Hi Ian
I went through your post and this reminded me what happened to me two years ago.
The symptoms were the same and being unable to find the reason of this breakdown drove me crazy for two days. I checked each and every pipe. As long as there were no counter pressure it was ok. Each time I connected all the pipes the pump did not work. The impeller was new. I changed the entire gasket set of the water pump. I checked every possible solution with no success until I fixed back the old impeller and, miracle everything went back to normal?
Incredible indeed. I took off again the pump impeller cover and while holding it between my fingers I realized that the inner cover face was uneven and slightly worn out. There was a difference of a couple of tenth of millimetres due to the impeller friction which left a print into the steel. In fact the old impeller was matching perfectly the small cavity conversely to the new one which did not. I then polished the plate surface to even it. I fix it back with the new impeller and miracle after two days of nightmare it worked perfectly. It has been working perfectly well since.
It was a very tricky issue which I managed to solve just by luck!
I hope this post will be helpful.
Erick

To: amelyachtowners@...
From: yahoogroups@...
Date: Sun, 4 Jul 2010 08:00:52 +0000
Subject: [Amel] Re: Genset Cooling Circuit Airlock




























Hi Ian,



I did not ask for the experience, but I have had a lot of experience with salt water flow and the Onan because I had 3 strange issues occurring at the same time:

(1) The stainless steel pipe had a suction leak.

(2) A high rate of impeller failure.

(3) Water-flow sensor failure.

At one point I thought they were related, but later found that they were separate. The water pump impeller shaft key was sufficiently worn causing the impeller to spin on the shaft, the pipe had a defective seam, and the water-flow sensor was filled with crud.



During these issues I found that the slightest amount of air entering on the suction side of the pump would cause a water-flow shutdown, a small amount of crud would cause a water-flow shutdown, and a missing or cracked impeller blade would also cause a shutdown. Recently, I found a 4th cause of water-flow shutdown:

"Y" fitting off the sea chest clogged very tightly with mussels.



The more I think about your problem, the more I am curious about your situation. I have disconnected the input hose to the salt water pump enough times to know that if the sea chest valve is open and the lines are clear of obstructions, gravity will cause water to flow freely through the hose when it is disconnected from the intake side of the sea water pump. Close the intake valve to the sea chest and the water flow at the intake to the salt water pump stops.



I believe that if water is flowing freely to the intake side of the salt water pump, it is impossible to have air replace the water in the salt water pump unless air is entering as the pump begins to operate, or possibly a cracked or broken impeller blade.



Like I said, I have done this drill more times than I can count. Here is what I would do:



1. Remove the hose from the intake side of the Onan salt water pump, holding the end over a bucket. Loosen the clamp on the hose where connected to the SS pipe. Twist the hose downward several inches. Water should begin to flow and continue to flow freely (no dribbles) until the sea chest valve is closed...If NOT, there is an obstruction. Clear the obstruction and go sailing. If this does not solve the issue, go to 2.



2. Buy a 2-3 meter piece of wire-reinforced 25mm hose. Disconnect the Onan 25mm hose from the "Y" on the sea chest. Using a piece of wire, inspect the "Y" carefully to ensure no obstructions then connect this new hose to the "Y." Connect the other end of the hose to the input side of Onan Salt Water pump. Go sail. If this solves the problem you either have an obstruction in the SS pipe or 25mm hose or you have an air leak in either or both (it is possible to have a small tear in the hose which does not leak water, but will allow air). If this does not solve the issue, leave the hose in place and go to 3.



3. Pull the Onan Salt Water pump. Change the impeller, carefully inspecting the shaft key to ensure it is not worn. Go sail. If this does not solve the issue, leave the hose in place and go to 4.



4. Remove the heat exchanger and have it properly cleaned; reinstall it with a new zinc (take care with the end caps as they are delicate and will probably need replacing). Go sail.



If this does not solve the issue, I have wasted your time, and for that I apologize. If it works, one Red Stripe, please.



I remain curious of your eventual solution.



Best,



Bill

BeBe, SM2k, #387

Currently Malaysia



--- In amelyachtowners@..., Ian Shepherd <sv_freespirit@...> wrote:

Hi Bill,

thanks for the suggestion. I have checked the stainless pipe and there

is no sign of any leaking water, though I have been unable to check the

aft end of the pipe yet as it is buried under storage boxes that I built

above the hot water tank.

If left at it's berth, the genset will always start normally with a good

supply of water. It's only when sailing that the water disappears. It

would seem that the forward motion sucks the water out of the system,

creating an air lock. Why water does not get forced back to the pump

when the boat is staionary and on an even keel is a mystery. Maybe if

the impeller vanes straddle the intake opening, then the air cannot pas

through the pump?

Two solutions come to mind. Install a scoop at the strainer intake.

(Haul out required and more drag), or install a non return valve just

below the impeller. I am dubious as to whether such a valve exists that

will withstand prolonged salt water immersion.

I have emailed Amel for help and will pass it along if they come up with

the answer.

Regards

Ian

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