Hi Eric,
As long as the same hull was utilized for different props I would think that the effects of the hull could be safely ignored. I like three blade props because they are inherently more efficient than four blades for a given diameter. Two blades would be better than three unless one ends up with the two blades being simultaneously in the shadow of the keel. Not certain what the limits of keel shadow are either so just to play it safe I'll settle on a three blade propeller.
I believe that I also understand the remainder of what you are saying but the lag in regen start up really intrigues me. Because sailboats all cruise at such low speeds it makes sense that the pitch would also be relatively small to reduce slip. When one gets the engine/motor near the top of the non-stressing rpm (or power curve) output it is best to have reached hull speed. I'd think that the larger the propeller the better but that an increased pitch (one that gets the boat to hull speed at a much lower rpm) might indeed work out quite a bit better in terms of regen. For me right now, the question is would an increased pitch allow the boat to begin regeneration at a lower speed? But then without more experimental data every supposition is nothing more than conjecture.
Just poking around on paper but IF a propeller is 20% less efficient in regen (i believe that someone recently wrote this and being due to slippage) then wouldn't one wish to increase the pitch in recognition that not doing so will never get an equivalent rpm at hull speed anyway? I would suspect that the resulting prop would begin to spin more quickly with an increased pitch.
I am going to re-read plot the data points you have provided and probably add my own into the mix as I am now looking to convert a 30' boat myself. Hmmmm, might be able to test out some of my pitch speculation with an electric trolling motor prop reconfigured in different ways.
Thank you for the comments and the data.
Michael
From: Eric <ewdysar@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Sunday, October 2, 2011 11:16 AM
Subject: [Electric Boats] Re: actual test data for prop regen?
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
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> does anyone know of any experimental test data that compares props for regen? wish someone would swap out props varying pitch and diameter so that there would be a starting point that was not theoretical
>
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
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> does anyone know of any experimental test data that compares props for regen? wish someone would swap out props varying pitch and diameter so that there would be a starting point that was not theoretical
>
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