# Estimating Electric Consumption

Discussion in 'Electric Propulsion' started by Jedidiah, Mar 18, 2018.

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### JedidiahJunior Member

Hello Fokes - First post here ...

Let's look at estimated consumptions for a conversion; I'm wanting to reality check a couple of assumptions I'm making.

There are a LOT of boats out there that can be had for not much. Some have been damaged to some degree or another - hurricanes, groundings, whatever. Others are owned by people who have suffered financial reversals & can no longer afford them, while still others are owned by those who have simply lost interest & are no longer taking care of them. Many are owned by marinas & yards that want only what they're owed to get them out of their way, etc.

So, if a guy's walking around on the dock or out in the yard eyeballing boats with a view toward picking up a cheapie for an electric conversion, it seems that the place to start would be the boat's appetite as-is. If a given boat has a history of or can be expected to burn, say, 6 gallons of diesel per hour at whatever given speed, we should be able to come up with an off-the-cuff estimate of how much electricity it'll use per hour if all other factors remain equal - length, beam, displacement, draft, speed, etc. Any significant changes, of course, would have to be factored in, but for the moment let's just rip out the diesel(s) & the tanks, then replace them with electric motors & bring the boat back up to it's existing displacement weight & trim with batteries.

At 6 gallons per hour it's approximately 1.2 gallons per hour that's actually moving the boat while 4.8 gallons get spent doing what internal combustion engines do best, which is to turn hard-earned cash into unwanted heat. All other factors remaining equal, can I estimate this boat's kWh/hr consumption as simply as ...

kWh/hr = (6 gph x .2) x (37.854 x 1.2)

Assumptions - older diesels are maybe 20% efficient, & a gallon of diesel fuel is worth 37.854 kW. The 1.2 is a generous allowance for the losses inherent in using AC power to charge DC batteries that power less than 100% efficient AC hardware, stepping voltage up or down, etc. Assumed values can be tightened up with specific hardware selections - for now they're worst-case (maybe).

I get 54.5 kWh/hr

I am looking at a particular boat, a very large classic motor yacht, & frankly I think 40 tons @ 10 knots & 6 gph is a Please-Buy-My-Boat story, but if the owner's not lying to me we're looking at a potential 1,250 kWh pack of 2170s that could power it for 20+ hrs & 250 or more statute miles of cruising - & that disregards the slight bit of solar & wind generation that could add eggs & bacon to my coffee along the way.

As for comments, let's just overlook the obvious stupidity of spending any money at all on a rickety old hull - I'm about to drop a goodly chunk into setting up for retirement (a chunk that I was competent enough to earn to begin with) & I won't be buying or building a featherweight carbon-fiber skiff to use as a retirement liveaboard. For the moment I'm just interested in a workable, off the cuff estimation of electric power required to push any given conversion of whatever boat happens to minimize or obviate any personal need for E.D. meds. In other words, I'm looking for useful data drivers that will inform personal decision making for myself, & hopefully for others as well.

Will this work as I look around at conversion candidates, or consider building my own specifically electric version of something I like?

Thanks Much

Last edited: Mar 18, 2018
2. Joined: Mar 2013
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It's easy for an existing boat. You take the boat out for a spin and record the rpm at different speeds. Let's say (total bogus example numbers) it makes 6kn at 2000rpm. Then you look up the rpm/power curve of the installed diesel. It says maybe 20kW (27HP) at 2000 rpm. Now you know how much power it needs with the existing prop and gear reduction (so you don't have to change that too) for a given speed (and sea state).
Calculating the battery bank is now a function of system losses and desired runtime. For example: series DC motor 82% efficiency, PWM controller 90% efficiency, (the manufacturer gives you his numbers). So the battery needs to supply 27kW for the motor to put out 20kW. If you want to sustain this speed for 1 hour you need a battery capable to supplying 27kWh. Since you don't want to discharge deeper than 80% (even with lithium) minimum battery size for 1h of 6kn is 34kWh. Depending on battery type and voltage used this number could go up.

If you are looking at something to convert you need to look at displacement type hulls optimized for low power. Meaning something long, lean and round bodied. Ask the owners what engine is fitted and at what rpm they achieve what speed. Since most engines have their stats published it's easy to calculate what power they actually use, and if their consumption figures are bogus or not.

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### DCockeySenior Member

This contains a fundamental, but not uncommon, error in interpreting engine power vs engine speed and engine torque vs engine speed curves. The curves show the maximum power and torque that is possible as a function of engine speed, not the power and torque produced in a particular situation. Unless the throttle/fuel control is at its maximum position the actual power and torque being produced by the engine is a fraction, and possibly a small fraction of the maximum possible. It is not possible to estimate the power and torque an engine is producing based only the engine speed.

Barry likes this.
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### JedidiahJunior Member

This is why I'm looking for a quick, off the cuff method that I can apply in my head while I'm standing there on the dock ... no tech specs or complex math required.

Good logs & (or) a day with the boat burning fuel should answer the basic question - is this boat a viable candidate Y/N. If we know how much of what fuel a given hull burns under given conditions, we also know how much electricaltricity the same hull will burn under the same conditions.

... & I completely agree with Rumars on the most likely hull configuration - this is the type of boat that I've been looking for. You're talking about hull forms that were the state of the art a century ago & more, when early powered vessels were posting impressive numbers with the very earliest of internal combustion engines.

Thanks Much

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Yes manufacturer data is usually provided at full load. But initial engine selection was based on the same data, and for a conversion one must asume the same full load. If one does not do that you come to the conclusion that "electric horses are bigger", underpower the boat and have problems when sea state and wind put additional load on the motor beside calm water resistance. What happens then is clear, the driver opens the throttle and either the batteries die or the motor (or sometimes the controller) overheats because it's not capable to take so much amps for that long. I consider my method valid for a quick estimation of needed battery capacity for a conversion.

For a dockside estimation you don't even need to drive the boat. For 10kn speed you are looking for a waterline over 60ft, L/B ratio of at least 4, curently powered with something small, in the range of up to 4HP/tonne. You will want the lightest boat you can find.

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### DCockeySenior Member

Two different "full load" conditions are being confused. Engine selection may be based on the maximum power the engine can produce at some engine speed. At slower speeds the power needed to move the boat will generally be less then the maximum power available from the engine. If the design speed with electric power is less than design speed with IC power then using the IC power curve at an engine speed below max boat speed will result in too high an estimate of power needed for the slower design speed.

Last edited: Mar 19, 2018
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### JedidiahJunior Member

The boat I'm looking at has a lot more power than that. It's a pair of supercharged GMC 6-71s that should make around 200+HP each. It also has an Onan 7.5kW genset, & all of the tanks are welded, 3/16" plate. The fuel tanks are 6 x 100 gallons & when I total all that up I get around 12,000 lbs of unwanted junk with fuel tanks full - water & holding tanks empty.

Given the weight advantage of an electric motor (& plastic tanks), that's a lot of batteries.

The engines need a rebuild. I'd rather put that money plus whatever I can sell them for (plus buttloads more, of course) into a conversion. There are other smaller, lighter options but none would be quite so livable, & today's best battery could be a whole lot better in as little as 3 to 5 yrs - so y'know, I'm thinking about it & just might spring for a survey.

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### JedidiahJunior Member

DCockey - I've been eyeballing the YASA 750 R & similar motors that can be stacked up on a common shaft. If 2 aren't enough, 2 more would certainly do the job. If I'm not mistaken they can be switched in & out as required, as in 1 per shaft just sits there freewheeling while 1 per shaft meets a lighter demand, like idling along in the Intercostal.

No - I have NO idea what they cost, & they do look spendy - but - anybody priced rebuilding or replacing a marine diesel lately?

... & after all that money, it's still a marine diesel.

9. Joined: Mar 2013
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Please explain what I am confusing exactly. Engine manufacturers publish data taken from a dynamometer at WOT. Boat manufacturers use that data to match an engine to a boat and propeller so that at average sea state and expected cruising speed the engine is loaded properly, meaning that in this conditions the engine operates around it's most efficient state without glazing the cylinders from underloading or exceedig operating parameters by overloading. Yes in reality sea conditions vary and the boat might need less or more power, I'm aware of that. What I'm saying is that when trying to do a conversion to electric the battery bank must be sized according to the same principles. The motor must be capable of a similar continuous duty and the batteries must be capable of delivering the amps without voltage sag. If one uses less power the only thing that happens is that range increases.

Jedidiah I think you have no ideea of how much the kind of performance you are talking about costs with electric motors. The battery management system alone will pay for rebuilding the diesels. And just think about how long a typical marina outlet will take to recharge such a big battery bank.

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### JedidiahJunior Member

8+ days for a full charge, or a couple of days to recover 6 hrs of easy cruising time.

Let's say it comes out to 60kWh/hr @ 10 knots, which is 1) too fast for confined waterways like most rivers & much of the Inter-Coastal (in such a large vessel, anyway), & 2) 8 to 10 hrs of charge time for every hour on the move, & 3) on the boat I'm interested in, at least, 500 sq ft of solar wouldn't even look weird - it's a flat-top canopy from the wheelhouse back to the stern.

Anchoring out for days or even weeks at a time would not be out of the question, & any marina capable of taking such a vessel will have 220V @ 30A, minimum. I can live with that.

Just as an example, a lot of Loopers do it start to finish in a year. Nobody moves every day, but if they did that would work out to around 16 1/2 miles per day. That's the pace I anticipate - 100 & some miles in a week, if that.

Last edited: Mar 19, 2018
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### latestarterSenior Member

The way you achieve different speeds on the water is by adjusting the throttle, so by definition, only at maximum speed will the engine be at WOT.
Therefore the manufacturer's power and torque curves are only valid at maximum speed.
The manufacturer is altering the resistance on the dynamometer to change the rpm.

If Jedidiah wants the electric boat to have the same maximum speed as the diesel powered boat you could use the power curves as a guide but it appears that he will be running it slower.

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I understand all that, and that the prop is more or less missmatched most of the time and so on. But the OP was asking for a quick estimative method for sizing a battery bank. One has to asume that sooner or later he will have to run WOT for longer than a minute so he needs to size for that.
On the other hand if the plan is just for short hops maybe it's doable with not so big batterys.

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### JedidiahJunior Member

Anybody looked at ELCO's systems? I'll not suggest that they're the be-all-end-all option, I do have my issues with them & will continue to look for better power densities, but they're certainly worth considering - particularly for a massive old power yacht that's so very much like ELCO's own pre-war cruisers.

First is weight. The EP-100 is a 700 lb motor - with the included inverter & disregarding any gear reduction, it's a 740 lb package. Ok, that's less than a third of a 6-71's total installed weight, but still pretty freakin' heavy. My next issue is with voltage. I'd prefer to at least double the EP-100's 144V system voltage. Third - ELCO is pretty tight-lipped about the details, & that makes it harder to short-list them.

In fairness to ELCO, they're designing & building systems that should be absolutely bullet-proof for decades, the voltages for their various packages are easy to achieve, & their marketing is just that - marketing. It's a small company with an interesting history, & serious inquiries would no-doubt be taken seriously. Getting an ELCO engineer on the phone for a discussion about prop sizing etc shouldn't be an issue.

What might seem surprising is that I'm not troubled by their approximate \$25,000 per shaft price tag. eMarine Systems is currently listing the EP-100 at \$22,570. That's a lot, but it does represent an approximate 10% price drop since I last looked. In any case, compare that to \$3 to \$4k per bore to rebuild a 6-71, & it looks pretty reasonable.

It's the cruiser-caliber battery that will knock a 6 figure hole out of a Roth account - all by itself - & this is probably why ELCO's promotional material leans on unrealistically small, lead acid installations - the 30kW generator that I don't want anywhere near my boat is much-more-cheaper.

In any event, the power & control electronics & related hardware are doable to the extent that it IS being done. My question remains, "which hull", & the answer probably won't make any sense to anybody who hasn't been working toward it for decades like I have been. My objective isn't to be the most efficient thing on the water, it's to have an agreeably comfortable liveaboard that will be efficient enough to completely eliminate any fossil fuel component.

EDIT: I just looked & the best price I've found for a reman 6-71 is \$17,000, & of course that doesn't include shipping or installation. They weigh 2,200 pounds not-installed but strapped to a pallet for shipping - no fuel delivery & conditioning, no induction, no exhaust system.

That beats as much as \$4,000 per bore (x 12) to rebuild existing engines, which also doesn't include removal, 2-way shipping, or re-installation. The only caveat here - you can do alot with a 6-71 as she sits in her bed, total rebuilds even or so I've heard, though I dunno how you accomplish 2-axis clean-up bores without pulling them out.

The bottom line is \$40 to \$60k for fresh engines. Me-thinks electric propulsion is becoming more price competitive by the day, & that cost doesn't yield two & a half tons of loud, stinking, vibrating junk with an insatiable appetite for Dino-Chow.

Last edited: Mar 19, 2018
14. Joined: Mar 2013
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You my friend are in another league compared to what Elco has to offer. If you prefer more open options look in the high power EV scene. Another solution comes from light traction (buses, trams). Last option is a dedicated marine high power system. There are a few firms doing this.
One Yasa 750 motor will set you back around 13K without tax. Similar PM motors from other manufacturers have similar pricing. The associated high power controllers have about the same pricetag. This are reseller prices from the EV scene. The big item is the battery. Besides the obvious cell price you will need a custom designed BMS with cooling and heating capability in a fireresistant and waterproof box. I would ask the cell manufacturers who has a proven track record with them for designing and building such items.
If you don't have sticker shock anymore the best hull is the one you love. If are going to spend half a million on a boat (guestimate, could be more) you better like it. Just make sure it's a true displacement hull and not semi-displacement or even planing.

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### JedidiahJunior Member

The EP-100 is adequate - they're meant to be replacement propulsion for precisely the types of vessels that I'm looking at. They say up to 80', & you yourself said 4 HP per ton. With the heaviest boat I've considered so far, a pair of EP-100s would provide all-but 5 HP per ton, peak.

An EP-120 or 150 that doesn't yet exist might be the more comfortable safety factor, but simple seamanship solves a lot of problems before they happen. When the weather's looking iffy, run for safety or just stay put to begin with. Don't try to run narrow, shallow, or otherwise obstructed water when the tide's running hard, etc. Be smart, be cautious, take care of the boat & the boat will take care of you.

... & yes, this is the kind of thing that a working stiff trades the house for, along with everything else that won't fit in the boat. It's not that uncommon, either. A growing population of people are cashing out & putting what they've got into a different kind of home, one that once paid-for doesn't cost that much more if ANY more than what they're leaving behind. Many are cruising all over the world on surprisingly modest retirement incomes, but what really surprises me is how young that community is becoming. I don't know how anybody does it without a long lifetime of asset building behind them, but many do & more power to 'em.

It's a matter of choice & what motivates choice. In my case I've wanted to do this for as long as I can remember, & I've prepared for it financially. The other motivator - along the you-never-know-what-people-are-thinking vein - my dad, who was just short of 21 yrs older than I am, he died a few days ago. I won't spend the next 21 yrs not doing what I want to do because it's hard or expensive.

Screw it - the kids are solid, the grandkids are coming along nicely, it's time to move on.

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