# Estimating Electric Consumption

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

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### BarrySenior Member

I am thinking that perhaps trying to figure out a relationship between diesels engines and electric motors and bypass ENGINE hp curves etc which is a subject beaten up frequently, and most often incorrectly
DC and others have it correct,

I would hazard a guess that you would want to take the boat out and record rpm vs speed.

The existing prop might/should have been installed correctly for the hull at normal cruising speeds.

I am not a prop selection guy, but I will table the possibility below and see where it leads with the contributors that are

Pick one rpm

Say 1500 and this gives 8 mph (mph just for easy math)

At 8 mph the inlet speed into the prop is 704 feet per minute

Look at the prop curve, and it should pretty much give you the horsepower that you need at that rpm to turn the prop with an inlet speed of 704 fpm

Figure out what you are comfortable with losses in controllers, motors etc, ( and lots of discussion on this forum and others about this) and then match that output hp for that rpm with your motor and you should get a pretty good idea of what you need for a size of motor to obtain the same shaft speed.

Of course, you would do select several spots along the speed that you want to run the boat at to decide what is the max hp you will need.

I am assuming that prop curves include a variable of inlet speed to determine hp requirements for particular rpm

So if a prop table shows 100 hp demand at 1500 rpm with an 8 knot inlet speed (ie the one example that I am using) and say you need a 120 hp electric motor before losing 20hp for controller losses etc ( a guess) , then it would be quite easy to do the math.

Alternatively, you could take a flow meter and do the same drill, build a fuel curve at different rpm, convert an accepted pound/mass per hour into horsepower into the shaft and size the electric motor that way.

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### BlueBell"Whatever..."

Barry,
The OP is gone.
Didn't like our input.
Natural selection is alive and well.
Cheers

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### IronPriceSenior Member

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### Captain CanuckJunior Member

I'm converting my boat to electric. It's been a long and interesting learning process, and I've had tremendous help from local marina people in various disciplines.

The first two questions you need to ask are "What is the hull speed of my boat?" and "Do I want to ever go that fast under electric power?" These two questions will drive all other decisions. If you want to go hull speed, you'll need a *much* bigger motor than you think, because you need the continuous power rating at hull speed, rather than the burst (1 min) rating. My motor is 12kw cont, 40kw burst, so I can go hull speed as long as I have charge remaining, as my 7.2kt hull speed is achieved with about 16 shaft hp. Since most waterways are flagged at 6kts or less, having a system that caps out at 6kts is perfectly OK. I wouldn't go any lower than that, though. Once you figure these questions out, you can find or build a kit that will meet your needs. I suggest just buying a kit - everything is already worked out beforehand, saving you quite a bit of time for not a lot more money.

The next consideration is batteries. Lead-acid batteries are easy to work with and are available everywhere, but very heavy. Lithium batteries are much lighter, but require a BMS and active management, and can be very expensive. A consideration not many people take into account is that some insurance companies do *NOT* insure boats with Lithium batteries onboard. I'm using Lithium, but it's been a challenge to come up with a design that's ABYC compliant. This is mainly because there's no actual ABYC recommendation for lithium batteries yet. The current recommendation is "use manufacturer recommendations", which sounds easy, until you realize they don't make any.

The next consideration is control(s). You can use your old throttle, but you're going to need some new gauges, like a power meter instead of a fuel gauge and battery temperature monitor with audible alarm. A real time power consumption monitor is also very useful. Every boat has a sweet spot where you get the most speed for the least power, and you'll need a power consumption monitor to find it.

Cable sizing is also important. Having 1AWG wire where you need 00AWG will cause a fire an burn your boat up. If anything, calculate what you need and go one size bigger. There are plenty of wiring charts for this.

Wear safety equipment and *NEVER* touch any electrical connections that are live. Get a pair of 00 rated electrician's gloves. Wear them whenever you are connecting or disconnecting anything, even if you're sure it's not live. You can also put electrical tape on all of your hand tools. You only have to be wrong once.

Now for the good news. Since all the parts are off-the-shelf, getting replacements is much easier than getting parts for a marine ICE. Or, you can carry spares without a big weight penalty. My Yanmar 2gm20 and transmission that I replaced were over 300lbs. The new motor is 35lbs. The new transmission is about 25. I can take both the motor and transmission out in less than 10 minutes. I can have pretty much any part of my system overnighted if necessary. Try that with your local Yanmar or Volvo Penta dealer. Marine motors are notoriously expensive to fix and parts are notoriously slow to arrive. And then there's the installation, which most people are unqualified to do.

Oh, and it costs me about \$1 to refuel my 8kWh battery pack. (12c/kWh here in MD).

I hope this doesn't discourage you too much. It's a lot of work, but most of that is planing. The actual installation is much less difficult than putting in any form of marine ICE.

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### DCockeySenior Member

Do you make an allowance for head winds and waves, or do you only use your boat on calm days?

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### Captain CanuckJunior Member

It's a sailboat, so it's pretty rare I use the motor outside of getting into and out of the marina. I'm also a weekender, so I'm not often far from my marina. The longest I've run my motor since I bought the boat is 2 hours, which is well within my cruising battery range. If I find at some point that's not enough, I can double the capacity fairly easily.

The motor I replaced had about 12 shaft hp, so the new motor is actually stronger than the old one. I also put a beefier prop on it, so I don't anticipate any issues in bad weather.

This season will be my first with the new system, so we'll see how it performs. I will probably need to make adjustments, but that's OK. New designs always have bugs.

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### gonzoSenior Member

The electric horse has been already beaten to death. If you need x power with a diesel, gas engine or sails to attain a certain speed, you will need the same x power out of an electric motor. Therefore, to replace an existing engine and get the same performance you simply match the power. If you decide to underpower the vessel, then compare the smaller electric motor to an internal combustion engine of the same power.

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### Captain CanuckJunior Member

Yes, and no. Just like electric cars, electric boats need a different powertrain design to work properly. If you simply slap an electric motor onto a diesel powertrain, it will work, but not as well as it could. To really realize the gains from an electric motor, you need to rethink both the prop and the transmission.

An electric motor can run a bigger prop at a slower speed to generate the same thrust, gaining efficiency in the process. Generally speaking, the less cavitation you can generate, the more efficient the prop becomes. Ideally, you'd run a big prop slow enough to not generate any cavitation at all, giving you the most efficient thrust possible. Most sailboats can't support a prop anywhere near that size, so you have fit as best you can.

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### JoakimSenior Member

Why? Most electric motor installations have a gearbox just like combustion engines, which also could use a higher gear ratio, if found necessary.

kerosene and Ad Hoc like this.
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### DejaySenior Newbie

Maybe different torque curves? Like electric cars accelerate way harder than ICEs even with transmissions. ICE has less torque at lower RPM. That would suggest you get more power out of the electric motor other than at the max brake power of the ICE. Also I'm not sure if you ever want to run motors close to their max break power. Sorry I meant to write brake power

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### keroseneSenior Member

Dejay - this has indeed been beaten to death.
E-motor has strong torque from low rpm BUT it is of no use in a boat where the resistance (required torque) goes up with the rpm. Need for torque is very low at lower rpm.

Car is very different. I drove a BMW i3 and it feels much more powerful than its 170hp suggest as the power is available at an instant and at any speed.
Boats cannot benefit from these factors.

And the efficient prop yada yada is exactly the same for a diesel boat.

Dejay likes this.
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### Captain CanuckJunior Member

Because a slower turning prop is more efficient than a faster turning prop, all else being equal.

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### Captain CanuckJunior Member

The big benefit is higher efficiency, both at the motor and with less slip at the prop. My motor is about 94% efficient and I should see about 60% efficiency at cruise, vs <40% and ~50% before. Also, I should get considerably more thrust at 600rpm (the fastest you can go before cavitation starts eating your efficiency).

I'll be taking measurements of all this once my boat is back in the water. I'm about 75% of the way through the installation, but I still have to scrape and repaint the bottom, so it will be a few weeks before I get to testing.

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### Captain CanuckJunior Member

The fact that there's a clutch on a diesel transmission limits the size of the prop. If you put too big a prop on it you'll stall the motor every time you shift. This consideration doesn't exist in an electric setup. Also, there's no forward or reverse - the trans is just a simple gear reducer. I use a 2:1 on my setup.

Boats do benefit, just not as much as cars.

Electric motors, trannies and props are far more efficient than diesels when properly set up. It's not even close to the same. I can recommend some papers on hydrodynamics of props if you're interested.

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### DCockeySenior Member

How does a propeller know if an electric motor or a Diesel engine is powering it if the shaft is turning at the same speed? As Joakim said a Diesel engine can have a reduction gear to turn the shaft at the same speed with the same torque/power as an electric motor.

What definition of "efficiency" are you using?

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