# Predict the speed of a towing propeller?

Discussion in 'Props' started by Jonahs, Sep 30, 2010.

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### JonahsJunior Member

Hi Everyone.

I am currently working on a electric generator for small yachts, based on the princip of towing propellers. Either on a long rope, or on a long shaft.

I know that it is possible to buy generators like theese, but they are expensive and i want to make my own.

Anyway, the proplem is to calculate the rpm of the towed propeller, based on any given design/propeller model and the boat speed.

It seems, that for all propeller calculations, you calculate the propeller dimensions based on the engine hp and the speed you want. I need the rpm, in order to choose a generator and to predict the amount of energy it will deliver to the batteries.

Any ideas of doing this?

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### baeckmoHydrodynamics

It is not clear wether you are using the standard prop in forward rotation, or you are going to use a prop in its turbine mode. If in std mode, you have to find data in the "overspeed" region of the propeller diagramme, ie with advance ratio J = is positive, and torque factor kq is negative.

If in the reverse turbine mode, you have to find data for operation in the "negative rotation, negative advance, positive torque"-operating quadrant.

To put it simple: this info is not available for standard props!

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### JonahsJunior Member

I see.

I'm not shure what exactly turbine mode is, however I was going to use a propeller designed/bought for the purpose (of being towed behind a boat). If a normal standart propeller is best for this, or a turbine propeller, thats what i will use.

Think there should be another way of doing this, even though you are right.

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### philSweetSenior Member

Just a word of caution- propellers and turbines are totally different animals and you must not try to understand turbines based on your experience with props. The engineering is very different, as are the definitions for things such as efficiency and other performance measures. At first glance, they both appear to be one moving part of similar design, but the fluid flow around the device is what makes the two different. You need to research turbine design to get where you want to go.

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### JonahsJunior Member

Hi philsweet.

It seems that, in turbines operation, there is no loss of speed in the fluid, only loss of pressure. where in propellers and wind turbines, there is a loss in pressure and in the speed of the fluid. So you are right, there is a big difference between the two. Not sure if i can use the any of the theory from turbines.

I'm still looking for some theoretical way of solving this problem. However, i figured out, that i can do without the rpm, as long as i can calculate the theoretical power extracted from the propeller, based on the design i choose.

I did figure out a way of calculating the power for a windmilling boat propeller, but only according to diagrams from standard propellers in "turbine mode". the problem is that these diagrams only works for certain propellers. I would like to figure out a method for calculating power on propellers with dimensions(diameter, pitch, blade number and design-torque and design-rpm) chosen by myself.

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### philSweetSenior Member

Jonahs, Your first statement is false. In order for the turbine to generate power there must be a pressure buildup on the forward face of the turbine, and this deflects flow around the turbine and reduces the near field flow. Betz made some simplistic calcs and set the optimum flow at around 69% of far field flow for a flat prop. More recently, Gorlov has a better method, but still pretty crude, but it does serve to show the upper limit of what is possibly. Propellers that aren't flat, such as the Gorlov rotor, may be able to do a bit better.

Last edited: Oct 21, 2010
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### philSweetSenior Member

oh, one other thing- if, in your pursuits, you discover a way to dissuade large omnivorous fish from gobbling your device, please let us know. It's been a problem in the past.

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### gonzoSenior Member

The calculations are the same as for a propulsion propeller. The main difference is that the value of the thrust is negative. The blades should be facing the opposite way than when installed for propulsion, so the pressure face is pointing forward.
That is usually done with a standard propeller with the point of the shaft forward.

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### JonahsJunior Member

Hi Phillsweet. Yes there is a change in speed in windturbines, but not in axial flow turbins like fx Kaplan. Because theese turbine type are ducted, the speed before and after the propeller has to be the same. I am aware of Betz's limit.

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### JonahsJunior Member

Hi Gonzo.

Thanks for the answer. Do you know if, what you are talking about, is described in detail somewhere?

I never heard about it before, but it sounds like it could be an option.

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### gonzoSenior Member

Any propeller manual has the calculations for it. Gerr's "The propeller book" will have all the basic calculations and a primer on propeller theory too. A turbine works in a confined tube and is very different from a propeller in open flow.

12. ### WickedGoodGuest

large omnivorous fish gobbling your device

What a great IDEA!

Take a 3 inch dia PVC Pipe and a 4 inch to 3 in reducing cone for the intake.

This will increase the water velocity tru the impellerby a factor of 3

Have an small DC Motor connected to a turbine and put two large fish hooks on steel cable off the ehaust of this device.

You connect one hook to pos & the other to Neg.

This creates a Field of Mystery Energy

that attracts large fish.

If you guys have some motors and turbine blades send them to me and Ill build some prototypes and troll them as a field test.

here is the same thing, but different

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### JonahsJunior Member

Field of mystery energy? what a wonderful thought! I wish i had some motors and turbine blades and could afford the shipping costs to Maine. Let me know if you are having any luck catching these omnivorous monsters

14. ### WickedGoodGuest

What My Rep Points become dimished by the Field of Mystery Energy?

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### BertKuSenior Member

I would consider to work it out backwards.
You download the program : PROP.CALC from Castel Marine LTD in the UK

http://www.castlemarine.co.uk

a) you enter the estimated speed.
b) you assume a displacment boat
c) you enter 1 HP = 750 watt
d) you enter the speed you estimate

and this gives you some propellor information.

Yes, it is a very raw calculation and not 100% accurate, but who can calculate an accurate situation in anyway, i.e. the water temperature, which makes the water more .... or .... less dense, currents etc.

But most important,
a) at what voltage do you want the output to be? 16 - 18 Volt to power a 14,4 Volt regulator for a 12 Volt lead acid battery?
b) any voltage which you with a upswing booster can power to a higher Voltage i.e. at very low speeds?
c) You could use a cheap 3 coil star configuration HOBBYKING.COM model aircraft 1 Kw motor for a few dollars. You need 6 diodes to configure the output. However they are 180 rpm per volt. i.e. for 18 Volt you need a rotation speed of 3240 rpm. Thus a small propellor and high boatspeed.

Or you use a normal old hairdryer 220 Volt motor, drill machine motor and experiment with it, by using a few diodes, to make it a DC generator.

But it may give you some idee's on how archieving your dream of getting DC power from a towing generator.

I made quite a few propellor blades from a piece of stainless steel sheet. Cut it out to the approx shape and bend it to the approx. pitch. You need to balance it however and that you can do by putting it on two ballbearings and see where the prop stops.
Bert

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