The Offshore Voyaging Reference Site

Real Numbers For Electric And Diesel-Electric Drives

spreadsheet
Note October 2020: Do not let the original date of this post and model dissuade you for using it and instead going with salesman BS and pretengineering that will destroy large chunks of your wealth. The basic physics have not changed since 2014 and you can change things like the price of batteries and efficiencies of solar cells to reflect changes since then, using the instructions Eric provides.

I have always believed that one of the most important things you can do in sailing, as well as life, is do some basic arithmetic before you make a decision. That would seem to be self evident, but it’s amazing how many of us (I have made this mistake too) rush into things just because they are cool and/or the latest fashion without subjecting them to a bit of arithmetic rigour.

I guess the classic error in this regard is whipping out a credit card and buying something without doing the simple arithmetic to figure out how long it will take to pay down the card or how much interest we will incur while doing so. In the same vein, investing large amounts of money in electric or diesel-electric drive options without doing the maths first can be expensive, as well as extremely time consuming and disheartening.


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More Articles From Electric and Hybrid Diesel-Electric For Cruising Boats:

  1. When Electric Drive Works For a Cruising Sailboat
  2. Electric or Diesel-Electric Drives for Voyaging Boats
  3. Real Numbers For Electric And Diesel-Electric Drives
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Chris

Ran my boat numbers through the model and no surprises. The output data passed my test of reasonableness.

The Series would cost me plus $1800 a year and the Parallel plus $600 over 20 years — doable. But I can’t find a reason to triple the complexity of my power plant (Diesel + Electric + Interfaces), aggravate my search for quality maintainers, and render resale very, very problematic.

I would really like to see the people pursuing this succeed in a manner which makes this a supportable concept from the maintenance and logistics standpoint as well as an energy conversion one. But as a thermodynamics instructor and a former aircraft maintainer, I would say it will be a while.

Laurent

A quick remark concerning diesel-fuel-to-propeller math in the pure-diesel and series-hybrid cases. I noticed that shaft-power-at-cruise figures were 13,15hp fore pure-diesel and 12.89hp for series-hybrid without too much explanations for those figures.
-Pure-diesel solution uses 270g diesel fuel per KWh, does not generate any diesel-based house electricity and has an expected life of 5000 hours (perhaps 20 years with 250 hours par year…). In that case diesel fuel consumption over equipment expected life should be 4202,96 gal. I didn’t see gearbox efficiency in the datas, so I guess that the 13,15 shaft-power hp are engine power and not really shaft- power, and that you took about 98% for gearbox efficiency, which means that shaft power is the same 12.89hp with both solutions.
-Series-hybrid solution uses an electric motor with 86,71% efficiency, a controller with 98% efficiency a generator with 82,15% efficiency, and generator’s diesel uses only 240 g. diesel fuel per KWh. Grand-total for global transmission efficiency is 69,80% (86,71% * 98% * 82,15% ), but, accounting for a better diesel-engine thermal efficiency (240 g. instead of 270 g., which makes a 11% improvement probably due to reduced running hours at higher loads, the batteries being used as buffers between engine & propeller…), you get about 77,47% relative efficiency of series-hybrid vs. pure-diesel solution (69,80% * 111%)(in fact counting pure-diesel 98% gearbox efficiency hybrid relative efficiency should be 79,02%…). So diesel consumption should be about 21% more (100 – 79,02…), which means 5085,58 gal. (4202,96 * 1,21). You got slightly less (4948,76 gal.), because you considered that batteries will provide on average 0,52 KW of propulsion (green ?..) energy when motoring.

First I am a bit surprised by your efficiency figures for electric motors, controllers and generators. UQM Powerphase systems, which are moderns, but not experimental-grade electric motors, controllers & generator have figures of 92% for motor + controller and also 92% for the generator (and its controller…), which makes an grand total of 84.84% instead of 69,80%. Adding your figures of 11% diesel-engine thermal-efficiency improvement (because of higher loads for shorter times…), 2% for the pure-diesel mechanical-gearbox (not used here…) and 2,7% to account for green or shore energy ending-up in propulsion, we get about 100,54% relative diesel efficiency of series-hybrid vs. pure diesel. This won’t pay for 20659.17$ of batteries but do you need that much batteries, in fact many series-hybrid have been built without batteries. Plus, UQM PowerPhase125 motor + controller or generator + controller are about 1500$ per unit, volume prices. Makes a total of #3000$, looks to be a far cry from your #14000$ quotation.

Then I don’t know were those values of 13,15hp & 12.89hp at propeller shaft come from. As pure-diesel gearbox efficiency is not mentioned I understand that you probably took the same 13,15hp at propeller shaft for cruising in both case. Point is that the main advantage of diesel-electric transmission is to be able to fulfill requirements (here enough bollard-pull at zero speed to be able to get out of a quite bad stormy and windy situation…), with much less compromise than a fixed speed gearbox. So in our case, the efficiency of the propeller that will be able to reach the required bollard-pull value with a continuously variable transmission will in most case have a much better efficiency in cruising conditions with the same transmission (typically twice better…). So relative efficiency is not 100,54% but more in the vicinity of 201,08% which means a 10021,45$ diesel-fuel saving over equipment life.

Then do we need propulsion batteries in the case of serial-hybrids ?. Batteries can give plugin, or alternate energies, capabilities, an can also help with thermal diesel efficiency and diesel engine life-time, because they can be used to run diesel engine nearer from their efficiency optimum by storing excess energy and shutting down the engine at slow-speed (using stored energy instead). Those functionalities have a cost, and it is not very difficult to conclude in each and every use case, and to add batteries, and the number of batteries, that make sense for each user.

Last, I never said that, in the world as it is, everybody should rush installing series-hybrids in every boat. I said that considering the technics and the costs we can get nowadays, it looks quite reasonable at least on paper, and I guess that we are on a trend, not a fad, so the guys who have somewhat deeper pockets or somewhat better technical knowledges could or should not hesitate much more (“early adopters”…). For the other guys (most of us…), it might take some time before the recreational equipment industry considers it is a safe ground for you (and for them…).

Laurent

PS. in my opinion parallel-hybrid and pure-electric concern mainly, or only, comfort of use and for pure-electric, only day-sailers.

Eric Klem

Hi Laurent,

I will try to answer your questions, hopefully I succeed.

There is a gearbox efficiency in the model. The model is broken into assumptions and inputs and this falls under assumptions so it doesn’t jump out at you as quickly but you can change it if you want. You are correct that this efficiency accounts for the difference in engine/generator shaft power. In some installations, there should be an efficiency number on the electric drive as well but most of those installations can achieve high enough efficiencies that they were deemed to be negligible. You are also correct in the end of your first statement about the little boost from the batteries. To make the model “fair”, all of the configurations end up at the fuel dock/charging station with depleted batteries. In practice, this could be difficult for some people to achieve but it gives the hybrids the best shot as a really skilled user could achieve this.

Regarding motor selection, the model provides inputs so that you can play with a few different motors. The motor that is in there right now is the motor used with the Beta hybrids and this was chosen as it is actually a configuration that someone can buy off the shelf right now. It is not the most efficient motor available as you point out but I would guess that it is one of the most commonly used motors in boats. Many people use Elco motors in marine installations and a bit of efficiency data is available for them but they do not appear to have a plot publicly available. To change the motor input, go to the motor efficiency tab and change the numbers in the left most matrix. For motors or anything else, if you are aware of options that are much more appealing than the ones included with the model, I would love to hear about it. You would just need to post efficiency numbers, size, weight and cost.

Regarding prices, these are single unit prices. For something like the Adventure 40, I hope that prices would be a lot lower. The prices in the model could be said to reflect the price of a single repower not a new production run where prices are normally more secret.

For propellers, you are welcome to put in any propeller curve you want in that tab. There is a graph that has several in there that you can pull data from. If you have data for a much larger propeller as electric propulsion proponents often suggest, you could certainly put that in there. If you manage to put in a propeller with such a high midrange efficiency that you don’t have enough torque, the model will not return values. This is very unlikely unless you choose a maximum speed that is far below hull speed so that you get onto the more linear part of the hull drag curve. It is my opinion that propeller selection is one of the largest areas for improvement in current propulsion selection.

If you don’t want batteries in the series hybrid (or parallel) part of the model, simply set the time at full power to near zero. These batteries are for peaking application so if you don’t do any peaking, they won’t need to exist. There is a limit to make sure that you don’t end up with less batteries for the hybrids than the straight diesel as that would not be fair.

Sorry for being long-winded, I hope that answered your questions.

Eric

Eric Klem

Hi Laurent,

I think that I might have said something slightly misleading in my last comment and wanted to clear it up. While it is true that you can eliminate the batteries for a series hybrid by setting the time a max speed to near zero, that is probably not the best way to do it as that messes with your use assumptions which you shouldn’t do. A better way to do this would be to turn up the “% Series Hybrid Overrate for Cruise”. This was actually specifically intended to let you do what you want to do. 100% rating (0 overrate) means the generation system is matched to supply the full power of the motor at cruise with the engine at WOT at the chosen RPM. If you overrate it, you get a bigger generation system and need less batteries to deal with your peaking requirements. Once you increase this generation system so that it can output the amount of power used by the motor at max speed, the model will not require any batteries to meet your propulsion requirements. This “knob” also lets you play around with finding the most efficient point in the fuel map for the engine.

Eric

Laurent

I think that the main question to be answered is: “might a sailboat hybrid aux. propulsion system pay for itself in saved diesel-fuel, considering different technologies and different use-case ?”.
The mainstream solution we are trying to beat is 2014 sailboat aux. diesel + mechanical gearbox with same technology level than the diesel-engines used in the hybrid systems we try to evaluate.
I think the documents you can find on Hybrid Marine site give a good approach of the problem. For instance their paper about “Propulsion methods for a 32′ auxiliary yacht”. In that paper they compare a classic “pure-diesel” system, a diesel-electric transmission (without propulsion battery), a serial-hybrid and a parallel hybrid with, as I understand it, the same diesel engine and the same propeller in those 4 cases.
They conclude (see fig. 5…) that :
– the diesel-electric efficiency is about 10% below pure-diesel in all cases (3 to 6.5 knots)
– the series-hybrid is about 60% more efficient than the pure-diesel at 3 knots and 10% less efficient at 6.5 knots
– the parallel-hybrid is about 60% more efficient than the pure-diesel at 3 knots and as efficient at 6.5 knots.

In this example, they didn’t try any “overrate”, that is, it looks like they used the same diesel-engine in the 4 cases, and this engine is powerful enough to generate all the electricity needed by the electric motor even at max. speed. I have no objection against overrating, but, if the intent is to evaluate the fuel-efficiency of different solutions in different use-case, I think that overrating makes things more difficult to understand without improving the fuel-efficiency of the solution. I understand that overrating might improve efficiency because it should increase diesel engine average load, but, you can get the same result through engine-cycling in way that is easier to understand and to simulate. I don’t mean overrating might not be part of a fully optimized solution, but I think it should not be part of a debate about the capabilities of sailboat hybrid aux. propulsion systems to pay for themselves, because it can not help to understand this point and it adds unneeded complexities.

I like Hybrid Marine approach, but I think their study is a bit flawed, mostly because diesel-electric transmission and serial-hybrid should not use the same propeller than pure-diesel and because it looks like they evaluated only fairly mature electric components instead of reasonably modern ones (electric high-efficiency from the 1960′ vs. electric high-efficiency from the 2000’…).

Eric Klem

Hi Laurent,

I agree that the approach shown in the Hybrid Marine study is a reasonable one. To me, the most important conclusion that they came to was that above ~4.5 knots (the exact speed is a little unclear, under the description they mention 4.25 for the parallel), the best system from an efficiency standpoint was a regular diesel. The parallel had equal efficiency but the motor was sitting idle so it was effectively a straight diesel. As we have discussed in the comments to previous posts, it really comes down to how you use the boat. On a 32′ boat as they used with a hull speed of around 7-7.5 knots (I don’t see a mention of the actual boat or LWL?), I can’t imagine motoring along at below 4.5 knots for any length of time. However, some people might really enjoy this type of motoring or they might really want to save the fuel and still have the ability to motor at high speeds.

In all of these studies, arguments can be made that either a diesel or hybrid system was not optimized. It seems that one area where you feel serial hybrids should be optimized further is the propeller. I assume that you want to go to a larger diameter propeller with a lower slip ratio. When comparing torque curves, modern common rail diesels and AC motors actually have very similarly shaped torque curves, DC motors are obviously very different but I don’t think that those are what we are interested in. You could potentially run a larger drive motor (higher cost, slightly lower efficiency at cruise) than generator to allow you to get more torque (and less speed) in bollard pull situations although this is not how the systems I have seen are designed so they are really not different than a diesel. From a practical standpoint, many boats simply don’t have the room to swing a larger prop and maintain the tip clearance required for efficiency. I am not a designer so I can’t answer the question as to whether it would be reasonable to design for this in new boats. For people who like fixed props, a larger diameter one would really have a lot of drag associated with it so I think you could only consider it with a feathering or folding one.

Eric

Laurent

Hi Eric.
I like Hybrid Marine approach of the problem and I also like their product offering, because I think they are nice pieces of engineering and probably quite appropriate for some use-cases.
This said, I also understand that their study is part of the marketing literature they published on their server and I suspect they went not very far discussing the issues where their products were at a disadvantage.
The first obvious shortfall I noticed in this study concern the electric motor/generator/controller technology they presented. High efficiency electric motors etc. have done huge progress in the last 50 years, and you still find electrical motors etc. dating from the 1960′, and sold in the 60′ as “high efficiency components” still sold today with that kind of arguments, while today’s “high efficiency” should relate to component with significantly lower losses (#92% efficiency instead of #85%, which means losses reduced by #50%…). I understand that their products use 1960′ criteria for high efficiency and that they also used the same level of efficiency for the electric-transmission and series-hybrid solutions they discussed, which is not quite fair in my opinion.
The second shortfall is that the main “raison d’être” of electric transmission and serie-hybrids as I see it, is to be able to take advantage of the corresponding continuously variable transmission, and to use larger propellers with carefully calculated characteristics to offer a very good propeller efficiency at motor-cruising speed and to fulfill all the constrains. I understand that they used the same propeller in all 4 cases, which I consider a serious flaw.
I don’ t think that the existing space for the propeller in existing sailboats should be a game-stopper. If, after system-optimization including careful propeller calculus, we find-out that this kind of problem does happen, it might only mean that sailboats hulls might need to be redesigned to be able to take full advantages of hybrid aux. propulsion systems. I think that current condominium-inspired sailboats are losing their marketing appeal, and I doubt that a return to 1960′ style sailboats might be a lasting solution for that problem. “Reduced environmental impact” equipments are fashionable and should remain so in the future, so I don’t think that a sailboat that would include some unusual architectural feature to obtain that kind of reduced environmental impact, should be very hard to sell.

Laurent

Laurent

I didn’t try altering your spreadsheet model, because I think that spreadsheet technicalities should be addressed only after more basic questions of equipments efficiency and general logics of hybrid propulsion design have been fully understood and explained in plain english.
Considering electric motors efficiency weight, cost etc…. You must understand that “high efficiency” industrial motors are typical factory-equipment motors improved to obtain a somewhat better efficiency than usual, but “usual efficiency” in industrial electric-motors is low or very low by current standards. Corresponding designs date from the 1950′, or sometimes the 1930′, and users were most concerned by reliability and life-expectancy, not by energy-efficiency. So basic industrial motor efficiency is typically around 75%, and “high efficiency” industrial motors efficiency is around 85%. Today ‘s good volume-produced electrical cars’ or buses motors are at 92% or better, which makes 1960′ “high efficiency” industrial-motors something of a joke.
Concerning Beta hybrids, I understand that, today, you have 3 kind of electrical equipment manufacturers :
– 1) very mature ones (technology from the 1930′ or 1950’…) who sell very sturdy reasonably priced motors & generator with motor-efficiency around 75 or 80%
– 2) “high-efficiency” standard industrial equipment manufacturers (technology from the 1960′ or 1970′), who sell more expensive equipment with same standard flanges etc. than preceding and motor-efficiency around 85%
– 3) specialized manufacturers : who sell much more advanced equipments (technology from the 2000′) generally with non-standard flanges, ratings etc., mostly intended for electric cars, buses, trucks etc.. , with efficiency in the 90-94% range. The good new is that those manufacturers have been reaching volume production levels lately, and that sticker price of their equipments is much lower than 10 years ago.
4) as a side remark, some specialized real-high-efficiency motors (more than 92% efficiency…) have been on the market for 20 years or more for diesel-electric passenger-vessels transmissions but those motors are very specialized and…….very large (Converteam corp…).

Beta-Marine Elco motors belong obviously to category number 2 (“high efficiency industrial”), which is a reasonable choice if you consider that Beta-Marine purpose was to allow slow-speed for a limited time without diesel-engine noise and smoke, for a better user experience, and not really energy saving for itself. Point is that if you have an electric motor + a generator 86%^2 makes 74% and 92%^2 makes 84,6%, which is a big difference (losses reduced by 69%….).
As for the appropriate electric-motors and generators, I understand that UQM corp. is probably today the only manufacturer currently engaged in a pure-player volume strategy for type-3 electric motors and generators manufacturing in the 45 KW range, so the UQM PowerPhase125 motor/generator looks like a very good choice today. I guess that we will see many other similar offerings in a near future.

For propellers, I think you missed the main issue about series-hybrid vs. pure-diesel in our case, which is that pure-diesel have fixed-ratio transmissions while series-hybrid have continuously variable transmissions, and, concerning aux. propulsion of sailing boat, the first requirement is, and has always been, to be able to start moving the boat in a demanding wind a sea condition, for instance, lifting an anchor in a gale. Because of that you need a bollard-pull (that is a pulling-force at zero speed..) that is superior to the force that will be exerted by wind and sea on your fully rigged sailboat in those conditions. So, in the case of sailboats, you must chose propellers and gearbox ratios that will give this kind of bollard-pull with limited engine power, which is a big constrain. The same propeller and gear ratio are generally a very poor choice when motor-cruising or motor-sailing because they are very far from the optimum propeller and ratio you should use for those conditions only. With the continuously-adaptable gear ratio you get from a series-hybrid, you need much less of a compromise for bollard-pull constrains (because you can increase the gear ratio at zero speed as much as needed) and you will have a much better usage of this less compromised propeller when motor-cruising, because you can lower gear ratio at will. I guess that, with both benefits you should get a propeller-plus-gearbox-system twice as efficient when motor-cruising with series-hybrid vs. pure-diesel. This point is mainly due to the high wind and sea resistance, plus limited aux. power of sailing boats. With lower wind resistance and higher specific power, like recent Dashew motor-boats, the corresponding advantage should be lower.

Considering batteries, I think that once series-hybrid motor, generator, diesel-engine & propeller configuration have been optimized without any battery in the system, it is no difficult to make a detailed spreadsheet to simulate the possible advantages of adding batteries for pluggin-capabilities, diesel-motor thermal-efficiency improvements (using cycle at optimum load instead of continuous sub-optimal loads) & green energy usage for propulsion. I guess that “some” batteries will prove worthwhile in most, if not every case, but I think we can set aside the battery issue when deciding about series-hybrids. Batteries, when (and perhaps if…) added can only improve figures, not degrade them.

Laurent

Laurent

Point is that Hymar research team studied a few hybrid or pure-electric #”off the shelf” products obviously not made for high efficiency, but only for added user comfort in some situation (less noise, fumes etc…) and needed about 50 pages to conclude that those systems were not made for high efficiency.

Looks like established recreational equipment manufacturers and distributors were asked if they should/could/would give parts of their market to newcomers and “demonstrated” that they would not.

What surprises me most in that kind of “research” is that I understand it has been paid with my taxes 🙁

Laurent

+ a diesel-electric transmission based upon UQM PowerPhase125 motors/generator, some batteries and a controller able to deliver a specified power on the propeller-shaft, adapting torque and rpms as needed, is a very straight and simple adaptation of modern serial-hybrid delivery trucks off-the-shelf technologies. There is no tweak in that system apart from the good efficiency of standard UQM components and perhaps a really optimized propeller calculus using a very basic fixed pitch propeller.
Controllable pitch propellers for sailboat aux. propulsion is very common in the 30m.+ boat and nearly non-existent in the field for sailboats less than 60′ in length. I understand that corresponding providers had ample opportunities to develop and demonstrate their offerings for those boats and did not succeed. I don’t think that none of them was both a good technician and a good salesman, so I understand that this solution was probably not that good for those boats (manufacturing costs, maintenance issues ?….).

CaperAsh

well, I just wrote a nice long comment asking if John or someone could plug in the Wylo II system into the attached SS and display a comparison study since I lack the skills to do so. But then the spam protection, which has two numbers, one in text and one in cardinal, does not accept a text reply, meaning that the entire comment was lost!

How does the parallel hybrid work?
During low to mid power cruising (around 2/3 of maximum hull speed) the electric motor can drive the vessel using the energy stored in the battery bank. When the batteries are depleted then the main engine is started to drive the vessel. At this time the electric motor automatically becomes a generator and recharges the batteries. The extra shaft load seen by the engine during this combined propulsion and charging period causes the engine to operate with higher efficiency. Once the batteries are recharged you can then switch back to electric drive. Alternatively you can keep the energy in your batteries to silently drive high power appliances when anchored at the end of the day.

When sailing, the propeller can be allowed to freewheel. The motor/generator will then rotate and regenerate electricity to charge your batteries. This charging process does add some drag to the vessel and so in light airs regeneration can be reduced or disabled via the control panel.

If you need to punch into heavy seas or motor against the tide then the full power of the diesel engine can be used to drive the vessel. Equally in calmer conditions you can slip your lines and motor in complete silence under electric drive, later in the day, perhaps after some regenerative sailing, you can return in the same way. In the evening you can use power stored in the battery bank to silently run standard domestic appliances.

How fast can I go using the electric motor?
The Hybrid is intended for low-medium power requirements – for slipping in and out of harbour silently and with the great control offered by the high torque and instant response of the electric drive – and will deliver up to about 2/3 of hull speed in calm conditions, about 5 knots for Wylo 35.5.

How far can I go on my batteries?
This depends on boat size, the size of the battery bank and how fast you want to go. The standard Wylo 35.5 installation will give 2-3 hours use at 4-5 knots. Increasing the size of the battery bank will increase these figures.”

Seems to me the folks making the Wylo II have put together a pretty good design with similar goals to the Adventure 40. Personally I like their presentation because it appears that
a) for most short-term uses the electric motor will do find – entering and leaving marinas etc. and
b) each is back-up for the other, which is also good
c) I just like electric motors mainly because I have never liked, nor understood, combustion engines.
(I am copying this second, less well composed version!)

Eric Klem

Hi CaperAsh,

That is a good question and the answer is that the model does not directly address the usage profile that is described in your comment. The parallel hybrid system chosen for the model is what I call a peaking system and is the most efficient way to use a parallel system that I am aware of. The advantage is that you can run a much smaller diesel engine for normal cruise which is more efficient (reflected in the better fuel consumption numbers the model should give you when compared to diesel). However, this only works when short burst of peaking are required as batteries have comparatively poor energy density so the system gets impractical if long periods of peaking are required.

The Wylo II system’s advantages to me are in noise reduction, fast house bank charging and very low speed maneuvering all of which the model does not address. From an efficiency standpoint, it will be lower than a straight diesel for most applications (exceptions being extremely high house loads or very low propulsion loads). The reason for this is that each time you convert energy, you loose some to heat so taking the diesel shaft energy and converting it into chemical energy (batteries) and back lowers your overall efficiency. As they state, the diesel efficiency goes up from the higher load but the important thing is that the incremental efficiency is below 100% so you can’t make up for the lost efficiency in the energy conversions. In fact, you would be better off leaving your batteries at whatever state they are and simply running the diesel when at a steady cruising speed.

If you want to model this yourself, you can do it as an energy problem and draw out a diagram of each step in the process and assign efficiencies. I am happy to give more detail on how to do this if you would like.

Eric

CaperAsh

No big deal, J. I just wanted to let you know in case others were having similar issue. (I did do the back button but the previous post was etherized.)
That spreadsheet is impressive..

Paul Newman

I am really thrilled to be studying this spreadsheet, not only for the numbers and what they may mean, but also for the example it sets in doing thorough planning. Thanks to all involved!