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Old 26-07-2011, 10:15   #121
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Re: Electric Propulsion

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Originally Posted by skipmac View Post
Did some further reading and research on the Thoosa electric systems and came across this comment on the spec sheet for their 12 KW system.

"12 KW continuous power. Replaces a 25 - 40 HP combustion engine"

So based on the manufacturers own statement I would need an 18 - 28 KW electric motor to replace a 58 HP diesel.
Hmmm, not sure I am following your reasoning. To me, if a 12 KW motor can replace up to a 40 HP combustion engine, then assuming the relationship is linear, a 58 HP ICE would be replaced by an 18 KW (or 17.4 KW) electric motor.

I suppose you COULD hang a 28 KW electric motor on that boat, but you would then be providing way more power than the original 58 HP. At least that's the way I read that spec.
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Old 26-07-2011, 12:30   #122
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Re: Electric Propulsion

This discussion is heading in a bad direction. You can't compare the two horsepower to horsepower. That's not surprising since the horsepower torque relationship is not easy to understand. What is most important is do you have enough torque when you need it. First, here is the mathematical relationship betwen horsepower and torque:

Horsepower=(Torque X RPM)/5252

In an internal combustion engine, torque is low at idle and builds to a maximum. In my Nissan SD33 this peak happens about 2000 RPM. Every I/C engine has such a curve. It's probably in the manual for yours. As you can see from the above equation, if both torque and RPMs are low, horsepower is low. Diesels are rated at PEAK horsepower.

In an electric motor torque is constant and is available from zero RPM to full rated speed, most commonly 1800 RPM. This means the electric motor torque (and therefore horsepower) starts higher than a similarly rated I/C engine and remains higher until they match at peak.

What this means is that at any speed except maximum, you must over rate your I/C engine to get mid range performance. If you need 25 HP at 50% speed you won't get it with a 25HP engine. There is simply more available power throughout the speed range with an electric motor.

This has two effects. First, if you need 25 HP to move your boat at maximum, a 25 HP electric will do the job. So will a 25 HP diesel. BUT the diesel won't have the output in the lower to middle RPM range. You have to provide more than you need at the top of the range to get what you need in the middle.

In summary, an electric motor is not more powerful than a diesel. It does have a better distribution that allows a smaller electric to do the same job as a larger diesel.

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Old 26-07-2011, 12:37   #123
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Re: Electric Propulsion

Any engineering drive manual will tell you.

2.0 diesel HP equals 1HP electric, (because of torque curve). 2.5 Gas HP = 1 HP electric.

Electric can be matched to provide optimum power over entire load range. Diesel's only provide peak power over a very narrow range.
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Old 26-07-2011, 13:09   #124
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Re: Electric Propulsion

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Hmmm, not sure I am following your reasoning. To me, if a 12 KW motor can replace up to a 40 HP combustion engine, then assuming the relationship is linear, a 58 HP ICE would be replaced by an 18 KW (or 17.4 KW) electric motor.

I suppose you COULD hang a 28 KW electric motor on that boat, but you would then be providing way more power than the original 58 HP. At least that's the way I read that spec.
This is my logic. The spec sheet from ASMO said

"12 KW continuous power. Replaces a 25 - 40 HP combustion engine"

So I made the following calculations (before anyone corrects me I know that the equivalent conversion is 746W / 1 HP) using their range of replacements.

12 KW for a 25 HP CE yields c. 2 HP per KW. This is the low end.
12 KW up to 40 HP CE yields c. 3 HP per KW. This is the high end.

So if I use the most conservative equivalent power estimate from ASMO of 1 KW replaces 2 HP I would need 30 KW electric to give equal preformance to a 60 HP diesel.

If I use the more optimistic 1 KW electric replaces 3 HP of CE power I would need 20 KW electric motor to give equal preformance.

Without getting into the differences between HP and torque, RPM, etc, etc, etc., using figures publised by ASMO, maker of the Thoosa electric drive systems, in their opinion I would need a 20-30 KW electric motor to give similar performace to a 58-60 HP diesel engine.

Yes there are lots of other factors to consider (eg. torque vs HP) but without buying a boat and 4-5 different systems to test I've got to start somewhere and I would think the manufacturer of electric drive systems for boats might publish somewhat reliable figures.
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Old 26-07-2011, 13:22   #125
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Re: Electric Propulsion

Ask Lagoon why all of their hybrid owners are repowering with Yanmars. Electric propulsion is nice on a daysailer, but not on a cruising boat.
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Old 26-07-2011, 13:29   #126
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Re: Electric Propulsion

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Yes there are lots of other factors to consider (eg. torque vs HP) but without buying a boat and 4-5 different systems to test I've got to start somewhere and I would think the manufacturer of electric drive systems for boats might publish somewhat reliable figures.
Sorry, but good luck with that.

I have seen in my marina two identical boats one powered by a 4HP outboard, the other by a 60HP diesel. The difference one could motor upwind at hull speed, the other could barely get into and out of slip.

With this large of difference in conventional propulsion is doesn't surprise me that there is a simular difference with a new technology. Like in all things bigger is better, but in electric, size and weight of motor will differ little between high and low power systems, (just make sure it can handle peak controller output).

The big bucks and variablility is in the controller charging, and batteries. If you over size one, you can scale the others if later you decide you need more power.

Another advantage is there is little efficiency loss in driving a big electric motor at lower loads and RPM's, (and it will last longer). So you could buy a bigger than needed system, and just not run at full output, (save the batteries).

The motor in my car is 12 inches long, 8 in Dia, and weighs 70 lbs. It replaced a 400lb V-4. It left a whole lot of empty space under the hood. (now filled with batteries). The controller is a small box the size of a toaster. With apprpriate guage wires these can be located anywhere.

I'm using my electric car conversion as an example as my electric boat conversion is not complete.
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Old 26-07-2011, 13:30   #127
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Re: Electric Propulsion

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Originally Posted by skipmac View Post
This is my logic. The spec sheet from ASMO said

"12 KW continuous power. Replaces a 25 - 40 HP combustion engine"
I think their Thoosa 17kW engine is in the range that'd replace a 58HP diesel. It may be a little under powered though.

But personally if I were going to spend 10-15k on a new engine setup, I'd be asking for a consult based on specs on my specific boat. I'd want to plot a prop curve and see exactly how much power my prop demands and where the electric engine falls into delivering that.
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Old 26-07-2011, 13:32   #128
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Re: Electric Propulsion

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Ask Lagoon why all of their hybrid owners are repowering with Yanmars. Electric propulsion is nice on a daysailer, but not on a cruising boat.

Diesels will always have their place. For one thing there are a lot more diesel mechanics in third world countries than electrical engineers.
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Old 26-07-2011, 13:48   #129
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Re: Electric Propulsion

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Originally Posted by Dick Pluta View Post
This discussion is heading in a bad direction. You can't compare the two horsepower to horsepower. That's not surprising since the horsepower torque relationship is not easy to understand. What is most important is do you have enough torque when you need it. First, here is the mathematical relationship betwen horsepower and torque:

Horsepower=(Torque X RPM)/5252

In an internal combustion engine, torque is low at idle and builds to a maximum. In my Nissan SD33 this peak happens about 2000 RPM. Every I/C engine has such a curve. It's probably in the manual for yours. As you can see from the above equation, if both torque and RPMs are low, horsepower is low. Diesels are rated at PEAK horsepower.

In an electric motor torque is constant and is available from zero RPM to full rated speed, most commonly 1800 RPM. This means the electric motor torque (and therefore horsepower) starts higher than a similarly rated I/C engine and remains higher until they match at peak.

What this means is that at any speed except maximum, you must over rate your I/C engine to get mid range performance. If you need 25 HP at 50% speed you won't get it with a 25HP engine. There is simply more available power throughout the speed range with an electric motor.

This has two effects. First, if you need 25 HP to move your boat at maximum, a 25 HP electric will do the job. So will a 25 HP diesel. BUT the diesel won't have the output in the lower to middle RPM range. You have to provide more than you need at the top of the range to get what you need in the middle.

In summary, an electric motor is not more powerful than a diesel. It does have a better distribution that allows a smaller electric to do the same job as a larger diesel.

Dick Pluta
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Hi Dick,

A long time ago, in a galaxy far far away, I got a degree in engineering. Did all the physics, thermodynamics, circuits, digital logic and math like I could not belive. I could speak Newtons and Joules, torque and horespower, Watts, volts, amps, all that esoteric stuff. Now it has been a while but I do remember enough that all the points you make are not lost on me and I am in agreement with your statements.

However, at some point in time, in the real, physical world, numbers of some form and units need to be applied to a real application.

For example, one could have two identical hulls, one with a diesel engine, one with an electric motor. If one of these identical hulls had a 58 HP diesel engine, to make the other boat perform the same in all conditions: calms and storms, low speed and high speed, short term bursts of speed and continuous motoring, what size (in HP, KW, or any other rating you prefer) electric motor would be required?
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Old 26-07-2011, 13:54   #130
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Re: Electric Propulsion

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I think their Thoosa 17kW engine is in the range that'd replace a 58HP diesel. It may be a little under powered though.

But personally if I were going to spend 10-15k on a new engine setup, I'd be asking for a consult based on specs on my specific boat. I'd want to plot a prop curve and see exactly how much power my prop demands and where the electric engine falls into delivering that.
Accoring to a statement on a Thoosa 12 KW spec sheet I would need an minimum of 20 KW. Since I consider my boat a little underpowered as it is with 58 HP I would not want to go with the low end.

I did try to call US agent for Thoosa and got a recording. Will try to call back and will post their responses.
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Old 26-07-2011, 14:17   #131
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Re: Electric Propulsion

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Originally Posted by skipmac View Post
This is my logic. The spec sheet from ASMO said

"12 KW continuous power. Replaces a 25 - 40 HP combustion engine"

So I made the following calculations...
I am afraid you are misinterpreting the ASMO sales literature. It's just an application guide to help you with the motor selection. All they are trying to tell you is that if your diesel engine is sized somewhere between 25 and 40HP, you should install their 12 KW motor.

The operative number is upper range of the spec, i.e., the 40 HP . They mean that their 12 KW motor will replace any diesel up to 40 HP. You could certainly use the 12 KW motor to replace a smaller diesel, but why? If you have a smaller diesel, say a 20 HP, ASMO will sell you a smaller system that will provide a more appropriate power match.

BTW, I don't even see a 12 KW system in the Thoosa product line. The closest one would be the Thoosa 13000, which is a 13 KW system.

In any case, to eliminate further confusion, here is the spec guide for the Thoosa product line from the NGC Marine website, with diesel HP equivalents:



Output Power (At prop shaft): 6kW (8HP)

Equivalent Diesel HP: 16HP




Output Power (At prop shaft): 7kW (10HP)

Equivalent Diesel HP: 20HP




Output Power (At prop shaft): 7kW (10HP)

Equivalent Diesel HP: 20HP




Output Power (At prop shaft): 9kW (14HP)

Equivalent Diesel HP: 28HP




Output Power (At prop shaft): 13kW (18HP)

Equivalent Diesel HP: 36HP




Output Power (At prop shaft): 18kW (26HP)

Equivalent Diesel HP: 52HP

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Old 26-07-2011, 14:30   #132
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Re: Electric Propulsion

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Ask Lagoon why all of their hybrid owners are repowering with Yanmars. Electric propulsion is nice on a daysailer, but not on a cruising boat.
How do you know they are ALL repowering with Yanmars? Got any backup for that?

IMO, Lagoon used an awkward electric propulsion system for their implementation. In addition, they appeared to have released it to the market before the early versions of the motor controllers were ready for prime time. But that doesn't affect any aspect of the electric HP vs. diesel HP comparison currently being discussed.
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Old 26-07-2011, 15:27   #133
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Re: Electric Propulsion

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Originally Posted by capn_billl View Post
Any engineering drive manual will tell you.
2.0 diesel HP equals 1HP electric, (because of torque curve). 2.5 Gas HP = 1 HP electric.
Whoa there! We just took a hard turn towards perpetual motion machines. Name an "any engineering drive manual."
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Old 26-07-2011, 15:42   #134
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Re: Electric Propulsion

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This discussion is heading in a bad direction....

Horsepower=(Torque X RPM)/5252
Agreed. So you just shot yourself in the foot? Your equation proves that one diesel HP is equal to one electric motor HP. Thanks.

I suppose what the electric drive fans should be saying is that "we have found that we can get reasonable performance with only 40% of the power of the diesel that was replaced." That's the same thing I would say about my experience replacing large diesels with much smaller diesels. "Reasonable performance, but nothing like the maximum power of the larger engine."
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Old 26-07-2011, 16:02   #135
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Re: Electric Propulsion

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How do you know they are ALL repowering with Yanmars? Got any backup for that?

IMO, Lagoon used an awkward electric propulsion system for their implementation. In addition, they appeared to have released it to the market before the early versions of the motor controllers were ready for prime time. But that doesn't affect any aspect of the electric HP vs. diesel HP comparison currently being discussed.
I have two sets of friends who own Lagoon 420 Hybrids. One has already converted to diesel, and the other is in the process of trying to do so. I have talked in detail with both couples about their ordeals. The first couple was able to get financial assistance from Lagoon in their diesel conversion. The second couple has not been as lucky, and will likely take legal action.
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