Don't forget when you are calculating your clearances, that your new
engine/transmission combination will likely result in the drive flange being
in a different position fore and aft than the old one. This is not
necessarily a bad thing, especially if you want the prop a bit further back
to get more clearance, and that is how it works out. A "drive saver" would
set the prop back a bit too.
Steve Thomas
----- Original Message -----
From: "Carl Gerstle via CnC-List" <[email protected]>
To: <[email protected]>
Sent: Sunday, November 09, 2014 9:59 AM
Subject: Stus-List 35-1 drivetrain replacement (prop, engine HP,transmission
ratio)
Having searched the archives and not found this info, I feel the need to
ask...
I am committed to a re-power this winter and am surprised at the
difficulty in choosing an appropriate prop which then leads to the
appropriate engine HP and transmission choices. I already have input from
two other 35-1 owners but am looking for a wider array of input. My old
engine/tranny/prop is the venerable Atomic-4, 1:1 tranny, 12x6 folding
prop. The new engine will be a BetaMarine... but either 20 or 25 HP
depending on how much power a prop that FITS can absorb through a choice
of a 1.45:1, 2:1, or 2.6:1 transmission.
As I really don't want to face the added expense of upsizing the shaft
from 7/8" to 1" (even a 14" prop can be safely driven on a 7/8" Aquamet
shaft) or worse - moving and replacing the strut to gain prop diameter -
please provide whatever real-world info you have on the prop (make,
diameter, pitch); engine HP, and transmission ratio that you are using on
your 35-1.
It turns out that the prop make/model is critical due to the effective hub
length. For instance, my current folder places the blades 5 5/8" behind
the strut (5" hub, with 5/8" exposed shaft). The prop I am considering
(Flexofold 13") has only a 4.5" hub length, while a Martec 14" appears to
be a beefy 9" from hub to blade pivot. So the length of the hub has a
large effect on tip clearance. As Dave Gerr (The Propellor Handbook)
recommends an absolute minimum of 10% clearance (a 14" prop thus needs 7"
+ 1.4" clearance from shaft center to hull) - I only have 8" clearance
with a 4.5" hub) - a longer prop hub allows for greater prop diameter (a
very good thing). So real-world experience - including your experience
with speed, prop noise, and vibration from using a small blade tip to hull
clearance would be invaluable to me.
The problem is complex enough that Flexofold actually refused to recommend
a prop, tranny drive ratio, and HP requirement; and recommended that I
work with a boat-builder to work out these choices.
Your actual numbers and knowledge will be greatly appreciated.
Thanks,
Carl Gerstle
Impulse
C&C 35-1 #149
Portsmouth, RI
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