T-Pain Is On A Boat: Re: [Electric Boats] Re: actual test data for prop regen?(for Femm)

Sunday, October 2, 2011

Re: [Electric Boats] Re: actual test data for prop regen?(for Femm)

Hey Femm,

I really like your reply but am not so aware of what is available in the RC world. I looked once before without being able to find higher pitched props. I believe one of the ones you pointed at had a pitch of 12. Do you know whether or not RC props with higher pitches exist?

Michael

From: Femm <femmpaws@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Sunday, October 2, 2011 2:57 PM
Subject: Re: [Electric Boats] Re: actual test data for prop regen?

Okay I'm going to sound off here.

The first thing you have to take into account is your brake out speed as in how fast does the water has to be moving before it can over come the mass and drag imposed on the prop and shaft...

Next you need to look at prop pitch... The more pitch the wheel has the more speed it's going to take to get to your brake out speed. So a fine pitched prop is going to work better. Two reasons, the fine pitch get's better bite in the water, and the water doesn't have to move as far to make one turn with the prop.

Because you are working at such low speeds the drag on the wheel is almost the same as the power you are getting out of it. using your numbers you are getting around 25 watts of power.... In my book that is a very poor return.

Now if you built something that was set up just for charging would have a different wheel design. With towed charging systems they have found a helix to work the best at low speeds. But there is another way to do it as well, by using a high aspect ratio prop.

What would you be looking for first, a large disk area so a 18 to 24 inch diamator would be a good start, then something with a fine pitch, my guess would be 6 to 10 inches of pitch.

Next where to find them? A hobby shop!
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCGK5&P=0
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHMK6&P=0
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXFGV4&P=0
Or
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXZ993&P=0
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXZ994&P=0

Now any of these props will absorb over 3 to 5 horse power in the air. If you take two of them and make a four bladed prop by stacking them on the same shaft you should have the blade area needed to pull 400 to 500 watts of power at your 6 or so knots. I would say that's a lot better output them 20 watts. These props are going to have less drag then a boat prop at this speed and that means you should be able to get more power out of them.

Now you are doing the how do I hook it up? Well the first thing that come to my mind is a curved shaft weed-eater setup. Better yet would be a straight shaft with a fairing pod over the little gear box. A prop spinner may be the slick ticket for your nose cone.

Now you could get fancy and get some streamlined tubing and use a right angle drill gearbox and a shaft up the tubing with some UHMW blocks to act as bearings so the shaft will not whip in the tube. You could also set it up so the water lubed your bearings and cooled them at the same time.

You can get replacement stators for the GM altanators that will put out 48 volts.

Kriss aka femm

--- On Sun, 10/2/11, Eric <ewdysar@yahoo.com> wrote:

Hi Michael,

You're asking for simple answer to a very complex problem. First of all you need to define the boat's underbody and where the prop is in relation to it. You would need to define how much resistance the prop would have to over come before any power would start to generate. You would have to set up a system that would provide consistant boat speeds for testing. And you would have to have a bunch of props.

One could start the process in a controlled water tank but I think that the results would be misleading.

But if you are willing to help fund the study, we could start with my boat. My prop is a 13 x 15.5 four blade prop and cost about $400. I think that we would need a decent collection to start some trending.

Acme makes models 674 and 470, both 12.5 x 15, one is three blade and one is four blade, so that you be a good test for comparing the number of blades for the same diameter and pitch. Model 1210 is a 12.5 x 12 three blade so we could see the effect of 20% less pitch. Model 650 is a 13 x 12.25 four blade prop, so that would be the same 20% pitch reduction compared to my prop. Maybe we could get a number of more conventional Michigan Wheel sailboat props to compare those as well

We could hire a diver to swap the props in the water. Then we could get a power boat to tow my boat at different speeds to collect regen data. We should be able to compare 4 props a day on one boat, once all the the props were ready to go.

At the end we would have some data, but it would be most relevant for a 30' full keel boat with the prop in an aperture. The results for a fin keel boat with an exposed prop might be considerably different. Maybe if we could get another converted boat like a Catalina 30 at the same place and time to swap the props (if they would fit) on it as well. Then we would get info for different underbodies too. We might be able to test 3 props a day on two boats at once. Of course, just swapping props isn't a fair test, the reduction ratio sould be matched for the different props, at least it makes a difference for propulsion, so it probably should be done for regen too.

If nobody wanted invest the time and money for that type of study, you're stuck with the objective regen observations that people have collected on their converted boats. Here's your first data point, my boat is 30' x 8.75', full keel, 48V system, the four blade prop is 13 x 15.5 and I get an average of about 0.3-0.4A at 6 knots. I think that Mike gets about the same results with Bianka, I don't know his technical particulars.

But his boat is 2.5' wider, has a fin keel, a spade rudder and an exposed prop. And he's getting the similar results at the same speed, so this may point to the fact that if the drive is geared correctly for the prop, the regen results are similar for a wide range of prop and boat configurations. You might be focusing on small percentages on what turns out to be a small amount of energy for most of our boats. A different prop might give you an extra 0.1A of regen at cruising speed under sail, or maybe not.

I know that it's fun to while away the hours figuring out little parts of a potential conversion, but this one may just be a distraction. You're not going to find a magic prop that generates twice as much energy and even that would only increase my regen to 0.6-0.8A at full speed sailing. It's kind of like trying to balance your annual household budget by focusing on what brand of butter that you use. You might cut your annual butter cost in half, but will that make a difference in making your house payment?

Fair winds,
Eric
Marina del Rey, CA