Spead through water

[Das_Boot]

I am trying to work out how to build a generator for towing behind my boat. I can buy the fixed magnet generator and all of the relevent bits off the shelf.
I dont have a clue as to how to work out the spead of the shaft at say six knots. I know my engine prop will be working at say 1800 rpm to get me to this spead. Does anyone know where I can start to research how fast a propellor will turn when dragged through the water at say 6 knots.
Any information will be greatfully received.

 

[Stemar]

Your engine prop is pushing water out backwards at a lot more than 6kts, but a generator prop is turned by the water, and "slips" through it, so it won't turn at the theorectical speed. The pitch of a prop is the disance it would advance through the water in one revolution - or how far it will have to be pulled through the water to turn one revolution - assuming no slippage.

Back of envelope calculation:

6kts x 6080 = 36480 feet per hour
36480 / 60 = 608 feet per min
608 x 12 = 7296 ins per min

Assuming an 8 inch pitch prop with 8 ins slippage, (if you've got other figures put them in here), you need to travel 16ins through the water to get one turn of your prop.

7296 / 16 = 456 rpm

Obviously, I've no idea what pitch prop you'll be using, so I've used that of my prop as an example. The amount of slippage is a pure guess. A lot will depend on the diameter of the prop and the load. The engineers among us will probably be able to say if it's reasonable or not.

HTH

 

[Das_Boot]

Thanks at least it is a starting point. I did not have a clue where to begin. I know the torque will play a big part but most of the off the shelf gennys use rpm in their graphs.

 

[Colvic Watson]

...Which means my old Macwester 28 goes at more than 40,000 feet an hour! You've made my day; I always knew the old girl had life in her, never dreamed she went that fast - thanks!

 

[roger]

Put it in millimetre per fortnight - even more impressive

 

[billcowan]

Mmmm, if you have the ENGINE running, why not just fit a bigger alternator. If sailing, just turn off all the electrical things you dont need (nearly all). 20 mins engine running coming into port usually recharges all you need after a days sailing.

 

[Joe_Cole]

Why not drive it off the shaft of your existing propellor rather than getting another prop to drag behind you. Or have I missed something?

By the way, where are you sourcing your generator? I've been trying to find one but haven't had much luck so far. I can either get massive ones, or little tiddlers. All I want is something turning out about 10 to 20 amps.

 

[Anonymous]

[ QUOTE ]
If sailing, just turn off all the electrical things you dont need (nearly all). 20 mins engine running coming into port usually recharges all you need after a days sailing.

[/ QUOTE ]Not quite the case if you are crossing the Channel at night in poor viz. Radar on, probably nav lights rather than the tricolour, and autopilot. We draw between 10A and 15A under those conditions which, after a 10 hour night passage, takes a good deal more than 20 mins of engine running to replace. Some sort of water powered generator seems like a good idea though wind has the advantage of working while not under way.

 

[William_H]

The question related to towed generators so he doesn't want to use engine charging or engine propeller. My instinctive feeling on this is that a longer pitch prop will give more torque with less tendency to stall than a normal; propulsion prop. However the flow angles improve when the prop is turning. I guess the answer is to try various pitches. You might do well with a bet or gear speed step up to get the voltage out of the generator you need. Obviously shaft speed is going to be quite low.
regards olewill

 

[Das_Boot]

I have considered gearing up to increase the revs but if you look at the Aquagen they dont so I assume it is possible to achieve the same results as them at a fraction of the cost with minimal DIY as all of the parts are available from the shelf. Also if you have the shaft on a direct drive from the prop it will be more efficient. If you are looking for generators type in Permanent magnet generators in google or try fixed magnet generators.
By the way if you do gear up you could use a car alternator I am not sure of the gearing required but it should be easy to work out.

 

[Gunfleet]

Sounds like you need a motorcycle alternator, Joe. Try a m/c scrap yard.

 

[Das_Boot]

The problem with a mc or car alternator or magneto is that they are built for higher revs. With the calc of the earlier post showing only 500 or so revs you need somthing slow spead high torque although tweeking the prop might achieve more revs. This is what I am trying to find out.