Strainer Flush Fitting Questions

[bdunigan]

My set up: twin QSB6.7 380 with SB flush fittings mounted to engine intake strainers. Sherwood pump P2708 w/ 27000K impeller. Flush fittings just installed. To be tested shortly. I have watched the SeaBoard video on ‘How to Perform an Engine Flush’ and have a question regarding the recommended method of leaving the seacock open and running fresh water into the strainer with engine running. The video states that I will get 80-85% fresh water into my engine (s).  A Sherwood chart shows the 27000K impeller pulls 10-12 gpm at idle (500rpm). Now let’s say the dock water being used runs at 5 gpm. The math suggests if the impeller is pulling 10-12 rpm at idle that it will use all of the dock water (5 gpm) provided and the difference in seawater…say 5-7 gpm…or 50%+ in seawater. At least on paper this sounds right but is it? I plan to first test the impeller demand at idle using the bait tank method to see if I can get an idea of the ‘real world’ pull rate.  

1 user thanked author for this post.

russkarns

Topic

[donald roth]

Simple test of dock water supply is to start the motor and watch the hose.         <br />If it collapses, your supply is probably marginal, but if the hose stays round, you’re good to go.

[bdunigan]

Brad, I like your idea but would have to see it in action. There are plenty of videos re saltaway for outboards but I haven’t seen anything for diesels like ours. I would also have to get some kind of endorsement of this process, a how to guide, and some assurance it works and doesn’t cause any issues. 

[bdunigan]

I may add the deck fitting later as I improve the overall system. For now, a hose goes to each strainer. These hoses are led through the aft engine bulkhead into the pod bay where they tie into a y valve. A single hose goes from the y valve to the dock. When not in use, that hose is simply stowed in the port cockpit lazerette (still connected to the y valve). To flush, I need only extract the hose and connect to dock water.   

[Rob Schepis]

Good stuff.  You took nothing for granted.   Are you attaching the hose direct to the strainers or did you go with deck fitting(s) ?

[bdunigan]

OK, here is an update. I bought a water pressure gauge and a water flow gauge. Using these, my dock water pressure is 60psi and pumps 15gpm…plenty to handle the 10gpm my QSB6.7 units demand at idle (600rpm). Once I plumbed 30′ of hoses, y valves, etc to the flush valves, I still had 13.5-14gpm. I used 3/4″ hose. My sense is that I am pumping more fresh water than the engines demand and the excess is going out of the open intake. A ‘taste test’ of the exhaust water is totally fresh water. I was concerned about putting too much pressure into the Groco strainers (plastic bowls) so I called Groco. They calculated that, with the intakes open, 15 psi at the flush valves translated into 7 psi in the strainers….way less than the 30 psi it takes to crack them.

 

[Brad Arrington]

Just a thought – perhaps use an in-line dispenser to add salt-away to the flush water.  That would help reduce the impact of any residual salt coming in from the sea cock.  

[bdunigan]

Thanks, Steve. I plan to do a test as soon as our marina dock water is turned on. I also plan to test with 25′ of 5/8″ hose as I suspect the flow is reduced proportionally to hose length and ID. I may also test with 3/4″ ID hose. I will also measure the gpm intake rate of the engine at idle using the bait tank method to see how fast the engine ingests a given amount of water (with seacock closed). I want to use the ‘seacock open’ method in the future and hope my fresh water feed will meet or exceed the gpm demand of the pump & 27000K impeller at idle. 

[Steve Lewis]

Just a thought…..

BD,

I wonder if your dockside water flowrate is correct?  Our dockside flow rate is actually ~10gpm.  I timed filling a 5 gallon bucket and it took ~30 seconds.  I understand that my situation might not be the same as yours but I figured I would see if you did the same.  I did that exercise to understand how much water I can put in our water tank each minute.  so we are flowing 10gpm and I can get the amount of water I want to carry accordingly.  If you are flowing water at the spigot close to the rate we have then your fresh water concentration will be a whole lot better than your calculations.

It might be a whole lot easier to time your dockside water than the water flow rate on the engine…..

 

[bdunigan]

Thanks, Gene, for the reply. On the 6.7, the sea water pump pulley goes to a smaller idler pulley. However, the idler is driven by the much larger crank puller so I suspect both the sea water pump and the alternator run at higher rpms than the engine itself. Not sure though.

[Gene Fuller]

I do not know the exact numbers without going to the boat and measuring, but I believe the raw water pump runs quite a bit slower than the engine RPM. Take a look at the belt photos on this site. Small pulley driving large pulley.

[Author]

Replies