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

[Bellinghamster]

I'm curious to know how you came up with the 3.5kWhr excess figure. how do you measure something you didn't use or store? (I can come up with several ways to measure it, but none that are simple). Just wondering if that number is accurate - it seems to me like it'd be higher with the panel capacity you have, but without knowing your boat loads it's just a gut feel.

So your watermaker uses quite a lot of power... is it an energy-recovery type? If not, it'd be interesting to consider changing to one that is to allow more collected energy to be used for the pods.

You're also doubling up storage capacity by adding dedicated motive storage - you could run inverter loads off the higher voltage bank which means you can down-size some cabling and potentially save some weight... just some rambling thoughts that popped into my head (before my morning coffee... coherence is not very high)

[Moontide]

Spending $17,500 to save $300/year doesn't sound like a very good payback to me. 17500/300 = 58.3 years to break even. Of course that calc ignores the other considerations. Also potential effect on resale value. Whether up or down depends on a lot of factors that are in the future.

[CaptainPete222]

Originally Posted by trifan

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.

From a financial standpoint, you’re definitely right. But, as my notes indicate, there are things that are valuable to me that don’t translate into dollars and cents very easily, e.g., entertainment value. Owning a boat never makes financial sense (unless it’s relatively small, inexpensive to purchase and maintain, and you live aboard full-time), unless you take into account its entertainment value. Although I didn’t make specific mention of the entertainment value of this project, it should have been fairly obvious from my notes that I find it very entertaining. It also has a lot of valuable social currency. Adding it all up, it was a close call for me (until I factored in a couple of important things that GRIT and It Depends pointed out).

[donradcliffe]

Nearly all diesels will produce 20hp for one hour from a gallon of fue. At 746 watts per hp, that is equivalent to 15 Kwhr.

[trifan]

Originally Posted by CaptainPete222

From a financial standpoint, you’re definitely right. But, as my notes indicate, there are things that are valuable to me that don’t translate into dollars and cents very easily, e.g., entertainment value. Owning a boat never makes financial sense (unless it’s relatively small, inexpensive to purchase and maintain, and you live aboard full-time), unless you take into account its entertainment value. Although I didn’t make specific mention of the entertainment value of this project, it should have been fairly obvious from my notes that I find it very entertaining. It also has a lot of valuable social currency. Adding it all up, it was a close call for me (until I factored in a couple of important things that GRIT and It Depends pointed out).

I know I shouldn't and I'll probably be chastised severely for this but as the Ferengi would say, "Where's the profit in this?" Anything else is chalked up to zealotry and that cannot have a price assigned to it. At that point you don't need to ask if it makes sense because it most definitely doesn't, you just have to go with what makes you feel good and don't try to justify your behavior.

[Adelie]

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.

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.

Diesel contains 37.95kWhr of energy. But running it thru a Diesel engine wastes about 2/3 of it.

As a rule of thumb, for diesel, you can expect 18hp/gal/hr at the shaft. That’s 13.5kWhr/gal. Tractor comparisons are a good source of info in this. A really good tractor right now makes 20hp/hr/gal. This is a huge selling point. Marine diesels are not going to be this good.
https://www.farmprogress.com/tractor...cient-tractors

Let’s be generous and say the generator is 95% efficient. That means for 1gal of diesel you can expect 12.75kWhr of electricity.

[zstine]

My 2 cents... Financially, as others have said, doesn't make sense. But there's more to life than money!

The $17.5k seems a bit high for 20kW storage and a pair of 6kW outboards.

I would simply use electric outboards on a stern bracket that allows them to be easily kicked up out of the water while sailing or removed completely and stowed. You can even use one for your dinghy! 500lbs of weight is offset somewhat by not needing to fill those diesel tanks. And with outboards, you won't have any extra drag in the water while sailing. Others may say the outboards won't work because the stern will lift them out in waves, but waves come with wind and you will be sailing in wind, not motoring. This is for calm conditions. Another option, remove an engine and replace with internal E-motor and batteries.

Elco 9.9hp equivalent outboard (~7kW) are under $3k, so <$6k for both. (probably need a high thrust prop)

20kw batt's is qty 4 of EG4, 48V 5.12kW LiPO batts, $1,500ea x 4 = $6k.
so $12k plus a few grand in install hardware, wire, connectors, etc. maybe $15k total.

You have room on the forward end of the cabin top for more solar, IMO. May need to modify the jib leads... why can't you put solar under the clew of the jib?

Obviously this is a 'cool' project and one that gives the boat more autonomy and cleaner running. If the money ain't that big a deal, just do it... teach us how it went


I own a motorcycle I ride for no other reason than fun. There's lots of people who spend thousands on toys for fun.

[Brewgyver]

Originally Posted by rslifkin

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.

That's more than just a BIG BUT (and I can't deny! )

IIRC, there is no such thing as a 100% conversion of energy from any form to any other form. People have been chasing that for a VERY long time. Google "perpetual motion."

[rslifkin]

Originally Posted by Brewgyver

That's more than just a BIG BUT (and I can't deny! )

IIRC, there is no such thing as a 100% conversion of energy from any form to any other form. People have been chasing that for a VERY long time. Google "perpetual motion."

Agreed, there's no way to prevent conversion losses. But if the system allows more optimal engine loading, the engine efficiency will be better, so the overall system efficiency (with the conversion losses included) can sometimes still be the same or better. Hard to know at a quick glance whether it'll come out better or worse overall though.

[CaptainPete222]

[QUOTE=zstine;3639031I would simply use electric outboards on a stern bracket... that allows them to be easily kicked up out of the water while sailing or removed completely and stowed. You can even use one for your dinghy! 500lbs of weight is offset somewhat by not needing to fill those diesel tanks. And with outboards, you won't have any extra drag in the water while sailing. Others may say the outboards won't work because the stern will lift them out in waves, but waves come with wind and you will be sailing in wind, not motoring. This is for calm conditions. Another option, remove an engine and replace with internal E-motor and batteries.

Elco 9.9hp equivalent outboard (~7kW) are under $3k, so <$6k for both. (probably need a high thrust prop)

20kw batt's is qty 4 of EG4, 48V 5.12kW LiPO batts, $1,500ea x 4 = $6k.
so $12k plus a few grand in install hardware, wire, connectors, etc. maybe $15k total.

You have room on the forward end of the cabin top for more solar, IMO. May need to modify the jib leads... why can't you put solar under the clew of the jib?

Obviously this is a 'cool' project and one that gives the boat more autonomy and cleaner running. If the money ain't that big a deal, just do it... teach us how it went


I own a motorcycle I ride for no other reason than fun. There's lots of people who spend thousands on toys for fun.[/QUOTE]

I posted about my experiment using a 6 kW outboard that was on my dinghy on a mount on my cat's port ama transom. In a nutshell, the experiment didn't work because I needed a long shaft to keep the prop from catching air when the motor was on the cat and the dinghy required a short shaft. (The 6 kW outboard experiment also turned out to be a bust because it was insufficient power for the dinghy and the range was very limited due to the present size and weight of lithium battery storage. I replaced it with a 15 hp Yamaha.) I did have some good moments with the outboard pushing the cat, but the sugar scoop doesn't permit tilting and I didn't want a bracket extending off the back of the transom that would interfere with the people using the sugar scoop to get on and off the boat. Moving the engine every time we were in port was also a PITA.

There have been plenty of occasions when auxiliary power has been handy and some instances necessary in rough conditions I've encountered, so having an engine that doesn't have a prop constantly catching air is essential in my book. I don't know if a long shaft would have addressed the catching air problem.

As to the cost, I don't buy batteries that are that inexpensive. That's an option, but not one that's acceptable to me. I also like lengthy and enforceable warranties given that they are a sizable investment. Hard to get an enforceable, lengthy warranty from the batteries in the price range you mention.

I don't put panels on areas where there is living space below as that heats up the living space. They are like little heating pads and the last thing I want is to increase the heat in my saloon and cabins.

As to the "is it worth it?" question, I own a boat so I'm very used to just blowing money for no particularly good reason. That's the very definition of owning a boat. :-)

[CaptainPete222]

Originally Posted by donradcliffe

Nearly all diesels will produce 20hp for one hour from a gallon of fue. At 746 watts per hp, that is equivalent to 15 Kwhr.

Thanks. Based on that figure, having pod drives to use my excess 3.5 kWh of power would have reduced my diesel consumption by 42 gallons. Since my total consumption was only 60 gals in six months, the pod drives would have been used the majority of the time I wanted/needed auxiliary power. One thing I didn't factor in is that I would need a considerably larger battery bank than the 20 kWh storage I proposed to maximize the ability to fully utilize the excess 3.5 kWh. There are many occasions when I'm cruising that I'm only using the diesels for very brief periods while anchoring or setting off. That translates into a lot of wasted energy unless I have a larger battery bank to store it. I don't know what that would need to be.

[peter57]

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.

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.

The old rule of thumb was around 2hp per pint as good fuel efficiency, that gives around 12 Kwh per gallon or 16hp per gallon. Play ball.

[CaptainPete222]

Originally Posted by Moontide

Spending $17,500 to save $300/year doesn't sound like a very good payback to me. 17500/300 = 58.3 years to break even. Of course that calc ignores the other considerations. Also potential effect on resale value. Whether up or down depends on a lot of factors that are in the future.

As to increasing resale value, I have a distinct feeling that trying to sell my present PV system in five years will be a lot like trying to sell a five year old Dell laptop. In the two years since I redid my 12V system, batteries are half the size and half the weight. Assuming that trend continues, no one will want a used lithium battery that weighs several times what newer batteries weigh and takes up much more interior space. I have no expectation on getting a return for my money on any of this stuff.

[Matt Johnson]

Originally Posted by CaptainPete222

Thanks. Based on that figure, having pod drives to use my excess 3.5 kWh of power would have reduced my diesel consumption by 42 gallons. Since my total consumption was only 60 gals in six months, the pod drives would have been used the majority of the time I wanted/needed auxiliary power. One thing I didn't factor in is that I would need a considerably larger battery bank than the 20 kWh storage I proposed to maximize the ability to fully utilize the excess 3.5 kWh. There are many occasions when I'm cruising that I'm only using the diesels for very brief periods while anchoring or setting off. That translates into a lot of wasted energy unless I have a larger battery bank to store it. I don't know what that would need to be.

That 3.5kw extra is daily. I'm guessing the 6 month of diesel use was more like an hour or more every few days. I don't think you can do a 100% swap of usage unless you can utilize every extra bit of solar as a direct replacement for diesel.

[CaptainPete222]

Originally Posted by Bellinghamster

I'm curious to know how you came up with the 3.5kWhr excess figure. how do you measure something you didn't use or store? (I can come up with several ways to measure it, but none that are simple). Just wondering if that number is accurate - it seems to me like it'd be higher with the panel capacity you have, but without knowing your boat loads it's just a gut feel.

So your watermaker uses quite a lot of power... is it an energy-recovery type? If not, it'd be interesting to consider changing to one that is to allow more collected energy to be used for the pods.

You're also doubling up storage capacity by adding dedicated motive storage - you could run inverter loads off the higher voltage bank which means you can down-size some cabling and potentially save some weight... just some rambling thoughts that popped into my head (before my morning coffee... coherence is not very high)

The Victron app on my phone shows how much power went unused. I averaged up the unused power month to month. You'd think that by May the numbers would really jump in the Bahamas compared to December and January, but April and May are historically cloudier months and were cloudier this year as well. There was a modest increase, but not as much as I expected.

I wasn't interested in seeing if I could reduce our energy usage, but did note differences in how much excess there was when we didn't run AC or didn't do laundry for a while or for some other reason weren't using as much water. There are ways to increase the amount of power that would be available for auxiliary power, but they don't interest me. We already live in something smaller than a tiny, tiny house and are pretty frugal. It seems pretty absurd to start being stingy on hot water, water usage, air conditioning on hot day, etc when you're already treading very lightly in general and minimizing use of your engines.

I have a 12 to 48V converter which I put in to boost power to my dinghy's 6 kW outboard when I had it mounted on my cat's port transom (as an experiment). It doesn't supply all that much power and can't give even one 6 kW engine the power it needs once the battery juice runs out. Now, if I were to do this all again, I'd set up the entire system on 48V with 48 to 12 converters where needed. That would give me more flexibility, but not all that much as my battery bank is just sufficient to get me through a day with no sun. In other words, I don't have excess house storage, so there really wouldn't much in the way of weight or cost savings.