This posting is rather a cheat since we merely kept the propellor on our boat as it was prior to the conversion(!), but I've included it since in researching our conversion I came across several articles and other comments concerned about the best propellor to use for an electric drive such as ours.
A common recommendation is to use a larger prop design with greater grip on the water. I personally cannot see the argument for this since the overall physics process is that you set a certain power input and then the electric motor will accelerate until "power out" = "power in" (less incredibly small losses - but these cannot be large or the motor would rapidly get blindingly hot and it doesn't!))
The propellor therefore speeds up until this power equation balances (actual revs are irrelevant)... when the boat will be being given a driving power exactly as it was for conventional drive. So, on this basis, if the propellor drove the boat "sensibly" before conversion, it still will in exactly the same way!
I think propellor considerations people make are really based in terms of maximally efficient connection between the prop and the water when the conversion is such one needs maximum range for a given drive battery charge. But this isn't a prime requirement in this case since one can choose to recharge at any time.
Clearly, any increase in coupling efficiency between prop and water would mean less re-charging time - but from my researches gains only seemed to be of the order of 5% at best... really quite trivial compared to the variations one would meet for meeting different river currents, etc. - not really of practical significance.
Incidentally, with our control set to maximum, the motor draws 14kW (rather than the designed 12kW) so it would seem that if anything we are "over-propped" by retaining the original propellor and its size should be reduced for the motor to run as specified!
All-in-all, our system seems to work entirely satisfactorily retaining the original prop!
Saturday 6 September 2008
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6 comments:
The reason for a larger prop is likely because electric motors have maximum torque at 1 RPM, so having a larger prop would provide for greater acceleration.
Really sorry I didn't see you left a comment earlier...
The acceleration available (as we've experienced) from electric drive is already phenominal compared to conventional diesel because the motor spins to speed far faster than conventional drive can. Albeit (in our case) with a 0.5 sec electronic delay which can be highly confusing - you move the lever and the response comes later like playing church organ with the pipes the other end the church...
I fully appreciate larger prop should make greater use of the torque available at low RPM. But contrariwise there is the argument the drive motor can be overloaded at high power for having too big a prop.
I think the real prop problem is - whatever the size - when cavitation sets in, and that's to do with design - and hull design. In our case it is clear that to run the prop above certain speeds in the various conditions can be absolutely pointless because once cavitation sets in the effect of the prop becomes relatively desperately inefficient.... although better than nothing if you really need the thrust for some reason.
On a canal, as I'm sure you realise, the huge problem is that they are so shallow you have to try to be sure you aren't using the prop to pump water up or down the canal rather than move the boat - and that effect is nothing to do with prop size.
I'm now involved as an consultant for electric boat drives as a result of my experience and reports so I'd be delighted to get into technicalites and observations if you like - by all means contact me at edwin.osborn@dsl.pipex.com if you'd be interested to discuss this further.
Hi Edwin, this looks like a really interesting project. I bet it was alot more work than you thought before you started. I should know, I just converted my Bristol 32 sailboat to run with an Asmo Marine electric motor system. What a pain! But well worth it. I'm a true believer in electric power for boats, especially sailboats. Your post about propellers particularly caught my eye, as I am researching the same thing and have come to similar conclusions after talking to a few "experts." It's abit complicated though on a sailboat because you don't want the drag of a big prop... but then again you do, so you can regenerate... Anyway, nice work on your blog, thanks for sharing! Please check out mine if you get a chance. Thanks, -Greg
Good to hear from you, Greg, and many thanks for your kind comments! I'm, naturally, pleased to hear you came to the same conclusions about props having consulted "experts"..
I've read/heard a bit about props on sail boats and potential use for re-charging - it seems to me the trick is to have a design you can CHOOSE whether the prop is active when (wind-driven) sailing - either via a system that moves the whole prop up into the hull or via a prop with adjustable pitch blades etc..
I've looked at your blog - congratulations on your efforts!! - both in adapting your boat and in writing up the blog (it's a real effort, isn't it?!! And so just wishes you could just SHOW people how it works!)
I have a comment about your batteries... but I pull it in a second "comment" because I'm losing track of this one.
Greg, as regards batteries.. I realise you've already fitted yours and wouldn't want to change them NOW, but....
Firstly, I'm somebody gets tired of people always fearing disaster.. but, actually, if "wet" lead acids "gas" (or spill) it can be quite nasty, so...
Secondly, when due for replacement, why not consider an AGM type (such as the "Odyssey" brand I eventually found and used?)I've found them just SO good - they absolutely don't spill and absolutely don't gas so long as the recharging voltage doesn't exceed 15V per 12V unit. We've had ours four years now, and I can't detect ANY deterioration as yet. They hold charge absolutely (at least, if left near full re-charge and with slight solar trickle-charge - not that I've tested if one doesn't have this!) Compare our previous "ordinary" wet units I used-to use for auxilliary functions which let me down desperately badly one winter!!
Anyway, just a thought. If you wanted to discuss further (or anything else) my direct email is edwin.osborn@dsl.pipex.com.
Incidentally, what's the rated power of your electric drive motor and your boat's "tonnage"? I ask because I've been drawn into so many "arguments" over requirements for this. Our "12kW" (only 16 bhp mumerically!) motor is definitely MORE than we need for our boat For normal use) and definitely more EFFECTIVE than the previous "42 bhp" diesel... but try persuading a diesel-fanatic it's more than enough short of them trying out our boat for themselves!!!! (which can be quite funny when the stern goes down and the bow goes up and one looses steering control in about four seconds flat... not that it doesn't amuse me to demonstrate to disbelievers!!)
Hope to hear.
Hi Edwin, thanks for the reply, the compliments and the insight. As for my project, I'm already in the mode where I just want to relax and enjoy what I spent so long working on. I can change the prop, that's easy but the batteries might be abit more of a pain at this point. I wished there was more guidance when I did my project. The biggest problem for me though is that I did this project in Japan (I'm American and am not fluent in Japanese) so the biggest challenge was getting all the right materials and knowing what to ask for here. The wet cell batteries are the best/ most cost effective that I could find in Japan. I know the gel or AGM type would've been better but my wet cell ones haven't caused any problems yet either. No gassing or spilling and it's been about 2 years so far. I'm hoping when these finally give out, Lithium Ion will be cheaper and more practical, since it would be really nice to reduce the weight... Anyway, thanks again and if you want to share stories and exchange ideas in another format, my email is ffmagellan@gmail.com but an even better idea- please join the forum that I'm starting on my website. I'm about to announce it to the world, so there should be alot more people on there soon. -Greg
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