exhaust elbow design - art or science

In chasing down a lot of steam from our port engine (raw water pump impellor) I checked the exhaust elbow - made of steel and badly corroded. Bummer! Up to my local stainless guy for a new one (as an aside, I dropped the old one in at 9.00am and picked up the newly manufactured heavy guage stainless one at 3.00pm!!) - he reused the base flange that bolts to the manifold) and it got me thinking about the effect of the design of the elbow could have on engine cooling.

My corroded one was not the original however we followed the design ie diameter of inside pipe - here`s the question - if the design of the elbow restricted the through flow of water ie the water space in the jacket was too small - would this reduced flow increase engine temp - and vice versa.

Are original elbows designed with any great science or are they basically the same - I have seen some with the water injected into the exhaust through a series of vents round the circumference of the inner exhaust pipe and the outer, or in the case of mine - no restrictions. Am I over thinking this?

Your question relates to the hydraulic water flow in the mixer. There is a lot more to it than that, but let me address the hydraulics:

Design for pressure drop in the mixer and number and size of holes or just an annulus is all pretty much based on experience as to what works. But there are some simple tests to see if it is working. First run the engine at idle and check the hose temps below the mixer with an IR gun. They should be in the low 100s, 150 tops. Then go on a seatrial and check it at slow cruising rpms, high cruising rpms and wot.

If your mixer keeps the hose at decent temps throughout the rpm ranges, then it is ok.

But there are geometry considerations that are at least as important as raw water flow: height above the water line and a self draining mixer. See the pics below which are courtesy of Tony Athens on boatdiesel.com.

I'm going to strongly suggest that you go to Seaboard Marine sbmar.com
and look up Tony Athens articles about exhausts. He has written and offers freely what he has learned about marine exhaust systems and how the poorly designed??? systems most builders put aboard boats can ruin engines and do so very frequently.

Use the info to avoid the the problems so many people face.

Hi C and Dj, thanks for taking the time to reply - the Seaboard Marine site is great. Happily, the overall design of my system ticks all the boxes.

Dj, you really answer my question re checking temps and as a rule of thumb, if the pipe is cool, the design is cool (groan - sorry).

I`ll run the check at the weekend.

Thanks again

On some boats the VOLUME of cooling water is too large for the system so engine power is required to push the water excess out.

A bypass valve with an overboard discharge is required to limit the volume sent to the exhaust ..