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Boat Propeller

Terms

Diameter – The diameter is the largest circle created by the tip of the blades in one revolution.

Pitch Ratio – The ratio between the Pitch and Diameter. Pitch in general is always larger than the diameter.
Pitch – The pitch is the amount of measured theoretical forward travel when the prop completes one revolution. Pitch Calculator.

Balancing a Propeller

It is important to note that most RC props in general do not come pre-balanced. In some cases they can be bought at a higher price if one does not wish to do the balancing by hand. The technique to do so after purchase will be described here.

A prop balancer will be needed. One can purchase a prop balancer at their local hobby store. The most common type that will work for plane props, boat props and small car tires is the magnetic balancer type.

When the prop is properly placed on the balancer, the heavy tip will tend to drop to the bottom naturally by gravity. This will alert the user to determine which blade to remove material from, in order to balance it. It is absolutely important to understand where to take material off from. Material should only be removed from the front of the prop. When looking at a boat from the rear you are looking at the side which performs all the work and turns rotation in to thrust. This side must be left alone. Material on the heavy will be removed from the opposite side on the face of the prop blade. Material can be removed from the entire face to thin the prop out. It will be required to constantly check the balance of the prop as you do not want to take too much material away as it may be hard to get back in to balance.

Remove material until the blade starts to rotate where the blade that is being worked on is now the lightest blade. Then repeat this process on the new heavy blade.

Repeat this process until the desired thickness is achieved. Also sharpen the leading edge of the prop to gain a knife edge. The leading edge of the will be thinner than the trailing edge from the factory. This should not be too hard to determine.

Once complete your prop should be true, thin, and sharp. Perfect for RC boating.

Choosing the Right Propeller

Choosing the right prop for an application is the hardest selection. Too large of a prop will certainly destroy electrical components. Too little of a prop will not get the boat on plane. It’s not voltage that destroys motors, it is current and maximum rotational velocity that destroys motors. Current is controlled by the load placed on the motor. 

It is recommended to seek advice if one does not know what a conservative prop would be for their specific application. Every combination is different!

Knowing which prop would work best for a hull, comes mostly from experience.

propeller2 cell LiPo  – 30mm – 40mm Diameter
4 cell LiPo – 37mm – 46mm Diameter
6 cell LiPo – 40mm – 52mm Diameter
8 cell LiPo – 45mm – 55mm Diameter
10 cell LiPo – 48mm+ Diameter

It is extremely important to understand this chart is very basic, and the best or most conservative prop for any given combination may very well be out of this range.
A larger prop increases load. Increased Load will result in an increased current draw. A smaller prop will decrease current draw. Higher prop pitch will increase current draw, a lower prop pitch will decrease current draw.

propellerLarger Props may create stability issues, make it difficult in corners and create torque steer. An advantage to using a larger prop at slower speeds is thrust. Acceleration on plane will be much greater. Efficiency is also higher for larger props as there is less possible slippage.

Smaller props may have higher levels of slippage, slower acceleration out of the hole but have several advantages. They are typically more stable handle well in corners, and don’t veer the hull of course under hard acceleration. (torque steer)

 

Advantages: 2 Blade vs. 3 Blade when diameter and pitch are equal. 2 Blade

- Higher Top Speed
- Less Load on motor
- Motor will turn higher RPM’s

3 Blade
- Higher thrust / Faster Acceleration
- Increased Stability and cornering
- Higher Lift Characteristics

OCTURA PROPELLER GUIDE

Octura props do have the best nomenclature for boat props. The first letter represents the type of prop as listed below. If the first letter is actually a number, this is the first digit in the pitch ratio. The second digit would then become the digit after the decimal. For example an X6** would have a pitch ratio of 1.6. An X4** would have a pitch ratio of 1.4. A 14** would have a pitch ratio of 1.4. A V9** would have a pitch ratio of 1.9 ect. The last two digits represent the prop diameter in mm. An X450 would have a 50mm diameter. The Pitch can be calculated using the Pitch Calculator.

 

Prefix Definition
12 Best used for submerged drive Setups. Low Pitch Larger diameter.
X4 are low lift general purpose props
M4 is similar to X4 but with the tongues removed to unload the prop
Y are similar to X series but have 10% more pitch
X5 are similar to X4 with medium lift and more pitch
X6 similar to X4 and X5 with medium-high lift
14 are medium-high lift commonly used for hydros
16 are high lift props for riggers and hydro’s
17 are the highest lift prop for riggers and hydros
P7 are similar to 17 series but more rake and slightly less lift
19 are high lift for riggers and hydro’s
V9 are moderate lift and high puitch suitable for riggers, hydros and SAW racing
20 are moderatley high lift with high pitch suitable for riggers and hydros
21 are high lift for riggers and hydro’s
22 are high lift, highest pitch, good straight line speed

 

PRATHER PROPELLER GUIDE 

All general Purpose Props with Pitch ratio’s near 1.5 ranging from 1.5 to

Prop # Diameter Pitch
215 40.6mm 63.5mm
220 43.7mm 66mm
225 46.7mm 71.1mm
230 48.2mm 76.2mm
235 50.8mm 81.3mm
240 53.8mm 83.8mm
245 57.1mm 88.9mm
250 58.4mm 91.4mm
255 61.0mm 96.5mm