Anti Syphon Valve

[HunterWanderer]

Do I need an anti syphon valve?
Engine is Yanmar 3GMF with exhaust outlet 6" above the water 12" off centreline. Its never had one and never had a problem but most systems seem to have one.
Are they needed?
Only seem to cause a problem if they jamb open and let sea water into boat.
Can't see how an engine would flood by syphon anyway.
I want to keep it simple but am I risking it.

 

[footsoldier]

There is a lot of inherited wisdom in the old adage "it it ain't broke, don't fix it".

 

[Tranona]

Do I need an anti syphon valve?
Engine is Yanmar 3GMF with exhaust outlet 6" above the water 12" off centreline. Its never had one and never had a problem but most systems seem to have one.
Are they needed?
Only seem to cause a problem if they jamb open and let sea water into boat.
Can't see how an engine would flood by syphon anyway.
I want to keep it simple but am I risking it.

The height of the exhaust outlet is irrelevant. It is the height of the exhaust elbow injection point that is important. The water that may syphon back is not from the outside but from the amount retailed in the waterlock. The 2 diagrams in this link explains. abcpm.co.uk/vetus/exhausts/

If your injection point is well above the waterline then there is no need for a valve, but if it is below then essential, just above highly desirable.

 

[B27]

The height of the exhaust outlet is irrelevant. It is the height of the exhaust elbow injection point that is important. The water that may syphon back is not from the outside but from the amount retailed in the waterlock. The 2 diagrams in this link explains. abcpm.co.uk/vetus/exhausts/

If your injection point is well above the waterline then there is no need for a valve, but if it is below then essential, just above highly desirable.

It's really more about water which will seep past the impeller when the engine's not running but the seacock is open.

 

[HunterWanderer]

Thanks for your replies.
Of course , if the water injection point is below the water line sea water will just flow into the exhaust pipe. The anti syphon valve is crucial in this case. I'm pretty sure I have the 150mm height above the waterline but I am going to check.
The forum is great.

 

[bluerm166]

If a sailing vessel you also have to check the heeled condition .A central position for the anti syphon valve will make it far easier to meet the requirement.

 

[Tranona]

Thanks for your replies.
Of course , if the water injection point is below the water line sea water will just flow into the exhaust pipe. The anti syphon valve is crucial in this case. I'm pretty sure I have the 150mm height above the waterline but I am going to check.
The forum is great.

No it won't if your exhaust is properly laid out with a waterlock and loop before the transom. You are confusing water flooding forward if the exhaust is straight through as on some powerboats with syphoning that occurs because when the engine is shut off the engine seawater circuit remains full which is what might happen if it is all below the waterline. The purpose of the valve is to introduce air into the pipework when the engine stops to break the potential syphon.

Not all boats need one either because the exhaust elbow and injection point is well above the waterline or a high rise exhaust has been added (as on my boat) to achieve the same. The valve does not have to be located as shown in the Vetus diagram. Many installations are easier (for example with Volvo engines where the injection point is built in) to locate the valve between the raw water pump and the heat exchanger. So long as it is downstream of the raw water pump it will work.

 

[HunterWanderer]

No it won't if your exhaust is properly laid out with a waterlock and loop before the transom. You are confusing water flooding forward if the exhaust is straight through as on some powerboats with syphoning that occurs because when the engine is shut off the engine seawater circuit remains full which is what might happen if it is all below the waterline. The purpose of the valve is to introduce air into the pipework when the engine stops to break the potential syphon.

Not all boats need one either because the exhaust elbow and injection point is well above the waterline or a high rise exhaust has been added (as on my boat) to achieve the same. The valve does not have to be located as shown in the Vetus diagram. Many installations are easier (for example with Volvo engines where the injection point is built in) to locate the valve between the raw water pump and the heat exchanger. So long as it is downstream of the raw water pump it will work.

I can see that a syphon break valve would work anywhere after the pump but I can't see how a waterlock and loop makes any difference. The waterlock stops the exhaust gasses escaping without the water but why does the loop affect anything except a following sea entering the system. Once the engine is stopped, the water in the exhaust system equalises. If the engine has stopped with a valve open and the injection point is below sea level water may flow in and flood the exhaust pipework and the engine?

 

[Tranona]

I can see that a syphon break valve would work anywhere after the pump but I can't see how a waterlock and loop makes any difference. The waterlock stops the exhaust gasses escaping without the water but why does the loop affect anything except a following sea entering the system. Once the engine is stopped, the water in the exhaust system equalises. If the engine has stopped with a valve open and the injection point is below sea level water may flow in and flood the exhaust pipework and the engine?

Your exhaust system should have a loop above the waterline before it exists, often called a swan neck. This is to stop water flowing forward through the outlet. It you do not have one then, yes you run the risk of water getting past the waterlock and running into the engine. As i said that is different from what the anti syphon does, which is to break the flow of water around the cooling system so that the water falls into the waterlock and prevents any syphoning past the impeller.

Your last sentence is wrong. The waterlock holds the water left when the engine stops. The swan neck prevents water floeing forward from the outlet in the transom - which may on many boats actually be on or below the waterline. As I said you are confusing 2 different things.

Go back to the link I posted in ~3 which explains it all