The diagram from DeniseO30 is great. The important question of how far below the manifold is the water injected into the exhaust was not answered.
If injection is too close to manifold, or not low enough below the level at the exhaust ports
(think.. gravity, hobby horsing, etc.), raw water
could reach up into the exhaust valves, (rust problem, hydraulic lock).
On some Perkins 4.108
, the mixing elbow water injection is just inches away and just below the exhaust manifold. If the aluminum
mixing elbow's internal baffle is somewhat corroded, then the water is really close to coming into the engine.
To remedy this, probably can use some threaded, black pipe 45 degree elbows, couplings and nipples threaded to fabricated carbon steel
plate flanges on each end, to make a "dry riser" as the original question was asked, (use one pipe size larger than the existing exhaust hose).
Consider this: One of the plate flanges, with a 45 elbow threaded into it, attaches to the exhaust manifold, going up with a pipe for as long as needed ( think water rising, volume of hoses and w.lift muffler, hobby horsing at anchor
, etc, not only necessarily above W.L. and not needing a vented loop here) and down again after a second 45 elbow with a pipe of at least 8" long which screws into the second flange which then attaches the original aluminum
mixing elbow and raw water
nozzle ( need a vented loop on this feed hose) and exhaust hose pointing to water lift
muffler. Sort of a 'goose neck' spacer with flanges between the manifold and the mixing elbow.
Of course, must wrap this new pipe and flanges with insulation
because this new section is not water cooled and will get very hot there.
Also, should be supported to the engine itself to avoid cracks from engine vibration. Flanges can be reused if ever needed to replace the pipe fittings later since everything is threaded. Use hi-temp. anti-seizing compound when threading things together.
The vented loop for the raw water injection should have some small tubing coming from where the original little valve is, through maybe the exhaust hose's down slope or even better, somewhere just above the water line, outboard
, where you can see it.
Maintain the large loop, minimum 12" above the loaded and moving W.L. before exhaust hose exits at the stern and add a flapper valve outboard