Predict the speed of a towing propeller?

[Jonahs]

Hi Bert.

Thank you for your interest. I had a look a the propcalculator, which seems like a good tool for choosing propellers for boats. The power of the towed propeller however, will have nothing to do with displacement, hull type etc, since it will be towed several meters after the boat. Also, i wouldn't expect the power to be more than about 100 watts with a 0.25 m diameter propeller.

I expect to generate more than 14 volt, thereby not having to convert to higher volts, and lose valuable energy. However, this is mainly a question of finding the a generator with the right specifications.

I have had a look at standard calculation methods, using David Gerrs propeller handbook, and other similar books. To be honest, none of the methods seems to be nearly precise enough. I know this, since i made a model, and tested it, by towing it behind a motor powered boat at different speeds. It seems like lifting line theory might be the only way of getting usable calculations.

With kind regards, J.

[BertKu]

Originally Posted by Jonahs

Hi Bert.

Thank you for your interest. I had a look a the propcalculator, which seems like a good tool for choosing propellers for boats. The power of the towed propeller however, will have nothing to do with displacement, hull type etc, since it will be towed several meters after the boat. Also, i wouldn't expect the power to be more than about 100 watts with a 0.25 m diameter propeller.

I expect to generate more than 14 volt, thereby not having to convert to higher volts, and lose valuable energy. However, this is mainly a question of finding the a generator with the right specifications.

With kind regards, J.

You are right, that program is not suitable for the 100 watt. My philosofy is that what is put in, must come out minus the losses. Maybe for higher power applications like 10 Kw, one could maybe manipulate it to get some information.

I am so sorry to give you the bad news, but for your standard Lead acid battery voltage regulator to work properly, you need at least 16 - 18 Volt. Even if you would make yourself a simple voltage regulator with a 2N3055 transistor and a zenerdiode, you need at least 15 Volt. Sometimes you may have to accept that you will have unnessesarry losses. Should you put your generator straight onto the battery for a long time, your battery will get a knock. Also you will have losses in your cabling to the towed generator.
Except if you make a system that charges up to 90% of the battery and then switch off. You could do that. Similar to the electric motorcar Leaf from Nissan. This gives the battery a much more longer lifetime. I was planning to do the same with my project. In that case you only need to switch it off at 13,0 Volt and on at 12,5 volt with an OpAmp. (depending on battery specifications)
Bert

[BertKu]

Originally Posted by Jonahs

Hi Everyone.

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?

I read your first thread again and I am wondering.
How smaller the prop, how higher the rpm. How larger the prop how lower the rpm. Thus you don't need the rpm. You need to proportionaly manipulate the information in PROP.CALC and make then the conclusions for 100 watt. i.e. you take 13 HP. engine and a 130 HP engine and then somehow make conclusions to apply to your 0.133 HP generator. By the way, the 100 watt is that exclusive or inclusive of the efficiency. I estimate to be 78% i.e. 78 watt out.
Bert

[BertKu]

Originally Posted by Jonahs

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.

Any ideas of doing this?

Hi Jonahs,

Maybe you should consider that excellent idee from Portacruise , on variable pitch props. (Thread third one down) He mentioned about two propellors on one shaft, a large one and a small one. What about such a solution?
Bert

[Jonahs]

Hi Bert.

You might be right. However, would like to see the calculations myself, instead of letting the propcalculator do it for me. The propcalculator is designed for propulsion propellers. I need to design a turbine propeller.

And yes, originally i thought that i needed a way to calculate rpm versus speed. Later i learned that, it is not as simple as that.

Power equals radians per second times torque. If you drag a specific propeller behind your boat, it will turn at a given rpm. If you increase the torque(for example by drawing more power out) rpm will decrease, but the power will stay the same. At least within given interval of speed.

Therefor you need to use a generator that induces the specific torque calculated, at the specific rpm.

The problem is to choose an optimal propeller pitch, at a given speed, and afterwards calculate two of the three factors in the formula of power, as a function of speed.

Once again. Thank you for your ideas and suggestions.

J.

[BertKu]

Originally Posted by Jonahs

Hi Bert.

Power equals radians per second times torque. If you drag a specific propeller behind your boat, it will turn at a given rpm. If you increase the torque(for example by drawing more power out) rpm will decrease, but the power will stay the same. At least within given interval of speed.

Therefor you need to use a generator that induces the specific torque calculated, at the specific rpm.

The problem is to choose an optimal propeller pitch, at a given speed, and afterwards calculate two of the three factors in the formula of power, as a function of speed.

Hi Jonahs,

I have the bad habbit on talking to people and suddenly everything fals into place and the solution comes out of nowhere. I hope it is also the case with you. Thus keep on talking.

Is it true that the power stays the same? If you load your generator by drawing more current (I don't know how you do that other than having equipment switched on) indeed the rpm drops. because power out, is power in, minus the losses.(or reversed reaction to the increased load) By dropping the speed, the pitch will be wrong, or better said, not optimal. If your boat speed drops, the wind is reduced or the skipper is not watching, your rpm also drops. What about 3 props on one shaft in your turbine funnel? I assume the biggest prop in front and when the turbine funnel narrows, your smallest prop. Or you have to consider an expensive VPP (Variable Pitch PROP) . Keep on talking, maybe yo have to run to the patent office tomorrow to lodge your solution.
Bert