# Weedless Propeller Specs

**URL:** <https://discuss.bluerobotics.com/t/weedless-propeller-specs/17165>\
**Category:** Thrusters and Motors\
**Created:** [June 13, 2024, 4:36pm UTC](https://discuss.bluerobotics.com/t/weedless-propeller-specs/17165 "2024-06-13T16:36:45Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Matt\_D](https://avatars.discourse-cdn.com/v4/letter/m/e495f1/32.png) [@Matt\_D](https://discuss.bluerobotics.com/u/Matt_D)\
**Post date:** [June 13, 2024, 4:36pm UTC](https://discuss.bluerobotics.com/t/weedless-propeller-specs/17165/1 "2024-06-13T16:36:45Z")

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Hi,

I’m looking to use the Weedless propeller M200 motor combo.

Is there anywhere I can find the pitch of the Weedless propeller?  
This would help for calculating the operating speed of my vessel.

From data I can find, I have calculated the propeller is 112x124 mm with a pitch angle of 25 degrees at 75% diameter. Is this even close to correct?

Thanks

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**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\
**Post date:** [June 20, 2024, 12:02pm UTC](https://discuss.bluerobotics.com/t/weedless-propeller-specs/17165/2 "2024-06-20T12:02:13Z")

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Hi @Matt_D,

I’ll preface this by noting that I wasn’t involved with the propeller design, but assuming I’m correctly understanding how the pitch angle is determined, then my result from a rough measurement on the CAD model is p\_{0.75D} \approx 15.5°

For reference, here are a couple of screenshots:

 ![prop-clip-side](https://us1.discourse-cdn.com/flex019/uploads/bluerobotics/original/2X/5/55ae5776ec588ddfa7aa2179b883edee741fbdf0.png)  
 ![prop-clip-top](https://us1.discourse-cdn.com/flex019/uploads/bluerobotics/original/2X/6/6b2ba72ccb2b0d49d841a69d8d89c40e3535059d.png)

I calculated the angle based on the measurement points I’d selected, which were close to the 75% diameter circle:

\begin{align} p\_{0.75D} &= \tan^{-1}\left(\frac{\text{rise}}{\text{run}}\right)\\ &\approx \tan^{-1}\left(\frac{14.08 - 7.05}{13.10 - -12.26}\right)\\ &\approx 15.5^\text{o} \end{align}

I’ve asked internally to see if I can get confirmation on the value, in which case we can hopefully include a value in the technical details. That said, given we had quite different values, I’m curious how you went about calculating yours? 🙂

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**Author:** ![Matt\_D](https://avatars.discourse-cdn.com/v4/letter/m/e495f1/32.png) [@Matt\_D](https://discuss.bluerobotics.com/u/Matt_D)\
**Post date:** [June 21, 2024, 3:18pm UTC](https://discuss.bluerobotics.com/t/weedless-propeller-specs/17165/3 "2024-06-21T15:18:06Z")

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Hi @EliotBR ,

Thank you for working that out for me and I really appreciate you talking to the team about this.  
If that is the same CAD uploaded to the website, I think it is quite different to the real propeller because it looks like a flat plate.

This was my method to work it out:  
Your website has some data on the propellers which I used:  
**Operating RPM @ 16V** : 2751 RPM  
Hence, rotations per second: 45.85 RPS

Then I estimate the flow velocity though the propeller:  
**Thrust @ 16V** : 5.63 kg (55.23 N)  
**Power** : 447.58 W  
**Propulsive Efficiency (estimate)**: 70%  
Velocity=Power/Force  
V = ( 447.58 W x 70% ) / 55.23 N  
Flow Velocity (Vf) = 5.67 m/s

**Pitch** = Vf / RPS = 5.67 / 45.85 = 0.124 m  
This is a flawed way of measuring pitch as it does not account for slip, although, at the time this was the easiest way I could think of.

D\_0.75 = 84mm  
Using a right angle triangle to work out the angle:  
Pitch Angle = TANH( Pitch/ Circumference\_D\_0.75 )  
**Pitch angle** =TANH( 124mm/(84mm\*PI) ) = 25 degrees

Subsequently I redid this with F=ma  
Force = 55.2 N  
Density (Rho) = 1000 kg/m^3  
Mass Flow = Area x Velocity\_exit x Rho  
Acceleration = Velocity\_exit - Intake Velocity (Intake is static = 0)

Therefore:  
F = (Disk Area(A) x V\_e x Density) x (V\_e)  
Rearranged:  
V\_e = sqrt ( F/(A x Density) )

Filling in values:  
V\_e = sqrt(55.2N / (0.009852m^2 x 1000kg/m^3) )  
**V\_e** = 2.37 m/s

Reducing this estimate for efficiency to 29% for the first method gives:  
**Velocity** = 2.35 m/s  
_This speed seems much more reasonable as I have heard this prop is calculated to operate at ~3 kts_  
**Pitch** = 0.051 m  
**Pitch Angle** = 11 degrees

This is only 4.5 degrees from your measured pitch. Because the calculated pitch angle comes from the real world data for the screws path through the water, it seems reasonable that this could be accounted for by the angle of attack of the flow to the blade. This gives the real pitch angle of 15.5 degrees.

I hope that makes sense, I’m not sure how to get notation into the post.

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**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\
**Post date:** [June 21, 2024, 5:27pm UTC](https://discuss.bluerobotics.com/t/weedless-propeller-specs/17165/4 "2024-06-21T17:27:18Z")

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> [@EliotBR](#):
>
> I’ve asked internally to see if I can get confirmation on the value, in which case we can hopefully include a value in the technical details.

Following up on this, I’ve been told my approach was indeed correct, but apparently the public CAD model has the blades slightly modified for IP reasons. The measurement from the actual design file has been confirmed as 14°, which I’ve now added to the technical details on the product page 🙂

> [@Matt\_D](#):
>
> This is a flawed way of measuring pitch as it does not account for slip, although, at the time this was the easiest way I could think of.

This is effectively a re-arrangement of my approach in [this T200 comment](https://discuss.bluerobotics.com/t/flow-rate-s-for-t200/4117/9), but with an included efficiency estimate (so you’re not the only one doing such calculations) 🙂

> [@Matt\_D](#):
>
> Pitch Angle = TANH( Pitch/ Circumference\_D\_0.75 )  
> **Pitch angle** =TANH( 124mm/(84mm\*PI) ) = 25 degrees

I’m assuming you meant \arctan here (i.e. \tan^{-1}), not hyperbolic tangent (\tanh)? That matches your calculated value, and makes follows your triangle logic description.

> [@Matt\_D](#):
>
> I’m not sure how to get notation into the post.

There’s a formatting section in the [How to Use the Forums](https://discuss.bluerobotics.com/t/how-to-use-the-blue-robotics-forums-please-read/10521) post, which covers the basics - the maths is essentially handled by LaTeX syntax, if you’re familiar with that 🙂

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<div class="post-metadata">

**Author:** ![Matt\_D](https://avatars.discourse-cdn.com/v4/letter/m/e495f1/32.png) [@Matt\_D](https://discuss.bluerobotics.com/u/Matt_D)\
**Post date:** [June 24, 2024, 2:58pm UTC](https://discuss.bluerobotics.com/t/weedless-propeller-specs/17165/5 "2024-06-24T14:58:17Z")

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> [@EliotBR](#):
>
> Following up on this, I’ve been told my approach was indeed correct, but apparently the public CAD model has the blades slightly modified for IP reasons. The measurement from the actual design file has been confirmed as 14°, which I’ve now added to the technical details on the product page 🙂

That makes sense, thank you for looking and posting that on the store page.

> [@EliotBR](#):
>
> This is effectively a re-arrangement of my approach in [this T200 comment](https://discuss.bluerobotics.com/t/flow-rate-s-for-t200/4117/9), but with an included efficiency estimate (so you’re not the only one doing such calculations) 🙂

Indeed, although mine doesn’t looks quite as neat, I’ll use the LaTeX approach if I need to do this again.

> [@EliotBR](#):
>
> I’m assuming you meant \arctanarctan\arctan here (i.e. \tan^{-1}tan−1\tan^{-1}), not hyperbolic tangent (\tanhtanh\tanh)? That matches your calculated value, and makes follows your triangle logic description.

Yes exactly, I mixed up my notation here, fortunately the results is the same.

@EliotBR thank you for your help.