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Perfect Pitch
By Eric Colby
Photographed by Boating Mag
March 2001
What’s the most cost-effective way to improve
your inboard-powered boat’s performance?
A good prop. Here’s how to get one.
We'd heard the stories and, to be honest,
they all sounded too good to be true. People were raving. After their
propellers went under the gun of the Prop
Scan system, customers said their boats ran faster, burned
less fuel, and were vibration-free. Heck, throw in the spray-on hair
in a can and we could do an infomercial. This was something we had
to see for ourselves.
Prop Scan is a computerized propeller measuring system that allows
a repair shop to tighten prop tolerances to degrees that aren't possible
with manual methods. Developed in Australia in 1980, Prop Scan was
introduced in the United States in 1995. Nationwide, 35 shops currently
use the system.
We put the program to the test on a 46' Post sportfisherman, Scirocco,
owned by Brian Geary, which was powered by a pair of 625-bhp Detroit
Diesel 6V-92TA engines with DDEC engine management systems. We ran
Geary's boat with the stock propellers and then took them to WildCat
Propellers of Chesapeake, Virginia, to have them put through
Prop Scan's diagnostics. The next day, we ran the boat again with
the tuned wheels.
The original four-bladed Nibral propellers were marked 26"-by-28"
and pushed Scirocco to 37.8 mph at 2390 rpm, burning 68.2 gph. After
the propellers were tuned, the boat ran 38.8 mph at 2380 rpm and burned
75.7 gph-an increase in speed and fuel consumption. But when we backed
down to the original top speed of 37.8 mph, our fuel consumption dropped
by 4 gph (from 68.2 to 64.2). And at the original high cruising speed
of 33.5 mph, fuel consumption dropped yet again, from 54.3 gph to
50.
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GOOD VIBRATIONS? NO SUCH THING.
Vibration is one of the primary complaints
customers have with their boats before using Prop Scan, says WildCat
Propellers president Larry
Carlson, who was the first to use the system in the United
States (see "Setting
the Standards"). Vibrations
happen when a propeller is out of balance. This condition also causes
a loss in speed or an increase in fuel consumption because it takes
more power to turn a jittery prop. Balance the prop, remove the vibrations,
and you should see immediate gains in performance, which is what we
were after.
The Prop
Scan device looks like a large record player. The propeller is secured
face-up on the rotating table, a turntable equipped with an optical
sensor that measures degrees of rotation. With the prop in place,
the Distance Travel Sensor (DTS), which looks like the record player's
needle arm, is placed on the propeller blade. The DTS' optical sensor
moves up and down as it rolls along the face of the blade from the
leading edge to the trailing edge.
Both the rotating table and the DTS are linked to a computer programmed
with Prop Scan software. The computer measures the vertical distance
that the DTS travels over one degree of rotation and gives you what
the exact pitch of the blade should be. For our test, WildCat used
the new Prop Scan Enhanced Propeller Standards software. "We're
seeing pitch progression across a radius from the root out to the
tip, and we're seeing the hydrodynamic shape of that propeller,"
says Carlson. "All of those things are critical to propeller
performance."
Both the rotating table and the DTS are linked to a computer programmed
with Prop Scan software. The computer measures the vertical distance
that the DTS travels over one degree of rotation and gives you what
the exact pitch of the blade should be. For our test, WildCat used
the new Prop Scan Enhanced Propeller Standards software. "We're
seeing pitch progression across a radius from the root out to the
tip, and we're seeing the hydrodynamic shape of that propeller,"
says Carlson. "All of those things are critical to propeller
performance."
Although there are no standards for propellers here in the United
States, there are classes set by the International Standardization
Organization (ISO) for accuracy. Class III is the least accurate,
Class II is medium, Class I is precise. Class S is the highest level
that ISO prescribes. Prop Scan claims to reduce inaccuracies by half
of ISO Class S, which Carlson says is the highest standard in the
business. Most props are either Class III or Class II.
Because all Prop Scan customers' prop specs are kept in a database,
any shop in the network can access them (a serial number is stamped
on the prop hub). If a customer is cruising outside his home area
and bends a prop, he can find a local Prop Scan shop and get his props
repaired to the precise specs.
FLATTENING THE BUMPS
Our props' diameters actually measured
25.75", about 0.25" shy of the original spec. That's from
normal wear and tear. After entering that figure into the computer,
it calculated where on the blade the pitch measurements would be taken.
The computer separates the blade into 10 radii from the center of
the hub out to the tip of the blade. For Class I or higher, measurements
start at radius 3, which is the area closest to the hub. The computer
tells the technician how far to move the DTS so it can take measurements
at radii 4 through 9.
The software revealed that our props were repitched from their originally
stamped 28" pitch to 29". The problem was the way the pitch
was changed. It wasn't continuous across the blades.
The Prop Scan computer displayed the pitch of our props in two ways.
The first was a series of bar graphs that showed the pitch of each
blade in different colors onscreen. On our first prop, the average
pitch for all four blades was 28.623", with blade 2 measuring
28.17" and blade 4 showing 28.872". The second prop had
a mean pitch of 28.839" with individual measurements that were
much closer. Blade 2 was the lowest at 28.75"; blade 3 was 28.845".
The second method was displayed on another screen, which showed the
pitch progression across the blade from the leading edge to the trailing
edge in a line of dashes. A straight line means perfect pitch progression.
On our test props' blades, the dashes were wavy, revealing a series
of convex humps at radius 5 on blades 1, 3, and 4.
When the water was hitting the humps on
the face of our props' blades, it was shooting straight out past the
tip, causing ventilation and reducing the blade's grip on the water.
Additionally, a concave (cupped) or flat blade grips better and reduces
ventilation.
Once the Prop Scan software determined that the diameter and pitch
of our propellers were actually 25.75" and 28.74", Carlson
punched the measurements into the computer along with the desired
maximum 2350 rpm. The Detroit Diesel 6V-92TAs in our test boat were
rated to achieve 2400 rpm. Carlson targeted 50 rpm below that because
few people run at wide open. Dropping down in rpm required more pitch,
so the computer established that our new pitch would be 29.23",
a change of about a half an inch.
BEFORE
TEST BOAT POWER:
Twin 625-bhp Detroit Diesel 6V-92TA DDEC in-line-6 diesel inboards
with 552 cid, 4.84" bore x 5.0" stroke, swinging 25.75"
x 28.74" four-bladed nibral props through 1.5:1 reductions.
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AFTER
TEST BOAT POWER:
Twin 625-bhp Detroit Diesel 6V-92TA DDEC in-line-6 diesel inboards
with 552 cid, 4.84" bore x 5.0" stroke, swinging 25.75"
x 29.23" four-bladed nibral props through 1.5:1 reductions |
Once the computer provides the measurements,
changing the shape of the propeller still requires manual labor. The
propeller is bolted to a stand and a technician pounds on each blade
face with sledgehammers to change the pitch. After he hammers on a
blade for a while, the propeller is put back on the Prop Scan equipment
and measured again. The closer the technician gets to the desired
pitch, the more frequently the measurements are taken.
The pitch on our finished props measured 29.175" and 29.186",
which is within .06" of the desired 29.23", and had a slight
cup. After the pitch was approved, our props were balanced on a rotisserie-looking
device and polished. The entire process takes at least three days
for two props, but the WildCat crew of Dave Waddington and Troy Erb
burned the midnight oil to finish our set overnight so we could test
in similar weather and sea conditions.
LONG-TERM SAVINGS
Reshaping our props cost $560.53 each.
I called a handful of propeller shops and asked what the cost would
be to do the same job by manual methods-the prices ranged from $400
to $475.
If you wanted to get the same performance increases by reworking your
boat's engines, Geary's mechanic, Tommy Snook of Western Branch Diesel
in Portsmouth, Virginia, said you'd need about 25 horsepower more
per engine for every mile per hour you wanted to pick up. That could
involve changing injectors or installing different turbochargers.
Capps Boatyard, where we had our boat hauled and returned to the water,
estimated a cost of about $4,000 per engine to pick up 3 mph. Or you
could be in Geary's situation. The 625-bhp 6V-92TAs cannot be souped-up
anymore, meaning the props were the only place Geary could turn to
boost performance.
| Model |
Post 46 |
| LOA |
46'9" |
| Beam |
15'9" |
| Draft |
4'4" |
Displacement
(lbs., approx) |
49,668 |
| Transom deadrise |
5° |
| Bridge clearance |
19'0" |
| Minimum cockpit depth |
2'1" |
| Max. cabin headroom |
6'6" |
| Fuel capacity (gal.) |
635 |
| Water capacity (gal.) |
120 |
By comparison, $1,121.06 for our two props
looks pretty good. Using less fuel on the way to an offshore fishing
ground could give more range. And the reduced vibration might mean
you don't scare off as many fish. Getting more efficiency out of a
cruiser could also mean one less stop on a long cruise.
As they say, your results may vary. But analysis and estimates are
free at any Prop Scan shop. Considering what you'd spend to soup up
or replace your inboard engines, at least getting your props checked
out by a Prop Scan shop is a smart place to start in your search for
more performance.
SETTING THE STANDARDS
Larry
Carlson was a boatyard owner who had difficulty finding propellers
that would keep his customers happy. "I called the manufacturers
to see if we couldn't establish some common ground about the quality
of the propellers," says Carlson. "I said, 'Do you build
propellers to any standards?' and they replied, 'We build propellers
to American standards.' I asked who set the American standards. They
said, 'We do.' "
When he asked what the American standards were, Carlson was told that
information was proprietary.
Wrong answer. Carlson had heard about a company in Australia that
had a computerized
propeller measuring system. The inventor, Terry Ryan, was in Canada
and agreed to come see Carlson. "He talked to me from about five
in the afternoon until two in the morning, and everything he said
made sense," Carlson says.
"The next day I asked my wife if she'd like to go into the propeller
business."
It's no small commitment. Ryan charges $56,000 for Prop Scan equipment
that can handle up to a 60" wheel and $25,000 for a smaller system
that can take a 20" model. Then you have to pay your way to Australia
for a two-week training session with Ryan at his shop, Propeller Dynamics.
Carlson says the investment was well worth it. "When we looked
at the books after the first year, we were making money with the prop
shop and doing just okay with the boatyard, so we decided to sell
the yard." back
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