- 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 doneby getting the right propeller. While choosing the best prop for your boatoften involves dedication to testing and comparing results, this single exploitcan be the most rewarding in terms of performance gains ‒ and it all startswith understanding what the variations in propeller designs can mean toperformance.
One of the most common questions I hear regarding propeller selection is‘what is pitch’? The engineering theory around ‘how a propeller works’ is reallya discussion for another article, but understanding ‘propeller pitch’ is key inundertaking your mission of propeller selection.
There are many variables that influence the performance of a propeller, includingprop diameter, the number of blades, blade shape, blade thickness andpitch. And there are some more-difficult-to-understand features, such as bladesection type, blade skew, blade contour, cup, camber, rake, hub design, rotationand disc-area ratio.
Then there are operationalissues like propeller cavitation and ventilationthat are based on prop selection and boat set-up. Also, to make things moreconfusing, 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 environmentand performance.
All of thesecontributors make comparing two different propellers somewhat complex. A majorperformance influence and common measure of different propeller designs on alltypes of boats and set-ups is ‘propeller pitch’.
Propeller pitch
Changing the pitch of your propeller can be the key to fine-tuning your boat’sperformance. Pitch is the theoretical distance the prop willtravel through water during one complete revolution. It is similar to thedistance a screw will travel in one revolution while penetrating a piece ofwood.
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 pitchreferences that you may see utilised. This can affect the ‘prop slip’calculation, since we might not really know which pitch number is stamped onthe hub of our propeller.
There are several pitchdimensions that are commonly referred to:
Constant pitch (alsocalled ‘true’ or ‘flat’ pitch) means that the pitch is the same at all pointsfrom the leading edge to the trailing edge of the propeller.
Progressive pitch (alsocalled ‘blade camber’) starts low at the leading edge and gradually increasesto the trailing edge.
Regressive pitchdecreases along the radial line from leading edge to trailing edge.
The pitch reference that we see for a prop should be the average pitchover the entire blade surface ‒ or effectivepitch.
There are even controllable pitch propellers, whichhave their blade angle mechanically varied. For example, the Land and Sea Torque Shift propeller shifts pitchautomatically from a low pitch, which is necessary for strong acceleration, to ahigh pitch for best top-end speed.
However, a 19in pitchprop never actually travels 19 inches in one revolution. This is because slippage occurs in the water.
Propeller slip
Prop slip is not a measure of performance but rather a calculation thathelps in comparing the performance of different propeller set-ups on a hull. Slipis the difference between the propeller’s actual distance travelled and thetheoretical distance of travel. Measuring slip is a good method forcomparing 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 orefficiency.
Figure 4: Prop slip is a calculation that helps in comparing the performanceof 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.
Pitch ratio
Another measure of pitch that is sometimes used tocompare different propellers is pitch ratio. The pitch of a propeller dividedby its diameter is called the ‘pitch ratio’. For example, a propeller with a 20indiameter and 20in pitch has a pitch ratio of 1.0, while a diameter of 20in and apitch of 15in gives a pitch ratio of .75, etc. Pitch ratio is usually ageneralised comparator used to define groups of boat type and operation. Forexample, the pitch ratio of boat propellers for working boats (trawlers) isusually in the range of .55 to .80; the pitch ratio for heavy cruisers can bein the range of .65 to 1.0; fast cruisers often use pitch ratios in the rangeof .80 to 1.2; performance runabouts can use a pitch ratio of .90 to 1.5; andfast (racing) boats use pitch ratios of 2.0 or more.
Pitch effect on engine performance
The selection of the pitch measurement on yourpropeller will significantly affect engine performance and long-termdurability. As a rule of thumb, the higher the pitch, the faster the top-endspeed, but the lower the RPM achieved by the engine; the lower the pitch, thefaster the acceleration, and the higher the engine RPM.
For a specified engine running at a given RPM, whena 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 forengine operation allows the motor to achieve the maximum specified RPM at topspeed.
If too low a pitch is selected, the engine RPM may betoo high (above the recommended RPM limit), putting an undesirable higherstress on many moving parts. You may realise great acceleration, but your topspeed will probably suffer, and your propeller efficiency certainly will.
If you select too high a pitch you may cause theengine to ‘lug’ at a low RPM (below the recommended range), which can bedamaging to your engine. Top speed may not suffer too much, but accelerationwill be diminished.
The speed predictionformula
Here is how propeller pitchworks with the other engine and operation parameters when estimating thepotential speed that a specific propeller can deliver.
The speed of your hull can always be calculatedusing an engineering formula. While some of the variables for the formula mayrequire some experience to apply, the formula works for all hulls, all designsof boats, all power and drive packages, and all speeds.
V = [RPM x PP x (1- S) x GR] / [1056]
(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 ourboat/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)
Then:
Vactual = [6650 x 26 x (1- 0.09[SB1]) x .535] / 1056 = 79.7mph
Engine gear ratio affects propeller pitchselection
There is an optimal propellerpitch and diameter that will do the best job for a boat with an engine of a specificHP 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 thelower unit. Engine manufacturers install lower-unit/drive gear ratios that enable themost efficient combination of propeller diameter and pitch that can result inthe best combination of thrust and speed for particular hulls and varyingoperating conditions.
Propeller performance isvery much dependent on the propeller shaft RPM at which it operates. Hopefullythere is a range of pitch/diameter propellers that best suit your engine, hulland application. It’s important to note that comparing the propeller sizing onone boat to another that has a different engine (possibly with a different gearratio) is impractical.
The bottom line
Propeller pitch,while just one of many propeller specification measures, is one of the primarymeasurements used in propeller selection. We can’t ignore the other influenceson propeller performance, but if we don’t have a good understanding of pitch,our performance optimisation will be challenging.
The easiest rule ofthumb to bear in mind for propeller comparison and selection is that more pitchwill reduce the top RPM, can allow the boat to go faster but has less ‘pull’ oracceleration, while less pitch increases RPM, gives the boat more ‘grunt’ andacceleration but can reduce top speed. The secret to propeller selection is tomatch the engine power/RPM to the optimum balance of propeller pitch. It’susually through propeller testing and comparison that this optimum combinationis discovered.
Safe performanceboating 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.
Linksfor reference:
- ‘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 aprofessional engineer with a mechanical and aeronautics background. Currentlyliving in Canada, he has done extensive aerodynamic research at the Universityof Michigan, OH, and the University of Toronto, Canada, and marine research atthe NRC water channel laboratory in Ottawa, Canada. His published works andpapers are highly acclaimed and are specifically related to the aerodynamicsand hydrodynamics of high-performance catamarans and tunnel boats, as well asvee and vee-pad hulls.
Russell hasdesigned and built many tunnel and performance boats. As a professional racedriver, he piloted tunnel boats to Canadian and North American championships.He has written powerboating articles for many worldwide performance magazinesand has covered UIM and APBA powerboat races. He has appeared on SpeedVision’s PowerboatTelevision as a guest expert on ‘tunnel hulls’, and was performance/designtechnical consultant on National Geographic’s Thrill Zone TV show andeditorial consultant on the Discovery Channel’s What Happened Next?
Russell is theauthor of the books Secrets of TunnelBoat Design and Secrets of PropellerDesign. His company designed and published the well-known powerboat designsoftware ‘Tunnel Boat Design Program’ and ‘Vee Boat Design Program’specifically for the design and performance analysis of tunnel boats, poweredcatamarans, and performance vee and vee-pad hulls. ‘Jimboat’ is recognised for hisadvanced aerodynamic and hydrodynamic research and consulting on powerboatdesign, performance analysis, safety analysis, accident investigation, expertwitness and education/training.
