Notes on adding two 6 kW pod drives to supplement or reduce use of

[CaptainPete222]

I thought I'd share my notes analyzing whether it makes sense to add two 6 kW pod drives to use the excess power produced by my solar system for supplemental auxiliary propulsion or occasionally in lieu of starting up my two 29 hp diesels:

1. My 3.4 kW PV system produced enough last winter to do without a generator for hot water, water making, washer, fridge, freezer, etc, but only left me with an average of 3.5 kWh excess per day (5 kWh if I skimped on water and didn’t run air conditioning).

2. There isn’t any remaining space for panels that wouldn’t heat up the interior of my boat, so that’s (presently) the limit for a catamaran as far as I can determine.

3. A gallon of diesel produces 40 kWh of power, so, if I had pod drives to use my average daily 3.5 kWh of excess solar power, I would have avoided burning 16 gallons of diesel last winter. If I skimped on water and didn't run air conditioning, I would have burned 22 fewer gallons of diesel by having pod drives available.

4. During the six months we were in the Bahamas, I burned a total of 60 gallons of diesel for auxiliary power (so adding roughly 40 hours to each engine).

5. Despite being fairly thrifty in my use of auxiliary power, my excess solar power would only be able to provide propulsion about 1/3 the time I want/need auxiliary propulsion.

6. The cost to add pod drives and additional 20 kWh of storage (in 48V batteries) would be $17,500, excluding labor.

7. Adding 40 hours to my diesels per year and having an increased fuel bill of roughly $150 would cost somewheres around $300/year.

8. Adding the pod drives and the 48V batteries would increase my cat's weight by 450 pounds.

9. A 450 pound increase in weight would result in a slight increase in how much diesel I burn when I want/need auxiliary propulsion and also slightly slow the boat down when under sail.

10. Even with feathering props on the pod drives, the boat would be slowed slightly when under sail or motoring from added drag.

11. I don't have enough information to compare my carbon footprint from burning 160 extra gallons of diesel over ten years against the carbon footprint of adding the pod drives and batteries during that same period. My guess is that the electric propulsion option would be less, but that would just be speculation on my part.

I haven't yet decided whether to move forward with the project. The ability to become more energy independent and produce less C02 are two of our goals and adding pod drives would be beneficial in helping us realize the first goal. I am not so sure about the second, but, again, I'd guess that that goal would also be achieved to some degree. Despite the fairly significant difference in cost (pod drives vs no pod drives), there is considerable value in being able to motor quietly and without having to smell/breath in exhaust fumes. However, losing some speed while sailing because of increased weight and drag would be a pretty big cost given that I'm sailing the vast majority of the time. Finally, there would be some social currency by adding the pod drives because it would, at the very least, make for good cocktail conversation and I haven't yet found a cruiser who isn't interested in talking about solar-power electric propulsion.

Them there are my notes. Curious what others think. (If you think it's incredibly stupid, please at least take some time to explain why you think that. Same goes if you think adding the pod drives is a good idea. I personally find naked opinions to be very boring and, while I'm a sailor and therefore quite used to boredom, I come here hoping to read something interesting ��.)

[PippaB]

On our cat we very rarely motor with both engines, pretty much the only time we use two is for maneuvering. If your use case is different the the following is not useful...

How about some of both? Add two pods, but remove one engine and replace it with the batteries? That way you don't have the weight penalty. For maneuvering you have the dual pods, and if for some reason you want to motor a long distance you can still turn on the single diesel. Getting really clever, if you put a big enough alternator on the diesel you might be able to drive the pod in the opposite hull enough to offset some of the unbalance you get from running on one engine.

If you generally motor with both, or don't want to lose the couple of knots you can get with using two diesels then it wouldn't work for you, but after reading through your post I think it might be the kind of setup I would consider.

[CaptainPete222]

Originally Posted by PippaB

On our cat we very rarely motor with both engines, pretty much the only time we use two is for maneuvering. If your use case is different the the following is not useful...

How about some of both? Add two pods, but remove one engine and replace it with the batteries? That way you don't have the weight penalty. For maneuvering you have the dual pods, and if for some reason you want to motor a long distance you can still turn on the single diesel. Getting really clever, if you put a big enough alternator on the diesel you might be able to drive the pod in the opposite hull enough to offset some of the unbalance you get from running on one engine.

If you generally motor with both, or don't want to lose the couple of knots you can get with using two diesels then it wouldn't work for you, but after reading through your post I think it might be the kind of setup I would consider.

Very good idea, but I'd need to swap out the one remaining diesel with something larger (in addition to taking a hit on selling off my diesels/saildrives). I'm already at the bare minimum for power with 29 hp diesels. But, if I was doing this from scratch, your idea is definitely the way to go.

[ItDepends]

Originally Posted by CaptainPete222

3. A gallon of diesel produces 40 kWh of power, so, if I had pod drives to use my average daily 3.5 kWh of excess solar power, I would have avoided burning 16 gallons of diesel last winter. If I skimped on water and didn't run air conditioning, I would have burned 22 fewer gallons of diesel by having pod drives available.

I am sorry, but this number seems to be very far from accurate.

NO WAY will you get 40kWh of electric energy from a gallon of diesel. The TOTAL thermal energy content of diesel is about 37.5kWh per gallon, but in small engines AT THE VERY BEST only about 30% of that is available as mechanical energy using any current technology.

We have a diesel driven permenant magnet alternator DC generator that burns 2.5 l/hr of fuel to generate 150A at 24V, or 3.6kWh/2.5 liters. That's about 5.5kWh per gallon of diesel.

You might want to revisit all the numbers in your analysis after fixing that.

[rslifkin]

I wonder how the numbers would look with larger electric drives and a diesel generator to help power them when there's not enough surplus electricity available?

[CaptainPete222]

Originally Posted by ItDepends

I am sorry, but this number seems to be very far from accurate.

NO WAY will you get 40kWh of electric energy from a gallon of diesel. The TOTAL thermal energy content of diesel is about 37.5kWh per gallon, but in small engines AT THE VERY BEST only about 30% of that is available as mechanical energy using any current technology.

We have a diesel driven permenant magnet alternator DC generator that burns 2.5 l/hr of fuel to generate 150A at 24V, or 3.6kWh/2.5 liters. That's about 5.5kWh per gallon of diesel.

You might want to revisit all the numbers in your analysis after fixing that.

I pulled that number off some website, but, now that I think about it, it's probably closer to 20-25 kWh based on how much diesel my 29 hp Yanmar burns per hour. Your number is way too low, but also illustrates why using a generator to power an electric engine is the least green option of all.

Unfortunately it's too late to make changes to the original post; the edit function has timed-out. Running the calculations based on 20-25 kWh per gallon of diesel, I would have saved around 28-30 gallons of diesel over the six months of cruising if I had the pod drives available to use my excess solar power. Still much more expensive to add the pod drives, but it does make it more attractive from the standpoint of how much I would actually be able to use my pod drives in lieu of starting up the diesels.

[CaptainPete222]

Originally Posted by rslifkin

I wonder how the numbers would look with larger electric drives and a diesel generator to help power them when there's not enough surplus electricity available?

The only thing I can say about that is that converting diesel to electricity to use for propulsion is highly inefficient, as compared to just using a diesel engine to spin the prop. It would be interesting to do an analysis based on real-world usage with someone who has one of the new "hybrid" systems to see which is more green (diesel vs hybrid). Obviously the more a boater uses their generator, the less green a hybrid system would be, so the outcome of the analysis really hinges on actual usage of auxiliary propulsion.

[ItDepends]

Originally Posted by CaptainPete222

I pulled that number off some website, but, now that I think about it, it's probably closer to 20-25 kWh based on how much diesel my 29 hp Yanmar burns per hour. Your number is way too low, but also illustrates why using a generator to power an electric engine is the least green option of all.

If you want to believe you can take half of the potential energy in diesel fuel and convert it to actual motive power, or electricity you are severely kidding yourself.

I'll bet you took fuel burn in your engine and converted that to the kWh equivalent of 29HP. BUT: That ONLY applies if you are running at full throttle. Back off a few hundred RPM and the power dissipated by a propeller drops dramatically. (Roughly proportional the the cube of the prop RPM). Look at the propeller power curve for you engine...

If you just make up numbers that "feel" right or pick them at random off of a website without really knowing what they mean, your analysis is worthless.

Bring data or go home.

[GRIT]

If you're thinking carbon footprint, you should also add the carbon released when building the motors, batteries, and cables too.

Financially, it's a loss, and I don't think you'd be doing much better with the carbon footprint, once you add manufacturing into the mix.

Where I really think you'll lose is reliability, and complication. It seems that electric motors on boats suffer from many issues, or at least that's what I gathered from the "great circle" blog. It's a 50 foot cat with inside steering, and electric motors. It seems he had several issues with the batteries, and at least once had problems with the controllers, and perhaps once with a motor too.

I think half the issues with electric drives tend to boil down to poor design, poor execution, or poor quality parts. Typical marine stuff...

Though those kids on a pearson (Uma?) seem to be doing ok, once they got the bugs out. I don't think they've had the new system in for more than a year.

The research I did back in 2014 didn't support having electric or hybrid drives, on our new build; and though things are more advanced today, I don't think your situation (as far as carbon) would be improved with what you suggest.

You'll save a lot more carbon, and cash, by simply sailing more, and motoring less. Adjusting your schedule, or expectations will really make a difference in the amount of carbon released.

As you say, it would be a great conversation piece, though an expensive one.

Keep me in mind if you do it; I'd like to have you over for sun-downers, to chat about your electric drives!

Cheers.
Paul.

[rslifkin]

Originally Posted by CaptainPete222

The only thing I can say about that is that converting diesel to electricity to use for propulsion is highly inefficient, as compared to just using a diesel engine to spin the prop. It would be interesting to do an analysis based on real-world usage with someone who has one of the new "hybrid" systems to see which is more green (diesel vs hybrid). Obviously the more a boater uses their generator, the less green a hybrid system would be, so the outcome of the analysis really hinges on actual usage of auxiliary propulsion.

I agree, diesel electric drive is less efficient on a basic level. But if it allows use of a single well sized diesel and some of the motoring time is done without the diesel (or you motor on battery and run the diesel at its peak efficiency point to recharge), it may be possible to gain enough to offset the conversion loss, etc.

[Cliffhanger]

I just love the idea of being able to "motor" into the marina using the pods. Silent and instant torque = best possible manoeuvrability.

[CaptainPete222]

Originally Posted by ItDepends

If you want to believe you can take half of the potential energy in diesel fuel and convert it to actual motive power, or electricity you are severely kidding yourself.

I'll bet you took fuel burn in your engine and converted that to the kWh equivalent of 29HP. BUT: That ONLY applies if you are running at full throttle. Back off a few hundred RPM and the power dissipated by a propeller drops dramatically. (Roughly proportional the the cube of the prop RPM). Look at the propeller power curve for you engine...

If you just make up numbers that "feel" right or pick them at random off of a website without really knowing what they mean, your analysis is worthless.

Bring data or go home.

To be clear, I did "bring data", just inaccurate data according to you. But, you're correct; my data is incorrect. However, so was yours. Data from what a generator produces in kW isn't particularly relevant to calculating how much diesel my excess solar power could have saved by running pod drives instead of the diesel engines. As you seem to acknowledge, diesels are more efficient in converting the fuel into propulsion than a generator/electric motor system.

So, how much diesel do you think I could have avoided burning last winter if I had pod drives to make use of my 3.5 kWh/day of excess solar power? We agree that's an important variable in the analysis, so I'd like to hear your thoughts if you're willing to share them.

[CaptainPete222]

Thanks to all for your contributions. GRIT's input was particularly useful, as I neglected to include in my analysis the issues of reliability and the consequences of adding yet another complicated system to my already very complicated systems. A diesel's reliability is very well known. The reliability of the 6 kW pod drives I have in mind using is unknown. The fact that they are relatively inexpensive compared to something like an OceanVolt system should be a red flag that, unfortunately, I didn't flag in my notes. (It's also unfortunate that CW won't let me go back and make amendments to my original post.).

ItDepends' observation that I erred with respect to my underlying data regarding how much propulsion can be generated by burning a gallon of diesel was also very helpful and is much appreciated. I'm still curious how much diesel I could have saved converting my excess solar to propulsion. But, that analysis is for another day (for me), as the lack of reliable information about the reliability of those pod drives and the fact that I'd be making my already complicated systems even more complicated are enough, by themselves, to justify shelving this idea for a few years.

[RAyDGus]

Depends, CP was right the first time - hw wasn't talking about generating electricity, but rather using electricity he already had. So the comparison of DRIVE power is quite reasonable. If he has to generate the electricity by diesel, then that conversino comes into play.

[trifan]

I thought I'd share my notes analyzing whether it makes sense to add two 6 kW pod drives to use the excess power produced by my solar system for supplemental auxiliary propulsion or occasionally in lieu of starting up my two 29 hp diesels:

<snip>

6. The cost to add pod drives and additional 20 kWh of storage (in 48V batteries) would be $17,500, excluding labor.

7. Adding 40 hours to my diesels per year and having an increased fuel bill of roughly $150 would cost somewheres around $300/year.

<snip the rest>


I would think that your numbers show in no uncertain terms why it doesn't make sense. Your boat will fall apart from old age before you could amortize the expense.