Cooling water - to screen or not to screen? - SailNet Community
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post #1 of 22 Old 10-05-2011 Thread Starter
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Cooling water - to screen or not to screen?

I don't currently have one and was wondering if any here have set ideas on whether engine cooling water through-hull strainers are a great idea or not. These things - and their brass counterpart:



I've heard arguments both ways:
For: They stop large rubbish (plastic bags, etc.) blocking the cooling system entirely.
Against: They're impossible to anti-foul properly and hence allow little nasties to accumulate, restricting cooling-water flow...

Which one is true in practice?!?

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"Honestly, I don't know why seamen persist in getting wrecked in some of the outlandish places they do, when they can do it in a nice place like Fiji." -- John Caldwell, "Desperate Voyage"
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post #2 of 22 Old 10-05-2011
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They're all true.

Now, would you rather use neverseize on the screws and clean that fitting out once or twice a season, or try to unravel plastic bags from your water pump?
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post #3 of 22 Old 10-05-2011
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Don't do it!!!

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They're all true.

Now, would you rather use neverseize on the screws and clean that fitting out once or twice a season, or try to unravel plastic bags from your water pump?
Firstly, I believe the "anti foul" comment referred to not being able to coat the INSIDE of the suction strainer against marine growth - which IS a concern, not frozen fasteners.

About a year or so ago, there was quite a thread on the "gear and maintenance" board about this very issue. Some very wise sailors (Mainesail among them) pointed out that if you have a screen on the outside of the hull, you cannot take the hose off and run a clean-out rod through the fitting from inside to outside (yeah, you will get a little wet!). You presumably have a suction strainer before your raw water pump, so it has plenty of protection. If you pick up a big plastic bag or something similar on the outside of the hull, the external suction screen will get clogged just as much as a simple through-hull opening will. Only difference is: you CAN'T rod out the through-hull from the inside with a suction screen blocking the passage on the outside!

After reading that past thread and giving this some thought, I removed my suction strainer on the outside of the hull. No problems to this point (knock wood!). Anyway, DON'T DO IT!!

Last edited by BELLATRIX1965; 10-05-2011 at 08:12 PM. Reason: mis-spelled name
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post #4 of 22 Old 10-05-2011
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"Firstly, I believe the "anti foul" comment referred to not being able to coat the INSIDE of the suction strainer against marine growth - which IS a concern, not frozen fasteners."
Two sides of the same coin. It is EASY to gob antifouling paint inside the fitting, but in order to paint it, as well as to clean out the little critters that don't care how well you painted it, it is still necessary to unscrew the strainer plate, at least during annual haul.
And you will not be able to unscrew it, certainly not with a slotted screwdriver while in the water, unless you've used neverseize. Or impact tools and the risk of collateral damage.
that would really be a place to have flush mounted dzus fittings instead of screws, if dzus fittings came in flush mounts.
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post #5 of 22 Old 10-05-2011
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To add to what BELLATRIX posted, as long as the inlet hose leading to the strainer in the engine compartment is above the waterline you won't get wet.

I will be removing my through hull strainer at next haulout.

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Living aboard in Victoria Harbour
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post #6 of 22 Old 10-06-2011 Thread Starter
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I actually thought that the idea of these things was to ensure that any rubbish that came past was more likely to be washed away in the slipstream than enter the cooling hose.. but perhaps you have to be doing +50kts for that to happen.

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To add to what BELLATRIX posted, as long as the inlet hose leading to the strainer in the engine compartment is above the waterline you won't get wet.
'Tis.. just.

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I will be removing my through hull strainer at next haulout.
I guess that's reason enough not to fit one then.

On the topic of rodding the hose out, how would I know there was something blocking the inlet? If I didn't get any water out the back end, my first thought would be to check the impeller - but that's after the strainer. Perhaps I should check the strainer first? Other than the cover being under serious vaccuum, how would I know to look elsewhere? Anyone had this actually happen??

I vaguely remember being told to take the hose off the strainer and blow - not try to shove something down the pipe..

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Last edited by Classic30; 10-06-2011 at 03:03 AM.
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post #7 of 22 Old 10-06-2011
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Keep an eye on the temp gauge, if it rises check for water in the exhaust. If the flow is less than it should be I would check the strainer first as it both easier and faster than checking the impeller.

If it is a straight run or close to it from the seacock to the strainer either a smaller hose or a rod of some kind can be used.
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post #8 of 22 Old 10-06-2011
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I replaced my thru hull strainer with an open thru hull last winter when I replaced my thru hull valve. I also added a large strainer and mounted it well above the WL. Finding space and building that mount was a challenge. The original thru hull was a flush bronze surface filled with small diameter holes. There was no strainer on the raw water side of the pump and I had added a small plastic one when I replaced the engine two years ago. It worked, stopping some grass. The new arrangement seems to work well. The strainer I now have looks large but it is easily serviced and is doing its job. I did push a small brush with AF paint up into the opening of the thru hull when I painted the bottom this spring.
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post #9 of 22 Old 10-06-2011
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Regardless of whether you install one of these screens or not, I strongly recommend you install a raw water flow detector. Should intake water be blocked for any reason the Raw Water Flow Detector sets off an alram instantly. By the time you notice the white smoke, rising temperature gauge or your overheat alarm goes off, much damage has already been done. Aqualarm makes a good one.
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post #10 of 22 Old 10-06-2011
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Some time back we sucked up a large gob of kelp on the outside screen and I had to dive in 55 degree water without a wet suit to clear it. It would have been a whole lot easier and warmer to undo a hose long enough to hold above the water line and push the kelp out with a rod or smaller diameter hose.

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