Towing a prop -- how to pick a prop?

Perhaps someone can help me sort out the issue of sizing a prop to a towed generator.

I'm trying to estimate the pitch required to achieve a certain rpm when towing a prop for a generator.

If I pick a boat speed, say 4 knots, that equates to 4857 inches a minute. Then if I choose a pitch that would spin at 500 rpm, the resulting inches traveled per minute would be 9.7 inch pitch for the prop. That assumes no prop slip.

Is this the right way to look at this problem? How would I estimate prop slip? Would it be a function of the torque and diameter of the blades?

 

Since I'm not getting any responses perhaps someone knows the proper place on the internet to ask this question?

 

This is the right place, Eric, welcome aboard! Unfortunately the forum is so busy that on occasion a new thread gets lost from the New Posts page before it garners any replies...
I suppose the formula you've come up with could be a starting point for experimentation, but I think what you'll find is that you will need to try several props to find the best one. It's hard to ignore slip, though. I can't think of any formula for a towed generator in water... perhaps someone else here can?

 

Thanks Matt. I'm not sure how much slip will occur. I suppose it depends on the torque created by the back EMF of the generator. For a starting point though, I think the slip would be low if it was just free spinning.

I have a 9" pitch prop so I guess I'll just start with that. I don't have time for much trial and error and I figured this problem would be easy for a prop expert to help guide me in the right direction.

 

I assume this is for a sailboat??? Why don't you run something off the prop shaft? Then you can tinker with all the variables with pulleys or sprokets or what have you.

 

Yes this is for a sailboat. I have a Yanmar engine and you can't let the prop freewheel on a Yanmar because it will burn out the transmission. Also I don't have enough room for adding a pully to the shaft.

I'm really just hoping for some information about the characteristics of towing a prop.

 

Is this question posted in the wrong forum?

Perhaps someone could suggest a book or article that might help me choose the right prop for this application?

 

I cant suggest a book Eric but I could suggest a little patience. You could go to the library if your in a rush, --do you have one in None.

 

Sorry if I seem impatient. No libraries here, I'm in mexico. I can't seem to find any information on this topic, which seems surprising. I must be making it too complicated. If I knew how a prop's torque and rpm are estimated based on free-flowing water stream, then maybe I could work with that.

All the information I find on the web relate the prop to the shaft's rpm because that's were the power is coming from, but I'm trying to work it backwards and not having much luck.

Once I leave this port, I doubt I'll have the opportunity to really tweak the chosen prop, so the best guess I can make now, the better.

 

Searching internet, I've found this book: "The 12-Volt Doctor's Practical Handbook" by Ed Beyn (Spa Creek Instruments, Annapolis, MD). I understand (Not sure) it describes how to do it.

 

Jack Rabit Marine sells a setup that has tiny outboard prop on a rope. Try google

FAST FRED

 

There's several on the market, but I'm making my own. I just need some info about designing the prop for this application.

 

Dear Eric,
Are you using a formula to define your pitch ?
If you are , pay attention to the units, they may be :
Speed foot per second
Power HP
Rotation per second.
I raised this question because you mentioned speed in inches per minute and rpm.Regards Amador

 

Look at "A Cruising Yacht", a paper by Curtze given at the Chesapeake Sailing Yacht Symposium in about 1976 or so and reprinted in Marine Technology. www.sname.org

 

Thanks. Is that Marine Technology Society or Marine Technology Magazine or something else? I was unable to find any abstracts under Curtze at both sites.