My Back-Of-The-Envelope Calculation

[dcs02d]

Hi all,

I am building a sort of solar sailboat. It's a small houseboat that will be all electric, including the outboard motor.




I'm hoping to first have someone review my 'Back-Of-The-Envelope Calculations' here in terms of batteries required with a given set of solar panels.

From there I'm hoping to get some advise on individual components etc.. But I thought I'd better make sure my high level understanding is correct prior to diving into the details.

I'm an old moron so please be kind if I've seriously miscalculated something!


Power requirements:
A powered cooler - Similar to a Dometic CFX3 45 - ~4300 Watt hours per day

Air Conditioning - Probably a Zero Breeze but there are other options. I figured 12 hours a day of run time to be safe but I think it will be far less than this with open doors and windows a bit off-shore with no bugs and a fan - ~1800 watt hours per day.

Misc stuff. This includes a tablet, led lights at night, a chart plotter while underway... 1200 watt hours per day.

A desalination device like the 12V Rainman Watermaker System. This is for toilet flushing, quick shower, and drinking water etc for 2 people.
- 1536 watts hours per day to make about 32 gallons of fresh water.

The motor - I'm planning for 2 hours of runtime per day. If emergency runtime is needed beyond that I am planning to have a portable gasoline powered generator on board and I'll be asking about that later... but for now planning on 2 hours of runtime is more than plenty for the trips I'll be doing off battery / solar. I hope I'm not breaking a rule with this link but this is the motor I'm looking at - https://www.elcomotoryachts.com/product/ep-20-electric-outboard/
17760 watt hours per day

Total - ~26596

I'm in Florida and planning to put 4 200 watt solar panels on the top. I'm planning for that to produce on average -
~5600 watts per day

So my battery capacity per day will need to be about 21,000 Watt hours per day

I was planning on 8 or 12 12V 206Ah LiFePO4 Lithium Iron Phosphate Battery that would be wired at 48Volts.

At a high level do these numbers seem at all close to reality?

Thanks for the help!
Don

 

[Alphacarina]

I don't see a mast, let alone any sails

How realistic is an air conditioning system that uses half as much energy as your refrigerator??

My wife and I have sailed for 15 years or so (34 footer) and then another 5 years on our little trawler (30 footer) and I can tell you we were averaging closer to 5 or 6 gallons of water usage per day, never anything close to 32 gallons - That's just not sustainable. Most boats flush the potty with seawater, be it fresh or salt and you take a 'Navy shower' every other day or so, using 1.5 to 2 gallons per person . . . . hopefully less once you get good at it. Wet yourself, turn off the water, then lather yourself and finally, quickly rinse yourself. Very few small boats make their own fresh water, as it's a very big energy hog to do so. A water maker on anything less than a 40 footer is pretty rare, so you're living on what's stored in your tanks and you make that last until another port of call

I think the 'reality' is, that your plan is consuming 26Kw per day and your solar provides a little over 5Kw (on a good day) which means you use the boat for one day and then wait 5 or 6 more days for your solar to get the battery fully recharged so you can use it for another one day, and then you wait 5 more days . . . .

When we were cruising Florida from Pensacola to the Keys and up to Jacksonville, we ran across a family on an old 40 foot ferro-cement sailboat that the guy had removed the masts from and covered every square foot of it with solar panels (which provided shade to the entire deck) and dozens of golf cart batteries and he said after 2 or 3 days of good sun, they could motor about 15 miles on the energy stored in the batteries. They were quite happy with the old boat's performance too. They were 'cruising' just like we were, but not very fast and not very far, week by week, but they were having a great time

Don

 

[time2roll]

Forget A/C
Cut water use by 70%
Misc cut in half
Skip the motor except for emergency or for in port docking etc.

I assume that is the easy part. Raising the sails will be the hard part.

I do hope this works for you.

 

[Forbisher]

Power requirements:
A powered cooler - Similar to a Dometic CFX3 45 - ~4300 Watt hours per day

Air Conditioning - Probably a Zero Breeze but there are other options. I figured 12 hours a day of run time to be safe but I think it will be far less than this with open doors and windows a bit off-shore with no bugs and a fan - ~1800 watt hours per day.

Misc stuff. This includes a tablet, led lights at night, a chart plotter while underway... 1200 watt hours per day.

A desalination device like the 12V Rainman Watermaker System. This is for toilet flushing, quick shower, and drinking water etc for 2 people.
- 1536 watts hours per day to make about 32 gallons of fresh water.

The motor - I'm planning for 2 hours of runtime per day. If emergency runtime is needed beyond that I am planning to have a portable gasoline powered generator on board and I'll be asking about that later... but for now planning on 2 hours of runtime is more than plenty for the trips I'll be doing off battery / solar. I hope I'm not breaking a rule with this link but this is the motor I'm looking at - https://www.elcomotoryachts.com/product/ep-20-electric-outboard/
17760 watt hours per day

Total - ~26596

I'm in Florida and planning to put 4 200 watt solar panels on the top. I'm planning for that to produce on average -
~5600 watts per day

So my battery capacity per day will need to be about 21,000 Watt hours per day

I was planning on 8 or 12 12V 206Ah LiFePO4 Lithium Iron Phosphate Battery that would be wired at 48Volts.

At a high level do these numbers seem at all close to reality?

Thanks for the help!
Don

Your 4300 watt hour power calculation on your fridge is Way off.
You dont say that the fridge is 12V / 24V but you have a 48V battery so are you running it on 120V?
My Iceco 45litre 12V / 120V fridge uses 250 watt hours per day so 4300 Wh is crazzeee.

Showing how you came to these results would be helpful.
I did not check any of your other results.

8 or 12 206A 12V batteries is a big difference.

 

[Alphacarina]

Forget A/C
Cut water use by 70%
Misc cut in half
Skip the motor except for emergency or for in port docking etc.

I assume that is the easy part. Raising the sails will be the hard part.

I do hope this works for you.

I agree - The big secret to all electric cruising would be . . . . to start with a sailboat, so the only motor power needed would be brief, getting into and out of port. When the wind doesn't blow, you take a day off and enjoy the anchorage. Even then, doing it all on only 800 watts of solar would be a real challenge

Don

 

[mrzed001]

Total - ~26596

Because LFP and you should use it between 10-90% , or even better 20-80% to get a bigger cycle life, so first you need to multiply it by 1,25-1,6.
So it is 33245 - 42553 Wh

I'm in Florida and planning to put 4 200 watt solar panels on the top. I'm planning for that to produce on average -
~5600 watts per day

With 4,2 kWp PV you can get 15-20 kWh/day if you sail near Florida or 8-15kWh near Portland
But where ? A 450Wp solar panel is 2x1 meter (6,6x3,3 ft). And for 4200Wp you need 10. That is 20 square meter (218 square ft).
I think there is max space to 2-4 panel on that boat. That is 0,9-1,8 kWp PV max. Can produce 4-8 kWh/day max

But it can work (charging every day like a Tesla). Here are leaks where only electric boats are allowed.
Like this 125 kWh (600 kg battery) boat with 150 kW electric motor (not on max power 2-3 hour):

 

[dcs02d]

This is all very helpful. I'll go back and re-calculate and include details.

It's not really a sail boat. There will be no sails.. it's a inland and coastal tiny little houseboat. I'd love to run an electric motor but it seems it's not at all practical.

I probably need to just use a good ol fashioned Gas outboard for travel and focus on enough power to just run the house. I'm also planning on a propane tank for on demand hot water.

 

[DIYSteve]

There are ways to cut your power expectations down a lot. Using 12v fans that run through a channel of 6-8 12v Peltier cells with the hot side being blown outside would help cool things down. Peltier cells can make a decent fridge so why not a mild air conditioner.
There are many ways to convert things to DC power rather than losing power in the process of inverting DC to AC. This would save heaps of power. Also a good rule of thumb is to have an extra 30% more than you expect.

 

[Alphacarina]

I'm also planning on a propane tank for on demand hot water.

Warm water for a shower is easy to come by in tropical climates. Consider that every gallon of water aboard your boat will be at essentially 'room temperature' already, so you don't need to heat it much more for a shower or doing dishes. Lay a shower bag on the deck in the sun for 15 or 20 minutes and you'll have a couple gallons of pretty hot water to use for a shower without expending any energy. Hang it on a hook and take your shower or wash your dishes

Amazon.com : PGYFIS Camping Solar Shower Bag Portable Solar Heating 5 Gallons/20L with On-Off Switchable Shower Head for Outdoor Traveling Hiking (Black) : Sports & Outdoors

Amazon.com : PGYFIS Camping Solar Shower Bag Portable Solar Heating 5 Gallons/20L with On-Off Switchable Shower Head for Outdoor Traveling Hiking (Black) : Sports & Outdoors

www.amazon.com

Don