# Thrust – Week 5

This week we developed several propellers with various pitches for testing on started designing the gearing system to attach to the drive shaft.

After advancing further upon using pre-existing CAD designs, we realised that this was not the best option as they were not as easily adjusted as anticipated, so we designed our own.

We came across the a thrust equation   [1], where F=Thrust, =density of fluid passing over the blades, A=propeller disc area, Ve=the velocity of fluid exiting the propeller and Vo=speed of fluid travelling towards the propeller. To calculate a simple  we derived this equation to calculate the potential speed of the exiting water, Ve=(RPM*prop.pitch)/60, and the speed of fluid travelling towards the propeller is just assumed as Om/s just for simplicity.

The RPM has been estimated to need to be around 300 RPM so there’s enough torque and so the propellers aren’t giving out too much thrust, as the ROV needs to move steadily to allow surveying on hulls or inside of fuel tanks. The propellers are also going to be in a tube like casing, which should therefore reduce slip and direct the thrust to go 1 direction. I used the density figures as water-1000kg/m^3 and diesel-835kg/m^3

we calculated a range of pitches thrust, varying from 0.36 inches to 6.72 inches.

 Pitch (inches) 0.36 3.14 6.72 Thrust in water (N) 0.005225 0.3976 1.8209 Thrust in diesel (N) 0.004364 0.332 1.5204

These calculations are obviously going to be different to the true value which is why testing is very necessary.

[2]

After these have been printed then we will attach it a motor and test it in a body of water with a pivoted arm attach to a newton meter.

From this we will be able to chose the ideal pitch for the ROV.

[2]

References

[1] – Hall, N. (2015) Propeller thrust. Available at: https://www.grc.nasa.gov/www/k-12/airplane/propth.html (Accessed: 15 November 2016).

[2] – Applegarth, S. (2016) MMU Babcock Team 1. own photos

## 2 thoughts on “Thrust – Week 5”

1. Michael Fox - Babcock says:

An excellent post demonstrating your research and referencing your sources. It may be worthwhile including everything you have considered in your decisions whether they have an affect or not. i.e. The effect of depth and temperature to the density of water and diesel.
Do you know how much force you require from each thruster rather than setting your specification to the RPM of the propellers? Once you know the force required to manoeuvre the ROV into place as well as crawling the surface you are surveying you can better select the size of the propeller required and then the motor.

The steps to look at are,

1. Speed Requirements (consider the tidal conditions in water, none in diesel)
2. Surface area and therefore effective resistance through different mediums
3. Model in CFD to work out Effective Force required
4. Propeller Efficiency (assumed or calculated)
5. Propeller and motor selection

Keep up the good work.

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1. ok thanks for the pointers. ill make a reassessment

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