A why is it so type question....

My stbd engine (SP135) has been producing a small amount of steam out the exhaust - nothing from the port. No other symptoms. It wasn`t a huge amount but enough to cause me thought. Checked the raw water impellor - good so I pulled the exhaust raw water injector and sure enough, it was cactus - severely corroded. I`m currently having a new one made out of stainless. I went through the same exercise with the port engine a year or so ago.

The old one had a hole corroded in the inner sleeve immediately opposite where the salt water was introduced into the outer casing. This meant that the water flowed directly into the exhaust gas stream rather than being introduced into it at the circumference of the inner casing (hope that is clear).

And my question is - why should this matter - why should being squirted directly into the exhaust gasses rather that sprayed in via a ring produce steam -way down the back at the exhaust; after all the amount of water is the same?

Just curious.

That ring of holes is called a shower or spray head. I'm sure there are other names.
It is meant to spray the water in the circumference around the hole the exhaust gasses exit so the water falls through the exhaust gasses.
That cools the gasses.
Without that circular spray the gasses will NOT be cooled and likely will burn parts of the rest of the exhaust system.

The spray also contracts the gasses and dampens exhaust noise.

It is not just the volume of water but how it is used that is important. Of course the volume must be adequate but it also must be sprayed effectively.

I also suggest you be very suspicious of the other engine shower head. Many engines are severely damaged when water from a leak such as you describe runs back into the engine. Can damage the turbo, rust a cylinder, hydrolock the engine causing expensive damage. Go after it.

When the shower head clogs or fails, it’s not unusual that the top of the wet exhaust hose burns through as it gets the 800 degree or so gasses without cooling. That causes your engine room and parts of your boat to fill with soot. It can be a terrible mess to deal with and stinks. There are worse outcomes, but this is the typical one.

The shower heads on my cat 3208 naturals are stainless, but the risers are cast iron and shed a little. I typically find that every 1.5 years an inspection leads to a little cleaning of the restrictions with a narrow screwdriver. I rarely hear people recommending this, but mine are reliably clogging at those intervals and lots of examples over the years on other engines.

Thanks C lectric and ghost

So........in summary. the water sprayed from the "shower head" mixes with the exhaust gas, cool, contracts and quietens it, and as a consequence the water doesn`t get a chance to turn to stream, rather than in a big squirt (like my busted one was) ,hitting the super hot exhaust which causes it to vaporise (into steam) and get pushed out the exhaust.

Got it - thanks

Check the top and bottom of your wet exhaust with a pyro and see how big a differential there is and if there are any hotspots. In addition, not all diffusers are equal. Some of the early Taiwanese ones were pretty hopeless.

Regards

Paul