# Reverse prop calculations

Discussion in 'Propulsion' started by ErikdeJong, May 11, 2012.

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### ErikdeJongJunior Member

I want to install a shaft generator on my 50' boat. We have a 4 blade propeller of 23" diam. I would like to maximize the power generated, but a trial and error approach would be very expensive.

What I was searching for is a calculation method with the prop. characteristics as input and a maximum generated power as outcome. Does anyone knows if this exists and where I can find it?

Thanks,
Erik de Jong

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### CDKretired engineer

I doubt you can find such a calculation method, but you can find out approximate figures if you know the speed to be expected, the fuel consumption of your engine at that speed and the specific consumption , expressed as fuel weight per horsepower.

From these you can calculate how many horse powers you engine must generate to obtain that speed. Assume 10% prop slip in either direction, so take 80% of the HP value as the amount of power that can be generated, multiply that by .736 to get the theoretical output in KW. Multiply that by .8 for a generator with 80% efficiency and you know what to expect.

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### jehardimanSenior Member

The size of the region where you can extract power is dependent on blade shape and section, but generally, again you will reach a point where the blade stalls so negative Kt will decrease and negative Kq will max out. This of course is all for RPM ahead. There is a similar set of curves for RPM astern( I.e. J positive RPM astern Va astern; J negative RPM astern Va ahead), but generally you get much poorer performance from the wheel becase the blade is generally optimized for "ahead" operation (and again, depending on blade shape, bending stress may limit operation). It is possible to build a prop with symetric blade shape and sections (think a Kaplan thruster) but it will be poorer performing as both a propeller and generator.

All in all, IMO, it is simplier and better to build a specfic generation system designed to be towed than to try to use the vessels propeller. Better on the running gear also.

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### FAST FREDSenior Member

We have a 4 blade propeller of 23" diam.

Try towing a 23 " diameter drogue to see the effect on your boats performance.

If the prop and charge system is optimized for charging , use a 30 inch drogue.

FF

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### johneckSenior Member

There is some 4 quadrant data out there that can be used. Some of the Wageningen B-series props were tested in all four quadrants. I don't have a link for the report, but I'm sure it is out there somewhere. You will also need to include the bearing/gear losses as well since they be a fairly important factor.

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### jehardimanSenior Member

Actually most of them and there is a TMB report (NSWCCD-50-TR-2006/004 April 2006)on smoothing the data. But that report is in terms of beta, CT and CQ, which is a little rough going for most people.

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### ErikdeJongJunior Member

Thanks guys,

I think that I will take a more practical approach on this one. I'm thinking of a wooden lever of a meter long for example that I will clamp around the propeller shaft and that has an adjustable slip by means of a bolt clamp. This way I can measure the resistance at different shaft speeds and different boat speeds, plot this in a graph and I see what is happening.

I'm not afraid of the wear and tear on the shaft or other propulsion components. The alternator will be mounted close to the axial bearing and according to the manufacturer, such a side load is not a problem at all. The gearbox is hydraulic and has been freewheeling since 1979 without any problem. The alternator will even slow down the rotation of the shaft which theoretically should give less wear on bearings and gearbox.

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### johneckSenior Member

I don't see how that is going to work, but I may be missing something. You could use strain gages to measure torque and RPM (power) at various speeds and RPM, but you need a way to apply a load to the shaft or you will just be measuring bearing losses. The amount of power available will be dependent on both speed thru the water (Va) and RPM. If you have looked at any of the data out there you have seen that it is dependent on those two factors to determine an operating point.

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### ErikdeJongJunior Member

My plan is to take a two by four of about a meter long, at one end, connect a short piece of two by four with a hinge to the long one, both ends are notched out with about 80% of the shaft diameter. The short one can be tigned by means of a bolt. This will be placed around the shaft. By tightening the bolt, the resistance on the shaft increases and the torque created by this resistance can be measured at the end of the piece of wood. If you tighten the bolt in small steps and therefore increase the resistance on the shaft while monitoring the rotational speed of the shaft can be plotted in graph. this all needs to be done during sailing and should be repeated at different boat speeds.

I thinks this comes closest to the real data since you have no assumptions to make and measure the real thing.

Do I make a thinking mistake or is this all that is required to determine the maximum torque created by a free-wheeling propeller? Is there a simpler way?

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### johneckSenior Member

Yes, now I see. It seems simple and workable; just make sure you stay clear of the lever! The torque can be quite large. Do you have a way to know the RPM?

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### ErikdeJongJunior Member

The RPM is low. I can count it myself. Without any extra resistance added, I can keep up with it.
I placed a sticker on the shaft with some thickness and when I press my finger against the shaft, I feel every passing of the sticker. Just keep on counting for a while and repeat that a few times. I think the average of three or four counts will come close to the truth.

I will stay clear!
the shaft is under a floor panel which gives me full access to every part of the shaft train without danger.
The clamp will be attached without resistance to start with and only be tightened step by step. I plan on fixing the end with a steel wire to a frame and a scale in between to read the torque.

I think this is easier and more precise than calculations. I will let you guys know if it works out or not.

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### DCockeySenior Member

A variation of a "pony brake". The power will be absorbed by the pieces of wood wrapped around the shaft. The wood will heat and depending on the amount of power may char and possible catch fire.

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### DCockeySenior Member

What is more important for your application, maximum torque or maximum power?

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### ErikdeJongJunior Member

If needed, I can put a piece of leather in between, that will not catch fire. It is not something that I will leave on for hours and worst case, I can cool it with some water.

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### ErikdeJongJunior Member

With the graph of torque vs rpm, I can go talk to some alternator suppliers and see what would be the best option to mount on the shaft.
If you know speed and torque, you also know power.

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