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post #51 of 283 Old 01-05-2019
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Re: Electric Conversion log for Kahleesea

I haven't had my coffee yet, so f'give me if I don't make any sense.

Quote:
Originally Posted by Captain Canuck View Post
Hi Sean,
If I'm going to run at 48V, there might be a better choice of motor out there.
You're looking at one of the best compromises of speed/torque/efficiency at your boat's requirements. Short of a switched reluctance motor and controller suddenly becoming available in the 5-10kw @ 48v range. And at <50v ABYC wiring standards. e-10 I think, you really are pushing a lot of current at 10KW. ~200 amps. 2/0 awg is expensive. 10kw is roughly the upper practical limit unless you want do deal with the isolated grounds and relative danger of higher voltages.

Quote:
Hull speed is more of a personal design challenge. I'm not looking to skip along the water on my wing keel, or maintain it for any period of time, I just want to be able to touch it and say "yeah, the boat can do that". I realize that I would suck the battery dry in no time. I'm going to start with 8kWh and see how far I can get with that. If I need more, I can always keep adding until I run out of space.
You can still get to practical hull speed with the above system by going above the continuous rating of the controller & motor. Thunderstruck says 600 amps for 1 minute into the motor. I imagine that means 300 for 2 or more minutes, then. Their recommended Sevcon 72/80 550 controller is rated for 2minutes @550 amps. So you have plenty headroom to overprop/overpitch the system for brief periods of hull speed, so long as you size your wiring safely. Set it up, and play with the props. Again something with variable pitch will be cheaper long run, props are expensive. You never know, maybe you have a slippery lightweight hull and can get the hull speed on flat water without over currenting... stranger things have happened. test test test!

Quote:
I have a basic knowledge of electricity, but I'm not a professional, and my knowledge of hydrodynamics is about as complete as someone who's two weeks into their first college course. What I don't understand is why the speed of the prop is an issue. If I get X thrust from a prop spinning at 1200 RPM, and the same thrust from a prop spinning at 800 RPM, what's the difference, regardless of weather conditions, other than the gearing necessary? Won't both props push the boat equally well? You can even argue that fewer RPMs means less wear on all moving components.
Specifically to address your 1200 vs. 800/same thrust question. Thrust at what speed? The efficiency curve of the 1200 rpm prop will not peak as high as the 800 rpm prop will at the particular speed you've optimized for but it will be broader and take less of a hit in different wind/wave conditions it's not optimized for, since the slip will vary less over that range of conditions (Simply because of the higher RPM/lower speed of advance/ratio against actual speed of vessel in prevailing conditions)

At another level you seem conflicted on what you want. You want hull speed -- by definition, inefficient. And you want biggest 2 blade prop for efficiency. wahh? Hey I get it I want fast cheap quality stuff too. But down here in reality we can only do one at a time. Experimenting is fun, and I'm all for it, knock yourself out if you've got the $$ to do so. Report your findings, we all want to know!

For me it's easy, my e29 has enough room for MAYBE a 14.5" prop. May have to modify the rudder to get enough aperture. So I have to go 3 blades. I'll do a kiwi prop, and optimize for headway against crappy conditions, and get my speed fix from my sails.

Quote:
What more risks are there at 96v vs 48v? I don't know much about DC, but surely the risks can't be as high as the risks for the 120VAC that the boat also runs? Are you talking about undersizing your cables, or is it something else?
Vs AC/shore power/inverter? You can't shut batteries off at the breaker when you're working on the interconnects. They're always on. Always a hazard. They never forgive you when you forget to take your watch off, or drop that wrench you were tightening that bolt with. Add to the 'always on' bit with you're quite a bit more likely to achieve a lethal current through your body's internal+skin resistance than at 48v. And you're in an environment (salt wet) that lends itself to testing the theory quite handily.

The arc gap is twice as long as it is at 48v, and you put twice the current into a given resistance, generating twice the heat into any 'accident' that occurs.

Oh and, by the way, you're welcome. Hope it's useful.


Sean
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post #52 of 283 Old 01-05-2019
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Re: Electric Conversion log for Kahleesea

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Originally Posted by boatsurgeon View Post
Long story short, I give that episode and their reasoning for electric propulsion an "F" grade on the sincerity scale.
I agree with you on Uma. They went into it kinda ignorant but at least honest cheapskates. I cringed a lot as they made what were obvious to me mistakes though, and then it went sideways when they didn't report honestly what they learned (Which would have dampened the enthusiasm for electric and their whole cooky eco warrior theme they have going on) But that tends to be the way I guess. Their system, originally, for all its flaws, WAS cheap and did work. objective reached! Admirable in that.

Quote:
I'm not suggesting electric propulsion is bad, in fact I recommend it to clients where it is a viable technology for their use case.

But there are a lot of factors to consider.

If I converted to a turnkey commercially available electric system (with only 8 hours at 5 knot range) ) I would have as much money and weight into it as I would a diesel system with 34 hours at 5 knot range.

What's more, the diesel system can be refueled very inexpensively in about 10 minutes, whereas the electric system will take at least 24 hours, and cost at least one night of marina fees.

On my cruising boat, after motoring 30 hours at 5 knots, I start looking for a marina with diesel as I have about 4 hours left.

If one will be doing a lot of motoring, like down the ICW, or just getting from one anchorage to another between norther's, an electric propulsion system is not a viable option, without an ICE generator onboard, and then what is the point?
I think there are some advantages to this arrangement -- Electric prime mover for the reliable quick anchor set/docking/get out of the way/short hop, and diesel genset for induction cooktop, microwave and air conditioning at anchor, and longer 30-40 hours of motoring @ 5kts. It's the direction I'm going.


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Re: Electric Conversion log for Kahleesea

Sean,

Electricity is like the sea. If you give it a chance, it will kill you. I'm pretty careful with my wiring, having learned it from a friend of mine who's been a professional electrician for over 30 years. He's helped me reno three of my homes. I've also had to go through the OSHA safety training course a couple time for various contracts I've worked on. I don't consider myself an expert, but I am extraordinarily cautious around electricity and am a firm believer in "too much safety is just enough". I will be testing the hell out of the system I build long before it gets on my boat. If anything is going to melt down, I'd rather it do so on my concrete workshop floor rather than inside my boat.

I think I'm not explaining myself well. Thrust dictates speed. Once the water resistance matches the thrust from the prop, acceleration stops and the speed remains steady. Whether that thrust comes at 800 RPM or 1200 RPM is probably irrelevant. I guess I'll find out. I can't put Khaleesea on the hard until some space frees up, so I don't even know how big my prop is right now. Probably 16x10 or thereabouts. Hopefully I'll be up on the hard soon so I can take measurements.

Point taken about overdriving the motor temporarily. The DC cables on Kahleesea are already 2/0. From what I've read, at 48V that's over 200A, more than enough for cruising speed. Since DC and AC systems converge in the starboard aft compartment, that's where I'll put all electric connections. Looks like the charger and the inverter are going to have to be separate, since I can't find an inverter/charger unit that works with lithium batteries.

Are you putting your genset where your old diesel was? I can't imagine there are too many places in a 29' boat where you can put something like that. I'd also be interested to see your wiring diagram if you have one.

I find it interesting that you're going with the Chevy Volt model of hybrid propulsion. If I needed a longer cruising range, that would be the direction I would go too, but with as much solar as I could cram overhead.
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Re: Electric Conversion log for Kahleesea

Your project is interesting and i appreciate your goal. Gotta learn and try new things...to reach further.

With your lifepo4 batts...
Any tool i use that goes down into where those batts reside is taped except for the extreme working ends.
I might sneeze or fumble...
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post #55 of 283 Old 01-05-2019
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Re: Electric Conversion log for Kahleesea

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Originally Posted by Captain Canuck View Post
Thrust dictates speed. Once the water resistance matches the thrust from the prop, acceleration stops and the speed remains steady.
I will take a stab at trying to explain why the above statement makes me cringe.

Thrust is not static. A given prop/rpm does not create say "500lbs of thrust" and when you reach "500 lbs of water resistance" you achieve equilibrium. This is not at all how this works, and is an especially misleading train of thought if you are attempting to armchair/roughly calculate efficiencies or select the correct prop.

A propeller thrusts a boat foward due to the lift generated by its blades' angle of attack relative to the water column it is spinning in (the AoA often times indirectly referred to as 'slip,' or the speed of advance of the propeller VS. the speed of the water column it is advancing in). Thrust, the force pushing forward on the prop shaft, is a dynamic force and depends on RPM, pitch of the propeller, speed of the boat. As the the boat speeds up, the angle of attack at a given RPM is reduced, and so is the lift, and so is the thrust. When the boat is not moving, at the same RPM the blades can begin to stall, create turbulence and possibly cavitate, generating very little thrust. When optimizing for efficiency, you can only optimize for a single point in the speed/thrust curve. Everything slower or faster than this particular point will not be at max efficiency of power in vs. power out of the system, and for thrust can be wildly different from what you expect.

A perfect example of this is the lowly trolling motor, which may have 100lbs of thrust at zero speed, but at roughly 4-5 kts, is generating zero thrust. You can put 5 100lb thrust trolling motors on the same boat and it will not go any faster. It can't. The propeller blades are effectively at zero AoA to the water column they are moving in.


SO let's say max efficiency is roughly 50% slip (Just throwing a number out there but this is a fairly commonly used point), you prop your boat so that at max RPM the props are going twice as fast as the water column. You're aiming for hull speed at say, 7.5kts, so you pitch it so the speed of advance of the prop is say, 15 knots. This will give you maximum thrust at 7kts, and will be great for your ideal speed run in calm water with no wind. however... when the 30knot winds on the nose are blowing, the chop is reaching 3-4 feet in the bay, your boat has slowed down, the propeller at max RPM will be at various states of stalled and will NOT generate maximum thrust where you now need it the most, to make meaningful headway against the conditions. It's like oversheeting your mainsail. You keep pulling but the boat doesn't go.

The conservative angle would be to pitch the prop for say, 10kts speed of advance. Maybe now you can't go faster than 6 kts, because you are not generating enough thrust to overcome your wavemaking resistance. But when the crap hits and forces your boat to slow.. you have a lot more thrust at 4-5 kts headway to push through the wind and chop, and your prop is not operating in a stalled speed regime.

When designing for a system with limited power -- speed is great, but it comes at the cost of low end grunt. Low end grunt comes at the cost of speed. This is the compromise you have to make when designing for efficiency, speed, or power.

Quote:
Whether that thrust comes at 800 RPM or 1200 RPM is probably irrelevant. I guess I'll find out. I can't put Khaleesea on the hard until some space frees up, so I don't even know how big my prop is right now. Probably 16x10 or thereabouts. Hopefully I'll be up on the hard soon so I can take measurements.
The higher RPM means that the change in AoA across the speed regime is less drastic, so the efficiency peak, while lower, is more broad (it's over a bigger range and you are less likely to stall at the lower end of the speed range than the higher pitched, slower prop)


Quote:
I can't find an inverter/charger unit that works with lithium batteries.

Are you putting your genset where your old diesel was? I can't imagine there are too many places in a 29' boat where you can put something like that. I'd also be interested to see your wiring diagram if you have one.
The boat came with an atomic four (ugh). I'm opposed to explosive things in a cave so want to avoid propane/gasoline/etc. The thunderstruck motor + belt drive take up little enough room that I can fit a 5kw genset roughly in the engine bay in front of the motor, if i build out the front of the engine bay out far enough to comprise the second step of the companionway 'ladder'. Since i had to rip the floor out to get the oil cleaned up from the ranwater flood that allowed me to pick up the boat for cheap... i'm rebuilding that, the galley, the bulkheads, etc. The new engine stringers are in now, though, and i'm rebuilding the compression post step/cabin sole now. Exciting! Take a look at the Victron Quattro/Multiplus/whatever they call them. They can adjust charging voltage in .1v increments, plenty close enough for lifepo4 @ 90-95% charge regime, and you dont really want to top lifepo4 off anyways.


Okay off to dinner...


Sean
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Re: Electric Conversion log for Kahleesea

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Originally Posted by RegisteredUser View Post
Your project is interesting and i appreciate your goal. Gotta learn and try new things...to reach further.

With your lifepo4 batts...
Any tool i use that goes down into where those batts reside is taped except for the extreme working ends.
I might sneeze or fumble...
You are a smart man!

Sean
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Quote:
Originally Posted by Captain Canuck View Post
I won't be concerned with 50% DOD. I'll be running Lithium batteries, so 80% DOD will be by cutoff. I always thought Peukert was a wanker.
You can discharge AGM's to 80% also. Lithium batteries big investment.
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Re: Electric Conversion log for Kahleesea

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Yes they did.

Kinda blows the primary factor to reduce carbon footprint outa the water.
I think the conversion to electric (for many) is less a "solution" and more an "intention" to move forward.

Here in coal-fuelled Alberta, owning an electric car is actually more carbon producing than a Honda Civic. Next door in hydro-powered BC it is way, way less. Doesn't stop concerned Albertans from buying electric cars, and IMHO it shouldn'tóthe more common it becomes the more likely it is that it will eventually make a significant difference.

So more power to the electric boaters!
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Re: Electric Conversion log for Kahleesea

Sean,

Thanks for the great info. I really appreciate it. No idea why I though the water would be a constant, but when I read it was dynamic I had a serious "duh" moment. OF COURSE it's dynamic. It's WATER. Too many years of car work, I guess. Anyway, thanks for snapping me out of that and pushing my knowledge just a little bit farther forward.

Looks like I should be making my most efficient point cruise, (say, 5kts) and leave about 25-35% more on top for emergencies. That sound about right?

Did you manage to sell your atomic 4? Seems like people either love or hate them.

Good on you for getting a scrapper and doing a total reno. That's a lot of work. I doff my hat to you, sir.
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Re: Electric Conversion log for Kahleesea

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You can discharge AGM's to 80% also. Lithium batteries big investment.
Big investment, but much cheaper than lead acid in the long run, with a much longer list of advantages than disadvantages. My two favorites are much less weight and giving Peukert the finger.
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