Re: [Electric Boats] power requirements - predicted vs observed

Hi Myles, Eric, Dave, Judith and all members of the Electric Boat Forum, 

this is a piece I submitted to the ABYC as an explanation to this issue.  

What is important to know is that this is new science.   The ABYC, nor Westlawn Institute or Design or anybody else, except this group, fully understand  this matter and will deny that electric horsepower is more capable than diesel.   A horse is a horse.   One flippant remark that I made that electric horses were stronger than diesel was met with great opposition.   So clearing this up is of utmost importance to furthering the adopting of this technology.

The reason electric motors can be smaller than diesel for equivalent performance is a result of the combination of several factors, with propeller size and speed probably being the most important.

I have corresponded with Dave Gerr about this and his conclusion above all is that most propeller installations are incorrectly matched to the boat and the propeller combination.

I have rebuilt this thread including all contributions and have CC'd  Dave Gerr for analysis.   One day it is my hope that he will take up a study at the Westlawn institute of Design to add some science to our observations.

I'm over is Europe now going to Perm, Kraeutler, Torqueedo and Sevcon and will surely have some interesting information when I get back.   One thing is for sure, the Europeans are adopting this technology faster than we are in the United States.

Thanks everybody for your contributions to this  subject!   It is very important that we keep this going!

James Lambden

Propulsion Marine

http://www.propulsionmarine.com

Diesel Horsepower VS. Electric Kilowatt Ratings.

 

Both Horsepower and Kilowatts are power ratings.    One horsepower is equal to 746 watts.   1 Kilowatt is equal to 1.34 Horsepower.

 

A Catalina 30 comes from the factory powered with a 21 horsepower Yanmar Diesel which is capable of obtaining 6 knots.   Our Catalina 30 obtains 6 Knots using 6 KW or 8 H.P.

 

In order to compare a diesel to an electric motor, the entire system must be evaluated, including the propeller.

 

The electric motor can develop maximum torque at all rpms.   Electric motors have a current to torque constant, and a voltage to rpm constant.     Diesel motors have a torque curve that is highest in the mid range and lowest at idle and wide open throttle.     Making this even more complicated is propellers requires exponentially more torque at higher rpm.

 

An electric motor is able to meet the propeller torque curve at all places by altering the voltage to current relationship.   As the rpm of the propeller increases, so does the voltage of the electric motor.   As the torque requirement of the propeller increases, so does the current of the electric motor.    The electric motor can provide the perfect amount of torque and rpm to the propeller at all rpms.  

 

Because the electric motor is capable of maximum torque at all rpms, it is capable of turning a larger propeller with a higher pitch.    Larger, high pitched propellers have less slip and significantly higher efficiency.   

 

By contrast a diesel motor produces maximum torque at only one rpm which is usually in the peak of the operating band of the motor.   But the diesel motor has to be sized to match a propeller at wide open throttle, which is usually higher than peak operating torque.   The propeller power curve and diesel engine power curve meet at maximum rpm.

 

The torque of the diesel above the torque requirement of the propeller is not utilized and is wasted.   If the propeller is too large, the diesel will smoke.    The propeller has to be small enough that the diesel motor is capable of turning the propeller at idle.    The result of the mismatch between diesel engine torque and propeller required torque is using a small propeller that turns at high rpms thus allowing the diesel engine to reach its peak operating rpm.   

 

Small diesel motors are usually sized two to three times larger than the propeller requirement due to allowances made for the mismatch in the torque curves,  allowances for two water pumps, one or two alternators, the belts and bearings that drive the accessories, and the decrease in efficiency of a small propeller.    In addition to these complexities, the small diesel motor rarely is installed with the correct propeller, with manufacturer's erring on the side of a larger motor than necessary to prevent overworking of the motor.  

 

Larger diesel motors on power boats and large sailboats usually have flatter torque curves, and less percentage of their power contributing to running the alternators and water pumps.   Hence the horsepower ratings of large electric motors and large diesels are more equivalent.   

I agree Kevin.

In my experience, virtually all ICE installations do have gear reduction. In fact, the gear reduction ratio is a prominant part of every propeller sizing calculations. But gear boxes and transmissions rarely introduce more than a 15% loss of efficiency, the gearbox in my electric conversion is 97% efficient, better than many belt drives. Remember that almost all friction losses show up as heat, if your ICE V-drive absorbed 20% of 50hp coming out of an engine, it would be a 7500W heater in your bilge. Now push that up to twin drives at 300hp a piece, and you got 90,000W of heat coming out of your tranmissions alone. So obviously, normal gearboxes are more efficient than that. 

My electric motor drives the boat more efficiently at slow speeds than the diesel ICE, and it drives the boat faster too. The old system was already optimized for the engine's torque curve and propped and geared appropriately. I dont think that the torque curves are what is throwing Gerr's formulas off.

Fair winds,
Eric

Begin forwarded message:

From: "Eric" <ewdysar@yahoo.com>

Date: August 18, 2011 3:39:13 PM PDT

To: electricboats@yahoogroups.com

Subject: Re: [Electric Boats] power requirements - predicted vs observed

Reply-To: electricboats@yahoogroups.com

Hi Kerry,

I was figuring something like that too. That means that in my case, the new propeller is 6 times more efficient than my old prop at 3kts of boat speed.

So if we assume that the electric drive and propeller is 100% efficient at that speed (just for this conversation) then Gerr was grossly overstating the power required to move my boat at 3kts. It also means that my old prop was less than 17% efficient at 3kts boat speed. Additionally, a simple propeller efficiency adjustment would put Gerr's formula back on track.

If that was true, then it also means that an electric boat conversion that doesn't change the propeller will be much less efficient (up to 6 times less efficient) than a conversion that does upgrade the prop. However, we can observe that this is not true, conversions that don't change the prop don't use significantly more energy to go 3kts than the ones with new props, if the proper reduction ratio is chosen. Remember that the gear ratio is before the propeller shaft (just like the ICE transmission), so it should not be part of the calculations.

Therefore, it is not just the prop. Partially perhaps, but that is not the whole story.

Fair winds,
Eric
Marina del Rey, CA

PS. If anyone sees any holes in my reasoning, in these or any other thread, please point them out. We're all learning this stuff together...

--- In electricboats@yahoogroups.com, "Kerry Thomas" <kjthomas@...> wrote:
>
> Hi
> 
> Don't have time to go into it at the moment, but I suspect it has to do with
> the larger, high torque slow speed propeller sizing for EP compared with
> smaller faster props for ICE, giving more efficiency at slow speeds.
> 
> Like running a tugboat prop at 5 knots compared with running a container
> ship prop, designed for 15 knots, at 5 knots.
> 
> -----Original Message-----
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of Eric
> Sent: Friday, 19 August 2011 5:33 a.m.
> To: electricboats@yahoogroups.com
> Subject: Re: [Electric Boats] power requirements - predicted vs observed
> 
> Hi hanermo,
> 
> Interesting point and I agree with each point, but it doesn't address the
> question. The Gerr formulas are stated to cacluate the propeller shaft HP
> required to push a hull. So it's supposed to be the HP applied to the
> propeller AFTER all of the parasitic drag and driveline efficiency losses.
> So regardless of the power source, the number is supposed to be how much
> power needs to be supplied to the prop for a given speed. Everything that
> you mentioned were losses before the power made it to the propeller shaft.
> 
> So all of this is just derailing the conversation. Accurate info, but not
> what we're discussing right now.
> 
> Let me try to simplify the topic to keep this conversation on track. The
> Gerr power formulas claim to quantify how much power the propeller needs to
> push a hull at a given speed. These numbers have been verified through
> years of observations with ICE engines and are accepted to be accurate.
> However, through observation, one can see that an electric drive can push a
> hull at the same speed with much less power applied to the propeller.
> 
> My boat at 3kts uses about 17% of the power via electric compared to what
> Gerr predicts. That is so far off, that I think that Gerr is predicting
> something other than what I am measuring. By 5.5kts, my observations are
> 42% of what Gerr predicts. Gerr must be using some assumptions that are not
> accurate for my boat.
> 
> My simple question was if Bill S. had a single data point to validate this
> question. What is the observed energy load on Barbara Ann's electric drive
> at 7.2kts?
> 
> It is physics, not magic, so I know that there is an explanation.
> 
> Fair winds,
> Eric
> Marina del Rey, CA