Simple really (well, nothing's that simple)... Higher speed engines put out
more power for less weight. The 100-hp Yanmars on Sno' Dog weigh a mere 571
lbs each (incl transmission). A John Deere (4045DFM) for example puts out
85-hp at 2500 RPM and weighs 960 pounds, similar to other slow-revving models.
True, there's some sacrifice of longevity, but not that much. How many
cruisers put 10,000 hours on their engines?
As for prop speed, yes, a larger, slower-turning prop is more efficient.
But the transmission can be used for this purpose by increasing the ratio.
Everything's a compromise, a large prop requires more draft. One of the things
I like about the PDQ is the shallow 2'-4" draft.
I think one reason the PDQ is so economical is its light weight. Of course,
there are trade-offs. A heavy trawler would probably outlast the PDQ in a
(very long) endurance test. But it might well cost more in the end if you
factor in initial cost plus added fuel consumption.
Henry
In a message dated 8/2/2006 12:33:52 P.M. Eastern Daylight Time,
mark424x@yahoo.com writes:
A little bit of a tangent, but I always wondered why some designs use high
RPM diesels, e.g. WOT=3800, vs many sailboats or passamakers with the high
torque low rpm diesels - I think these guys redline at <2500 rpm and generally
cruise in the 1200-1800 rpm range.
It would seem you'd get better efficieny with a slower turning prop with
large diameter and/or more pitch, especially if your wot speed is <18-20. Also
less noise, stress on the bearings, etc. Is it cost, power-to-weight, or ??
Thanks.
D C Mac Macdonald k2gkk@hotmail.com (mailto:k2gkk@hotmail.com)
Simple really (well, nothing's that simple)... Higher speed engines put out
more power for less weight. The 100-hp Yanmars on Sno' Dog weigh a mere 571
lbs each (incl transmission). A John Deere (4045DFM) for example puts out
85-hp at 2500 RPM and weighs 960 pounds, similar to other slow-revving models.
True, there's some sacrifice of longevity, but not that much. How many
cruisers put 10,000 hours on their engines?
As for prop speed, yes, a larger, slower-turning prop is more efficient.
But the transmission can be used for this purpose by increasing the ratio.
Everything's a compromise, a large prop requires more draft. One of the things
I like about the PDQ is the shallow 2'-4" draft.
I think one reason the PDQ is so economical is its light weight. Of course,
there are trade-offs. A heavy trawler would probably outlast the PDQ in a
(very long) endurance test. But it might well cost more in the end if you
factor in initial cost plus added fuel consumption.
Henry
In a message dated 8/2/2006 12:33:52 P.M. Eastern Daylight Time,
mark424x@yahoo.com writes:
A little bit of a tangent, but I always wondered why some designs use high
RPM diesels, e.g. WOT=3800, vs many sailboats or passamakers with the high
torque low rpm diesels - I think these guys redline at <2500 rpm and generally
cruise in the 1200-1800 rpm range.
It would seem you'd get better efficieny with a slower turning prop with
large diameter and/or more pitch, especially if your wot speed is <18-20. Also
less noise, stress on the bearings, etc. Is it cost, power-to-weight, or ??
Thanks.
D C *Mac* Macdonald _k2gkk@hotmail.com_ (mailto:k2gkk@hotmail.com)
