Propellers 

The page bellow is an attempt to stress the importance of the propeller in r/c    boat propulsion. It results from my lectures on the Web and information posted on Jim's board, mainly by Jim Nissen, Paul Govostes and Mike T. When you finish reading this, hopefully you will be interested in learning more. Go to Paul Govostes page: http://rcprops.com/ He is one of the best on this matter, the only problem is that he doesn't modify props for sale; you can't buy one of his props, you just have to look to them on his page and dream. 

When I started thinking on a page like that I posted some question at Jim's Board; the answers just made me sure that I had a lot to learn. So, when I finished the page  I asked Paul if he could look at it and correct my mistakes. Well, he not only corrected the mistakes but ad some good information. It's just a matter of justice register that this is a Carlos/Paul page and I am most honored with such company. Paul, thank your very much for your kindness and for sharing your knowledge with us.

Introduction:  

The prop is responsible for transforming the movement of the motor�s power. It accomplishes this task by displacing a mass of water when turning. One of the first laws of physic states that with each action there is an opposite and equal reaction of the same intensity, and of contrary direction. As a result, the water movement is displaced backwards, and the prop is pushed forward. There is a rigid link to the hull, that results in the boat moving forward in a speed proportional to the volume of water being displaced. The prop rests in the water and moves forward, by the amount of it's pitch � in theory. As water is not a solid mass, the prop slips and the forward movement is less then the theoretical forward motion.  

So, the larger the pitch, the higher the speed, as a result of increased forward movement at the same time unit. This is true to a certain point. Sometimes, the pitch is so great, the engine can't reach it's maximum RPM. And less RPM, means less forward movement within the same time unit, = less speed. Compared  to your car, a higher gear reaches higher speed � in normal circumstances. On a stepped hill for instance, the load on the engine causes a reduction in RPM and a decrease in speed. Under these conditions, the 3^rd gear may be faster. Like the car gear box, smaller pitch has more acceleration and less top speed, while greater pitch has more top speed with less acceleration. 

Conversely, too little pitch makes the engine over-rev, with possible costly engine damage.  

As your boat's engine doesn't have a gear box, the prop has to be carefully chosen to overcome these restrictions.  

Diameter is also of prime importance when choosing the right prop: larger diameter = more thrust, and greater capacity for moving a heavy hull. Increasing the pitch generally demands a reduction in diameter, lessening the drag and increasing speed. Again, this has a limitation: a minimal diameter is required to guarantee the necessary thrust. 

Moreover, the diameter helps control slippage, larger diameter is less prone to prop slippage, until the larger diameter causes too much drag and reduces engine speed. In a way, diameter directly effects speed, but it's major influence is on acceleration. 

The number of blades is also important. The ideal prop should only have one blade, eliminating   drag of the other blades that are not producing useful work. As this kind of prop would be impossible to balance, 2 bladed props are standard. On some type hulls, 3 or even 4 blades may be used. Mainly on boats that demand lift at the transom, because they run too louse. Outriggers are the best  example of this kind of hull. It is also commonly accepted that a 3 blades prop has a smooth acceleration, because 1 blade is always into water. And more: considering the same pitch, a 2 blades prop may have 1 mm more in diameter and will load the engine the same as a smaller 3 blades prop., But a 3 blades prop has more prop walk and torque roll.  

Blade Thickness (the thinner the best). A thin and sharp blade cuts the water and reduces prop-walk tendency (walking to the right as the transom goes to the left, because of the props turning effect). 

Preliminary Definitions:

Blade: is the prop part that cuts the water and thrusts it back. In R/C speed boats we use 2 or 3 bladed props, with very few 4 blade props. 

Hub: central portion of the prop, where the blades and prop shaft are attached, The Hub provides most of the strength of the propeller.

Diameter: prop size, measured between the tips of the blades. Or 2 times the measurement between the tip of the blade and the center of the hub. Larger diameter props are normally used on large and heavy hulls. With R/C gas boats, 65 to 80 mm is the standard. Like pitch, a larger diameter means more water is moved per each revolution.

Pitch: theoretical distance the prop travels on each rotation. This is theoretical because water is not a solid medium and the prop slips. 10 to 30% is normal, the lower numbers are found only on hi-performance props, specially prepared.

Constant Pitch: the pitch is the same across the entire propeller�s working surface, or blade face.

Progressive Pitch: the pitch is lower at the leading edge and increases progressively along the trailing edge.

Consider a prop that runs in a liquid media with a constant pitch, the tip of the prop rotates faster than the hub. Conversely, if the angle at the tip is lower, the water speed on all prop surfaces will be the same. In this case, the performance is far better. Progressive pitch props offer better planning performance.

NOTE: The ability to turn a large diameter or high pitch prop at a desired RPM is determined by the amount of torque provided by the engine. Bigger is not necessarily better.

Aspect ratio: (a definition by Paul Govostes): The relationship between pitch and diameter, is measured as the "Aspect Ratio". (Pitch divided by Diameter).
(eg) The Octura 1475 has 4.13"Pitch/ 2.95"Dia. The relationship between Pitch4.13" divided by Diameter 2.95" = (1.4 Aspect Ratio). Higher Aspect Ratio = More Pitch/ Less Diameter.  And lower (AR) = Less Pitch/ More Diameter.
Generally, a faster prop has a higher (AR), more Pitch, with less Diameter. However this holds true to a certain extent, as adequate Diameter is necessary to sustained push!

Leading edge: the side closest to the transom.  

Trailing edge: the side furthest from the transom.

The bellow drawing (adapted from an original posted by Wayne Rathbun) shows a propeller parts:

Same parts, on pics this time:

High lift: a prop which tends to lift the transom when the boat is running. 
Low lift: a prop which does not have this tendency. 

With Octura props, the X series (an X before the number) indicates a low lift prop. Props without the X are high lift props.

Ex.:X470 Octura prop
X= low lift prop.
4= pitch ratio
70= 70 mm diameter 

1667 Octura prop
lift prop
1.6 AR
67mm diameter
So: pitch divided by diameter= (AR) or pitch/67=1.6 or pitch=1.6 x 67= 107.2 mm diameter or 4,2" 

Cavitation: is water vaporizing due to the extreme reduction of pressure on the back of the propeller blade. Many props partially cavitate during normal operation, but excessive cavitation can result in damage to the prop's blade surface. Note: Cavitation is often confused with Ventilation.

Ventilation: often confused with cavitation, it's normally induced by an external source, not by the prop itself. It occurs when air is directed on the prop surface. Eventually, it may help by reducing cavitation. On real boats, this is sometimes accomplished   by directing the exhaust over the prop. On R/C boats this is not a common practice; nevertheless, it looks like some guys in Swiss are trying this. Look at the pic bellow:  

Cupping: Is a curvature added at the props trailing edge, normally very slight, towards the blade center, shaped like a spoon. It reduces cavitation and maximize thrust, increasing the effective prop pitch. Additionally, it lifts the bow and narrows the thrust cone. It looks like a magic medicine, the solution for all our problems but, like any medicine, it has to be taken carefully. Besides, cupping has to be equal in all blades, or you will have an imbalanced prop, that may damage your engine. 

Camber: Arching curve from leading edge to trailing edge, like a spread out cup. It makes a progressive pitch at the prop. 

Rake: Is the angle of the blade attachment to the hub, and the degree that the blades slant forward or backwards. It's normally use to correct ventilation or cavitation situations. Furthermore, rake lifts the bow and so the speed. Typically, low rake props are used on sub-surface drives with heavy hulls. Higher Rake blades with surface drive application, tend to re-direct and condense the thrust cone further aft.

Questions on which prop to use are recurrent at Jim's Board. The selection bellow is a compilation of props indicated.

Manufacturer: not necessarily the manufacturer of the original prop. In some cases, the listed manufacturer is the one who provides the prop modification.

Use: no indication means that the prop was listed as a general purpose prop, one that you should have just in case.

Additional features: additional modifications made to the original prop.

#	Manufacturer	Use	Features
H-31	ABC	1. AC Lazer 40 -Prop Comp Motor	1. -
		2. Insane mono � Zenoah RC230	2. -
H-50	ABC	1. Multi-Use depending on cut shape & pitch	1/3. Cut and pitched, Dia. 2.45 > 2.59", Pitch 5 > 6.0", Cup > 6.5"max Spooned Shape Blade/ Lead Cut-Back
		2. Stryker Sprint Cyclone Cats etc.
		3. 41>50"/ Hydros/ Riggers/ 23>35cc Gas
		4. Cyclone cat. modded RC230	4.2.45"Dia x 5.5"Pitch
1475	Andy Brown	1. Cyclone/ QD 35cc	1. Sharpened & Balanced Cup 7.0"
		2. Cyclone/ QD 25cc	2. Sharpened & Balanced, Cup 6.2", cut down to 72mm
		3. Apache/ Zenoah	3. Sharpened & Balanced, Cup 6.2"
		4. Apache/ J&G	4. Sharpened & Balanced, Cup 6.6"
		5. Zenoah gas monos	5. Cut and pitched
		6. Seaducer.	6. 6.3" pitch
		7. Seaducer2/QD 25 cc	7. Some added pitch
		8. RC Boatworks F-41 Stryker cat/QD 25	8/10. -
		9. F-41Stryker/Zenoah 230RC mod
		10. AC Lazer 40 - Prop Comp motor
		11. F-41 Stryker/QD 25	11. mod
		12. F-41 Stryker/Zenoah 230RC	12. cut and balanced lift out and pitched
		13. 44" Twincraft/Midwest Super Mod G-23	13, -
		14. Stryker Wide Cat � Mid mod Zenoah	14. -
1667	Andy Brown	1. RC Boatworks F-41 Stryker cat/QD 25	1. -
		2. Cyclone/ QD 25cc	2. Sharpened & Balanced, Cup 6.4"
		3. Seaducer2/QD 25 cc	3. Some added pitch
7016	Andy Brown	1. F-41Stryker/Zenoah 230RC mod	1. Cut and balanced lift out and pitched
2014/3	English Prop Shop	1. -	1.
2016/3	English Prop Shop	1. -	1.
7014	English Prop Shop	1. -	1. -
7015	English Prop Shop	1. Zenoah gas/ cats	1. -
		2. Cyclone cat. modded RC230	2.Dia reduced to 68mm / Pitch increased to 5.5" @ Cup.
7015/3	English Prop Shop	1. Cyclone cat. modded RC230	1. -
7016	English Prop Shop	1. General	1. -
		2. Cyclone cat. modded RC230	2. Star-cut mod.
		3. 44" Twincraft/Midwest Super Mod G-23	3. -
7018/2	English Prop Shop	1. Stryker Cat/ QD 25	1. -
1475	Octura	1. -	1. Couple of AB,s with different mods
		2. -	2. Several cut and pitched
		3. F-41Stryker/Zenoah G-23  mod	3. -
1667/2	Octura	1. -	1. Cut down and pitched somewhat like the 1475's
7016	Octura	1. F-41 Stryker/mid mod Zenoah   mod	1. -
7016/3/4	Octura	1. -	1. A good and sharp with mods
7018/2/3	Octura	1. -	1. A good and sharp with mods
7018/3	Octura	1. F-41 Stryker/mid mod Zenoah	1. -
		2. Mono 48 QD35	2. -
7020	Octura	1. F-41 Stryker/mid mod Zenoah   mod	1/3. -
		2.  F-41 Stryker/QD
		3. QD25 cat
7020/2/3	Octura	1. -	1. A good and sharp with mods
		2. Mono 48 QD35	2. back-cut the trailing edge
P-967	Octura	1. AC Lazer 40" � Pro comp motor	1. Sharpened and balanced
X-470	Octura	1. Zenoah engine	1/3. -
		2. Crackerbox + surface drive 3. Crackerbox with stock air cooled Zenoah
X-470/3	Octura	1. F-41 Stryker	1/3. -
		2. AC Lazer 40" � Pro comp motor
		3. Aeromarine Avenger � Zenoah modded
X-472	Octura	1. F-41 Stryker/mid mod Zenoah	1/12. -
		2. AC Lazer 40" � Pro comp motor
		3. Apache/stock Homelite 25cc
		4. Zenoah engine
		5.Homelite engine
		6. Cat 43"/Homelite
		7. QD25 cat
		8. 44" Twincraft/Midwest Super Mod G-23
		9. Aeromarine Avenger - Zenoah modded
		10. 56" TS Mono � Zenoah full modded
		11. PI 56" fountain - full mod rc 230   12. Crackerbox with stock air cooled Zenoah
X-482	Octura	1. -	1. good and sharp  BC and cut down
		2. Mono 46/JT Mod 25cc Homie	2. -
		3. 44" Twincraft/Midwest Super Mod G-23	3. trimmed.
X-570	Octura	1. -	1. -
X-572	Octura	1. -	1. Good and sharp and  mods
		2. Cat 43"/Homelite	2/3. -
		3. Aeromarine Avenger - Zenoah modded
X-670	Octura	1. -	1. -
		2. Seaducer	2/6 . -
		3. Stryker Cat/QD 25
		4. PI 56" fountain -  full mod rc 230
		5. Aeromarine Avenger- Zenoah modded   6. Crackerbox with stock air cooled Zenoah
270	Prather	1. Aeromarine Avenger � Zenoah stock engine	1. -
275	Prather	1. -	1. A good and sharp S/Steel
		2. -	2. SS, cut down some
		3. -	3. Bronze, stock
		4. Crackerbox	4/6. -
		5. Challenger 48
		6.  Homelite engine
		7. Crackerbox + surface drive	7. much modified Homelite 25 cc surface drive
		8. Cat 43"/Homelite	8/13. -
		9. RC Boatworks F-41 Stryker cat/QD 25
		10. Apache/stock Homelite 25
		11. Mono 46 JT Mod 25cc Homie
		12. QD25 cat
		13. Aeromarine Avenger � Zenoah stock engine 14. Crackerbox with stock air cooled Zenoah
280	Prather	1. RC Boatworks F-41 Stryker cat/QD 252	1/2. -
		2. Zenoah engine
		3. -	3. Bronze, stock
		4. -	4. SS, cut down some
		5. Cat 43"/Homelite	5. Sharpened and balanced
6716/3	Prop Shop	1. hybrid Zenoahs	1. -
7015	Prop Shop	1. 25cc Quickdraw with low exhaust timing	1/4
		2. - 56" TS Mono � Zenoah full modded
		3. Prop Shop
		4. hybrid Zenoahs
7015/3	Prop Shop	1. -	1. Cut down and pitched somewhat like the 1475's
7016	Prop Shop	1. QD25 cat	1. -
		2. Stryker Wide Cat � Mid mod Zenoah	2. -
		3. General	3/4. -
		4. Zenoah engine
		5. Cat 43"/Homelite	5. Back-Cut & tweaked
7016/3	Prop Shop	1. Zenoah engine	1. Zen-Cats best friend
		2. 56" TS Mono � Zenoah full modded	2. back cut
		3. 23cc Zenoah PUM with a Hanson wrap to center pipe	3. -
7018	Prop Shop	1. -	1. Cut down and pitched somewhat like the 1475's
		2. Cat 43"/Homelite	2. Back-Cut & tweaked
		3. Cat 43"/Homelite	3. Back-Cut & tweaked
		4. RC Boatworks F-41 Stryker cat/QD 25	4. For heat racing
		5. QD25 cat	5. -
		6. Stryker Wide Cat � Mid mod Zenoah