boatmike
Well-Known Member
The physics of this are not difficult to understand. In the majority of cases yacht engines are mounted so that the heat exchanger is above the waterline. This is usually the high point of the engine and when you open the seacock water will not rise to this level unless it is pumped. On the exhaust side the water is usually injected into the exhaust directly from the heat exchanger. The exhaust usually has a wet silencer or waterlock on the exhaust.
OK. When the engine is started it pumps water through the heat exchanger and into the exhaust. Fine. But when a hot engine stops there is a possibility of drawing water up the exhaust pipe from the silencer and up the cooling water inlet pipe which can flood the engine so that next time you start it you bend a conrod because the water has got into the valve chamber via the exhaust manifold. To stop this happening you have to let air in AT THE TOP OF THE SYSTEM by some means. If you have a pipe it only needs to be capable of letting air in and technically it doesn't mind where it gets it from. if you have a valve, theoretically you only need it open to atmosphere inside the boat. If you don't have a valve the pipe will always let some water out so you need to ensure it goes overboard and that is the only reason you need to have a skin fitting. It needs to be well above the WL so that on any tack you draw air not water when the engine is not running. How long the pipe is and if or not it has to be routed down for part of its travel is immaterial. There will be very little water in this pipe as it is so small Obviously the most direct route is best. Now when the engine stops the water in the pipe will join that in the system and the water level will quickly revert to the WL level outside the hull. Thats all there is to it.
The only thing to remember is that the valve or tee piece should be mounted higher than the rest of the system as the objective is to let air in at the top. Where it is in relation to the WL outside the boat is immaterial but where it is in relation to the water in the engine is important.
To be entirely clear then if you have a tee piece it should be mounted as high up on the engine as possible. The other end can be routed by any means to a skin fitting anywhere above the WL outside the boat but as high as possible. Usually somewhere just above or to the side of the exhaust outlet is a good place. If you do have an engine mounted with the heat exchanger below the WL the whole setup gets more complex because you can flood the engine by just opening a seacock. Forget it. I am pretty sure you won't have, but just to make sure no one screws up, what I have said only applies to engines where the manifold and valve gear is mounted above the WL
Hope this helps.
OK. When the engine is started it pumps water through the heat exchanger and into the exhaust. Fine. But when a hot engine stops there is a possibility of drawing water up the exhaust pipe from the silencer and up the cooling water inlet pipe which can flood the engine so that next time you start it you bend a conrod because the water has got into the valve chamber via the exhaust manifold. To stop this happening you have to let air in AT THE TOP OF THE SYSTEM by some means. If you have a pipe it only needs to be capable of letting air in and technically it doesn't mind where it gets it from. if you have a valve, theoretically you only need it open to atmosphere inside the boat. If you don't have a valve the pipe will always let some water out so you need to ensure it goes overboard and that is the only reason you need to have a skin fitting. It needs to be well above the WL so that on any tack you draw air not water when the engine is not running. How long the pipe is and if or not it has to be routed down for part of its travel is immaterial. There will be very little water in this pipe as it is so small Obviously the most direct route is best. Now when the engine stops the water in the pipe will join that in the system and the water level will quickly revert to the WL level outside the hull. Thats all there is to it.
The only thing to remember is that the valve or tee piece should be mounted higher than the rest of the system as the objective is to let air in at the top. Where it is in relation to the WL outside the boat is immaterial but where it is in relation to the water in the engine is important.
To be entirely clear then if you have a tee piece it should be mounted as high up on the engine as possible. The other end can be routed by any means to a skin fitting anywhere above the WL outside the boat but as high as possible. Usually somewhere just above or to the side of the exhaust outlet is a good place. If you do have an engine mounted with the heat exchanger below the WL the whole setup gets more complex because you can flood the engine by just opening a seacock. Forget it. I am pretty sure you won't have, but just to make sure no one screws up, what I have said only applies to engines where the manifold and valve gear is mounted above the WL
Hope this helps.
