Sven
Seglare
After our recent non-sailing experiences I've become somewhat obsessed with figuring out not only how much fuel we have but how much we are using.
In a recent discussion with a very experienced cruiser he questioned our cruising RPM, he thought 1,900-2,200 RPM was way too high for a Perkins 4-108M. I went back and re-checked ...
47 SHP @ 4000 RPM for high speed pleasure craft
45 SHP @ 3600 RPM for pleasure craft
37 SHP @ 3000 RPM for commercial
so my recollection of 3000 was correct. 80% of that would be 2400 which should mean that we are running at low RPMs if anything.
So, assuming that we have a 2:1 reduction in the transmission and using our 8" pitch propeller ...
Inches per revolution 8
Prop RPM 1000
Inches per minute 8000
Inches per hour 480000
NM per hour (knots) 6.58
Of course, that's assuming 100% pitch to speed conversion and in flat water we're probably seeing 5 knots at 2000 RPM (1000 RPM at the prop) which would mean about 75% of the theoretical pitch to speed conversion.
So, are we under-pitched or right on target ?
If our tank is really only 55 Gallons instead of the 80 gallon spec we have another problem too, but it sure would be nice to get some of these numbers under control
-Sven
In a recent discussion with a very experienced cruiser he questioned our cruising RPM, he thought 1,900-2,200 RPM was way too high for a Perkins 4-108M. I went back and re-checked ...
47 SHP @ 4000 RPM for high speed pleasure craft
45 SHP @ 3600 RPM for pleasure craft
37 SHP @ 3000 RPM for commercial
so my recollection of 3000 was correct. 80% of that would be 2400 which should mean that we are running at low RPMs if anything.
So, assuming that we have a 2:1 reduction in the transmission and using our 8" pitch propeller ...
Inches per revolution 8
Prop RPM 1000
Inches per minute 8000
Inches per hour 480000
NM per hour (knots) 6.58
Of course, that's assuming 100% pitch to speed conversion and in flat water we're probably seeing 5 knots at 2000 RPM (1000 RPM at the prop) which would mean about 75% of the theoretical pitch to speed conversion.
So, are we under-pitched or right on target ?
If our tank is really only 55 Gallons instead of the 80 gallon spec we have another problem too, but it sure would be nice to get some of these numbers under control
-Sven