Am I going to create a syphon?

[Gixer]

I'm going to replace my engine raw water intake soon. The one I have at the moment has a strainer mounted on top of the valve which is below the waterline (I think). When I fit the new one I'm thinking of having the strainer separate like the Vetus ones.
Vetus says it has to be mounted above the waterline which is fine but I'm wondering if this could create a syphon to the engine.
I don't have an anti syphon loop in my system and never had a problem. the exhaust exits the engine and is then a pretty flat to the exit on the hull.





Any thoughts?

 

[vyv_cox]

I made just the same change years ago. No problem at all.

It is quite difficult to establish a syphon but especially through a vane pump!

 

[Gixer]

Thanks Vyv_cox.

This is what I'm thinking, is this ok?

 

[srm]

My current and last boat had Vetus filters mounted so they were on the waterline such that seacocks open and lids off there would be one or two centimetres of water in the bowl. This ensured the engine always had a water supply.

On a charter yacht my enthusiastic friend did all the engine checks every morning. After about fifteen minutes engine running I shouted at him to stop the engine, just as he was clearing the breakwater. He was reluctant to do so, until I pointed out that he risked melting/burning the exhaust system.

He had opened the Vetus filter, that was significantly above the waterline, allowing air in and dropping the level on the inlet side. He may also have not put the lid back on with a good seal. Once I had closed the sea cock while priming the system and put the lid back on he restarted the engine and we had a good flow of cooling water.

 

[Gixer]

What's the danger with mounting it below the waterline like my current strainer?
I guess its just not being able to open it with the seacock open. I will have a ball valve between it and the hull fitting.

 

[vyv_cox]

Thanks Vyv_cox.

This is what I'm thinking, is this ok?
View attachment 152325

That looks ok assuming there is a water pump there somewhere My exhaust drops down to a trap below the seawater level, then up to a silencer and down to exit just on the water line. Never had an anti-syphon loop.

 

[VicS]

What's the danger with mounting it below the waterline like my current strainer?
I guess its just not being able to open it with the seacock open. I will have a ball valve between it and the hull fitting.

It is not the height of the strainer which determines the risk of creating a syphon. It is the height of the injection point into the exhaust elbow, relative to the water line.

Vetus recommend a vented anti syphon loop if the injection point is below the waterline or less than 15cm above it

 

[Gixer]

VicS - I thought that was the case but thanks for clarifying.

 

[Tranona]

Is it a 1GM? If so no problem with it above the waterline. Good if you can mount it immediately above the intake.

 

[Gixer]

Is it a 1GM? If so no problem with it above the waterline. Good if you can mount it immediately above the intake.

Yes, it is a 1gm, thanks Tranona.

 

[thinwater]

What's the danger with mounting it below the waterline like my current strainer?
I guess its just not being able to open it with the seacock open. I will have a ball valve between it and the hull fitting.

One risk of mounting it directly to the valve is that the leverage on the valve is severe, particularly if something strikes it.

Mounting far above the waterline increases the risk of vacuum leaks and poor cooling. I think right at to a few inches above the waterline is about the best chioce. Try to avoid pockets where air can accumulate (in rough going it is always possible to get air bubbles into the intake). It is worth getting it right.

 

[Gixer]

I’ve got a bulkhead I can mount it to under a bunk but this might be just under the waterline. It would be at the same level as the top of the strainer I have now in the photo. I guess that should be ok.

 

[Tranona]

This is where mine is mounted


You can just make out the hose going directly down to the intake valve. appreciate you might not have space to do this, but it is good to try and keep hose runs as short as possible and easily accessible. Originally the water intake was behind the engine under the cockpit floor accessed through a hatch and the filter behind the bulkhead so virtually impossible to get the cap off.

 

[mattonthesea]

This is where mine is mounted
View attachment 152349

Mine is very similar. I had the op's strainer mounted on a gate valve. As I couldn't get stuff out of the damaged filter and I don't like gate valves I redid the whole caboodle with the new filter about the water line.

 

[Gixer]

I’m backtracking on this as I can’t find anywhere to mount the strainer above the through hull, this would mean difficulty in roding.
What I’m thinking now is to use a trudesign skin fitting and valve but reuse my current strainer.
Couple of questions.
it’s ok to mount a metal strainer directly on a plastic valve right?
I can’t tell what my current strainer is made of, should replace that as well?
The trudesign skin fitting is the standard round type where as I have a grated one now. That’s fine?

thanks guys

 

[fisherman]

Man who built narrowboats said he couldn't understand why everyone didn't have a standpipe on the seacock, so the top is above W/L
The only excuse i had was that I could take the top off the strainer and remove the gauze then the pressure would flush out the weed etc.
Then I took the idea along, and wondered if one could have a prop inspection trunk, with the sea water inlets in the side of it. I know it's another hole in the hull, but look at all the others from the bow thruster onwards. I would have the removed piece of hull in the bottom of the trunk with holes in as a strainer

 

[Gixer]

I think my strainer is one of these
Water strainer "Tirreno" series with Grilamid TR55 see-thru cover
Nickle plated brass - I guess this is ok to reuse?