T&T: props

[John Sikora]

I don't know if PropScan expertise differs from location to location but I
used them on my Ocean Alexander a few years ago and they improved the
performance significantly. Well worth the expense. I also found them very
knowledgeable and willing to explain the details. I worked with the Chicago
location to have my work done.




________________________________
From:
"JHWardJr at aol.com" <JHWardJr at aol.com>
To:
trawlers-and-trawlering at lists.samurai.com
Sent: Thursday, April 30, 2009
10:21:32 AM
Subject: T&T: props

Something other than anchors!!

I am in the
prep stage of evaluating and improving my propulsion system  
(long story) on
my 42' Present (semi-displacement 35,000# full up hull).  I  
have not found
prop shops to be very communicative nor helpful by phone or 
email  (I guess
they perceive this is a waste of time - I understand), so I am 
appealing to
the list for some advice/guidance.  I know a little, but  
certainly not
everything.

1. My twin FL T160s do not pull equally (150 rpm difference in
single  
operation - tachs are accurate).  The engines seem fine - I suspect
the  props.
2. I am taking them off next month and having a shop inspect them
for  
actual pitch with a special computer system (PropScan) that claims to be
the  
endall for prop measurement.
3. I expect that my props are way off
(given other things I have found  POs 
did to my boat) and will be buying
new-to-me ones as they are not even  the 
same profile/design.
4. a fairly
sophisticated program I downloaded from Castle Marine leads me  
to further
confusion about sizing.  Says I need 24x16P props - I am  
(theoretically)
running 24x20P.
5. I will probably need to repitch and possibly re-bore any
used props I  
buy.  I hear you can change pitch by up to 2", but is
re-boring  expensive?
6. would love to have variable pitch, full feathering
props for single  
engine operation, but I am sure cost is just WAAY over the
top.  My boat  
actually runs pretty well with one engine windmilling, but I
am sure the drag is  
significant.  And yes, I have confirmed nothing untoward
is happening to  
the drivetrain.
7. I rarely push the throttles to the
desired power range (60-80%) for  
when twin engines operating efficient hull
speed (8 kts) is at sub-1600 rpm  
(that's one of the reasons I run single
engine at times).  I get 2kpg at  this 
speed as long as bottom is reasonably
clean, no wave/wind, etc).  I  do, 
however, push them every hour or so to
'blow them out' and get the turbos  
going.  Wastes fuel, though.  Should I
load the props up at cruise and  
acknowledge full rated rpm (2450) is not
attainable?  Seems it would bring  the 
prop/power curve closer at my typical
operation and be better for the  rings.

Anybody have any input?  Any prop
gurus amongst you?  Anybody  with similar 
setup (38' waterline, 35,000
weight, 320 hp in twin, 2.0:1  reduction, 
'geezer' speed cruise!) that can
share prop info?  Got a lead  for a pair of 
props (24x20P 1.75" shaft)?
Thanks in advance to all.  Jim in NC
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