serious discussion about electric power in large boats

[john61ct]

Charge sources other than mains or ICE powered will be insignificant contributors to the total energy required for propulsion usage as most owners expect.

Not just relatively, but basically useless.

Unless waiting for days to gather an hour or two's motoring seems OK to you.

[GrowleyMonster]

Originally Posted by CarlF

GrowleyMonster

I generally agree with your comments. To get reasonable efficiency out of a marine diesel you have to be running at the right spot in the torque curve (50%-60% max RPM typically) and be comfortably below hull speed. But the same has to be said of diesel-electric. The genset has to be matched to the load and speeds have to be kept down.

I don’t agree that the current grid delivers low carbon electricity in most cruising areas. In many islands (Nantucket, Bahamas) it’s diesel.generated - although offshore wind has changed this on Block Island and in ten years on Marthas Vineyard. Also, utilities have to generate well above demand for voltage and frequency stability - and this is 24 hours a day. Of course the demand is spread across many users but when you charge from the grid it’s only fair to include some loss factor for this 24 hour standby availability of shorepower. With an engine (or diesel electric) it only emits when running the boat - rarely more than 2-3 hours a day for a cruising sailboat with a Captain who actually likes to sail.

A problem I’ve noticed with some diesel electric boat designs is the genset is oversized in the same way most boat propulsion diesels are oversized. This is inefficient at normal cruising speeds and even more inefficient when using the genset for hotel loads. Of course, you could install a big and small genset but that is an added expense and complexity.

But I agree with you that the diesel-electric has advantages and could make sense with the right design. My main problem with the the OP’s question is the frequent assumption by people looking for electric propulsion that a cruising sailboat could power 100 miles a day from solar panels or shore power charged batteries. I expect we agree on this

Yes your last paragraph won't get much of an argument from me. Sure, it would be POSSIBLE, if you were monomaniacally and singlemindedly pursuing as your only goal, a 3 figure range, forsaking all other considerations. But who wants to motor at a half knot? Clean the bottom every time you take the boat out? Tilt every solar panel just so, adjusting all the time? Or literally fill the boat with batteries? All I am saying is everyone should be realistic. Nobody without any sort of engineering background should go in for DIY EP in a cruising boat unless he has sailed on an electric boat. Real world conditions do not yield the same results as the tables and spreadsheets and sales brochures. Systems, ready to rock, are generally overpriced and misrepresented. OTOH going electric is not necessarily a stupid idea, either. It is a fool's errand only if it is a fool who is doing it. And it is never going to be the holy grail or the pot of gold under the rainbow no matter how many times you click your ruby slippers together.

[Auspicious]

I'm late to the party. Having read the entire thread (I had to take a couple of breaks to get my blood pressure down) it looks like a number of people who don't know what they're talking about arguing with people who do.



It's a little painful.


I called Tom to get the answers to some questions I think go to whether electric drive is practical for HIS application. From a subjective perspective I found him to be a very nice person whose story of how he came to need a new engine/motor/whatever will make you weep. That is his story to tell.



In my opinion (naval architect and marine engineer, Webb '82, mostly military, civil government, and commercial - currently working recreational boats on the front lines) electric propulsion has a place for day sailors. Energy density falls short even for long weekend cruisers and certainly for ICW cruisers, coastal cruisers, and passage makers. Sailing Uma, referred to in this thread, proves that to the educated eye. They simply take too many risks for my thresholds. YMMV. Their math doesn't add up either.



Tom and his bride are long-term liveaboard cruisers. US East Coast, Bahamas, Leeward and Windward Islands.



Thoughts in the context of previous posts:

Originally Posted by tom jones

options to date: rebuild the Lehman replace with newer power plant ie Beta 50

In the process we continue to run into discussions re: electric power

Looking at options is good. It's easy to replace like for like. It's easy to give in to religious fervor. It is more difficult to consider options, do a rational comparison, and make a decision.

Originally Posted by benvanstaveren

to bring up Sailing Uma, they've had a few close calls IIRC (I recall at least one) where having had a diesel would've turned it into a total non-issue.

Agreed. Sailing Uma is the poster child for religious fervor. They have the answer and work backward to the question. For any practical installation of solar and wind and accounting for provisions and water and the sustaining electrical load for systems things can get unpleasant in doldrums and downright dangerous in heavy weather.

Originally Posted by ramblinrod

And after they declared all of their “green” reasons for going electric, ...

The situation is worse than that. I think photovoltaic (PV) solar is a wonder for cruising sailors. It is however neither green nor renewable. The rare earth minerals necessary for manufacturing PV panels are most assuredly not renewable. The mining process is similarly not green. It surely isn't healthy for the miners. The manufacturing process is a horror. The service life of panels is not that long and recycling or disposal is not green either. Now we must consider energy storage. Most people are aware of how toxic lead is. The fairly limited reserves of lithium are less well recognized. Again, not renewable. More mining, less green.



There is nothing better for the cruising sailor than lots of PV solar and the biggest battery bank you can fit into your boat and out of your wallet. Do not kid yourself that it is green or renewable. It simply is not.



Let's not forget who owns most of the rare earth mineral deposits on the planet.

Originally Posted by mdenize

I think this a very unkind statement and fairly typical of this forum of flat eathers. As an EE I can say that here on this side of the round earth a watt/ hour is just as vaid as a kilowatt/ hour ( Just smaller units). Anyone with electrical knowledge would recognise 300w/hr as 0.3kW/Hrs per hour. Anyway just saying that stum's statement makes perfect sense to me😁

Originally Posted by mdenize

Btw I do realise that the correct abbreviation is W Wh and kWh but I just did not think putting a "/" in changed the intent of his statement at all

Add me to those who say just "NO." Keeping units straight is critical. When you make a mistake finding the units not being right is a clear indication that you erred. Who doesn't make a mistake from time to time? Carry the units through. Anyone that abuses units by writing 'Watts/hr' or 'amps/hr' (coulombs per second per second? really?) when they mean watt·hrs or amp·hrs has demonstrated that everything else they write is suspect. There have been some posts that say "but we all know what they mean" which are just wrong. 1. We don't know for sure. 2. If they get something so simple wrong what else is wrong? This is NOT nitpicking. This is science.



Read the link StuM posted in post #41. Then read it again.



Since I am on a roll I'll repeat something I've pointed out before. There. Is. No. Such. Thing. As. Hull Speed. Just give it a rest. There is no knee much less a brick wall in the speed-power curve. Hull speed is an empirical measure that had some merit for comparing one hull to another when hulls were all shaped fundamentally the same. The short version is that longer vessels go faster. Part two of the short version is that going faster leads to diminishing returns (fuel consumption or sail loading). Like most such ratios hull speed is only useful if you know what you are looking at. Consider the "hull speed" of a boat with long overhangs under power, upwind, and downwind. Anyone who hasn't sailed their boat well above "hull speed" should take a class in sail trim.



Okay I feel better now.



For poor Tom, based on our phone conversation I think his best bet is to stay with diesel. The choice between rebuilding his Lehman, buying a rebuilt Lehman, or finding a Perkins or Beta that fits and for which engine mount modifications are not onerous is dependent on cost, not function. I can think of a few simple adds that make sense while everything is taken apart (paint everything (I'm sure this is on the list), exhaust valve, dual alternators, maybe reorganize engine space for service needs, fire suppression, dewatering, ...) but many won't be right for him. I only know him from our phone call.



Did I mention I think he is a really nice guy? If you get to the West Coast of Florida stop by and buy the guy a beer.

[a64pilot]

I agree with just about all that you have said, except about hull speed, it exists, and further more is very easy to demonstrate.
Just simply record RPM and speed, you will find a speed at which it takes significant increases of power to exceed, yes it can be exceeded, if you have enough power and or if you have a planing hull.
My boats hull speed is just under 8 kts, I can hit or slightly exceed 8 kts under power and once when surfing under sail would exceed 10 in bursts.

However the power to achieve 6 kts on smooth flat water under power is about half of what it takes to make 7 kts, and 8 kts takes all there is.

Roughly 1600 for 6 kts, 2200 or so for 7, and 3300 or so for 8 kts, I can’t quite get to 3600.

This power chart shows how much difference in HP there is between those numbers, you’ll see there is quite a large difference. I normally cruise at 1800 which is a good balance for me between noise, fuel consumption, speed and range. That gives me about 6.5 kts

[john61ct]

Great post

Originally Posted by Auspicious

photovoltaic (PV) solar is a wonder for cruising sailors. It is however neither green nor renewable. The rare earth minerals necessary for manufacturing PV panels are most assuredly not renewable. The mining process is similarly not green. It surely isn't healthy for the miners. The manufacturing process is a horror. The service life of panels is not that long and recycling or disposal is not green either. Now we must consider energy storage. Most people are aware of how toxic lead is. The fairly limited reserves of lithium are less well recognized. Again, not renewable. More mining, less green.

There is nothing better for the cruising sailor than lots of PV solar and the biggest battery bank you can fit into your boat and out of your wallet. Do not kid yourself that it is green or renewable. It simply is not.

Those living aboard and cruising long-term

who are not flying frequently, nor also maintaining dirt housing and vehicles at the same time

have a **much** greener overall footprint per decade than the average first-worlder, especially compared to not-poor USians.

I totally agree that neither that lifestyle, nor electric propulsion, nor PV - in isolation - is any kind of "solution" to our species downgrading our habitat

but anything that helps enable more people to lower their overall consumption is IMO A Very. Good. Thing.

[Corvidae]

Originally Posted by Michael Gunning

Speed & Power QuietTorque 30.0LC 48v Liquid Cooled dual powerhead Motor System
Kts Amps Watts
1.6 10 480
2.6 20 960 This is what a portable 2kW generator would push the boat
3.3 40 1920
3.7 60 2880
4.1 80 3840 23NM with a 400ah 48v – 19kWh battery pack
4.4 100 4800
4.7 120 5760
4.9 140 6720 15NM with a 400ah 48v – 19kWh battery pack
We would recommend an 8 to 12kW genset to provide continuous cruising speed
5.4 170 8160
5.9 230 11040
6.4 290 13920
6.9 370 17760
7.0 390 18720
7.6 490 23520
8.0 590 28320
30000 Max power of 30kW motor

Hull speed 7.93


Mike

Electric Yacht

That chart pretty much matches the performance I get. 200ah 48v or 9.6kWh instead of the 400 you have listed there, so instead of 15Nm I get about 7 and change. The 12kw gen set I have also pushes us at right around 5 knots continuous. I probably should have gone with water cooled, and I might upgrade at some point. The ME1115's do get a bit hotter than I'd like. Hopefully I can get some fans and some passive cooling on them at some point.

We haven't been cruising much so I can't really speak to the longevity, but so far the agm batteries really seem to be the best bang for the buck. I haven't noticed any change in charge levels over the year we've been using them, and honestly if they make it more than 3 years between replacements, I'll have to consider that a way better deal than the lithium.

[Auspicious]

Originally Posted by a64pilot

I agree with just about all that you have said, except about hull speed, it exists, and further more is very easy to demonstrate.

It doesn't exist in any meaningful way.


Hull speed or speed-length ratio was used hundreds of years ago when all hulls were very similar in form and there was no repeatable way to measure power from sails. We could measure approximately hull resistance (recognizing scaling effects - see Froude, Reynolds, et al) but not power. As hull forms advanced the constant factor in speed-length ratio was modified. When I was in college we inherited nomographs, all empirically derived, to determine what constant between 1.05 and 1.5 to use in calculating "hull speed" the only purpose of which was to compare similar hulls. By that time the value of speed-length ratio was more in the constant than the "hull speed" number. In that same time frame numerical models began to develop which barely kept up with more and more variations in hull forms.



From the beginning professionals realized (or had it beaten into us with splines) that the numbers were only representative and did not provide any meaningful absolute value.



The clearest demonstration of this is to look at a simple speed-power curve that shows speed as a function of power (you can use fuel consumption vice power for a particular engine if you like). What you'll see is a curve that is more or less linear (there is a slight bow to the curve) from zero (0) speed to some point where the curve starts to bend upward a good bit over a range of speed after which it becomes mostly linear again with a slight bow. In the early portion of the curve hull resistance is dominated by static effects (wetted surface area and windage). In the last portion of the curve hull resistance is dominated by dynamic effects, principally wavemaking resistance. The transitional region--and it is a region not an inflection point--is more or less correlated with the waterline length approximating the developed wave length, which is itself dependent on both dynamic effects of the hull moving through the water and environmental conditions. The flat water curve is different from one in fully developed seas. Regardless it is a region and not a point and in fact is quite a broad region. See Figure 7.4 of https://www.usna.edu/NAOE/_files/doc...hapter%207.pdf and tell me just where the hull speed is?



In the transitional region for any fixed increase in speed the required marginal increase in power gets larger. For a moderate sized recreational boat the transition region is likely to be on the order of 20% the achievable speed range for the hull. That transitional region makes the concept of "hull speed" really silly. It's pure and simple marketing and nothing more.



There are of course contributing effects. One is fuel consumption curves in modern engines. These will, I believe, tend to compress the speed-power curve transition region when reflected as speed-fuel consumption. This isn't significant and the whole curve is truncated by the maximum power output of a given engine. At some point you can keep pumping more fuel through the engine and it won't generate more power. *grin* Second is changes in lift and drag under sail as a boat heels over. These are deterministic effects and for modern hull forms are why you generally go faster if you don't heel the boat more than about 15 degrees. Specifics of course vary with hull form. Lets not squabble over +/- 2 degrees okay?


In the real world there are other effects that contribute to how fast a boat will go. The "stuff" cruisers strap above deck creates windage. Drag from furled sails. Sail trim ability. Seakeeping thresholds. Sail condition.



It is worth noting that modern sail plans (and I should note that as a naval architect I look back of a great deal of history, so let's say modern sail plans cover the last fifty years, and modern sail materials cover the last twenty years) generate a prodigious amount of power when trimmed well. THAT is why you exceed the "hull speed" under sail, not because you're surfing the boat. Sail power on most boats can drive you well into and even through the transition region. That isn't surfing. It's pure power.



Still in the real world and for a moderate size boat (say 40-48 feet) and a modern engine like a Yanmar 4JH or a Volvo-Penta D55 or any other high speed diesel engine with power output between 55 and 75 HP (naturally aspirated or turbo) you end up with a "sweet spot" around 1800 HP where speed, range, and fuel cost are more or less optimum. That's a subjective choice of course - I've had customers who burn twice the fuel to arrive 35% sooner, or 50% more fuel to arrive 15% sooner. How deep is your wallet.



So yes, given a geosim hull, a 45' waterline boat will be about 5.7% faster in the same conditions than a 40' waterline boat. Absent a geosim (i.e. almost all boat to boat comparisons) there are too many other important factors. A naval architect can look at lines and make an assessment but we'll look at ratios like block and prismatic coefficients but we'll also compare the fullness of sections forward. Non-geometric considerations are important also like tankage, stability (CG and GM foremost), and the use cases. "Hull speed" simply doesn't matter.

Originally Posted by john61ct

Those living aboard and cruising long-term
who are not flying frequently, nor also maintaining dirt housing and vehicles at the same time have a **much** greener overall footprint per decade than the average first-worlder, especially compared to not-poor USians.

No question at all in my mind. I agree. I often tell people with 200-300A service to their homes that they have no idea what conservation is. Live on 30A (or less)--assuming 117VAC, half that for our ROW friends on 220VAC--for a while. Now in fairness most of us do avail ourselves of things like laundry and near-unlimited hot water when we can get them. *grin*


/rant/ We should be looking at unplugging things instead of adopting the IoT. Do you really need a "smart" refrigerator with WiFi that orders milk from Peapod because the 'best-by' date has passed?/end of rant/

Originally Posted by john61ct

but anything that helps enable more people to lower their overall consumption is IMO A Very. Good. Thing.

Maybe. I would say that working to reduce overall consumption is very good. Some things are easy - unplugging unused devices (any charger that isn't charging, anything in 'stand-by' to be turned on by a remote, anything with an electronic control panel). Some things, including PV solar, require thought and awareness of what we are doing to the planet. I don't have a better solution (well...a 15kW nuclear reactor would be nice) than solar and batteries for a cruising sailboat; I would like to think cruisers realize their solution has a foundation that is neither green nor renewable.

[a64pilot]

Most of your posts was frankly over my head.
However I’ll attach a screen shot from the link you provided.
I’d say what I refer to as hull speed is pretty clear, it’s the point where resistance vs speed goes almost vertical.

[Auspicious]

Originally Posted by a64pilot

Most of your posts was frankly over my head.
However I’ll attach a screen shot from the link you provided.
I’d say what I refer to as hull speed is pretty clear, it’s the point where resistance vs speed goes almost vertical.Attachment 200538

Look at Figure 7.16. What is magic about 1.34 vice 1.2 or 1.5? The bend in the curve is a region that is tantamount to 20% of the speed range of the boat! It just doesn't matter.

[a64pilot]

This is 7.16. To me as a JIm Bob type of guy and by no means a math guy, it looks the same.
I think your contention is the there is no exact, specific number that is an absolute limit? I absolutely concede that, but again as a JIm Bob type who never got past College Algebra we aren’t wrapped up in is it 1.3 or 1.5 or 1.57.
We just know that at some point we are pulling the guts out of it and expending a lot of fuel which kills range, for very little benefit.

Now on my boat the “theoretical” hull speed is 7.9 kts I think, but it seems really to be about 7.5 kts under sail, and when motoring I find that on my boat that 6.5 kts is a sweet speed for me, now admittedly a lot of that is NVH, or noise, vibration, and harshness to use an automotive term. Meaning that above 1800 RPM it sounds too much like I have a lawnmower in my salon and I just don’t like that.

But there is absolutely a speed at which with enough power you absolutely can exceed,but it takes more and more power to go faster.

You see it all the time in the ICW, that boat that passes you rolling that big wake is expending quite a lot of fuel to roll that big wake, the energy to make that wake of course comes from the fuel tank.
It’s not a wall, with enough power of course you can blow through it, but it requires quite a lot of power and fuel for very little extra speed.

[a64pilot]

Now in 7.15 the end of the chart where the curve actually decreases is I’d bet the point where that hull begins to plane

[a64pilot]

To me, using your supplied graph, it would seem that 1.1 is the “smart” speed to motor, for us Retired types that will get there, when we get there.
Interestingly 1.1 on my 33’ water line length boat gives 5.8 kts.
It would seem that if range is paramount I need to run at 5.8 kts?

[sanibel sailor]

I think if range is paramount, there is a fuel consumption curve for an engine that will also help find maximum efficiency. Is graph 7.16 for planing craft as shown in diagram above?


Also what's with the hump? Does that apply to sailboats as well?

[ranger58sb]

Originally Posted by a64pilot

To me, using your supplied graph, it would seem that 1.1 is the “smart” speed to motor, for us Retired types that will get there, when we get there.
Interestingly 1.1 on my 33’ water line length boat gives 5.8 kts.
It would seem that if range is paramount I need to run at 5.8 kts?

If your 1.1 is a multiplication factor for SQRT(LWL) -- as in the 1.34(etc) formula -- then yes, likely somewhere between 1.0 and 1.2 would give you best economical speed... according to Beebe in Cruising Under Power, and others.

IOW, 1.34 is a theoretical max (I think) and less than max is better.

-Chris

[a64pilot]

Originally Posted by ranger42c

If your 1.1 is a multiplication factor for SQRT(LWL) -- as in the 1.34(etc) formula -- then yes, likely somewhere between 1.0 and 1.2 would give you best economical speed... according to Beebe in Cruising Under Power, and others.



IOW, 1.34 is a theoretical max (I think) and less than max is better.



-Chris

I’ve used fuel flow meters and GPS speed in calm water and graphed fuel consumption over speed for three different boats.
All three were planning boats, two were Sportfishermen, one Diesel and one gas and one CC with an outboard.
I didn’t find that bump around 1.0, but my data points weren’t likely as numerous as would be needed to find that bump, and I think that is actually a very small bump. I’m not saying it’s not there, I’m sure it is, just my testing methodology wasn’t good enough to find it.
Anyway what I found on all three boats was the the most economical as in greatest range was idle, and any faster cut into range, there was no sweet spot like what is shown on these charts with the bump, and if you look at the bottom of the bump, it’s still higher than lower speeds.
Now all of this ignores wind and current which further complicates things of course.

The little CC I tied into the engines bus and picked all kinds of data from it, one of those was of course fuel consumption, the Garmin 740s, one of its features was it would compute fuel mileage continuously, so you could see the effect of trim tabs, motor trim and of course RPM on the fly.
As much as I wanted there to be, I couldn’t find any speed really that got better fuel mileage than a lower speed. Now right before she broke onto plane with the bow in the air was of course a bad speed for range, but just before it and just after it were better of course.

I expected that right after it got onto plane that it would actually get better fuel mileage than it did at a speed less than plane, but that wasn’t the case.

So in other words if you disregard wind and current cause I think that just too complex a problem to make general statements on, the slower you go the better the fuel economy.
But who can idle all day?

The other observation I made that seems to make sense is to observe your wake, if you have much of one, your burning excess fuel, slow down until you have no real wake and you’ll burn much less fuel.
Of course you may choose speed over fuel,that is of course your decision to make.

For whatever it’s worth, there is absolutely a sweet spot for any airplane, speeds less than or faster than will burn more fuel per distance covered, I expected a boat to have the same, but it doesn’t.