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.
beej67, Checkered Past, 1980 32' Pearson 323, Panama City FL
Last edited by beej67; 06-25-2008 at 10:15 AM.