- 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 performance.
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 and performance.
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 wood.
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.
Regressive pitch 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 pitch.
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 efficiency.
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 speed.
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 formula
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] / 
(Note: 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)
Engine gear ratio affects propeller pitch selection
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 operating conditions.
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
Propeller pitch, 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 is discovered.
Safe performance boating and wear your kill cord!
- Read more about performance vee-hull and tunnel boat design and set-up in Secrets of Tunnel Boat Design (ISBN# 1-894933-30-3).
- See more about the ‘Tunnel Boat/Vee Boat Design Program’ software at www.aeromarineresearch.com/tbdp6.html.
- ‘Vee Boat Design Program’ software – www.aeromarineresearch.com/vbdp.html
- ‘Tunnel Boat Design Program’ software – www.aeromarineresearch.com/tbdp6.html
- Secrets of Tunnel Boat Design book – www.aeromarineresearch.com/stbd2.html
- Secrets of Propeller Design book – www.aeromarineresearch.com/secretsofpropellerdesign.html
- ‘PropWorks2’ software for speed prediction and propeller selection – www.aeromarineresearch.com/prop2.html
- All the above are available on the AeroMarine Research website at www.aeromarineresearch.com.
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.
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 and education/training.