Buying Fireflys and Solbian - can someone check my math?

[Cthoops]

We're ready to pull the trigger and purchase two Firefly batteries (yes, I realize there is a significant backlog) and Solbian solar panels. Before we do it, I wanted to check my work. I tried to be overly generous in usage so as not to cut it too close, and we have a very basic boat - i.e. no electric windlass, no refrigerator:

Amp calc for 24 hours at anchor:
Interior LEDs - 4 lights at .3 amp hours each x 3 hours = 3.6
LED Anchor light - assuming 12 hours on/12 hours off - 3.6
Nature's Head fan - 24 hours - 3.84
Caframo fans - 3 fans at .54 amp hours each x 12 hours max - 19.44
Miscellaneous charging for computer, iPad, etc. - 5 amps
total = 35.48

Amp calc for 24 hours at passage:
Interior LEDs - 4 lights at .3 amp hours each x 3 hours = 3.6
Nature's Head fan - 24 hours - 3.84
Caframo fans - 2 fans at .54 amp hours each x 24 hours max - 25.92
Miscellaneous charing - 5 amps
chartplotter - .99 x 12 hours (assume standby for the other 12) - 11.88
Wind instrument/display - 1 amp x 24 hours = 24
LED Nav lights - .6 x 12 hours = 7.2
Tri color LED - .3 x 12 = 3.6
VHF - .55 on standby x 24 = 13.2
VHF AIS receiver - .9 x 24 = 21.6
compass light - .5 x 12 hours = 6
total = 125.84

We are planning to purchase 2 Firefly batteries which will give us a total of 220 ah, so 110 ah at 50% (which they are reputed to easily take).

For solar panels we were thinking a Solbian sxp154 which gives 36 amps (at 3 hours/day) and a SXp60 which gives 14 amps (at 3 hours/day). On sunny days this would give us 50/day which is more than enough to cover us at anchor. We would obviously fall short on passages but we'll have a 70 amp alternator so the engine would supplement. I did do calculations for hard panels (Renogy as a sample), but the cost difference for equal output was only $500 with the Solbians being 45 pounds lighter which is a consideration on our Bristol 29.9.

For anyone who has made it this far, thank you. I do wish that we could cut down the usage on passages, but I don't see how unless my estimated usage hours are way off.

Do you see any issues with my planned usage and output assumptions?

[dwedeking2]

I would think fan usage while under sail would be minimal. I never felt the need to run fans unless I was becalmed.

[Suijin]

No fridge?

You’ll spend far more time at anchor than on passage. And on passage you can bulk charge with the engine for an hour each morning and be fine. Design for time on the hook.

Ideally you’d put off the upgrade and get some practical experience on the boat with your usage patterns and energy needs. It’s always better to design based on real world experience than a theoretical budget.

[w32honu]

Have to admit right up front that I am no expert. But I would guess that you can reduce your on passage amperage use significantly.

Looks like you subscribe to the simple and basic approach with regard to systems, which I completely agree with....

Reconsider your calculation for the Nav lights and the Tri light. You have them both listed but really the Tri light is the only one in play. Nav lights will typically burn with the engine running and a steaming light which takes it out of the equation. The Tri light serves as your nav lights while sailing. In either case it will be one or the other but not both at the same time.

Fans: same scenario as above. My guess is that on passage with a nice breeze you could cut down the usage of any fans significantly.

Maybe take another look at the usage of the VHF and AIS. Are they combined?

Wind Instrument: This is just my opinion and others may not agree. Consider your wind instrument. I would guess that if you are on the edge with the amount of amperage draw you could eliminate that all together. A "windex" at the mast head and or a tell tail or two on your shrouds will do the trick with regard to wind direction. A wet finger held high and or a cheek or two will say plenty with regard to wind speed.

My guess is that with your boat set up as you describe you can get your on passage usage down significantly.

Just a guess....

[Cthoops]

Originally Posted by dwedeking2

I would think fan usage while under sail would be minimal. I never felt the need to run fans unless I was becalmed.

Yes, I wasn't sure about how stuffy it would get down below with the hatches closed, but I suspect I could decrease the fan estimate and be fine. Thanks - the real world perspective is helpful.

[Cthoops]

Originally Posted by Suijin

No fridge?

You’ll spend far more time at anchor than on passage. And on passage you can bulk charge with the engine for an hour each morning and be fine. Design for time on the hook.

Ideally you’d put off the upgrade and get some practical experience on the boat with your usage patterns and energy needs. It’s always better to design based on real world experience than a theoretical budget.

Yes, we may end up changing our mind about the fridge but with the way we eat I don't really think we'll miss it. We thought we'd start without it and add it later if necessary.

We don't have any real world cruising experience with the exception of several two week trips while on vacation, so I'm basing things like lights on that. It's not ideal, but we'd rather upgrade now while it's easier.

[Cthoops]

Originally Posted by w32honu

Have to admit right up front that I am no expert. But I would guess that you can reduce your on passage amperage use significantly.

Looks like you subscribe to the simple and basic approach with regard to systems, which I completely agree with....

Reconsider your calculation for the Nav lights and the Tri light. You have them both listed but really the Tri light is the only one in play. Nav lights will typically burn with the engine running and a steaming light which takes it out of the equation. The Tri light serves as your nav lights while sailing. In either case it will be one or the other but not both at the same time.

Fans: same scenario as above. My guess is that on passage with a nice breeze you could cut down the usage of any fans significantly.

Maybe take another look at the usage of the VHF and AIS. Are they combined?

Wind Instrument: This is just my opinion and others may not agree. Consider your wind instrument. I would guess that if you are on the edge with the amount of amperage draw you could eliminate that all together. A "windex" at the mast head and or a tell tail or two on your shrouds will do the trick with regard to wind direction. A wet finger held high and or a cheek or two will say plenty with regard to wind speed.

My guess is that with your boat set up as you describe you can get your on passage usage down significantly.

Just a guess....

Simple and basic definitely works for us. We like to keep the fun-to-suck ratio appropriate.

Good point on the nav lights and the tri light. I'll make an adjustment for that. The VHF and AIS are combined so unless I misread the specs I think it is what it is, and I'll make an adjustment for the fans based on feedback so far.

We could certainly reduce the amount of time the wind instrument is on during passages, so that would help. The display is an all-in-one but the chartplotter will also show speed and depth so turning the display off wouldn't be a big deal.

Thanks for the suggestions! They are helpful.

[Dave22q]

if your AIS and VHF share an antenna using a splitter you are correct in combining both but in effect your AIS will be receiving whenever a target is in range. When you are near commercial routes that will be most of the time and your draw will be much greater than is less crowded waters.

[Nickodemus]

Your post does not say where you plan on cruising -- but I'm guessing to warmer climates. Just wanted to let you know that if you are planning on keeping things in an icebox (since you indicate no fridge), that the cheapest bag of ice we found in the Bahamas was $6 a bag, and blocks are very difficult to come by. You can spend a lot of money very quickly that way. Plus you won't always find it easily - most of the time it's only at the major marinas - they don't always sell ice at the few and far between grocery stores.

[Rapanui]

I don't know what Wind instrument you have but I would expect something in the order of 100-200 mA draw with illumination not 1A. Also the compass illumination would probably blind the helmsman at 0.5A (6 Watts!). Even a filament bulb should be of the order of 100mA. So the good news is that you have conservatively estimated the current draw, which in actuality should be a bit less.
I notice you have not identified an autopilot. If you do intend to do 24 hour + passages, then you will probably find you need one, particularly short handed. It frees up the on watch to sail the boat and navigate better while being better rested. Just my 4p worth.

[guyrj33]

If you were more precise with your units, I think your calculations would make more sense.
for example
Interior LEDs - 4 lights at .3 amp hours each x 3 hours = 3.6
rewritten as
Interior lights LEDs - 4 x .3Amps each x 3Hours = 3.6AAH or Amp Hours.

Your estimates seemed OK to me and while I'm sure firefly batteries really are very capable with a 24 hour load consuming 50% of your capacity I think you're cutting it a little close. I suggest a third battery.

The solar panels will be a great source of charge when the boat is lightly used and will reduce the current the batteries need to supply when the boat is being more heavily used. If you use 125AH per day and have a 70A alternator I suspect you'll end up running your engine close to 2 hours a day to fully charge your batteries each day. It will depend on your use patterns and the sun.

With a third battery you wouldn't be discharged as deeply and may be slightly more efficient recharging with the alternator, extra capacity will also increase the solar panels contribution to the stored energy. Meaning you could leave the batteries at 90% charged and let the panels finish the charging for you without worrying about discharging the batteries too far.

We have 2 FLA Golf Cart batteries in our house bank with a capacity of 225AH and similar loads to what you have and we've rarely needed more capacity. But we don't typically sail for more than 12-14 hours at a stretch.

[Cthoops]

Originally Posted by Dave22q

if your AIS and VHF share an antenna using a splitter you are correct in combining both but in effect your AIS will be receiving whenever a target is in range. When you are near commercial routes that will be most of the time and your draw will be much greater than is less crowded waters.

Excellent point - thank you!

Originally Posted by Nickodemus

Your post does not say where you plan on cruising -- but I'm guessing to warmer climates. Just wanted to let you know that if you are planning on keeping things in an icebox (since you indicate no fridge), that the cheapest bag of ice we found in the Bahamas was $6 a bag, and blocks are very difficult to come by. You can spend a lot of money very quickly that way. Plus you won't always find it easily - most of the time it's only at the major marinas - they don't always sell ice at the few and far between grocery stores.

Yes, we're heading down to the Caribbean and then we'll see where we go from there. Thanks for the info on the cost of ice in the Bahamas. We'll see how it goes and add a refrigerator later if we feel we need one.

Originally Posted by Rapanui

I don't know what Wind instrument you have but I would expect something in the order of 100-200 mA draw with illumination not 1A. Also the compass illumination would probably blind the helmsman at 0.5A (6 Watts!). Even a filament bulb should be of the order of 100mA. So the good news is that you have conservatively estimated the current draw, which in actuality should be a bit less.
I notice you have not identified an autopilot. If you do intend to do 24 hour + passages, then you will probably find you need one, particularly short handed. It frees up the on watch to sail the boat and navigate better while being better rested. Just my 4p worth.

The wind instrument calculation also includes the display (B&G) but I'll double check to make sure that I've got the right numbers on that one. I had to guess on the compass light because I couldn't find any info on it. It's a filament bulb so if it's less than estimated, that's great.

We are going to have a windvane but will likely add a small tillerpilot for when the wind is too light for the vane, which I forgot about. Thanks for the reminder.

Originally Posted by guyrj33

Your estimates seemed OK to me and while I'm sure firefly batteries really are very capable with a 24 hour load consuming 50% of your capacity I think you're cutting it a little close. I suggest a third battery.

The solar panels will be a great source of charge when the boat is lightly used and will reduce the current the batteries need to supply when the boat is being more heavily used. If you use 125AH per day and have a 70A alternator I suspect you'll end up running your engine close to 2 hours a day to fully charge your batteries each day. It will depend on your use patterns and the sun.

With a third battery you wouldn't be discharged as deeply and may be slightly more efficient recharging with the alternator, extra capacity will also increase the solar panels contribution to the stored energy. Meaning you could leave the batteries at 90% charged and let the panels finish the charging for you without worrying about discharging the batteries too far.

We have 2 FLA Golf Cart batteries in our house bank with a capacity of 225AH and similar loads to what you have and we've rarely needed more capacity. But we don't typically sail for more than 12-14 hours at a stretch.

Thanks for the suggestion. We'd like to avoid a third battery if we can, and it looks like not needing one might be more likely when I make the adjustments to the passage calculations based on everyone's comments. But you're right, we don't want to cut it too close so we'll have to see.

[lordgeoff]

Originally Posted by Nickodemus

Your post does not say where you plan on cruising -- but I'm guessing to warmer climates. Just wanted to let you know that if you are planning on keeping things in an icebox (since you indicate no fridge), that the cheapest bag of ice we found in the Bahamas was $6 a bag, and blocks are very difficult to come by. You can spend a lot of money very quickly that way. Plus you won't always find it easily - most of the time it's only at the major marinas - they don't always sell ice at the few and far between grocery stores.

A €50 12V fridge with a cigarette lighter socket from Lidl will keep a few things cool for you........ICE is a PITA.

[Cthoops]

Originally Posted by Cthoops

Amp calc for 24 hours at passage:
Interior LEDs - 4 lights at .3 amp hours each x 3 hours = 3.6
Nature's Head fan - 24 hours - 3.84
Caframo fans - 2 fans at .54 amp hours each x 24 hours max - 25.92
Miscellaneous charing - 5 amps
chartplotter - .99 x 12 hours (assume standby for the other 12) - 11.88
Wind instrument/display - 1 amp x 24 hours = 24 should be 3.6
LED Nav lights - .6 x 12 hours = 7.2 the engine will be on when in use
Tri color LED - .3 x 12 = 3.6
VHF - .55 on standby x 24 = 13.2
VHF AIS receiver - .9 x 24 = 21.6
compass light - .5 x 12 hours = 6
total = 125.84 98.24

Thanks to Rapanui for the comment on the amps for the wind instrument/display. I found a big error in my original calculation which brings it down to 3.6 amps vs. the 24 that I had. Between that and w32honu's comment on the LED Nav lights vs. tri-color, I was able to bring the 24 hour passage calculation down to 98.24 before making any adjustments for the Caframo fan useage or the AIS receiver not always receiving. The numbers are looking a lot better. I'd edit my original post, but I can't anymore so I changed it in the quote above.

I really appreciate the comments I've had on this. I wouldn't have caught these errors otherwise.

[Kelkara]

I'm going to query the AIS too - if it is one of the combined VHF/AIS receiver units.

eg. the GX2200 advertises:
standby: 0.55A
receive: (full volume) 0.9A
transmit: 5A (High) 1A (low)

The extra power needed for receive is to generate all the noise ... In standby the receiver is still scanning all your priority channels, powering the display etc ... the tiny amount of digital electronics used to process the AIS signal will be trivial.

As pointed out your compass light is way over estimated probably closer to 0.1A.

You probably won't use the cabin lights as much on passage ... the off-watch crew will probably either be sleeping, or trying to preserve night-vision.