hpeer wrote in news:494ecf5f$0$5535
:

Now that you have me thinking about it, I wonder if the strainer had
gotten air in it? I don't know.

Thanks for the response.



Isn't the seawater pump under the waterline? Is there anywhere in the
hoses leading to the seawater pump that's above the waterline from the thru
hull fitting? That would keep it from priming properly and might get an
air bubble trap that can't be overcome.

If the thru hull valve is open, and I pull off the hose from the seawater
pump, I wanna see seawater come gushing out of that hose, unattended. See
if yours does it. I want the strainer underwater, too. Damned air
conditioner installed in Geoffrey's old Endeavour sloop used to lose its
cooling water prime because of where the idiots mounted the strainer. I
moved it all under the waterline...problem solved. Pull off any hose and
prepare to be DROWNED.

hpeer wrote in news:494ed38b$0$5541
:

I do (at least at times) get water flow through the system with the
engine off. This MUST be due to a siphon effect. I can't swear that it
happens when I am having the overheating problem.



There you go. You don't want ANY water system aboard depending on
something running on a siphon to make it go. That's just an accident
waiting to happen.

On Sun, 21 Dec 2008 22:50:10 -0500, hpeer wrote:

Well now, you've got me digging into Calder. While the text does not
specifically say to install the strainer below the water line the
diagrams, although fairly diagrammatic, surely tend to indicate that is
the way to do it.

I've never seen one installed any other way.

Since that engine is worth a lot more than a strainer, I'd ditch the
home made one and get the real thing while you are at it.

"hpeer" wrote in message
...
Capt. JG wrote:
"hpeer" wrote in message
...
I have Volvo MD7A, 13 hp two cylinder unit with salt water cooling.

Runs perfectly with one exception, it occasionally overheats and I can't
figure out what the heck is going on. I bought the boat used and the
previous owner had fitter her out himself. Every time she overheats, I
find something to "fix" after which she is fine for a spell then she
will overheat again. Clearly I'm not getting to the root cause.
Generally she will seem to be running along just fine and all of a
sudden the pressure gauge just goes up, up, up. It seems to be a
bi-state kind of thing, it either works perfectly well or not at all.
Its almost as if there is a ball valve or something somewhere that is
sticking. It's obviously not a bad gage.

The plumbing goes like this:
Through hull fitting
Salt water strainer
Tap to the transmission/engine
Tap to secondary pick up from bilge
Tap to sinks


Here is some of the history and what I had done, in each case I have
also checked/changed the impeller. They usually look just perfect.

1: Recently fitted salt water strainer seemed to be blocked with debris
from its construction. It is home built out of some interlocking pieces
of PVC pipe, the inner piece has a zillion small holes drilled in it. I
dumped this stuff out and she was fine.

2: Found a small leak in the copper pipe from the transmission to the
impeller. I thought that maybe it was sucking in air and the impeller
was getting air bound. Repaired the leak.

3: Moved and lowered the salt water strainer. This was mounted pretty
high on a bulkhead with real long hoses. I moved it to the engine
compartment and lowered it. I thought that maybe there was just too
much suction head to get the water going.

4: Threw thermostat overboard.

Any clues?

Many thanks,

Howard


Do you have water flow out the back when it overheats? Is it normal flow?

Also, I'm thinking about a failing raw water pump... not the impeller.
It's driven by a belt... is the belt snug?


No, the engine is not sucking up water. The bilge intake is used when
winterizing so that I can put anti-freeze into the engine. The last time
this issue/problem occurred was during layup and I was not able to get the
engine to suck up anti-freeze.

This particular water pump is on a little shaft on the back of the engine.
I suppose the pump could be failing to turn intermittently, I had not
considered that possibility. Seems kinda far fetched, but then again this
whole problem feels weird.


Seems like something that intermittent wouldn't be related to restricted
flow, since if it's restricted, it's not going to get better.



--
"j" ganz @@
www.sailnow.com

This installation diagram from Vetus on their water strainer
specifically shows that the strainer be at least 15cm / 6" ABOVE the
waterline.

http://www.vetusweb.com/manuals/file...01%2007-07.pdf

MW

On Sun, 21 Dec 2008 17:43:45 -0800, "Capt. JG"
wrote:

"Bruce In Bangkok" wrote in message
.. .
On Sun, 21 Dec 2008 15:38:40 -0500, hpeer wrote:

I have Volvo MD7A, 13 hp two cylinder unit with salt water cooling.

Runs perfectly with one exception, it occasionally overheats and I can't
figure out what the heck is going on. I bought the boat used and the
previous owner had fitter her out himself. Every time she overheats, I
find something to "fix" after which she is fine for a spell then she
will overheat again. Clearly I'm not getting to the root cause.
Generally she will seem to be running along just fine and all of a
sudden the pressure gauge just goes up, up, up. It seems to be a
bi-state kind of thing, it either works perfectly well or not at all.
Its almost as if there is a ball valve or something somewhere that is
sticking. It's obviously not a bad gage.

The plumbing goes like this:
Through hull fitting
Salt water strainer
Tap to the transmission/engine
Tap to secondary pick up from bilge
Tap to sinks


Here is some of the history and what I had done, in each case I have
also checked/changed the impeller. They usually look just perfect.

1: Recently fitted salt water strainer seemed to be blocked with debris
from its construction. It is home built out of some interlocking pieces
of PVC pipe, the inner piece has a zillion small holes drilled in it. I
dumped this stuff out and she was fine.

2: Found a small leak in the copper pipe from the transmission to the
impeller. I thought that maybe it was sucking in air and the impeller
was getting air bound. Repaired the leak.

3: Moved and lowered the salt water strainer. This was mounted pretty
high on a bulkhead with real long hoses. I moved it to the engine
compartment and lowered it. I thought that maybe there was just too
much suction head to get the water going.

4: Threw thermostat overboard.

Any clues?

Many thanks,

Howard


I am the process of rebuilding my own raw water cooling and have the
following comments about yours:

The usual configuration for the engine cooling is Thru-hull to
Strainer to xmission cooler to water pump to oil cooler to engine to
exhaust elbow and overboard.

You used the word "tap" to indicate connections to the transmission
cooler. This should be the entire flow from the strainer to the pump,
i.e., hose from strainer to xmission cooler to pump. The hose from the
thru-hull to the strainer to the cooler to the pump and onward to the
engine should be the full size of the thru-hull (if possible).

If you are going to install "taps" in the line for uses other then
engine cooling you MUST be sure that these taps are isolated with
valves and not used while the engine is running.

The main strainer should/must be located below the water line so that
with the sea cock open the strainer fills.

If this is an old engine and it has not been flushed after every use
in salt water, which is seldom/never done you probably have scale
built up inside the engine that could be blocking water passages. This
scale will not dissolve in fresh water, by the way. There are acid
flushes which will remove it but I do not have a formula. Perhaps
google if you feel this might be a problem.

Regarding water coming out the exhaust. It is not an accurate
indicator of anything except that water is getting through. In my own
case there ~appears~ to be normal amounts of water out the exhaust but
the engine overheated over 1800 RPM due to restrictions in the inlet
line to the pump.

By the way, you DO want a strainer in the line to keep stuff out of
the system. I have removed plastic bags from the main strainer as well
as small creatures, weeds, mud, small sticks and innumerable things
that you probably don't want in the engine or pump.
Cheers,

Bruce
(bruceinbangkokatgmaildotcom)


He said it was an intermittent problem... so does your situation reflect the
same sort of over-heating?


Yes it was intermittent... until the gunk built up so much that it
became continuous.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

On Sun, 21 Dec 2008 22:50:10 -0500, hpeer wrote:


If you are going to install "taps" in the line for uses other then
engine cooling you MUST be sure that these taps are isolated with
valves and not used while the engine is running.

There are valves and I will heed this admonition.



The main strainer should/must be located below the water line so that
with the sea cock open the strainer fills.

The original installation of the strainer was most assuredly ABOVE the
water line. I believe that it is still slightly above but I am not
positive.

Well now, you've got me digging into Calder. While the text does not
specifically say to install the strainer below the water line the
diagrams, although fairly diagrammatic, surely tend to indicate that is
the way to do it.

While a rubber impeller pump is self priming I believe it is advisable
to make the system as much self priming as possible. Perhaps I should
have said it is better to have the strainer below the waterline...
The sooner you get water to the pump the less chance of overheating
the rubber impeller.

I do (at least at times) get water flow through the system with the
engine off. This MUST be due to a siphon effect. I can't swear that it
happens when I am having the overheating problem.

You should have a vented loop somewhere in the system to prevent
siphoning when the engine is off. I don't know how your engine is
plumbed but most systems have a vented loop between the engine and the
exhaust elbow. The top of the loop is above the waterline with a vent
to some distance above the loop. If you don't have this it is likely
that sooner or later you will fill the exhaust hose and then the
engine with salt water.... which is a headache that aspirin won't cure
:-)

Well I now have one MORE thing to put on my summer to do list.

Make sure strainer is below water level.

Ideally the system from thru-hull thru the pump will be under water
and self priming.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

On Sun, 21 Dec 2008 22:45:50 -0800 (PST),
wrote:

This installation diagram from Vetus on their water strainer
specifically shows that the strainer be at least 15cm / 6" ABOVE the
waterline.

http://www.vetusweb.com/manuals/file...01%2007-07.pdf

MW
Yes it does but I can see no reason for it. To clean the filter you
turn the sea cock off so that can't be the reason and the way it is
specified every engine start is with a dry impeller in the pump.
Doesn't make sense to me... unless you like to change pump impellers.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

"hpeer" wrote in message
...
This particular water pump is on a little shaft on the back of the
engine. I suppose the pump could be failing to turn intermittently, I had
not considered that possibility. Seems kinda far fetched, but then again
this whole problem feels weird.

I have had this trouble on a petrol engine in my 22' motorboat and the cause
lay in the rubber impeller in the pump.
It is easy to examine the impeller and, seeing no wear on the ends of the
lobes, to conclude that it is Ok for further service.
However after some time the rubber goes soft and this means that the lobes
cannot spring back quick enough after passing the cam to allow water to come
in through the inlet.
You may get away with a soft impeller at low speeds but when you increase
revs you will lose suction..
Also, with a soft impeller the pump becomes more sensitive to back pressure,
which in extreme cases can allow back leakage past the ends of the lobes.
Your system seems likely to create quite a bit of back pressure especially
if the taps you have in the system are not gate valves or ball valves which
allow full bore passage through them.
You need this sorted out quickly because you are salt water cooled and every
time you get temperatures over 140 degrees deposits will come out of
solution and ultimately cause severe problems.

In message , Edgar
writes

"hpeer" wrote in message
m...
This particular water pump is on a little shaft on the back of the
engine. I suppose the pump could be failing to turn intermittently, I had
not considered that possibility. Seems kinda far fetched, but then again
this whole problem feels weird.

I have had this trouble on a petrol engine in my 22' motorboat and the cause
lay in the rubber impeller in the pump.
It is easy to examine the impeller and, seeing no wear on the ends of the
lobes, to conclude that it is Ok for further service.
However after some time the rubber goes soft and this means that the lobes
cannot spring back quick enough after passing the cam to allow water to come
in through the inlet.
You may get away with a soft impeller at low speeds but when you increase
revs you will lose suction..
Also, with a soft impeller the pump becomes more sensitive to back pressure,
which in extreme cases can allow back leakage past the ends of the lobes.
Your system seems likely to create quite a bit of back pressure especially
if the taps you have in the system are not gate valves or ball valves which
allow full bore passage through them.
You need this sorted out quickly because you are salt water cooled and every
time you get temperatures over 140 degrees deposits will come out of
solution and ultimately cause severe problems.



Another problem I have had with impellers is - they look OK, but the
central bronze part becomes un-bonded from the rubber, and starts to
spin without turning the rubber. This might be intermittent by having
different friction at different speeds.

--
Spike