T&T: props

[JHWardJr at aol.com]

Mark - I am sorry - the post was so long I left some things out:
1. max rpm is 2450 port, 2300 starboard.  9.5-10 kts with reasonably  clean
bottom and usually towed dinghy left at dock.  No smoke, higher but  in the
green temps.
2. yes, port tranny is 2.0 ratio and 1.9 starboard.  This is due  to the
failure of the original Velvet Drive tranny running in reverse to achieve
contra-rotation.  Design flaw that is not uncommon for '80s boats.   The
props
are reportedly 19 and 20P to make the math work, but I think they are  way
off.  Further, the prop DARs are very different.  I suspect one  was replaced
w/o regard for compatibility, only that it 'fit'.
3. kpg, my incorrect shorthand for nmpg.  sorry.  I was  distracted.  Jim

I still need a lead on a pair!


In a message dated 5/1/2009 12:00:51 A.M. Eastern Daylight Time,
trawlers-and-trawlering-request at lists.samurai.com writes:

Jim,
You didn't give us the most valuable clue about the proper  props for your
boat:  the max RPM you can achieve at wide open  throttle, and the speed
achieved thereby.

Also a note regarding  your mysterious mismatched RPM's on port and
starboard; many Lehmans came  with port and starboard transmissions with
slightly differing gear  ratios.  Ask Bob Smith at American Marine in
Kilmarnock, VA about  this phenomenon.  You may need slightly different
pitches on the two  props.

If you want to really delve into the subject of props, get Dave  Gerr's
book,
Propellor Handbook.

What's a KPG?  Perhaps you  mean nautical miles per gallon, or NMPG?

Mark Richter, Mechanical  Engineer



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