- Even though we don’t have a large choice of what gear ratio our lower unit has, knowing how the gear ratio works with propeller sizing to optimise our performance is a benefit.
- Getting the right prop for your boat can be achieved with some disciplined testing of a range of propeller sizes.
Jim Russell, of AeroMarine Research, describes
how drive gear ratio works with propeller sizing to optimise performance …
Every engine has a maximum RPM where the engine
will achieve its horsepower rating. Manufacturers set the lower-unit (drive)
gear ratio and recommended propeller size to allow the engine to reach this
maximum RPM and engine HP.
The design, weight and length of the boat,
horsepower and torque of the engine, gear ratio of the drive and size of the
propeller all work together to affect the boat’s performance. Prop shaft torque and propeller RPM determine which propeller designs will operate
best. For example, more torque at the propeller is needed to turn larger-diameter
and/or larger-pitch propellers.
That’s a lot of variables,
which makes the whole issue of gear ratios and propeller sizing somewhat complex.
There is an optimal propeller
pitch and diameter that will do the best job for a boat with an engine of a specific
HP at a certain prop shaft RPM. The prop shaft speed is determined, in part, by
the gearing between the engine drive shaft and the propeller shaft in the lower
unit. Engine manufacturers install lower-unit/drive
gear ratios that enable the most efficient combination of propeller diameter and
pitch, which can result in the best combination of thrust and speed for
particular hulls and varying operating conditions.
For example, larger-diameter propellers can provide
better acceleration and less slip, and are more economical on heavily loaded
boats, but they generate more drag and require more engine torque at the
propeller shaft. Higher-pitch propellers can deliver
faster top speed and reduce engine RPM, but have increased slip
and poorer hole shot. Propeller performance is very much dependent on the
propeller shaft RPM at which it operates. Hopefully there is a range of
pitch/diameter propellers that best suit your engine, hull and application.
What do the gears do?
In the simplest of terms,
the propeller is driven by the prop shaft, and the propeller shaft is driven by
the engine (drive shaft). The gear case (transmission) can change the speed
that the propeller shaft rotates at compared to the engine drive shaft.
The propeller rotates at
the same speed as the propeller shaft, because it’s fixed to the shaft. The propeller shaft turns slower than the drive shaft due
to reduction gears between the drive shaft and propeller shaft. The purpose of the gear ratio is to reduce the
speed of the propeller shaft so that an optimum range of propellers (pitch and
diameter) can be used efficiently.
The terms ‘gear ratio’ and
‘gear reduction’ can be confusing.
A gear ratio of 2.5:1 means
that there is a reduction, so that the prop shaft turns 0.4 times for every revolution
of the engine drive shaft.
A higher gear ratio (say,
2.5:1 compared to 1.75:1) has more reduction, and means a comparatively slower
prop shaft and more torque at the prop shaft.
A lower gear ratio (say,
1.92:1 compared to 2.08:1) has less reduction, and means a comparatively faster
prop shaft speed and less torque at the prop shaft.
Here’s how it
‘Gear ratio’ refers to the
number of drive shaft revolutions for one revolution of the propeller. So if an
engine has a 2:1 gear ratio, it means that for one revolution of the propeller
the engine drive shaft turns twice.
We can find the
propeller shaft RPM by dividing the engine drive shaft RPM by the gear ratio.
For example, if
our engine operates at 4000rpm with a 2:1 gear ratio, its prop shaft will turn
Prop shaft RPM
= 4000rpm / 2.0 = 2000rpm.
If our engine
operates at 4000rpm with a 2.50:1 gear ratio, its prop shaft will turn at 1600rpm.
Prop shaft RPM
= 4000rpm / 2.50 = 1600rpm.
(numerically) gear ratio means that the speed of the propeller shaft is slower and
thus there is a lower number of propeller revolutions than engine drive shaft
So why aren’t all gear
ratios the same?
Perhaps if all engines used
low gear ratios (say, 1:1), the prop shaft would a have higher RPM and our
boats would go faster. Well, it’s more complicated than just getting faster
prop shaft rotation. Very small gear ratios generate high prop shaft speed, which
results in much lower prop shaft torque, and that means we can only use very
small-diameter propellers – creating very little thrust. And we need the
propeller thrust to push the boat along. So usually, we find that higher-horsepower
motors have lower gear ratios than lower-horsepower engines, which typically
have higher gear ratios.
diameter and pitch must be matched to the hull design, weight, engine power,
operating conditions and desired speed. There is a huge range of propellers
with different diameters and pitches available, but it’s not unlimited.
The purpose of
the gear ratio is to reduce the speed of the propeller shaft so that a proper-diameter
propeller can be used, which will give the appropriate thrust that our boat
needs to move.
Gear ratio availability
It’s much easier to change to a
different propeller than it is to change the gear ratio of our lower unit. Outboards
sometimes have a choice of gears, but manufacturers generally offer only a single
gear ratio that they think will suit most applications. Sterndrives usually
have more options for gears. For example, the Mercury Bravo One SM drive comes with
gear ratios of 1.35:1 and 1.50:1. Some outboards have choices too – Seven
Marine 627 outboards, for example, are offered with a range of gear ratios from
2.33:1 to 1.47:1. Mercury Racing engines offer different lower-unit designs
with options for different gear ratios, and Yamaha offer some of their
outboards with standard and ‘High Thrust’ gear ratio versions.
right gear ratio/propeller?
Getting the right prop for your boat can be
achieved with some disciplined testing of a range of propeller sizes. Ideally,
you’d like to have a gear ratio and propeller sized so that your boat can take
the fullest advantage of the engine HP and then torque generated at the
propeller. Normally, when your boat is going at full speed (wide open throttle),
the engine speed should be at the maximum RPM specified by the manufacturer.
If, at WOT, the engine speed (on the tachometer) is
lower than the OEM-specified maximum RPM level, it means that either the gear
ratio is too low or the prop is too large (pitch or diameter or both). While
the larger prop can improve your hole shot, you’ll suffer at top speed because
it has insufficient torque to achieve full engine RPM. This situation will also
overload the engine and can eventually cause failure. This is similar to
driving your car up a steep hill in fourth gear instead
However, if, at WOT, the engine speed (on the
tachometer) is higher than the OEM-specified maximum RPM, this indicates that
either the gear ratio is too high or the prop is too small. This is similar to
driving your car at 70mph in first gear. This set-up can also make it more
difficult to get up onto the plane. It’s possible that a higher top speed might
be achieved, but a higher propeller RPM can also cause cavitation
(inefficiency) and even blowout, thereby ruining performance.
Some applications do really
well with higher gear ratios. For example, Yamaha High Thrust 4-stroke outboards
are built for jobs that call for more muscle, like pushing a heavy pontoon
boat, adding extra kick to a multi-species boat or manoeuvring a sailing boat
through a crowded marina.
Their larger lower units and higher gear ratios
produce more thrust than conventional outboards of the same horsepower using
standard propellers. The 60hp Yamaha conventional 4-stroke engine has a gear
ratio of 1.85:1, whereas the High Thrust version comes with a 2.33:1 gear
ratio. The higher gear ratio results in lower propeller RPM, which generates
more torque, allowing the engine to use higher-diameter, lower-pitch propellers
for real ‘grunt work’. In the case of this version of the 60hp engine, the gear case is larger, generating more drag and lower
available top speed – so this engine would not be suitable for lightweight
boats with a desire to go fast.
Sometimes prop sizing just
can’t do it
Sometimes the right prop
just doesn’t seem to be available. In these situations, a different lower-unit
gear ratio can change the prop shaft RPM and torque, which can open up an
opportunity for a different range of pitch/diameter propellers.
There are some cases where
optimising propeller diameter/pitch can only partially improve an incorrect
gear ratio. If a set-up has a drive
with too high a gear ratio (2.14:1 vs 1.61:1) and is compensated by using a
more aggressive propeller, the desired performance may be achieved but
excessively high torque on the drives can cause premature failure. Similarly,
if a boat set-up has an engine lower unit with too low a gear ratio (1.68:1 vs
2.08:1), it could be difficult to find a propeller that is large enough to
optimise performance, even though the engine has HP to give.
In any of these cases, a different gear ratio
selection would improve the propeller selection range available, and optimising
prop sizing and engine power/RPM is achievable for better performance.
How to find your gear ratio
If you don’t know what gear ratio your engine has, and can’t find the
specification with the manufacturer, here’s how you can check the gear ratio
To be safe, remove the ignition key to avoid accidental starting. Remove
the spark plugs and shift the motor into forward gear. Place a piece of masking
tape on one of the blades and draw a line on it with a black marker. Slowly
rotate the flywheel clockwise (while looking down at it) until the mark on the
blade reaches the 12:00 o’clock position (centred underneath the motor’s trim
tab). Now mark the flywheel for reference and rotate it exactly three turns
clockwise. The propeller will turn at the same time, so now observe the new
propeller blade mark position.
location of the propeller blade with the starting position to determine the
lower unit’s gear ratio. For example, if the propeller shaft has turned one
full revolution and then stops at about the 4:00 o’clock position, then the
shaft has turned 1.33 times compared to the engine’s three revolutions, so the
gear ratio is 2.25:1 (or a .44 reduction at the propeller).
The speed prediction formula
There is one last piece of information that can be
very valuable when estimating the potential speed that a particular gear ratio
and propeller size can deliver.
speed of your hull can always be
calculated using an engineering formula. While some of the variables for the
formula may require some experience to apply, the formula works for all hulls,
all designs of boats, all power and drive packages, and all speeds.
V = [RPM x PP x
(1-S) x GR] / 
RPM = 6000rpm (rev/minute)
PP = 24 (inches)
S = 0.12 (12% slip)
GR = 1.87:1 gear ratio
(0.535 reduction at prop)
= [6000 x 24 x (1-0.12) x .535] / 1056 = 64.2mph
The bottom line
The performance effect of lower-unit gear ratios is
often a confusing subject. Even the terminology used is sometimes baffling.
purpose of the gear ratio is to reduce the speed of the propeller shaft so that
an optimum range of propeller sizes can be used efficiently. Higher gear ratio drives have more ‘reduction’ and
cause the propeller shaft to turn slower. Sometimes this higher ratio allows
the use of a range of bigger propellers that are more suitable for a boat’s set-up.
Even though we don’t have a large choice of what gear
ratio our lower unit has, knowing how the gear ratio works with propeller
sizing to optimise our performance is a benefit.
Safe powerboating! Wear your kill cord!
Jim Russell is a professional engineer with a mechanical and aeronautics
background. Currently living in Canada, he has done extensive aerodynamic
research at the University of Michigan, OH, and the University of Toronto,
Canada, and marine research at the NRC water channel laboratory in Ottawa,
Canada. His published works and papers are highly acclaimed, and are
specifically related to the aerodynamics and hydrodynamics of high-performance
catamarans and tunnel boats, as well as vee and vee-pad hulls.
Russell has designed and built many tunnel and performance boats. As a
professional race driver, he piloted tunnel boats to Canadian and North
American championships. He has written powerboating articles for many worldwide
performance magazines and has covered UIM and APBA powerboat races. He has
appeared on SpeedVision’s Powerboat Television as a guest expert on
‘tunnel hulls’, and was performance/design technical consultant on National
Geographic’s Thrill Zone TV show and editorial consultant on the
Discovery Channel’s What Happened Next?
Russell is the author of the books Secrets
of Tunnel Boat Design and Secrets of
Propeller Design. His company designed and published the well-known
powerboat design software ‘Tunnel Boat Design Program’ and ‘Vee Boat Design
Program’ specifically for the design and performance analysis of tunnel boats,
powered catamarans, and performance vee and vee-pad hulls. ‘Jimboat’ is recognised for his advanced
aerodynamic and hydrodynamic research and consulting on powerboat design,
performance analysis, safety analysis, accident investigation, expert witness