• Mobile App For Android Now Online!

    Download it here. The app is searchable in the Google Play Store under Sailboat Owners.

    Sorry iPhone/iPad users, we are still waiting on Apple. :(

    Click the X in the upper right corner to make this go away

M25 loss of raw water suction after sailing and/or rough weather turn - Catalina 30, 1983

Apr 10, 2018
3
Catalina 30 Newport
Hello,

Why does my engine loose raw water suction after a day of sailing and/or a fast turn in rough weather?

The most recent was last Friday 9/6. Dockside raw water discharge OK. Departed harbor enroute to nearby port at distance of 2 miles, raw water discharge OK, engine rpm 1800. Prior to entering destination harbor, I made a full turn to port (360 degrees), rpm 1800. Sea waves 3-4 feet, wind 20. At end of turn I notice no discharge of raw water from exhaust. Engine temp rising steadily. Stopped engine, quickly raised storm jib, and sailed into port. Inspection of the sea strainer, raw water pump (gear driven) and impeller OK. It took about 30 minutes to inspect system. I started engine and again had raw water/exhaust discharge. I returned to home port at 1500 rpms w/o further suction loss. All hoses brand new, fittings tight. Heat exchanger dipped and cleaned - works properly. Many articles refer to broken impeller, improper mounting, loose connections, etc., I have inspected all, all are OK. Suction inlet is unobstructed.

Please help. Thank you!
 
May 17, 2004
2,099
Beneteau Oceanis 37 LE Havre de Grace
Strange. The raw water impeller would've been my first guess, but evidently you checked it and it was ok. How long did it run after you noticed that it wasn't pumping? I'm surprised the impeller would look ok if it was pumping dry for any length of time. Is it possible that the impeller isn't the right size for the pump, and isn't properly contacting all sides of the pump body?
 
Feb 26, 2004
20,801
Catalina 34 224 Maple Bay, BC, Canada
Some rw pump impellers have been known to spin on their own shafts. Take the impeller out and check.

What pump? Oberdorfer or Sherwood?

I have the same engine. I refuse to use Globe impellers because of this issue. And it happened to a friend of mine, too.

I have an Oberdorfer and use only the black Universal impellers. Have for 21 years, no issues.

They call 'em Run Dry impellers, supposedly because they can run dry without disintegrating. 'Cept what happens is they run the pump without pushing any water and disintegrate YOUR ENTIRE ENGINE!!!!:banghead::banghead::banghead::banghead:
 
Last edited:
Jan 7, 2014
58
Beneteau 45F5 51551 Port Jefferson
Look at the inside of the cover to the impeller, if it has grooves in it sand and polish them down. Those little grooves are enough for the pump to lose prime.


1568340427238.png
 
  • Like
Likes: Sefuller
Nov 7, 2011
2,568
Catalina 30 Mk II Barnegat, NJ
Do you have a strainer on the outside of the raw water inlet? If so it may be partially clogged in a way that a change in water flow over it interrupts in the intake. Maybe?
 
Jan 6, 2006
2,276
Beneteau 423 Mt. Sinai, NY
Check the intake water line..... most optimal would be up with no areas that could catch air and cause an airlock..... good luck!
 
Mar 6, 2008
467
Catalina 1999 C36 MKII #1787 Coyote Point Marina, CA.
What happens if you turn off the engine then turn it on again. Do you get water flow?
To avoid costly engine repair I would change the pump as a first step eventhoug it is expensive step.
 
Jan 20, 2005
783
Nauticat 321 pilothouse 32 Erie PA
it is a well known problem with some brands of double-wall hose that after some years the 'inner' wall separates from the outer wall. in high suction demand situations (running at WOT , outrunning a storm etc . ) the suction causes the inner hose wall to separate from outer, deform and thus reduce the diameter of the hose that's carrying your raw water into the pump-- insufficient raw water to the pump = overheat. note that when the hose is not subjected to high suction the inner layer returns to look normal, so this problem is hard to diagnose. RX replace the raw water supply hose every 250 hours or so, and do it when you have elminated the other potential causes of your overheat. (and carry and extra length cut to fit when you replace it this time.) NB an overhat may also be d/to a shortage of closed circuit antifreeze, yan mars are particularly sensitive to proper antifreeze cooling volume.
 
Aug 31, 2017
102
Marshall 22 Falmouth, ME
I would check the external strainer as it sounds like the interruption occurred from a change in water flow under the boat rather than a change in RPM (where one would suspect a collapsing hose), and you have inspected all other parts anyhow (I think).

If you have a scoop style strainer (which is really meant for a powerboat) they can cause cavitation, a partial clog can increase the odds of this. The turn (change in flow) combined with waves (possible interruption of water over the strainer) could exacerbate that possibility.

It is also quite possible you ran over something that blocked the external strainer (patch of eel grass, a plastic bag etc) so when you turned off the engine, the suction stopped and the offending debris fell away. Starting after resulted in normal operation, this is one of the benefits of having an external strainer.

Obviously I cannot be sure that this is right at all, but having checked all the other links in the chain it would at least be a good use of time to inspect the one you did not.

I would recommend replacing the impeller if it ran dry for any amount of time, it’s cheap insurance. I would also recommend if you have a scoop style to replace it with a round one. They suck up less seaweed.
 
Last edited:

SG

Feb 11, 2017
1,536
J/Boat J/160 Annapolis
If you have a scoop style strainer (which is really meant for a powerboat) they can cause cavitation and hydro locking of the impeller, a partial clog can increase the odds of this. The turn (change in flow) combined with waves (possible interruption of water over the strainer) could exacerbate that possibility.
I believe should NEVER have a scoop style inlet on an engine intake, especially on a faster hull like a powerboat. You can induce flow to the elbow, then you can unintentionally back flush the engine through the valves when it's off.

As for our TSACat: I think the issues are pretty well laid-out above.
  • The first issue would have been a blockage of the intake;
  • Then, if you have a strainer which is plugged or has a leak in the lid ( lid that should be at the waterline so you can service the strainer, sometimes is above waterline); then
  • I'd go to the impeller and it's lid. Usually if you have a leak there, you'll see some water. If its the pump lid, sometimes you can REVERSE the cover so the eroded side is out and the "fresh side" becomes the wearing side. Then there is the impeller (which is usually the culprit).
  • It's possible that you have an issue with the raw water pump.
  • After that, I don't think its a question of loosing "the prime", because it should be all "uphill" from there.

  • The water pump is a "positive displacement pump" -- i.e., it will SUCK up water. and really should be able to overcome "losing its prime". What suffers first is the impeller which usually loses its vanes and become useless. The impeller needs water around it to stay cool and lubricated. (The non-petroleum grease, usually something like Superlube teflon grease, is intended to keep the initial water=less impeller lubed until you are "sucking" water to the impeller after you change the impeller.
  • It's possible you have a blockage downstream of the impeller (say the heat exchanger, or elbow, or exhaust hose). Usually you'd get flow, but not enough to cool the engine properly at speed, but not "nothing".
I've mostly repeated what's above -- but the point is: Start at the beginning and go systematically through the closed system. You'll find the problem.
 
May 6, 2010
410
1984 Oday 39 79 Milwaukee
Lots of good suggestions but I will echo what @TimFromLI said. We have an Oberdorfer pump and I had a bonehead moment where I ran the engine for about 20 minutes with the fresh water intake closed. After that the pump would not prime, even though the impeller looked fine. I called Depco to order a new pump, and they suggested replacing the pump cover and impeller. I had already replaced the impeller, so I took the cover off, and there were very slight grooves worn in it which I sanded out using sheets of sandpaper on a flat surface. That solved our problem.
 
Aug 28, 2015
182
Oday 28 St Joseph, MI
I would suspect the 360 you made when you discovered the lack of cooling water was just a coincidence. I also suspect you’re missing something with the recent work done on the cooling system. Has the problem repeated? It’s very possible you had a piece of debris stuck on the water intake and which was held there by the vacuum created. Do you still have your old hoses? It’s possible one of the new hoses is delaminated but I would think the problem would show up more often.
 
Feb 8, 2014
1,107
Columbia 36 Muskegon
I would check the external strainer as it sounds like the interruption occurred from a change in water flow under the boat rather than a change in RPM (where one would suspect a collapsing hose), and you have inspected all other parts anyhow (I think).

If you have a scoop style strainer (which is really meant for a powerboat) they can cause cavitation, a partial clog can increase the odds of this. The turn (change in flow) combined with waves (possible interruption of water over the strainer) could exacerbate that possibility.

It is also quite possible you ran over something that blocked the external strainer (patch of eel grass, a plastic bag etc) so when you turned off the engine, the suction stopped and the offending debris fell away. Starting after resulted in normal operation, this is one of the benefits of having an external strainer.

Obviously I cannot be sure that this is right at all, but having checked all the other links in the chain it would at least be a good use of time to inspect the one you did not.

I would recommend replacing the impeller if it ran dry for any amount of time, it’s cheap insurance. I would also recommend if you have a scoop style to replace it with a round one. They suck up less seaweed.
It's counterintuitive but a scoop style through hull should face AFT, not forward. That way the eel grass or plastic bag is forced away from the scoop by the boats motion through the water. Your engine's installation manual should show it that way, mine did.
 
Jan 19, 2010
326
Catalina 34 Casco Bay
This may seem obtuse, but did you happen to note a decrease or lack of exhaust with the cessation of discharge water ? If so, the issue might be in the exhaust riser..