Towing a prop -- how to pick a prop?

I think your logic is as good as any.

From the desired power output you can estimate the required blade area, pitch will give RPM.

The only propeller that I know of that are designed for towing are for taffrail logs and they look nothing like a propeller used for driving a boat.

 

Torpedo like propellers (or should we talk here about impellers?), like the Walker style posted by RHough, are designed specifically to be mounted in a rope and move small power devices, such as the Walker's clock. I do not have info on this kind.

You could use a conventional type one to run a generator, if mounted in a rigid axis. Although I do not know if it is feasible, as propellers intended for pushing a boat use to efficiently work in a rpm range much bigger than what we can get when towing a propeller when sailing. But, anyhow, you may try the following:

You need as input the power needed to run your generator (HP that must to be delivered at its outcoming axis, or shp), the top speed (in knots) intended to be measured, and the desired rpm of the device at that speed. To estimate power to be delivered by the propeller, allow for some loses in shaft, bearings, transmissions, etc. Let's say a 5%.

Then use the following:

Required Blade Area: 100*shp/v^3/2
With this you should go to commercial propeller's Blade Area charts (2, 3 or 4 blades, and several Disc/Area ratios) and choose the one that fits to the required area and the available space at your device (or desired size).

Convert knots to inches/minute by multiplying times 1215,6. Divide this speed by the required rpm and you'll get what your propeller needs to advance with each turn to cope with that speed, in inches. You shall now allow for slip, which is a tricky bit, but for an speed in the range of 6 knots we may asume it's going to be around 50%; so multiply times two the advance figure and you'll get the required propeller pitch, in inches.

Finally, to get the required diameter to cope with the power, you should go into a nomogram or chart specifically drawn for the chosed propeller type, and enter there with shp and rpm. You'll get the required diameter.

As you see you're going to need specific technical info on propellers (charts, nomograms...) and I do not know if you may get those easily. If you let me know the required power and rpm needed, I'll be most happy to try to help, without any guarantee that the device will work, of course (Or even that I'm able to give useful advice at all!). Send me a PM.

Cheers.

 

Thanks for the information! That makes sense to me. I think I can tell you the bits your looking for, but you're right, I don't have access to any technical details about props.

I think the nominal case might be the best to analyze, because that's where I'd like it to be most optimal. So at 4.5 knots, the generator should produce about 65 Watts. Since the specifications for the generator are only from second hand measurements, I believe it will produce 65 Watts at about 600 rpm. But it is really getting about 5% more power from the prop, let's just call it 70 watts to cover the electro-mechanical losses.

At 4.5 knots: I get 4.5 knots * 1.15 mph/knots * 5280/60 feet/min * 12 inches/feet = 5464.8 inches/min (using your factor of 1215.6 it is 5470 in/min, so it's the right range).

So at 600 rpm, that would advance 5464.8/600 = 9.1 inches. Does this look right for the pitch with no slip? Would a 50% slip mean reducing the pitch or increasing? 50% seems like a lot for just dragging through the water, but I have no idea....

Ok, so then to the second part where you lost me: Required Blade Area. I don't know what the term "shp" is. Would the result be in inches^2?

The prop I have now is a 9" pitch, 3-blade, 9" diameter outboard prop with cupped blades and an exhaust hub in the middle. I'd like to try to run the numbers on it to see if it is even worth trying. (Props are hard to find in Mexico, so experimenting is tough).

 

Even making the required power 150 W, that's a mere 0,2 HP.

'shp' means shaft horse power.

Using the formula I posted, entering with 4,5 knots and 0,2 HP, you get a required Blade Area of 0,45 sq inches. This calls for a very small propeller (Well, I'm not sure if formula works well for such low numbers, but let's asume it does).

50% slip means you need a 18" pitch instead of the 9" one, but that slip was for 6 knots. For 4,5 knots it will probably be somewhat bigger; but let's tight to 50% for simplicity.

We only miss diameter now, which I have no way of estimate with this info, but for such an small Blade Area for sure it will be pretty small too.

Are you sure about the 65 Watts figure? Would not that 65 figure be Amps rather than Watts? Most probably we should be talking about powers in the range of 2 HP and not 0,2 HP, in my opinion.

Cheers.

 

Yes the amperage is roughly proportional to the boat speed and it's very low power compared to an application of an engine pushing a boat.

Would a bigger blade area help reduce the amount of slip, especially with the shaft torque is probably really low for 0.2hp? Just from an empirical survey of people who have built similar designs the blade area for a 9" diameter 3-blade prop was what others had used. However, they didn't know what the pitch was.

 

By the way, one other person who built his own used a 9"x9" prop. I'm not sure all of his information is correct, and he didn't know what RPM he was getting out of the system:

Here's a link to his answer to my question at SSCA:

http://64.70.221.24/DiscBoard/viewtopic.php?t=265&highlight=tow prop

 

65 Watt at 12V means generator is producing only 5,4 Amps....
For that power you'll need (if you want efficiency), a long, low diameter, big pitch propeller: A Walker style one, that's it.
I think you should follow Antuka's suggestion a SSCA forums and do some trial-and-error tests. Surely you'll be able to move the thing with your 9x9 propeller, even if efficiency is low. But probably efficiency is not a big issue here.

 

In his Electrical book Nigel Calder gets well into the subject.

All the math for prop sizing and alternator gearing are given. At a Library?

Too long for this hunt & peck typist to copy.

FAST FRED

 

I can probably find a copy of that from another cruiser. Thanks.

When I'm experimenting, what are the general guidlines for tweaking it?

Say I need more RPMs -- less pitch? How does the diameter have an effect on the rpm's?

 

He recomends a 14 inch prop as a minimum, slip is figured at only 20% , the most commom alternator step up is 4-1 to get the alt to spin.

You will suffer the same speed reduction as towing a 14 inch pail astern.


FAST FRED

 

A Cruising Boat
Authors: Curtze
Subject: Sailing Yachts
SY January 1975
Pub MT Date October 1976

http://www.sname.org/index_of_publications/pubs_index.pl

 

Nigel's technique probably needs a bigger prop because it is in the turbulence of the boat's hull. And it is probably putting out 5x more power because it's driving a 20-30 amp alternator. However a 4:1 ratio would mean that the shaft speed is probably around 500 rpm to get the alternator going at about 2000 rpm. Most of the commercially available towed props (which produce about 70watts nominal) are around 9" in diameter, but they don't publish the pitch information.

And I was unable to get that link to work about Curtze's article.

 

Eric,
I'm not sure about the applicability of this here, but the rule of thumb states that when you change 1" propeller's diameter, you should change 2.5" pitch. So, to get a reduction from 18" to 9" in pitch, you should increase diameter in 3.6"

Fred,
I find 20% slip quite low for the considered 4.5 kn speed.

 

I appreciate everyone's help on this. I have a guy doing some machining for me and making some of the parts so in a month or so when I depart I'll be able to test it out.

The machinist had some good ideas about how to make the parts so I could easily swap props on it, without having to do any machining. I'm hopeful it will turn out as well as he expects it to.