Watermaker

[colemj]

Originally Posted by DotDun

The major difference I see is the 12v power recovery systems are considerably more effcient than the AC based systems. Does an extra pint of diesel per XX gal. of water generation make a difference?

I don't think I understand this - if you are running a generator while making water, you are running a generator regardless of the watermaker. Efficiency isn't applicable here - output is. So if running the generator, one would want the most water made during that time as possible.

Efficiency only comes into play when not running the generator, and really only relates to DC systems.

Mark

[Pelagic]

Originally Posted by colemj

I will go a bit further and say that if your needs are greater than ~12-15gph, AC systems are the only way to go - no DC system makes sense here.

Mark

..Not arguing Mark as I am just trying to understand the "energy math" in this?

Say you need a 1.0 HP electric motor to run the 30 gph pump for your system.

If you have lots of stored battery energy. . (say 2080Ah @ 12vdc) you have the choice of running either a 1hp AC Motor through an Invertor or just bolt on a 1hp DC Motor instead.

Will that DC motor consume more stored energy than an AC motor via inverter?

What am I missing here?

[colemj]

Originally Posted by Pelagic

..Not arguing Mark as I am just trying to understand the "energy math" in this?

Say you need a 1.0 HP electric motor to run the 30 gph pump for your system.

If you have lots of stored battery energy. . (say 2080Ah @ 12vdc) you have the choice of running either a 1hp AC Motor through an Invertor or just bolt on a 1hp DC Motor instead.

Will that DC motor consume more stored energy than an AC motor via inverter?

What am I missing here?

Hopefully one of the experts will step in here, because I am not one.

A 30gph watermaker, AC or DC, is going to need ~1200 watts (I think). This will translate into ~100A of DC power.

If you run a DC pump at 100A draw, your batteries must be able to supply this without a voltage drop. Most cannot, and most DC watermakers are spec'd at 13.8V which makes it worse.

If you can supply that power, then a DC motor will use a bit less power than an AC motor run through an inverter. The difference will be the efficiency of the inverter.

Mark

[a64pilot]

I believe a Clark pump can do it for considerable less power than a simple piston pump, but whatever the current draw would be for a 30 gl an hour Watermaker, for me to supply that kind of current I'd need to run a Generator or the main engine. I won't have the bank big enough to do that, and maybe just as important I wouldn't have enough Solar to replace it if I did.

So I believe you run a DC watermaker at considerable lower output in order to keep the current draw down to more acceptable levels, but for me it really doesn't matter that much, the total amount of current required to make say 60 gls whether it's in 2 hours or 10 hours will still be in excess of what I could put back in with just Solar, so I'm running a generator or the main engine.
If I'm running a generator anyway, why not use the power directly, and the excess to top up batteries, wash clothes or even heat water?

So unless I don't understand the concept, if you have a generator an AC Watermaker makes sense, but if you have an excess of 12V DC, then a DC watermaker makes sense, but you have to have the capacity of creating the power, whether it be Solar or Wind or an Alternator. I won't have enough Solar, and to me it's illogical to burn Diesel to charge batteries to make water.
I'd rather have a high volume AC watermaker so I don't run the Generator for nearly as long


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[LeaseOnLife]

Originally Posted by colemj

Hopefully one of the experts will step in here, because I am not one.

A 30gph watermaker, AC or DC, is going to need ~1200 watts (I think). This will translate into ~100A of DC power.

If you run a DC pump at 100A draw, your batteries must be able to supply this without a voltage drop. Most cannot, and most DC watermakers are spec'd at 13.8V which makes it worse.

If you can supply that power, then a DC motor will use a bit less power than an AC motor run through an inverter. The difference will be the efficiency of the inverter.

Mark

Pelagic mentioned way back he has a 24V system, his DC pump motor will run at 50A then. Above he mentioned 2080Ah @ 12vdc, I assume 1040Ah @24V. Running 1 hour would deplete his bank by 5% of capacity, not too bad I think.

A DC motor with commutator and brushes can be more efficient than a squirrel cage AC motor. Real world data from CruiseRO would help compare.

Dirk

[DotDun]

Amount of diesel burned generating power is related to amount of the power draw. Hence, you'll burn more diesel running a less efficient WM, the point I was trying to make is how much more fuel burn per XX gal. of water produced?

Probably not measurable.


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[a64pilot]

Actually I think from a Diesel burned perspective I believe the way to conserve is go high output, even at a loss of efficiency, that way you reduce the hours on the Gen, and therefore fuel burned.
The little 3.5 KW Nexgen is supposed to burn .2 GPH average and highest fuel burn possible is .4 GPH, so fuel burn is sort of irrelevant, I can live with 5 hours run time per Gl of fuel.
At that rate 1 gl Diesel gives me 120 gl of water, probably more.


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[a64pilot]

I believe from an energy efficiency standpoint, not much comes even remotely close to a DC Spectra, certainly not Cruise RO's but since the little Honda will run it with power to spare, why spend the extra money to increase efficiency when you don't need it?


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[CAELESTIS]

Another factor to consider is making water while underway. For a mono, where would you place the Honda gen while heeled? On the deck where you might be inhaling CO? Swim step where it could get wet/partially submerged? I believe the thought is to make a lot of water at anchor in a short period of time (the CRO does this). If you go the Spectra route, it is efficient but your solar may not produce enough for the longer run time, and end up having to run the engine/genset anyway.

They both have their trade offs and advantages.

[colemj]

Originally Posted by DotDun

Amount of diesel burned generating power is related to amount of the power draw. Hence, you'll burn more diesel running a less efficient WM, the point I was trying to make is how much more fuel burn per XX gal. of water produced?

Probably not measurable.

Yes, I doubt it would be measurable. The difference would have to be double or more of the load on the generator to make a very slight difference in fuel consumption.

In the case of comparing DC and AC units on generator fuel consumption (assuming the DC needed the generator), the generator load would be the battery charger, so run time would be the consideration, and that comes back to output.

Mark

[colemj]

Originally Posted by CAELESTIS

They both have their trade offs and advantages.

Yes, agreed. Just to be clear, I am not making a blanket case for an HO AC watermaker. My case is that the only DC units that make sense are the energy-recovery type. The flip side is that low-output AC units do not make sense either. Only in rare occasions do non-energy recover DC types make sense.

When choosing between energy-recovery DC and HO AC, there are a set of variables to consider, and you bring up some of those. For sure, AC systems are not a good choice for a lot of boats/people.

Mark

[Terra Nova]

Originally Posted by a64pilot

...If I'm running a generator anyway...

This is the slippery slope we face when rationalizing a preference, rather than clearly visualizing the options.

Let's say that a cruising sailboat like yours has its auxiliary running X-hours per year for the purpose of motoring (especially when there is no wind), motor-sailing, picking up and setting the anchor. With a 12-volt watermaker you can be making FREE water for X hours/year.

If you are clever you will use these "free" hours to supplement your solar so you don't need to feed your generators all that much. A high output alternator can do a lot of your charging during times you'll need to run the main, anyway.

[DotDun]

Originally Posted by Terra Nova

This is the slippery slope we face when rationalizing a preference, rather than clearly visualizing the options.

Let's say that a cruising sailboat like yours has its auxiliary running X-hours per year for the purpose of motoring (especially when there is no wind), motor-sailing, picking up and setting the anchor. With a 12-volt watermaker you can be making FREE water for X hours/year.

If you are clever you will use these "free" hours to supplement your solar so you don't need to feed your generators all that much. A high output alternator can do a lot of your charging during times you'll need to run the main, anyway.

With an inverter, you can also run an AC WM while running the auxiliary.... get the same FREE water. No???

[SV THIRD DAY]

Originally Posted by Dhillen

Rich Boren not only sells watermakers, he saves marriages.

I showed that to my wife and she said well....when do I start saving Our marriage...ha ha ha...man a guy can't get a break.

Originally Posted by Terra Nova

Let's say that a cruising sailboat like yours has its auxiliary running X-hours per year for the purpose of motoring (especially when there is no wind), motor-sailing, picking up and setting the anchor. With a 12-volt watermaker you can be making FREE water for X hours/year. .

Ah grasshopper....
....but what if while motoring you run the AC HO water make through your 2000W inverter and make over 30GPH?

Here's how we have been using our SM30:

1.
At anchor if not really needing any other major AC loads (water heater, washing machine, etc) then we use the Honda 2000 and make 33GPH while charging the batteries at 40A DC at the same time to give us that battery bank top off that you rarely get living on the hook.

2.
If we have some need for some larger AC loads, then we fire up the 8KW genset and make water, hot water, charge batteries, do laundry, and my wife irons her clothes....ya I know....she's sick like that.

3.
We are underway and motoring or motor sailing but the alternator is spinning. So once the acceptance of the batteries dials down the alternator output, we flip on the 2000W inverter and turn on the water maker. We are now making that "free 30GPH" of water while motoring. But as we all know, nothing is for free on a boat, but we like to call it free to make ourselves feel good.

About the power usage from the Honda and how the rational goes.
I get the question all the time, so I made a little YouTube Video explaining it:

https://www.youtube.com/watch?v=ROer7Bec9vw

Basically, the Honda will power the 9.3A @ 115v AC water maker and 40A DC charger load for 5 hours in a single gallon of gas. So you can then do the math of how much gas you really would need on board to make what quantity of water. Don't want to run the DC battery charger, and you get more run time.

I have to honestly say that I'm feeling a bit embarrassed and blushing at the nice comments. As a fellow cruiser, by goal is to treat clients like I would a buddy at anchor next to me (that of course didn't anchor on top of me) so getting the positive feedback from my fellow cruisers makes it all worthwhile for me. Plus I'm now going to make my wife read this thread and cut me some slack when she comes back to the boat after a 12hr shift of seeing patients at the Clinic and the first thing she asks is:

"What have you been doing all day? I don't see any new teak area varnished or nothing crossed off your boat to do list? Breakfast dishes are still in the sink and you are still wearing your pajamas!?!"

[Terra Nova]

You're already at the bottom of the slippery slope once you rationalize installing and operating an inverter large enough to easily start and safely run an A/C watermaker.