

Second, I am concerned about "pressurizing" the raw water side if I try to use the garden hose connection on the strainer. Assuming I were to close the seacock and turn on the garden hose, couldn't I force water past the impeller (before the engine is started, and after it is shut down) and into the engine through the exhaust side, damaging the engine-kinda like installing a scoop intake for a genset and then running the boat without the gen running?

Would it be better to run a short length of hose from the gate valve to a bucket, and let a hose fill the bucket instead (assuming the engine doesn't need more water then the hose can supply)?

 

[Four Suns]

I've thought through this and this was what I determined in my own head (right or wrong). I'm also going to refer to this Sherwood plot (I have 18000 series and I think you have 17000 series):

If your impeller is good, there is little chance you are going to have a significant amount of leakage past the water pump. You can see that the rubber fins would have to bend and I think that is more pressure than the hose is going to put out.

Even if there is no leakage, I don't think it's a good idea to run these bigger pumps/engines with a garden hose hooked up to the raw water cooling. IF you have the 17000 series, at idle the pump is going to be trying to put out ~100 lpm (26 gpm) per the chart above and I know here at home, my garden hose puts out about 20 gpm. For my engines, the pump is trying to put out ~46 gpm which would really starve the pump. When these rubber impellers are starved for water, the blades will distort and flip direction and usually not go back to their correct orientation (I've done this experiment on my generator last year when trying to find the flow restriction from crap in the line). The short story is that any starvation of water can, at a minimum, distort the impeller and shorten it's life. Worst case is the impeller heats up and melts or sets shape a little... Here's my great picture of not opening a sea cock:

Even if you have use an open bucket and have a bigger hose that won't starve the pump, it would have to be one big honkin' bucket...

The only reason I thought about doing this is to extend the life of the zincs. I think it's viable on the generator as that heat exchanger has a short zinc and only lasts about 30 days so doing a fresh water flush would probably help that if the boat is going to sit a few months. I think the mains, at least on mine, have pretty heavy duty metals and keeping the zincs fresh should be good enough.

My 2 cents.