Exhaust height

[cmedsailor]

Sorry if this is probably a stupid question but is due to lack of knowledge (thank God this forum exists). In some boats the exhaust is quite low on the side of the boat so in case of e.g motor sailing at some angle this will lie below the water level. Could this damage anything? I mean how could gases come out of it?

 

[Bilgediver]

Not a stupid question at all. Although the exhaust comes out low there should be a swan neck inside the boat as high as possible to prevent water backflowing to the engine,.

One of the biggest dangers of operating the engine with the exhaust submerged comes when stopping the engine when it has been running hard. It is possible for the cooling exhaust gases in the exhaust to create a partia vacuum and draw water from outside over the swan neck!!!

I have seen this happen on survival craft where the exhaust exits below the boat and no vacuum release was fitted.

The engine has no problem pushing the exhaust out through the water as the back pressure is very low.

 

[chas]

Yes, the problem is not so much exhaust getting out but water getting in. My exhaust system is a bit low - it does loop above the waterline but not by much and the hull fitting is under water much of the time, so I have put a non return valve just before the hull fitting for peace of mind.

 

[Csail]

Blew my old engine so make sure you incoporate a Vetus anti syphon valve. Quite cheap compared to the new engine.

 

[AlanPound]

I had loads of problems over many years with water getting into the engine oil (and had such an anti-syphon valve).

Many investigations (and loadsa money) into the possibility of cracked heads, bad heat exchangers, coked up exhaust elbows, etc.

The final definitive explanation was that the anti-syphon device had some kind of spring loaded valve within it, and this was sticking or blocking.

Engineer that identified it said he had come across problems with that type on a number of occasions - basically it looks like there is an anti-syphon device, but from time to time it fails and allows syphoning to occur, and water gets into the oil with all the panic, distress, inconvenience, cost (and corrosion?) that implies....

Replaced with plain type and problem has not recurred. A hard lesson learned.

 

[anniebray]

A few years back I fitted a Beta28hp & recall that Vetus supplied 2 anti syphon devices. One that used a non return valve allowing air to be sucked in thus preventing vacuum but with the possibilty that the valve might eventually stick due to corrosion. I chose the other type which consisted of a bleed off from the highest point connected by a small bore tube to a bulkhead fitting in the transom. Simplicity with the added benefit of a tell tale jet of water to show that the cooling water was flowing.

 

[Tranona]

The Moody 33s I looked at when I was thinking of buying one had a riser cum mixer close to the engine which seemed to be well above the water line. Seemed quite a good arrangement although I understand prone to corrosion.

 

[maxi]

No problems with the engine running, but have just filled my engine with water following a very fast channel crossing, under sail, with the outlet deeply immersed, This despite a very high swan-neck, high lift exhaust elbow and a water trap. First time is 27 years !!

My solution is to install a simple clamshell moulding over the outlet, so that water cannot be driven in, and where hull motion actually creates a 'suck' effect.

 

[chas]

This is exactly what I had to replace as the baffles that stopped the cooling water flowing back down the dry exhaust had corroded away. I could not find a unit that would fit into the space available so redesigned the exhaust (with the help of an engineer). It now loops up above the waterline, down through a mixer then a waterlock and finally a non return valve. This has given no problems for a couple of years now.