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  #1  
Old 06-25-2008
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raw water intake

Can air in my raw water strainer have an adverse effect on the rate of flow of sea water into my water pump? I've checked all hoses and seacock for obstructions and found everything clear..no kinks, no foreign objects. Right now, all I get from the raw water side before it enters the impeller is a moderate trickle. Secondly...how much exhaust water should be exiting my exhaust at, say, 1000 rpm? It is hard to find a definitive quantity, just lots of adjectives, like steady or sufficient. Thanks...
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Old 06-25-2008
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Originally Posted by rchad View Post
Can air in my raw water strainer have an adverse effect on the rate of flow of sea water into my water pump? I've checked all hoses and seacock for obstructions and found everything clear..no kinks, no foreign objects. Right now, all I get from the raw water side before it enters the impeller is a moderate trickle. Secondly...how much exhaust water should be exiting my exhaust at, say, 1000 rpm? It is hard to find a definitive quantity, just lots of adjectives, like steady or sufficient. Thanks...
I think that it can.
I never figured out why but with my old engine and water intake setup, the engine would sometimes overheat when heeling to starboard.
It did this once in a very precarious situation and caused a great deal of grief.
I bypassed the raw water strainer and it solved the problem.
I suppose it was a vapor lock or something.
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I recently replaced all the piping between by raw water intake valve and the raw water pump. It was totally worth the effort. Here's what I found when I did it:

Problem 1: the PO had used hose that wasn't rated for the appropriate pressure, so it was collapsing under suction. Not enough to completely impede the flow, but enough to impede it somewhat. It didn't *look* particularly collapsed, but the interior wall of the hose had separated from the exterior wall of the hose, so it was collapsing without us being able to notice. The right type of hose fixed the problem.

Problem 2: we'd got an accumulation of barnacles and other crap in the length of hose between the raw water intake and the strainer. Not enough to completely block the flow, but enough to seriously impede it. Replacing the hose, and digging around in the hose fitting of the strainer with a screwdriver fixed it.

Problem 3: carbon buildup in the mixing elbow. Didn't know this until recently, but mixing elbows are supposed to be replaced as part of routine maintenance, after about 8 years +/- of service life. Ours was right around that 8 year mark, and had undergone some corrosion, and a fair amount of carbon buildup where the raw water interfaces with the exhaust. Replacing the mixing elbow with a new part fixed the problem.

By the time we were done, instead of a slow trickle and an occasional splash, and occasional overheating when a clog developed in the mixing elbow, we had a continuous very heavy stream with a frequent almost full-pipe gush, even while idling. We also checked our raw water impeller for damage, and it was fine, but we replace that every year or two depending on what it looks like.

Others can correct me if I'm wrong, but I've always been under the impression that a properly operating raw water pump, with clean intake lines and discharge lines, will easily suck whatever air might be in your strainer out with no problems. I suspect if bypassing a strainer solved a flow problem, then the source of the problem is either blockage in the strainer or a serious air leak, not whatever air bubble might be in the strainer when you start your engine.

If I had a flow problem when heeling to one direction but not the other, the first thing I'd look at is how far you were healing, and what side of the boat your raw water intake is on.

Raw water intake systems are really pretty easy to troubleshoot, even for someone with zero engine repair experience. I'd suggest checking everything meticulously. You never know what kind of weird crap might be in the line someplace giving you the problem. Don't forget to doubleclamp.

To the top poster - if you've checked all the pipes and you're sure they're all clean, and the strainer is clean, and the hosebarbs on either side of the strainer are clean, check the thru hull valve itself. Close it, detach the hose, and open it for a few seconds to see how much water comes through. Should be quite a lot. If not, see if there isn't some seagrass or barnacles or something clogging the through hull fitting. Might be worth going over the side with a snorkel and looking from the outside too. Also obviously check your impeller if you haven't yet, since that's the most common source of raw water intake problems.
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Last edited by beej67; 06-25-2008 at 10:15 AM.
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One would think air in the fresh water intake should go away pretty quickly.

I've a length of clear hose I hook to our normal intake for pumping anti-freeze through the system prior to winter lay-up. Naturally, when I first hook that in, it's got nothing but air in it. Soon after starting the engine the entire length, about three feet, is solid purple all the way. I can't even tell the water is moving by looking at the hose.

In the case of a strainer, I guess it would depend on where the intake and output are. If the intake is on top, where the bubble is, I guess I can see it remaining. Unlike Steve, tho, I can't see it much affecting water flow rate.

Jim
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Where were you able to find pressure rated hose that's clear?
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Problem 3: carbon buildup in the mixing elbow. Didn't know this until recently, but mixing elbows are supposed to be replaced as part of routine maintenance, after about 8 years +/- of service life. Ours was right around that 8 year mark, and had undergone some corrosion, and a fair amount of carbon buildup where the raw water interfaces with the exhaust. Replacing the mixing elbow with a new part fixed the problem.

There was a very knowledgeable lady at the Yanmar dealership in Oakland years ago where we went to purchase spares for the boat before leaving the country. She insisted that we purchase a mixing elbow. It wasn't even on my list.
Listening to her proved to be one of the smart things I did in preparing for that trip.
It was a hot, dirty job and my whole engine compartment was cover in soot, but at least I was able to fix it.


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If I had a flow problem when heeling to one direction but not the other, the first thing I'd look at is how far you were healing, and what side of the boat your raw water intake is on.

That's exactly what I checked out first. There was no way the intake was out of the water. Though it may have gulped some air. It was pretty bouncy.
The thing is, after bypassing the strainer, the problem went away.
I have a new engine and the same strainer now but the hoses are led differently. I haven't experienced a recurrence of the problem yet, but I really haven't been it that rough of a situation with the new engine yet.
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It was a hot, dirty job and my whole engine compartment was cover in soot, but at least I was able to fix it.
I got a hold of a guy from Sound Marine Diesel LLC through the Pearson mailing list, who coached me through replacing mine via phone/email. It was pretty easy, I thought. I got a mixing elbow, an exhaust riser, and the coupler in the mail. Bought a bronze hose barb to fit in the mixing elbow down the street. Put them all together with a vice and a pipe wrench in my garage. Sound Marine guy also sent some exhaust-friendly thread locking agent with the parts for free in a ziplock bag. The hardest thing about my installation is that my muffler is right near my exhaust elbow, so there's not a lot of pipe running between them, and that exhaust pipe is a major PITA to bend. Also need to take great pains to avoid cracking your muffler. Hate that.
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I got a hold of a guy from Sound Marine Diesel LLC through the Pearson mailing list, who coached me through replacing mine via phone/email. It was pretty easy, I thought. I got a mixing elbow, an exhaust riser, and the coupler in the mail. Bought a bronze hose barb to fit in the mixing elbow down the street. Put them all together with a vice and a pipe wrench in my garage. Sound Marine guy also sent some exhaust-friendly thread locking agent with the parts for free in a ziplock bag. The hardest thing about my installation is that my muffler is right near my exhaust elbow, so there's not a lot of pipe running between them, and that exhaust pipe is a major PITA to bend. Also need to take great pains to avoid cracking your muffler. Hate that.
It probably would have been a lot easier if I hadn't been anchored off a rather small town in Costa Rica.
I sure wouldn't have wanted to have had one sent to me there. The other cruisers were saying that it took six weeks or more to get things sent from the states and it wasn't cheap.
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Yeesh. Be glad you could get the part at all. I think at that point I might have pulled the elbow and tried to figure out some way to clean it (pressure wash?) long enough to last me until I got back to the States.

Good story though, which is all that matters, right?
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Yeesh. Be glad you could get the part at all. I think at that point I might have pulled the elbow and tried to figure out some way to clean it (pressure wash?) long enough to last me until I got back to the States.

Good story though, which is all that matters, right?

That was my point. I had the part on board. If that wonderful woman hadn't insisted that I buy it before I left, I would have been screwed. The old one had rusted completely through.
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