Waterline vs Raw-Water Strainer

[sildene]

I am installing a new raw water strainer for my 3cyl diesel inboard. The engine is under the waterline.

I plan to install the strainer entirely under the waterline. This is to avoid a lengthy detour to get it above the waterline.

I currently believe the only disadvantage in locating the strainer below the waterline, is spillage (when you need to open the strainer, you close the seacock, but back flow from the heat exchanger and engine may spill). To stop the back flow, I plan to install a manual valve just upstream of the strainer.

Question: The Vetus strainer I'm using has product literature instructing installation 6 inches above the waterline, but they don't say why. Is this location merely for convenience (i.e. no spillage), or am I overlooking some more important considerations like 'air locks' or other?

Thanks
Martin

(My forum search did not reveal any previous threads that addressed this issue, but I will be very grateful if anyone can refer me to such).

[David M]

I have seen either way work on boats. Right now I have my sea strainers above the waterline which means I need to prime it by putting a hose down it and filling it up, screwing down the strainer end cap and then opening the seacock. I have also seen raw water strainers at least 10 feet below the waterline on larger vessels.

Below the waterline generally means you prime it by opening the seacock, open the strainer end cap, letting it flood until the water is overflowing a little, shut off the seacock, screw down the end cap, and then open up the seacock. Many times you don't even have to do that, the water pushes all the way to the raw water impeller.

Both ways work and I don't think it makes much difference other than in the way you prime your raw water cooling system.

My guess is that Vetus wants it above the waterline so as not to see you risking flooding your boat by forgetting to put on your strainer end cap or if it breaks ...so they "require" that it be above the waterline. Once all the air is purged, your engines raw water pump can't tell any difference.

Putting a valve on the outflow side of your strainer might be a good idea as it might help keep your prime. The downside is it adds complexity. My philosophy is that you want to keep it simple for things that have the potential of sinking your boat.

[Chief Engineer]

Follow the manufacturers suggestion for installation.....Why add another valve? More stuff to go wrong.

[Boracay]

When I put the Forespar raw water strainer into Boracay I first put it six inches above the waterline.

The water flow that resulted was full of bubbles and did not look good.

So I located it just below the waterline and there have been no problems.

It seems like positive pressure is necessary, possibly to avoid dissolved gases from coming out of solution.

[Celestialsailor]

Perhaps the reason they want it above the waterline is to prevent any liability of sinking your boat should the plastic crack...which they have. Personally I would not have a plastic strainer on the boat. A bronze one allows me to sleep at night.

[Joli]

We must be the only people in the marina that close all seacocks when we are not aboard.

[Steve Pope]

Nope, we close all of ours when we leave the boat, it is No 1 on our check list with the battery isolator 2nd and the gas bottles 3rd, etc etc.

[MidLandOne]

I don't recall actually ever seeing a valve as you describe, but no doubt it is done. Personally it is something I would never do, if only because of the extra risk of forgetting to open BOTH it and the seacock .

Normally, with small marine diesels, you should find that by taking care with understanding the static hydraulics when arranging the hose between the strainer and the raw water pump then the only drain down you get is for a short length of vertical rise in that hose (and not the heat exchanger, etc).

Our raw water filter is below the waterline, as is the engine, and we get negligible backflow into the filter when the lid is lifted (and the filter is plastic , but there again so are the hoses, the seacock and the through hull fitting too, so it is not alone in that - and some boats are even made of plastic too ). Usually the trapped air volume between the top of the filter and the top of the inlet/outlet connections is big enough to hold the backflow so there is no spillage. But that air dissolves into the water over extended time and if that has happened I just hold a small container under as I crack the lid to catch any drops.

PS Re the seacock closing comments - we always close ours too if the boat is unattended for more than a few hours. We also have a bilge alarm in a small sump that goes off if we get more than about half a pint of water in the boat, that in case of leaks when we are on board.

[Pblais]

Our last boat had a nice bronze strainer below the water line with a though hull valve very close by and cleaned easily but you did close the valve when you cleaned it.

This boat has a plastic Vetus that is quite high above the water line in side the lazerette. You can clean it easily while sailing and not leave the cockpit. Open the cover then close it. I would not install a plastic strainer below the water line. They are not meant to be installed that way.

[MidLandOne]

Originally Posted by Pblais

I would not install a plastic strainer below the water line. They are not meant to be installed that way.

Whether one personally likes the idea or not there are in fact plastic strainers that are stated by reputable manufacturers to be ok below the waterline. So it is incorrect to sweepingly say "They are not meant to be installed that way". Indeed not all are ok below the waterline, but there again I would not regard all metal strainers one might come across as being suitable below the waterline either. In either case I do not deal in junk.

As I said, I believe that they actually make boats out of plastic - not that I would personally have one of those though .

Like all strainers, no matter what material, the recommendation is to install strainers above the waterline if possible - but, of course, it often is not possible and doing so can introduce other problems (eg loss of suction and thus raw water if a drain down).

All strainers are something to be watched no matter what they are made of and where they are. There have been plenty of failures of strainers of all materials on vessels and even serious ones on land - in a past life I saw many of the lower galleries of a hydro power station dam accidently flooded through a strainer top failure and that definitely wasn't plastic (the pumps failed to start and in case not known such flooding can cause the failure of the dam so was considered very serious - I was contracted in to ensure that it could not happen again).

[Pblais]

MidlandOne,

The Vetus 150 is not designed to be used below the water line and are not stated to be used below the water line. The bottom of the bowl is to be located 6 inches above the water line per the manufacturers instructions. Given the specific nature of the question I will stand with the previous reply not with standing all your experience with power stations.

I have a Vetus 150 installed. Frankly, the top is not designed to be secure enough below the water line. t's held on with a ing nut on a venter shaft and a seal around the clear pastic top. I've found the strainer performs well and is easy to clean. The large top with a clear cover makes it easy to see right away if there is debris in the bowl. If the top is on tight it supplies water well. Below the water line I can't see how the top would hold up.

[MidLandOne]

I am not familiar with the Vetus strainers and accept what you say with respect to them (in fact if they are of similar poor quality as the Vetus water lift mufflers then they are junk, whereas Centek, for example, also make plastic waterlift mufflers which are not junk).

But your post read as making the claim that ALL plastic strainers are not meant to be installed below the waterline.

Perhaps interestingly, the only issue we have had with our own plastic strainers (engine raw water and sea water services) are with their metal components - the welds on the stainless steel filter baskets tend to corrode (was given free new ones and spares on which I coated the welds with epoxy and all is now fine - epoxy being a plastic of course ).

[Pblais]

I'm not sure how this has anything to to do with this thread. If I figure out how I will let you know.

[sildene]

Thank you to contributors for all the input - I'm feeling much more at ease with a below-waterline installation, seeing that for some members it improved water circulation. I've taken on board the cautions re. plastic below the waterline, I may go and look for a sturdier strainer than the Vetus (bronze or marelon, hopefully nice and compact). And always shut down the sea cock when not in use. Paul, regarding your last post where you say "Below the water line I can't see how the top would hold up", can you elaborate a bit? I interpret you to be saying the force/pressure on the cover plate is much higher if the strainer is located below the waterline than above. Or are you simply distinguishing between pos (not running) and neg (engine running) pressure?

[Pblais]

Paul, regarding your last post where you say "Below the water line I can't see how the top would hold up", can you elaborate a bit?

It's not that sturdy a structure. Basically the water gurgles through the bowl as it is installed. It works great but is never subjected to the idea of water pressure pushing against it all the time. As designed the water temp stays where it is supposed to be so it does the job as designed. It's not an expensive installation but then it's not a solid bronze strainer. I had one of those before. I can't say it worked better but is did lie below the water line for almost 20 years. This one is doing fine after 17 years.

Hey what do the folks at Vetus know about an inexpensive sea strainer. If you think it works below the water line then go for it. Its not a requirement in the design.

A Groco Bronze is nice below the water line. I would bet my boat on one. I had three on the last boat. After 20 years they don't look all that new however.