T&T: Detroits, impellers and keel-coolers

[Truelove39@aol.com]

IME, Detroits will run clean as long as they are  well-loaded. Unfortunately, 
by virtue of their scavenging design, if they  are run at low load or idled 
for prolonged periods, they will run "cold"  (internal temperatures-not cooling 
water temp.) and wet-stack, carbonize the  ports, crowns, chambers, valves, 
airboxes and nozzles. The rings will stick  and they will need a top overhaul 
in fewer than 10,000 hours; sometimes as  soon as 7500. Unfortunately, Detroits 
get a bad rap in some circles because of  overpowering. It helps if the 
injectors are properly-sized; some boats  have N80s when N55s would do if the extra 
power isn't  needed. But a 4-stroke engine (even turbocharged) will always be 
more  forgiving when idled and is a better choice if you will prolong light 
loads  or idle speeds. It's my personal rule-of thumb that the load should be  
kept within 40-80% for most of the time. 
 
I can't see the sense of going to a dry exhaust simply on account of  
eliminating impellers. Impellers, like other nondurable parts, are  fine if you 
simply change them, before they fail, at scheduled  intervals. Keel cooler 
installations of the protected, Fernstrum-type  or better, a simple split pipe or 
channel welded to the shell plating make  a lot of sense on a steel boat. But I 
wouldn't have one on a 'glass boat because  I don't think they can be adequately 
protected.
 
Regards,
 
John
"Seahorse"

> Dave Cooper said "Detroit's are not know as the cleanest burning  engines 
but 
again maybe I screwed something up in the design and building of  this 
system." and John Tones commented: "Our ex commercial fish boat has a  dry 
exhaust / keel cooler system and I would not give it up. It has to be  the 
most maintenance free system around for this application and in 15 years  we 
have not had a problem
including the cold run soot problem"

We  also have an x-fish boat running a Detroit 6-71 & dry exhaust and never,  
in five years, had soot problems either on start-up or running at any speed.  
As an aside we don't have soot problems with our Dickinson  stove  either.

It's been my observation it is a design (brand) of the engine, at  least on 
start-up which causes this.  We were side-tied to a big, new  crab boat in 
Wrangle some years ago (with our sailboat) when he started  up.  In fairness 
he did warn us that his engine blew soot when first  started -- wished I had 
moved :>).  I would not want a dry exhaust  if this might be a problem since 
the resulting mess would be a nightmare to  clean up everytime you started. 
In our case, like John Tones, I would 't  trade -- Our dry exhaust is simple, 
fool-proof and I hate impellers  also.