[PCW] Sno' Dog performance

[HClews@aol.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)