T&T: air circulation

[Richard Tomkinson]

My Taiwan Trawler, like most others in the 32 to 45 range is built with two 
transverse bulkheads which more or less isolate the hull space into three 
compartments. The space between the hotel room (people space) and the hull 
is mostly not ventilated except for the engine space as the external air 
entry is generally let into the engine room and the engine then draws from 
that.
This means that the engine breaths hot (~95 degrees in my case) air, and 
that if the engine did not draw the air in this way, the engine space temp 
would be much higher.
Putting these two together it seems to me that a system of having the 
external air enter and move through the enclosed space (which is about raw 
water temp) before it comes into the engine room, then draw some of that air 
at the cold temp directly to the engine would do the following:
increase engine efficiency or at least greater HP per given RPM,
decrease stale air / condensation problems,
reduce engine room temp,
increase vessel longevity due to reduced environment for dry rot and other 
afflictions of the stringers.

This is all so easy that there must be a reason the mfgr does not design 
this in.
Are there reasons that the compartments must not be linked (more that they 
are)?
Sound issues are manageable, fire spread issues are manageable (and required 
if in 'passenger' service).
Any comments? Anybody actually make changes to address the above? Are the 
promised greater engine efficiencies fiction?
Richard Tomkinson