Need help deciding an Inverter

fc3 said:

That makes no sense. 4 golf cart batteries weigh about 280 pounds. A Westerbeke 3kw generator weighs 165. Why would you put so much weight into the stern of such a little boat? The foot print about 11 X 28 is close to the Westerbeke's 13 X 25. The inverter might be cheaper up-front but there is not much advantage over the long run, either. The batteries require periodic replacement.

An inverter is good for little jobs, but running AC isn't one of them. If there is a logical advantage to using an inverter to power air conditioning, I don't see it.

Best regards,
Frank

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JuliaValentine said:

Factor in Fuel?? Wasn't there a recent post about costing $140 a weekend to run the genny?

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Plus the initial buy in fee.... Plus the noise. It may not work for everyone, but for some, it may.

Doug

The $140 comment was for a weekend... not a few hours. My genny burns about 1 to 1.5 gph so a 4 hour run is $16 at $4/gallon.

This reminds me of the argument about how environmentally friendly electric cars are. If you draw the analysis and control volume to include the dirty coal fired electric plant, inefficient power transmission over the lines, the cost of charging and replacing the batteries every few years because of the constant cycling, you are better off driving an SUV. off topic.

I'm just dying to see the results... go do it.

Let's do the arithmetic.

$140 / $4/gal = 35 gal.
35g / .4 gal/hr at full load to run a 3kw genset = 87.5 hours

How many batteries does he need to get 87.5 hours of run time?

Alternately
4 hours of battery life * .4 gal/hr full load 3kw genset * $4/gal = $6.40

Useful life of a genset - 20+ years
Typical lead acid battery life - 3 to 5 years.

Still doesn't make sense. Genset is cheaper over the long run.

Best regards,
Frank

fc3 said:

Let's do the arithmetic.

$140 / $4/gal = 35 gal.
35g / .4 gal/hr at full load to run a 3kw genset = 87.5 hours

How many batteries does he need to get 87.5 hours of run time?

Alternately
4 hours of battery life * .4 gal/hr full load 3kw genset * $4/gal = $6.40

Useful life of a genset - 20+ years
Typical lead acid battery life - 3 to 5 years.

Still doesn't make sense. Genset is cheaper over the long run.

Best regards,
Frank

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How do you squeeze 87.5 hours into a 48 hour weekend?

Doug

douglee25 said:

How do you squeeze 87.5 hours into a 48 hour weekend?

Doug

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I don't.
You and Julie picked the $140 figure which referred to a boat with a larger genset. I used the published full load fuel flow figures from Westerbeke. It is the appropriate genset for a small boat like that 240 with the inverter and the four heavy golf cart batteries.

Considering that the MVP represents more than a 100 lbs weight saving, there's probably also an additional efficiency to that. The main engine would burn less fuel while running since it's carrying less weight.

Best regards,
Frank

fc3 said:

I don't.
You and Julie picked the $140 figure which referred to a boat with a larger genset. I used the published full load fuel flow figures from Westerbeke. It is the appropriate genset for a small boat like that 240 with the inverter and the four heavy golf cart batteries.

Considering that the MVP represents more than a 100 lbs weight saving, there's probably also an additional efficiency to that. The main engine would burn less fuel while running since it's carrying less weight.

Best regards,
Frank

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I didn't pick anything. Someone else threw out the figure, not me.

Doug

So....

I'm curious to what the results of running the AC on some batteries were... Anyone out there?

I ran my A/C on the batteries for a little over 45 minutes and every thing checked out fine. The draw was about 30 amps at 12 volts. Now my A/C unit is pretty small compared to the big boats. I have not done an endurance run on the batteries so I do not know how long they would last under real world conditions.

Let me see if I can summarize our joint inverter project boat.

We start with a relatively small but sleek, fast and versatile sport cruiser. Maybe it has a small (noisy) generator. Part of the reason it's noisy is because the manufacturer hasn't enclosed it for sound control for both cost and space reasons - and didn't really worry too much about exhaust location - whatever was easy.

So we add maybe 500 pounds of batteries so we can run the AC for an hour. If we go over that the TV goes off too. Even so, we will routinely run these batteries so low that they will crap out after a year, maybe two, and have to be replaced.
Space was a little tight down there, so all the batteries were installed on one side and now the boat just goes in circles and leans way over.

We had to spend over $1000 to get an inverter that would do what we wanted, at least for 60 minutes. Hopefully, we installed it close to the batteries so the 12v wiring is short.
We never talked about recharging these batteries, although we concluded how much we will save over the cost of running the genny. That analysis was like a 1 legged accounting because there was no cost for recharging. I guess we just get free electricity with our marina contract. (Good thing this is a small boat, large boats often have electric meters at the dock).
We really should have figured in the cost for a battery charger. After all, that is a lot of big batteries we put down there. And, how long will it take to recharge them? Any chance they'll be recharged fast enough for 2 consecutive days of boating?

Let's go over one more time how we interfaced this into the factory connection from the generator to the AC unit. (We'll come back to that).

Maybe we should just check our main engine performance with all this installed, after all, we added 500 pounds at the rear of a small boat. Could we really have to include a new propeller with less pitch into this inverter project?

Well, anyway, we're done. Our small nifty cruiser now plows along a little slower, leans to one side, won't get up on plane with more than 3 people on board and is totally inaccessible for engine service. But, it's one more feature for the description list when we go to sell it, which may be soon.

Wow... that looks like something I would have wrote before the "new rules" were applied.

I like this guy!

I'm jealous

Sometimes things don't pan out like we'd hoped...
Your candidness will provide a lot of insight to others on the board that might be considering a similar project.
I'm sure you're not the only one that's added a big bank of batteries not realizing the impact of all that weight and it's effect on the hull's performance...

Divide by ten is good, then multiply by 1.25 for loss. Add a big altenator with a proper regulator to replenish the batteries. Some of the ac mfg's are touting their small cuddy units (5-6k btu) as being efficient enough to run on about 300 amps. The deep cycle magic number is the 20-hour rating not cranking.

I would like to add an inverter just to power the fridge and freezer. I don't like the idea of leaving the generator running all night while I am sleeping. It would also be nice when ashore while on a mooring. My initial thought was to use a 24 Volt Xantrex Prosine 3.0 Inverter/Charger and replace the Pro Mariner charger for the bow thruster. It has an automatic transfer switch that I could hook up to the fridge circuit.

I could essentially put it 'inline' in the circuit from the circuit panel to the fridge/freezer. The only weird thing would be the switch labeled 'Refrigerator/Freezer' would then power the bow thruster charger. The 'BOW THRUSTER CONVERTER' switch would be dead.

I was originally thinking of adding an additional battery bank, but didn't like the idea of the additional weight and clutter.

Thoughts?

Why not run the power in parallel to two switches... one for the fridge/freezer and one for the thruster?

Doug

douglee25 said:

Why not run the power in parallel to two switches... one for the fridge/freezer and one for the thruster?

Doug

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Not sure I follow that. I actually meant the AC switches, but thinking about it again, I could feed the AC side of the Inverter from the existing bow thruster converter line, then run the AC output from the inverter back to the input side of the refrigerator/freezer switch.

That might be a little better, but I would still need to have the bow thruster on for the fridge to work. But it would allow me to turn the fridge off, and keep the bow thruster charger on. I have no idea why or if I would want to do that.

OK, I think I'm even more confused now.

For an inverter - DC in (batteries) to AC out.

So -

1. DC to switch to thruster.
2. DC to inverter/charger to switch to fridge/freezer.

No? Or am I still missing something? Maybe a sketch would help.

Doug

Ok, so it's not pretty, but it is a sketch...

The boxes in the AC panel are the switches.
The 24 volt lines to the actual bow thruster are not shown, but would come right off of the batteries.
Install would be pretty easy, the only lines I would have to run is the AC out line from the inverter to the panel, and the comm cable to the inverter control panel.

Does that make it more clear? or more confusing?

OK, I agree with the drawing but I'm unsure why your thruster switch goes the inverter/charger? AC into the inverter/charger should come from the charger switch. DC into the thruster should come from another switch.

Doug

I would remove the existing bow thruster charger (converter) and install the inverter/charger in the same location. The AC bow thruster converter switch currently switches the AC input to the bow thruster converter. That switch would now switch power to the AC input of the new inverter/charger.

The DC part of the thruster circuit would not change, there is a switch on the helm that controls a big honkin contactor by the batteries. From there it goes on to the thruster itself.