Anti siphon on exhaust water line

Lou_tribal · Jan 25, 2018

My fellows TFers, greetings from the frozen land.
This morning I was curing my lack of boat by reading some docs from vetus. I have a waterlock from vetus on my exhaust line. In their doc they mention that an anti siphon valve should be set on the water line at a minimum of 40cm above the waterline. My exhaust water injection point is at the water level or few cm above and my ER does not allow to have an anti siphon valve at the mentioned level.
So the following questions
Is it required (or best) to have an anti siphon on the water injection line?
If yes what do you think about the 40cm?
How about the waterlock setup?
Note: my setup has a flap on the exhaust to avoid water ingress, a 1 to 1.5 feet rise before the exhaust output and the waterlock 1 foot below the exhaust from manifold. Never had an issue cruising in fresh water but thinking about this (or over thinking about this )in sea waves.

L

O C Diver · Jan 25, 2018

The issue has to do with water syphoning through the raw water pump into the exhaust of the engine after shutdown. Do you have an exhaust riser that prevents the water from syphoning? Is there a lift muffler between the exhaust elbow and transom exhaust outlet?

Ted

Lou_tribal · Jan 25, 2018

Exhaust line is as follow:
From manifold the elbow with injection is going down.
1 foot below there is a waterlock (if I am not mistaken, 10.5l capacity)
From the waterlock go straight to an elbow
The elbow it aroung 16 inches high
Then straight to the exhaust through hull
Then out a 90 degre to right and 2 feet to the exhaust flap and end. (Exhaust end is on port side)
Exhaust output is at around 3 inches above water when fully loaded.

L

O C Diver · Jan 25, 2018

It would help to describe the elevation. Does it go down from the exhaust manifold to waterlock, then up to an elbow, then down hill to where it exists the transom? What is the drop from the manifold to the waterlock? What is the rise to the elbow? Where is sea level in relation to the water injection point, the waterlock, and the elbow?

Ted

Lou_tribal · Jan 25, 2018

O C Diver said:
It would help to describe the elevation. Does it go down from the exhaust manifold to waterlock, then up to an elbow, then down hill to where it exists the transom? What is the drop from the manifold to the waterlock? What is the rise to the elbow? Where is sea level in relation to the water injection point, the waterlock, and the elbow?

Ted

From manifold to waterlock drop is around 1 foot.
Then from waterlock it rises 16 inches up then same down to the output.
Water injection point is more or less at sea level so water lock 1 foot below elbow after 16 above and output 3 inches above sea level
(Approximately at 1 inch or 2 precision)

L

O C Diver · Jan 25, 2018

If the injection point is more or less at sea level and the elbow after the waterlock is at or above the manifold, it would be possible to fill the waterlock with water and then have the excess go back into the exhaust manifold. Consider trying to start the engine; it turns but doesn't start. While it's turning, the water pump is running and filling the exhaust system.

A syphon break would be a very good idea. There are 2 types. One goes before the raw water pump. One goes after the raw water pump. Both types work well.

Ted

HenryD · Jan 25, 2018

Ted,
Do you have a brand or model fo the two types of siphon break? I am not happy with my setup and looking to improve it.
What I have now is a U loop up in the ceiling of the engine room with a Scot vacuum breaker. From time to time, the Scot vacuum breaker sprays water which I want to eliminate.
Thanks
Henry

O C Diver · Jan 26, 2018

HenryD said:
Ted,
Do you have a brand or model fo the two types of siphon break? I am not happy with my setup and looking to improve it.
What I have now is a U loop up in the ceiling of the engine room with a Scot vacuum breaker. From time to time, the Scot vacuum breaker sprays water which I want to eliminate.
Thanks
Henry

Hi Henry,
Is your syphon break on the suction or pressure side of the pump?

Ted

Ski in NC · Jan 26, 2018

Siphon break can not be placed in sea water pump suction line. In that case it will feed air to the pump.

Siphon break is critical if the injection point on the exhaust mixer is below the water line. If injection point is above water line, then no need.

Siphon break generally mounted in the line between last heat exchanger an mixer injection point, but can be anywhere between pump and mixer.

Lou- Once you splash, use a water level to measure exact elevation of your injection point. Good chance that it is higher than WL.

Water lift muffler is good protection from water backing up from tailpipe, but will not prevent weepage through pump if inj point is below WL.

sunchaser · Jan 26, 2018

Ski in NC said:
Siphon break is critical if the injection point on the exhaust mixer is below the water line. If injection point is above water line, then no.

+1

Lou, determine WL elevation to confirm or allay your concerns.

O C Diver · Jan 26, 2018

Ski in NC said:
Siphon break can not be placed in sea water pump suction line. In that case it will feed air to the pump.

There is a syphon break that closes on vacuum instead of pressure. The poppet valve is pulled closed by pump suction and opens when the pump stops and suction is reduced.

Ted

sunchaser · Jan 26, 2018

O C Diver said:
There is a syphon break that closes on vacuum instead of pressure. The poppet valve is pulled closed by pump suction and opens when the pump stops and suction is reduced.

Ted

Also one can run a line from the siphon break to the top of the water lift muffler.

Lou_tribal · Jan 26, 2018

Thank you guys I will precisely measure the injection point in regards to the water level.

Ski in NC · Jan 26, 2018

O C Diver said:
There is a syphon break that closes on vacuum instead of pressure. The poppet valve is pulled closed by pump suction and opens when the pump stops and suction is reduced.

Ted

To break a siphon, device has to open under vacuum. If a vacuum did not exist a siphon would not occur. Never seen what you describe on a marine engine. But I learn new things every day.

angus99 · Jan 26, 2018

sunchaser said:
Also one can run a line from the siphon break to the top of the water lift muffler.

+1.

Here’s how mine looks, with a Vetus anti-siphon valve above the waterline. The blue hose irrigates the prop shaft tube/cutless bearing.

O C Diver · Jan 26, 2018

Ski in NC said:
To break a siphon, device has to open under vacuum. If a vacuum did not exist a siphon would not occur. Never seen what you describe on a marine engine. But I learn new things every day.

From what I remember, the poppet valve is spring loaded open. The pump creates a significant vacuum which pulls it shut. When the engine is turned off, the vacuum is less without the pump's flow suction and the valve opens. The spring / required suction on the poppet valve is adjustable. Seen them on smaller sailboat engines, so maybe that's why it seems odd. :hide:

Certainly using the anti syphon U with a very small breather line to the exhaust seems a better choice with no moving parts. An 1/8" line will allow a very small amount of air in the raw water system, but quickly break the suction syphon on shutdown.

Ted

O C Diver · Jan 26, 2018

angus99 said:
+1.

Here’s how mine looks, with a Vetus anti-siphon valve above the waterline. The blue hose irrigates the prop shaft tube/cutless bearing.

Assuming that's after the water pump as it pressure feeds the shaft tube.

In that application you really don't want to feed the syphon break down into the lift muffler. It's creating another syphon path. Ideally the vent hose goes only horizontally to its point of discharge, often out the hull.

Ted

sunchaser · Jan 26, 2018

O C Diver said:
It's creating another syphon path. /QUOTE]

Won't happen in Angus's case as once engine shuts down drainage to nearly empty water lift of a pint or two of water occurs, thus breaking any latent suction. Mine's nearly identical.

PS - SIPHON

angus99 · Jan 26, 2018

O C Diver said:
Assuming that's after the water pump as it pressure feeds the shaft tube.

In that application you really don't want to feed the syphon break down into the lift muffler. It's creating another syphon path. Ideally the vent hose goes only horizontally to its point of discharge, often out the hull.

Ted

Also, there’s a valve in the Vetus that only opens when the pressure is removed, thus breaking the siphon.

psneeld · Jan 26, 2018

for general consumption...

Vetus comes in several styles...

some have a small rubber valve, some dont.

O C Diver · Jan 26, 2018

angus99 said:
Also, there’s a valve in the Vetus that only opens when the pressure is removed, thus breaking the siphon.

I understand. Believe Tony Athens has an article about this on his website. Would rather have the system without the valve going out the hull. One less saltwater mechanism to remember to maintain.

Ted

angus99 · Jan 26, 2018

O C Diver said:
I understand. Believe Tony Athens has an article about this on his website. Would rather have the system without the valve going out the hull. One less saltwater mechanism to remember to maintain.

Ted

I assume you mean the blue hose going to the stern tube, because the vent line goes to the muffler, not out the hull.

Yeah, I’d rather not have those blue lines either, and there’s some debate about whether they’re really needed on relatively slow-spinning shafts. They may serve to introduce oxygen into an area that could be starved of it where the shaft penetrates the hull and goes straight into a cutless bearing. It’s an almost cave-like space. My thought is they’re a means of preventing crevice corrosion on stainless shafts and my stuffing boxes always run cool, so I kept them.

HenryD · Jan 26, 2018

O C Diver said:
Hi Henry,
Is your syphon break on the suction or pressure side of the pump?

Ted

Ted,
The hose from the heat exchanger, goes up to a U pipe with a Scot Vacuum breaker valve (with a little spring) and then down to the exhaust elbow. The anti-siphon value is about 13" above the top of the elbow. The anti-siphon value is about 7" about the waterline.

What I don't like about the Scot vacuum breaker is that when salt or anything gets in the valve, it sprays saltwater in the roof of the engine room.

I have considered taking the breaker off, and running a 3/8" tube across the ceiling (5 feet) , into the aft wall and up a few feet to a vent screen. My concern with putting a tube/hose horizontal is that it could sag and create a water/airlock.

psneeld · Jan 27, 2018

Use something rigid or tubing in something rigid like a piece of pvc or pvc conduit.

Although I think if a siphon started, that tiny bit of water in a thin tube would get sucked right out and break the siphon.

The only thing I hate about the tube, is I tend to get a bit of staining on the hull like the bow brown mustache...

O C Diver · Jan 27, 2018

HenryD said:
Ted,
The hose from the heat exchanger, goes up to a U pipe with a Scot Vacuum breaker valve (with a little spring) and then down to the exhaust elbow. The anti-siphon value is about 13" above the top of the elbow. The anti-siphon value is about 7" about the waterline.

What I don't like about the Scot vacuum breaker is that when salt or anything gets in the valve, it sprays saltwater in the roof of the engine room.

I have considered taking the breaker off, and running a 3/8" tube across the ceiling (5 feet) , into the aft wall and up a few feet to a vent screen. My concern with putting a tube/hose horizontal is that it could sag and create a water/airlock.

A few engine room pics would help.

Would it be possible to run a 3/8" line from the top of the syphon break and vent it into the top of the exhaust tube after the lift muffler?

Ted

Anti siphon on exhaust water line

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