- Propeller performance is very much dependent on the propeller shaft RPM at which it operates.
- 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 particular prop shaft RPM.
- Measuring slip is a good method for comparing two set-ups on the same rig.
- There are many variables that influence the performance of a propeller, including prop diameter, the number of blades, blade shape, blade thickness and pitch.
Jim Russell, of AeroMarine Research, explains the concept of ‘propeller pitch’ …
Optimising the performance of your boat is often most successfully done
by getting the right propeller. While choosing the best prop for your boat
often involves dedication to testing and comparing results, this single exploit
can be the most rewarding in terms of performance gains ‒ and it all starts
with understanding what the variations in propeller designs can mean to
One of the most common questions I hear regarding propeller selection is
‘what is pitch’? The engineering theory around ‘how a propeller works’ is really
a discussion for another article, but understanding ‘propeller pitch’ is key in
undertaking your mission of propeller selection.
There are many variables that influence the performance of a propeller, including
prop diameter, the number of blades, blade shape, blade thickness and
pitch. And there are some more-difficult-to-understand features, such as blade
section type, blade skew, blade contour, cup, camber, rake, hub design, rotation
and disc-area ratio.
Then there are operational
issues like propeller cavitation and ventilation
that are based on prop selection and boat set-up. Also, to make things more
confusing, everything changes with different boat set-ups/operating conditions,
such as trim angle, speed variation, water conditions, trim tabs, hull weight,
engine power and engine height, all of which affect a propeller’s environment
All of these
contributors make comparing two different propellers somewhat complex. A major
performance influence and common measure of different propeller designs on all
types of boats and set-ups is ‘propeller pitch’.
Changing the pitch of your propeller can be the key to fine-tuning your boat’s
performance. Pitch is the theoretical distance the prop will
travel through water during one complete revolution. It is similar to the
distance a screw will travel in one revolution while penetrating a piece of
By convention, props are identified by diameter and pitch: for example,
12″ x 19″. This means the prop has a 12in diameter and a 19in pitch. Typically,
this information is stamped on the hub.
The pitch measurement can be confusing, because there are different pitch
references that you may see utilised. This can affect the ‘prop slip’
calculation, since we might not really know which pitch number is stamped on
the hub of our propeller.
There are several pitch
dimensions that are commonly referred to:
Constant pitch (also
called ‘true’ or ‘flat’ pitch) means that the pitch is the same at all points
from the leading edge to the trailing edge of the propeller.
Progressive pitch (also
called ‘blade camber’) starts low at the leading edge and gradually increases
to the trailing edge.
decreases along the radial line from leading edge to trailing edge.
The pitch reference that we see for a prop should be the average pitch
over the entire blade surface ‒ or effective
There are even controllable pitch propellers, which
have their blade angle mechanically varied. For example, the Land and Sea Torque Shift propeller shifts pitch
automatically from a low pitch, which is necessary for strong acceleration, to a
high pitch for best top-end speed.
However, a 19in pitch
prop never actually travels 19 inches in one revolution. This is because slippage occurs in the water.
Prop slip is not a measure of performance but rather a calculation that
helps in comparing the performance of different propeller set-ups on a hull. Slip
is the difference between the propeller’s actual distance travelled and the
theoretical distance of travel. Measuring slip is a good method for
comparing two set-ups on the same rig. While it can help identify a ‘big-picture’
issue, it’s not always helpful as a stand-alone measure of performance or
Figure 4: Prop slip is a calculation that helps in comparing the performance
of different propeller set-ups on a hull.
Slip is a necessity for a propeller to work. If there is zero slip, then the propeller can’t turn at all, thereby generating no force and thus no MPH. So while less slip is generally more efficient than more slip when measuring power efficiency, it does not indicate that a boat will go faster just because of less slip.
Another measure of pitch that is sometimes used to
compare different propellers is pitch ratio. The pitch of a propeller divided
by its diameter is called the ‘pitch ratio’. For example, a propeller with a 20in
diameter and 20in pitch has a pitch ratio of 1.0, while a diameter of 20in and a
pitch of 15in gives a pitch ratio of .75, etc. Pitch ratio is usually a
generalised comparator used to define groups of boat type and operation. For
example, the pitch ratio of boat propellers for working boats (trawlers) is
usually in the range of .55 to .80; the pitch ratio for heavy cruisers can be
in the range of .65 to 1.0; fast cruisers often use pitch ratios in the range
of .80 to 1.2; performance runabouts can use a pitch ratio of .90 to 1.5; and
fast (racing) boats use pitch ratios of 2.0 or more.
Pitch effect on engine performance
The selection of the pitch measurement on your
propeller will significantly affect engine performance and long-term
durability. As a rule of thumb, the higher the pitch, the faster the top-end
speed, but the lower the RPM achieved by the engine; the lower the pitch, the
faster the acceleration, and the higher the engine RPM.
For a specified engine running at a given RPM, when
a higher pitch is used, the boat can go faster but may sacrifice ‘pulling power’
or acceleration. A lower pitch can improve ‘pull’ (such as for waterskiing/boarding)
or acceleration but won’t achieve as high a top speed. The optimum set-up for
engine operation allows the motor to achieve the maximum specified RPM at top
If too low a pitch is selected, the engine RPM may be
too high (above the recommended RPM limit), putting an undesirable higher
stress on many moving parts. You may realise great acceleration, but your top
speed will probably suffer, and your propeller efficiency certainly will.
If you select too high a pitch you may cause the
engine to ‘lug’ at a low RPM (below the recommended range), which can be
damaging to your engine. Top speed may not suffer too much, but acceleration
will be diminished.
The speed prediction
Here is how propeller pitch
works with the other engine and operation parameters when estimating the
potential speed that a specific propeller can deliver.
The 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] / 
There are other things that can affect the speed prediction, such as expert ‘cupping’
of the blades, which increases the effective pitch of the operating propeller.)
Here’s an example:
Let’s say our
boat/engine has been set up to achieve:
RPM = 6650 (rev/minute)
PP = 26 (inches)
S = 0.09 (9% slip)
GR = 0.535 (1.87:1 drive gear ratio)
Vactual = [6650 x 26 x (1- 0.09[SB1] ) x .535] / 1056 = 79.7mph
Engine gear ratio affects propeller pitch
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 particular prop shaft RPM. The prop shaft speed is determined, in part,
by the gearing between the engine driveshaft 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 that can result in
the best combination of thrust and speed for particular hulls and varying
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. It’s important to note that comparing the propeller sizing on
one boat to another that has a different engine (possibly with a different gear
ratio) is impractical.
The bottom line
while just one of many propeller specification measures, is one of the primary
measurements used in propeller selection. We can’t ignore the other influences
on propeller performance, but if we don’t have a good understanding of pitch,
our performance optimisation will be challenging.
The easiest rule of
thumb to bear in mind for propeller comparison and selection is that more pitch
will reduce the top RPM, can allow the boat to go faster but has less ‘pull’ or
acceleration, while less pitch increases RPM, gives the boat more ‘grunt’ and
acceleration but can reduce top speed. The secret to propeller selection is to
match the engine power/RPM to the optimum balance of propeller pitch. It’s
usually through propeller testing and comparison that this optimum combination
boating and wear your kill cord!
About AeroMarine Research
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.
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 and education/training.