Exhaust Check Valve Siting

[biscuit]

The vertical distance between the injection point of my exhaust and the water level in the water lock is close to the minimum recommended, despite having an exhaust riser in place.
The antisyphon loop is as it should be.
Having had some experience of water ingress and expensive engine damage in a previous boat (bent con-rods), I want to have belt and braces security against this happening again.
I have a gate valve at the exhaust outlet, for use in extreme conditions, and am about to fit a Cetrek Check Valve (from ASAP supplies): a simple, but apparently effective exhaust non return valve.
The question is: what would be the most effective location: between the elbow and water lock, or well downstream of the water lock? I can see pros and cons to both.
I have also wondered about putting a flap valve on the exhaust pipe itself. These seem more popular on motor boats than yachts.

 

[VicS]

Centrek you mean presumably.

The decription on ASAP's website reads

Centek check valve is designed to be installed in the wet exhaust hose line to help retard the ingress of water surge from the transom.​

To my mind that implies down stream of the water lock, towards the transom.

I assume you are fitting this because you are unable to fit an effective swan-neck in the final section before the transom outlet ??

The height of the injection point above the water lock is not relevant to the siting of the check valve surely ?? The checkvalve is to prevent water entering the exhaust system via the transom outlet. Something normally prevented by the swan-neck

 

[Rossynant]

If waterlock is too high for comfort - drain it when leaving. Then wet valve at transom or after waterlock will then do the job.
It is possible to find a check valve for hot part of exhaust, if this is what you need, but such thing is seldom used. A valve for 'wet part' of exhaust must be after injection point, but should be possible to fit before waterlock - it just needs this water for cooling.
I understand this "minimum distance" is from riser top to water level? Injection point is not relevant here as it can be much below riser top. Easier way of prevention would be raising the riser if space permits.