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Old 29-07-2020, 15:02   #1
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Could someone check my math please?

We are kitting up the boat for a North Atlantic circuit and I am now getting around to the electrical system.

My back of the envelope calculation is that under sail we will use about 150-180 Ah a day. The big energy hog is the hydraulic autopilot, followed by the fridge, radar,, the twin plotters, and on down the line to charging phones.

At anchor my calculation is only 40Ah as then it’s just the fridge (which is a very efficient 12v compressor with cold plate) LED lights and phone charging.

We presently have 375 Ah of FLA for the house bank and 75 Ah FLA for the starter all charged off an internally regulated 55A alternator on our 90 hp diesel.

Our program aside from the odd extended trip is mainly anchorage hopping in daylight hours.

After reading many threads on this forum on this topic my plan is as follows:

1. Increase the domestic bank to 600Ah.

2. Put a watt and sea 600w pod on the hull.

3. Put a 90W solar panel on an existing pole on the transom.

The solar panel supposedly manages 4.6A at max output so I calculate it will give about 30Ah a day on average.

After contacting Watt and Sea they reckon that at our average cruising speeds a pod will give about 20-30A. Our alternator probably averages 30. So let us say that between them they average 25A. I’m reckoning that on a circuit the average trip length will be about 10 hrs. So we will make about 250 Ah enroute.

On longer trips the pod will obviously meet all our needs but in the anchorage hoping scenario it will only meet the needs of the trip and then maybe a day and a half at anchor. Lets say each stay at anchor is 4 days on average.
Then we need 5 days of anchor supply (including portion of trip day spent not sailing) plus around 80Ah for the trip. Rounding up that is 300Ah in total. The Watt and sea plus alternator give us 250 of that. The solar over four days gives us a further 120Ah . That gives us 370 Ah of generation to meet 300 Ah of expenditure. Further our batteries should never go below about 85% charge as they will be fully charge on arrival and only bleeding about 10Ah a day after that over what is being put back by the 90W panel.

My questions are as follows.

Are my usage numbers realistic for a North Atlantic circuit?

Are my assumptions about sailing profile realistic for such a trip?

Are my generation numbers realistic?

How many Ah are usable in a 600Ah FLA battery bank?

Many thanks for any and all input.

Best from Na Mara
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Old 29-07-2020, 15:16   #2
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Re: Could someone check my math please?

I believe your 90W solar estimate of 30Ah/day is optimistic. If the panel can be rotated toward the sun and unshaded all the time then perhaps 30Ah/day. Don't know about the Watt and Sea but I would install a windvane and reduce my energy budget.
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Old 29-07-2020, 15:46   #3
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Re: Could someone check my math please?

We ran on and off shore power over seven years on the US West Coast from San Diego to the PNW. We had a wind gennie, diesel gennie and 55A and 125A alternators.

We updated from 240W of solar to 1050W and from 400Ahr FLA to to 400Ahr Carbon Foam batteries. We also lived and worked aboard during this time. I'm an engineer so logged tons of data.

Our findings in summary are:

Solar accounted for 96% of power generation except for the PNW where the generator was dominant.

Solar calculated max power and summer actual matched with good MPPT controllers and flat solar panels.

Solar generation in winter drops to 20% of peak capacity and this lasts for days. Mainly due to sun angle and daylight hours and amount of cloud cover.

Wind generation was rarely useful unless in open water.

Our carbon foam batteries had double the usable capacity of FLA. So the 400Ahr bank acted like an 800Ahr FLA bank. Very good value for money. But you can't charge them as quick as Lithium so you need to reset MPPT controllers off of early float. We heated our hot water over midday.

Multistage regulators, on your alternator / generator tend to go to float too quickly leaving your bank undercharged. They are not designed for low resistance batteries like Lithium and Carbon Foam. We heated hot water off our inverter and this would reset the so called smart chargers or regulators.

Victron MPPT controllers improved solar charge by up to 60% over PWM cheap solar controllers especially on cloudy days.

We halved parasitic loads by moving to direct 12V DC. Any AC home appliances are power hogs.

Calculations are easy. Log data and you'll see what the real world gives you.

Your 55A alternator will take forever to charge your bank.

The towed generator, we had an old variant, are a waste of money unless you are moving all the time. Solar is the best value for generation. You can never have enough. The maintenance is zero for solar.

You will use more than you calculate and generate less than you want.
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I believe your 90W solar estimate of 30Ah/day is optimistic. If the panel can be rotated toward the sun and unshaded all the time then perhaps 30Ah/day. Don't know about the Watt and Sea but I would install a windvane and reduce my energy budget.
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Old 29-07-2020, 15:58   #4
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Re: Could someone check my math please?

I agree with leftbrain. You generator will almost certainly not produce 30AH - it is a 55 amp generator and more than likely it will quickly ramp down to something like 20 amps. You have a 90 horse engine - swap the generator out for a 160 or even 200Amp

Solar is the only way to go - we have 700 watts (two big sunpower panels) and an outback MPPT controller. That gives us most of the power we need.

I'd look closely at your projected usage figures. If the seas are rough - your autopilot will use over 10AH. Your fridge will probably average out at 2-3AH. Radar,will average 3-5. your chart plotters will draw 5 each (although you can turn one off or put them to sleep).

I guessing you should be thinking a bit over 200per day
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Old 29-07-2020, 23:31   #5
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Re: Could someone check my math please?

Thanks all for the very valuable input!

Aside from a difference of opinion about the usefulness of wind, which we arenít going to do due to noise in the master cabin, you all pretty much agree that my calculation on the solar panel is optimistic (it should be between 8Ah a day in winter and 40 in the summer) and that my figures for usage are a little low (I forgot about winter needs and generation). Say 200Ah a day for sailing worst case and 100 Ah a day at anchor in colder climes (I forgot about forced air heating).

I had already figured out that avoiding any need for inverters was the way to go but itís nice to hear real world experience confirming that. The only real sacrifice this entails for us is hot water for showering is limited to a day or two after we run the engine or to minimal 5 liter showers using a camping shower heated by the sun or water from the stove. We can live with that.

Beyond fitting a refleks stove or similar (I have no where to put one of these) we arenít going to be able to reduce our at anchor usage much and at sail usage is pretty fixed.

So going back through the numbers after what has been said. I need to distinguish two scenarios and cover both. 1 winter cruising in the northern latitudes and 2. summer cruising around the equator. In the latter case my figures and strategy work out. Our usage will be about 300Ah+-50 between trips and we will meet this with the setup proposed. However, during winter in the North we will need more like 600Ah between trips and we will only generate 300Ah on the proposed setup. In this latter scenario, adding solar wonít help much according to leftbrain

To cover life on the diesel heater when we go north we need about 100 Ah a day at anchor.

My choices to meet that need seem to be.

1 fit a diesel generator.
2 fit a wind generator (itís windy in the winter months in the north but even so that might not be enough)
3 add a second alternator to the engine ( loosing 4 hp to a 160 Amp alternator out of 90 ainít going to kill me as carstenb points out)
4 Fit another pod.

Option 1 is a non starter. I have space in the engine bay for a generator so it would be relatively quiet, but I donít see the point in lugging around two ICEís when the existing one is man enough.

Iím loathed to do option two as too many people have shared leftbrains opinion that they donít give you much bang for buck or for that matter the noise and nuisance they represent.

That leaves upgrading the alternator so I can put 100-150 amps back into the bank when running the engine and or doubling the pods.

Doubling the pods would let me generate around 500Ah on a normal 10hr hop. Adding an alternator would also let me generate about 500Ah on a typical 10 hr trip in conjunction with the single pod. Either would meet the trip and anchorage needs even on the worst case if we can stow all that energy. Iím minded to do a second pod rather than the second alternator but that will cost 2-3 times the alternator.

Either way I will need about 400 Ah of usable electrical stowage as with this set up we will be generating hardly anything when stopped In northern climes. My 600Ah bank wonít be big enough. Iíll need to go to 900Ah by my reckoning.

So second alternator or second watt and sea pod? Which would you pick and why?
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Old 30-07-2020, 01:11   #6
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Re: Could someone check my math please?

I have sailed on and off for weeks at a time with friends of mine in their previous 46 footer. The skipper is an old marine pro with decades of very extensive and eclectic nautical experience including 2 circumnavs. Here is a quick rundown of his then electrical scheme, to the best of my recollection.

The engine - 75hp Yanmar.

Alternator - don't recall but probably something substantial, at least 110A or higher.

Batteries - 8 golf cart 6v Trojans 115, (both in parallel and in series). They were hand downs about 3-4 years old when he got them from a friend for free. They lasted on my friend's boat another at least 5 or 6 years, may be 7, when he handed the best 6 of them to me and they lasted on my boat, albeit in light seasonal use, another 2-3 Summers, giving me about 50-75% capacity. I just replaced them this May when they were at least 12 years old. Amazing. But I digress.

Charger - Xantrex 60.

Solar - 6X150W panels flat mounted on the hard top.

They lived aboard but mostly on anchor so no shore power for extended periods of time. Most of the power was hogged by robust tiller autopilot when sailing, 50" flat screen TV, usually in the AM for news and in the PM for entertainment, 2 fridges and various kitchen appliances. I recall that only occasionally, after hours of TV watching and on cloudy days at that was there a need to run the engine. Most of the time the batteries never got below 12.5v. I know that since the Xantrex (Schneider) charge controller with the voltage display was in my cabin which I had to use to plug in my CPAP machine.

This Spring I did go back to 3 12v 27 batteries only because 6X6v set up at 65#s per battery was too much weight for my 36 footer and the logistics of placing them where they least affected my trim were daunting. Otherwise if one's on a budget and has room either Trojans or Decas are the best bang for the buck in the long run IMO.
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Old 30-07-2020, 06:37   #7
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Re: Could someone check my math please?

You need to be aware that in daily use - FLA batteries shouldn't be run down more than about 25-30% - so your 600AH is, in reality, only 175 aH at best. AGM batteries will let you run them down to say 35% maybe 40%(but not every day).

There are some new batteries, Firefly, i believe that claim you can run them down a lot further but I'm not personally familiar with them. Lith-iron batteries will let you run them way down - much more bang for your buck.

There is nothing wrong with having an inverter (we have a 2000W) as long as you use it intelligently.

Were it me - I'd ramp up as much solar as I could and add the big generator. Solar requires almost no maintenance. we've sailed around in Scotland and now in the PNW and the solar charges our system just fine. ON really cloudy days, we have to fire up the engine for about a half hour to an hour. If you use the engine you also get hot water.
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Old 30-07-2020, 06:51   #8
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Re: Could someone check my math please?

"So second alternator or second watt and sea pod? Which would you pick and why?"

Definitely not a second watt & sea pod. Hydrogeneration only works in motion and at decent cruising speeds. If you are motoring while it does "work" it really is a super inefficient motor driven alternator at that point. Hydrogeneration is a good way to diversify your energy portfolio but in terms of energy per dollar it is by far the most expensive. I would upgrade the alternator not even sure you need the added cost/complexity of two and add more solar.

Solar works when you are anchored, it works under sail, it works when motoring. Compared to anything it is dirt cheap. I know you said you are going to put in 90W of solar but I would start by revisiting that and see if you can add 150W or 200W or 300W of solar. Over a year of mixed motoring, sailing, and anchoring a 200W solar system will outproduce a 600W hydrogenerator at a small fraction of the cost.

BTW did I mention solar.
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Old 30-07-2020, 06:57   #9
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Re: Could someone check my math please?

That’s all really good advice but my boat doesn’t lend itself to lots of solar. Where would we put them where they won’t be ugly, expensive, in the way or all three?

The 90 w panel would go over the radar dome on the pole between the antennae.
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Old 30-07-2020, 08:35   #10
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Re: Could someone check my math please?

Quote:
Originally Posted by Na Mara View Post
Thatís all really good advice but my boat doesnít lend itself to lots of solar. Where would we put them where they wonít be ugly, expensive, in the way or all three?

The 90 w panel would go over the radar dome on the pole between the antennae.
To add solar you could add a pair of davits and it would be very easy to install a 350W panel athwartship. I do think that your radar dome and pole would cast a shadow on the solar panel and reduce it's efficiency significantly. A 350W panel with a MPPT controller could realistically give you 90Ah/day.
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Old 30-07-2020, 09:14   #11
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Re: Could someone check my math please?

much bigger alternator with multi-stage smart charger.
maximize battery capacity.
move radar to front of mast.
install stern arch
maximize solar panels on arch, wire in parallel to mitigate shading.
mount solar panel on cabin house top between hatches.
towed generator will slow you down, spend the money on more batteries.
all batteries (some more than others) have a middle range where they will accept their maximum charge, try to build a system that will be able to meet that demand. As others have suggested run engine in the morning to supply bulk charge and heat water, use solar to finish charge during the day.
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Old 30-07-2020, 09:56   #12
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Re: Could someone check my math please?

Unless you have a money tree, solar is the most cost effective solution. Solar panels are $1 per watt. The Watt n sea 600W is about $4,000!!! Almost 7 times more, That's a toy for the rich! also, makes for a paperweight at anchor. you have space on your dodger/cabin and davits or an arch would be nice to lift the dink and motor,... and mount solar. You'd still spend less $$ than the watt n sea and you'd have those nice to have supports... what are you going to do with the dink? Not tow it all the time I hope. What a mess and PITA that becomes!
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Old 30-07-2020, 10:06   #13
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Re: Could someone check my math please?

You say your total usage is 40aH per day at anchor. This is about 1/2 of what I have experienced for refrigeration only, with a built-in system of refrigeration (with freezer). If you have measured the usage that is a good first step, but if you did the measurements in a temperate location then better add a healthy increase for tropical conditions.
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Old 30-07-2020, 10:28   #14
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Re: Could someone check my math please?

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Originally Posted by Na Mara View Post
That’s all really good advice but my boat doesn’t lend itself to lots of solar. Where would we put them where they won’t be ugly, expensive, in the way or all three?

The 90 w panel would go over the radar dome on the pole between the antennae.
Looking at your pictures, "ugly" is in the eye of the beholder. (Edit here. That didn't sound right. What I was "implying" was that what I think would look awesome/fine/good on that MIGHT not be what you think, and wouldn't be what my wife thinks etc. NOT trying to say anything about your boat, cause I actually really like it. Also love the look of a non cluttered boat, but the function trumps that non cluttered look. Sorry if it sounded insulting.)
Personally, I think function trumps "ugly" EVERY SINGLE TIME. But, my wife's not reading, soooo.
I would put a flexible panel on the dodger roof, maybe two. Yea, shade, BUT you know what? 50% of 200-400 watts is way better than 0 watts.
If you don't want to do a large davit assembly over the transom, you could easily toss two "wings" on the side railings that fold down when needed. 300wX2 (one on each side) and then two mppt controllers. one for flexible, 1 for wings, and you're up around 800-1000 watts of solar for 1100$ ish.
The boat we're getting has 320w on it now on a large arch. I'm going to move those 160w panels to the wing method, and then put 300w panels up there, then add two 150-200w flexible panels, one that slides from the hard dodger for more shade and one just on the dodger. It has a wind generator, but, "meh" I figure 1000w+ will be sufficient to not have to think about it as much.
"math" is easier when you just hit it with a big hammer. Solar has gotten cheap and easy enough to just pad the numbers quite a bit. Based on the math, and short trial, we "should" be good with closer to 600w, but padding it will mean we can convert the dinghy to electric and charge it easily.
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Old 30-07-2020, 10:43   #15
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Re: Could someone check my math please?

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Thatís all really good advice but my boat doesnít lend itself to lots of solar. Where would we put them where they wonít be ugly, expensive, in the way or all three?

The 90 w panel would go over the radar dome on the pole between the antennae.

Everything is a compromise and you got to boat your own boat. Personally I would look towards and arch of flexible panels on the dodger.

Maybe in your case it won't work but I would give it a LOT of thought before rejecting it. Everything will be significantly easier with 300W of solar instead of 90W.
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