T&T: Engine efficiency: fixed oversize prop

[Robin Brueckner]

Two items:

(1)  "...Diesel engines, on the other hand, have no throttle. They suck in the
same amount of air each revolution..."

That doesn't sound right for DD. I sure don't understand if it is. DD natural
engine air is pumped in by gear driven blowers (directly RPM dependent) and
turbo engines are  pumped with air by the turbo(s), admittedly only at higher
RPM's

(2) On the other hand, lower RPM and increased loading does offer some
improvement for DD. Here is what a Hatteras owner said when he investigated
and overpropped his 650 HP 12V71TI's on a 61ft Hatteras motoryacht:

"...There is also a gain in turning the engine slower. If you take a look at
the fuel consumption curves on these engines they run about 18 HP per gallon
per hour at WOT fully loaded. When you reduce the RPM to 1200 they run about
19.5 HP per gallon per hour fully loaded..."  (about an 8% improvement)

His overall fuel consumption at 10.5 knots was decreased from 19 to 15 gallons
hourly running at 1150 RPM instead of 1500 RPM. (about a 21% improvement.) A
larger prop ( 35" x 35" five blade from 32" x 31" prior) increased his turbo
exhaust temp from the old 375 degrees at 1500 RPM to about 575 degrees at 1150
RPM.
 He picked the particular props because another owner sold them at a good
price and he lacked clearance for larger diameter ones.

So in this case it appears the engine provided about 1.5 GPH saving, the
bigger prop about 2.5 GPH...so the prop contributes 2.5/4.0 or abut 63% of the
improvement...

He "derated" his engines to protect them by adjusting the governor to a lower
maximum allowable RPM. This is easy on DD as they have variable speed
governors.

Rob Brueckner
1972 Hatteras YF
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