Going Electric: Will the batteries explode ?

[SDChristian]

Oversize the prop

[Jorgen Moller]

I have helped configure and install 10kw BLDC motors in a 26' Nonsuch and a 31' Herreshoff. My own 26' Nonsuch uses a 5kw BLDC. The two boats each has 200ah LIFePO4
batteries, the third 200ah Firefly carbon foam batteries. For the two 26' 4 knots under normal conditions use about 35A. The 30' a little more. Battery voltage is nominally 48V.

Using full throttle is not recommended, because even my 5kw will cause prop slip. I limit mine to 60A. The two 10kw installations also have 2kw Honda generators which allow them to reach 3 knots on generators alone. When using both batteries and genset keep an eye on the total amps in and out of the batteries to ensure you are not running the batteries flat. Don't rely on the battery voltage.

[dlymn]

can you get a chart for you em of current draw against rpm in the original car? Boats need much more power to achieve hull speed than to potter along. if you put that against a chart of your original diesel of speed against rpm, you should be able to get a basic idea of your cruising speed against current draw. EM is great for that 1 hour in and out of port, but there are also times when you need to go25-50 miles and not necessarily in calm conditions. That is not an unusual situation in the Med where I sail. A diesel has plenty of reserves to motor for 5-8 hours into a headwind and then anchor out
I guess that using a combination of battery and diesel generator, you should be able to draw 50 amps for 5+ hours, but after the batteries are depleted, you can only draw less than 30 amps. My concern would be if the weather changed and you had to depend on 30 amps to travel against strong winds. If your batteries are depleted and you anchor out, you'll be running the generator for 4-5 hours to recharge and then a bit the next day to top them up after house use.

It's all about personal safety and it would be great to have the power and storage reserves of a diesel. Anything less will mean you might be disappointed by the sailing/motoring experience. For day sailing your setup is fine but for day long cruising to distant destinations you might need to accept compromises for your own safety. yes, I know that it is a sailboat, but we often motor for 5+ hours at a time to get to a destination in the time we wish to. Waiting for calm days or for the wind to be blowing in the right direction can be annoying.

[hamburking]

Your motor is too big (powerful), but thats not really a problem, so long as you keep the throttle down. And I think you already know the answer...not enough batteries...too fast a drain.

I'm guessing you removed a heavy 50hp engine, transmission and fuel tank. Surely you've got space to double up the batteries?

There are so many variables, so many power losses. IMHO (and I'm a mech eng if that means anything to you) you should go ahead with the install as recommended by your car guy. Then do some tests. Make some graphs. Monitor current draw, rpm, speed through the water, and battery temp.

IMHO it doesn't hurt (except the wallet) to go bigger on the cables...no sense wasting any power to heat losses.

My guess is that the system will actually work pretty well, but the batteries will fail prematurely due to the heavy load. You may have to run your gen all the time while motoring. Maybe that will give you a better idea of just what type and how many batteries you need.

FWIW, I think electric drive is awesome and well worth the effort. Your results will help shape the future of sailboat electric drives.

[Maker42]

AGW batteries have a low internal resistance and do not have the reputation of overheating (exploding) that Li-Ion batteries do.

That being said, what if something goes south with them (like a short circuit in the system, or even internally to the battery)? Personally I think *any* large battery bank should have a temperature readout in the cockpit, with an alarm for exceeding a set temp.

I have the *impression* (maybe mistaken?) that personal watercraft are largely under-monitored. How many do we have out on the water with not even a bilge alarm? By that I mean a smart alarm that will go off if either the bilge runs for something like more than a minute OR if the bilge is running but the water level is still rising OR if the bilge runs for more than an average of x minutes per week.

[TrentePieds]

Quote: "(I speculate that in normal conditions i will need only a fraction of the available power, so that i will be going on batts for at least two hours and with the range extender around 8 hours)"

Quote: "Can't answer your questions but what would be interesting is a graph showing kw of power or amps, against speed and therefore range/time."


Interesting question, this. FWIW, using LWL = 30 ft; WLB = 10 ft.; moulded depth 3.5 ft; displ = 16,500lbs and a propeller RPM of 2,000, I find that a 3-bladed 14"D x 7"P prop will require:

.75 kW to attain 2 knots
1.5 kW to attain 3 knots
3.75 kW to attain 4 knots
7.5 kW to attain 5 knots
13.5 kW to attain 6 knots
21 kW to attain 7 knots
and
25 kW to attain hull speed of 7.5 knots.

The propeller doesn't care whence the power comes.

OP should therefore be able to get to 5.5 or 6 knots with the draw calculated by him. The remaining 1 or 2 knots to get to hull speed become MUCH more expensive, in fact THREE TIMES as expensive as the first 5 knots, and TWICE as expensinve as the first 6 knots.


At 6 knots we get: 13,500W = 84 x A —> 13,500/84 = A —> 161 Amps, and for hull speed, 298 Amps, so OP's notion of a ½ hour range at 16kW is in the ballpark. That translates to a distance THROUGH THE WATER of 3 nautical miles. That is the distance across Lake Tegel form Borsighafen to Tegelort and back again. On the Mighty Muddy Fraser you'd get nowhere at all if you were headed upstream, but the Havel is, of course, a bit sluggish.


And since the OP specifically asked for opinions :-): I don't quite understand why anyone would have a 30-odd foot steel boat on the Havel, unless the ultimate intent is to hit the Elbe and eventually do some real sailing. And while the electric setup will no doubt do the job within the landlocked confines of the Brandenburger marshes, It doesn't seem quite apposite for any real seafaring. For my money – Dänische Kroner — even the Baltic would be too much of a handful with so little duration on the auxilliary power


Sei das Glück mit dir :-)!


TrentePieds

[valhalla360]

Originally Posted by ZULU40

ought be about torque unless one intends to run flat out all the time
Significantly different power curves, where electric motors produce max torque at near zero RPM

I'll try to explain it for you.

It is about HP but you have to understand that just because a motor (gas, diesel, electric, turbine, doesn't matter) is rated at 20hp, doesn't mean it is always putting out 20hp.

HP = Torque * RPM

At low RPM, internal combustion engines (ICE) generally put out low torque.
Low Torque * Low RPM = Low HP.

At low RPM, an electric motor still can put out it's max torque.
Max Torque * Low RPM = Significantly higher HP compared the ICE

The result is for motors of the same HP rating, the electric will accelerate faster because it has more HP at low RPM.

But...

At ideal RPM (such as once you are up to cruising speed)

20HP Electric = 20HP Diesel = 20HP dolphins in harness

If one motor has less torque, it must by definition spin faster but if you have a desired RPM for the prop, you just run it thru gears to get the desired RPM.

Further reducing the impact of torque on a cruising boat:
- For a car to get maximum acceleration, the car must accelerate for the engine to spool up and get to that higher HP output.
- For a boat, the prop will slip in the water allowing the engine to spool up faster than the boat accelerates

So while nice, torque doesn't really come into play when selecting the desired HP for a cruising boat.

As previously stated, 20HP, while a little low, is still reasonable for a 35' boat but no it won't perform equivalent to a 50HP engine. 50HP was a bit overkill for most 35' boats, so a bit of a downgrade is reasonable. I would have suggested something on the 25-30hp range.

[valhalla360]

Originally Posted by TrentePieds

Quote: "(I speculate that in normal conditions i will need only a fraction of the available power, so that i will be going on batts for at least two hours and with the range extender around 8 hours)"

Quote: "Can't answer your questions but what would be interesting is a graph showing kw of power or amps, against speed and therefore range/time."


Interesting question, this. FWIW, using LWL = 30 ft; WLB = 10 ft.; moulded depth 3.5 ft; displ = 16,500lbs and a propeller RPM of 2,000, I find that a 3-bladed 14"D x 7"P prop will require:

.75 kW to attain 2 knots
1.5 kW to attain 3 knots
3.75 kW to attain 4 knots
7.5 kW to attain 5 knots
13.5 kW to attain 6 knots
21 kW to attain 7 knots
and
25 kW to attain hull speed of 7.5 knots.

The propeller doesn't care whence the power comes.

OP should therefore be able to get to 5.5 or 6 knots with the draw calculated by him. The remaining 1 or 2 knots to get to hull speed become MUCH more expensive, in fact THREE TIMES as expensive as the first 5 knots, and TWICE as expensinve as the first 6 knots.


Sei das Glück mit dir :-)!


TrentePieds

Good stuff but there are a couple of considerations:
- Those numbers assume 100% efficient system with perfectly clean hull ad prop. What is the actual draw? What about when you get some crud on the bottom?
- What about when you face a stiff headwind?


Out on a calm day, 20hp is probably plenty but 4kts going into a 20kt wind may need as much power as doing 7kts...then when the batteries go flat and the 5kw generator that might only be supplying 3.5-4.0kw might only get you 2-3kts...or less.

[Dockhead]

Originally Posted by ramblinrod

Ummm at 1C discharge, he is going to see no where near the .05C (20 hr A-hr rating).

Exactly, and this is the key point. There are a couple of erroneous calculations above, based on the 20 hour capacity.

We can't tell you how much power in kW/h you have at 1C -- you have to look at the discharge curves for your particular battery. But as a rough guide you will usually have no more than 50% or 60% of the 20 hour rated power.

Your specific batteries will have a table like this which will tell you:



So you won't get half an hour at full power -- you'll get more like 15 minutes, and that's only if the batteries are at 100%.

And full power is only 30kW, so may be needed in a wide range of conditions, much wider range than you will have needed all 75PS of the diesel.

LiFePo4 batteries have a Peukert factor of basically 1.0, and besides that can be discharged down to 10% or whatever, so they make a lot of sense for this kind of system. Might even be cheaper than AGM per kW/h of real usable capacity at C1 or C/2 or whatever.

I would want a much larger, heavy duty generator, if I had an electric boat.


I don't have experience with electric propulsion, so take this with a grain of salt, but it sure sounds to me like you don't have nearly enough battery capacity and/or the generator is too small. Unless you are planning to really sail all the time and use the motor only for emergencies or getting in and out of harbor. But in a river?? What if you have to get upstream? What if the wind is against you -- you can't be sure of being able to tack, where the river is narrow and/or there is traffic.

[Dockhead]

Originally Posted by ZULU40

ought be about torque unless one intends to run flat out all the time
Significantly different power curves, where electric motors produce max torque at near zero RPM

Keeping the boat moving is about power, not torque.

A lot of torque can accelerate you faster, IF you have a prop which lets you use it, but at the end of the day, it's power, not torque, that moves the boat, and one horsepower of diesel power equals one horsepower of electric power.

And the OP is using his original prop, so the low speed torque of his electric motor doesn't come into play at all.


EDIT: I would not have written this, if I had seen Valhalla's excellent post above, which goes vastly deeper into the subject. Sorry.

[TrentePieds]

Yes, Valhalla, your #23 is spot on, of course :-). My mutterings about the Tegelsee and the Havel were in recognition of just those deficiencies in the argument I made :-).

It is the propeller that drives the boat, and that has to be our departure point. An EM putting out a single liggle HP (.75 kW in a 100% efficient system) can turn a directly driven 14x7 prop, but what use is that? 7" of pitch at 100 RPM with 50% slip will take you the enormous distance of thirty feet in that minute, or 1,800 feet/hr. That is a whole third of a knot! To get the 2 knots in my table in #21 you would need a pitch of 6 x7 = 42"! Such a pitch is obviously nonsensical as the foil would stall at such an initial angle of attack.

It's the propeller that drives the boat, not the motor. and it's the hull parameters that determine how much HP (or kW) is required to attain a given speed. The motor "merely" has to supply the propeller with the HP required to drive the boat, and the propeller then has to be able to transmit that HP to the water at the speed you want to go, i.e. the "swept area" (the "disc") must be great enuff to transmit the required HP, and the pitch must be appropriate for the chosen speed. That's why variable pitch props were invented, but even in their case we are still left with the problem of "slip" which no naval architect or marine engineer has ever been able to get a satisfactory grip on. We just "assume" that it's "about" 50%.

That's why "propeller calculators" always leave a bit of a business opportunity for a "propeller adjuster" :-)

One way or another, you can never get away from the fact that energy has to be put into the propulsive system from an external source and then stored aboard till it is used since the whole objective of sailing is to be away from sources of energy, other than the original, unimproved "solar" power, and for the time being "energy dense" stuff like diesel fuel does a far better job of doing that than do agitated electrons. Diesel fuel is also far less likely to give you nasty surprises like fire. And it's easier to store! :-)

Cheers

TrentePieds

[Screwdiver]

Wow! Really learned a lot already. Thank You! and Mange Tak!

• General setup is considered sufficient within the limits of electric propulsion.
• Battery Size is way to small.
• The Danger of explosion is not imminent, but Batteries will not last long.
• Will publish my test drives.

As you guessed, this boat is not designed for the Havel. I hope, some day in a view years will take me on the barefoot route aimed for polynesia. Baghira is over 30 years old and was definitly cheap. The actual battery size (and the budget for it) is for testing, i hope to get more kwh for the same money in a few years. There will be an additional advantage: Free energy from the sun to move very slowly in extended calms.

In the many years when i sailed the mediterranean there was one (1) situation, where my new setup would not have done the task equivalent to the former diesel: Lowering tension on the anchor during a severe storm on bad anchor ground. On all other occaisons either my electro would help as well as the diesel or the diesel woulnd't help neither (esp. strong headwinds).

[Alan Mighty]

Nothing to do with your batteries, but have you considered your nervous system?



Read:


https://www.newscientist.com/article...gnetic-fields/

[valhalla360]

Originally Posted by Screwdiver

There will be an additional advantage: Free energy from the sun to move very slowly in extended calms.

Sorry but it's so little that it really doesn't justify claiming it as an advantage.

On a modest sized monohull, you will be lucky to get space for 500w of solar panels (possibly much less)

I'm sure you are aware you won't get 500w 24/7 out of the panels. General rule of thumb is you may get the EQUIVALENT of 4hrs at the rated output (early and late day, output will be much lower and at midday you may get 70-80% of the rating) or about 2kwh but in reality, with sails up and not being able to angle them ideally, you will be lucky to get 1.5kwh per day. Assuming house loads are a modest 0.5kwh per day. That leaves you around 1.0kwh for propulsion

Going off TrentePieds figures, that leaves you about 2 nautical miles per day at 3kts...assuming no headwind or waves which would leave you basically at a standstill.

Assuming a 3 week crossing, that's about 42 miles extra. A 5gal jug of diesel (seperate from the main tanks) will buy a similar diesel powered boat about the same range but at 4-5kts. Slow down to 3kts and it goes significantly further. Plus you can use that stored power any time or continuously during the voyage.

You could take the rig off and cover the whole deck area with panels and might get you up to 3-4kw of panels or about 12-15kwh per day. That would buy you about 10hrs at 3kts or about 30miles per day...far below typical day under sail.

Where electric power can make sense is if you only want to get in and out of port...a couple miles each way but are otherwise purely a sail vessel. In that case low speed is not a big issue nor is range plus if you are in port for several days, yes, solar panels can recharge the battery bank. But for actually making miles under power, it just doesn't work.

[Dockhead]

Originally Posted by valhalla360

Sorry but it's so little that it really doesn't justify claiming it as an advantage.

On a modest sized monohull, you will be lucky to get space for 500w of solar panels (possibly much less)

I'm sure you are aware you won't get 500w 24/7 out of the panels. General rule of thumb is you may get the EQUIVALENT of 4hrs at the rated output (early and late day, output will be much lower and at midday you may get 70-80% of the rating) or about 2kwh but in reality, with sails up and not being able to angle them ideally, you will be lucky to get 1.5kwh per day. Assuming house loads are a modest 0.5kwh per day. That leaves you around 1.0kwh for propulsion

Going off TrentePieds figures, that leaves you about 2 nautical miles per day at 3kts...assuming no headwind or waves which would leave you basically at a standstill.

Assuming a 3 week crossing, that's about 42 miles extra. A 5gal jug of diesel (seperate from the main tanks) will buy a similar diesel powered boat about the same range but at 4-5kts. Slow down to 3kts and it goes significantly further. Plus you can use that stored power any time or continuously during the voyage.

You could take the rig off and cover the whole deck area with panels and might get you up to 3-4kw of panels or about 12-15kwh per day. That would buy you about 10hrs at 3kts or about 30miles per day...far below typical day under sail.

Where electric power can make sense is if you only want to get in and out of port...a couple miles each way but are otherwise purely a sail vessel. In that case low speed is not a big issue nor is range plus if you are in port for several days, yes, solar panels can recharge the battery bank. But for actually making miles under power, it just doesn't work.

Hard to beat the power density of fossil fuel.

I agree with this analysis, but taking it one step further -- if you are a pure sail vessel only needing power in and out of port, then why bother with a shaft and large solar installation and inboard propulsion at all? Why not just use an outboard? Surely vastly cheaper, vastly lighter, less windage, simpler in every way, and at least as effective.

For a river, like the OP has, and canals, give me a reliable inboard diesel every day. For the South Pacific, you might not need any mechanical propulsion at all (the Pardeys did pretty well without it).