Table of Contents
Frequently Asked Questions And Inboard Tech Tips
CHOOSE YOUR PROP!
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Choosing The Prop For You
First we would like to say that
propping an inboard is not an exact science, the same prop moved
from one boat to another may not have the same results. There are
many factors to consider, for example engine HP and tune, hull
Usually the boat manufacturer has
optimized the diameter and pitch to match the engines
horsepower/transmission ratio to the maximum allowable RPM (+/- 200)
at wide open throttle. It's a safe bet to replace your prop with
the same size and pitch as the factory recommendation.
The first number of an Inboard prop
designation is the prop diameter the second is the pitch (example 13
X 14, 13 = diameter and 14
= pitch). You also
will have prop rotation to consider, LH left hand (counterclockwise)
or RH right hand (clockwise). This is determined by standing at the
rear of the boat looking forward. Another consideration is shaft
diameter, Inboard ski boats
will have a 1" or 1-1/8" shaft. They both use the same keyway,
prop nut and puller. Prop
designation is usually stamped around the prop hub near the nut.
Some earlier props were stamped on the hub between the blades.
All Johnson (OJ) or Acme props that are sold by Discount Inboard
Marine are standard cupped.
In the 70's and 80's prop selection was
simple compared to today's choices, it was the 3 blade NIBRAL (nickel,
bronze and aluminum). A few stainless props appeared but there is no
performance advantage like their would be on an I/O or outboard.
The 3 blade standard not CNC (computerized
numerical control) is a good choice if you use the boat for cruising and
general water sports.
= 4 Blade
During the 1990's the 4 blade (non CNC) was
seen as an upgrade to the old 3 blade. The fourth blade provided more blade
surface for better hole shot and sustained speed as well as a reduced
rooster tail with some boats. The fourth blade also added a better balance
to the prop for reduced vibration. It is common with PCM 1.23:1 ratio
transmissions to reduce diameter from 14" to 13" when switching from a 3
blade to a 4. Early Nautiques 89-96 may over rev at WOT when doing this but
they really come out of the hole strong and it is usually not an issue
unless you run the boat close to wide open throttle for extended periods.
Best = CNC Machined 3 or 4 Blade
The choice was simple back then -
choose the standard 3 blade or upgrade to the 4. Enter the CNC Acme prop
that is hailed by some as being revolutionary. CNC is a machining
process that mills small amounts of metal from the propeller to ensure
repeatable exact tolerances. This machining leaves small tool lines on
the face of the propeller and hub where the high spots of metal are
removed. In theory every prop would be perfect out of the box being
smooth and vibration free. Acme took this to a new level and designed a
3 blade with more swept blade surface to equal the 4 blade in
acceleration and increase top speed. (3 blades are theoretically more
efficient than 4). Most reduction gear transmissions still benefit from
running the 4 blade CNC prop. If you have a 1:1 ratio then we
would consider the 3 blade. CNC props are measured exact and not rounded
off like the non CNC, for example a 13 X 16 non CNC would have the same
RPM range as a 12.5 X 15.5 CNC.
(OJ) countered Acme with
their version of the CNC prop the XMP. These props have been around
since about 2004 and are getting very good reviews. The 4 blade seems to
be a good wake board and ski prop but the 3 blade with larger blades and
less pitch is supposed to provide even more acceleration for adding
ballast water tanks. If you
have a late model boat you may want to
check some of the online forums for recommendations on the latest
and greatest prop for your boat.
If you are not sure call or email us
and we will be happy to advise you on prop
selection. To help us determine what's best for you please
provide your current prop size and wide open throttle RPM. We can then
recommend a prop to utilize your boat's
engine RPM to match your application. Discount
Inboard Marine offers a 30 day evaluation period for any prop we sell.
We will exchange an undamaged prop within 30 days of purchase if you are
unsatisfied with the performance.
Discount Inboard Marine, LLC
2113 1/2 Wessinger Road / Chapin / SC 29036
Phone / (803) 345-0996 / Fax / (800) 510-9174