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Old 07-27-2017, 01:13 AM   #1
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What size water tank with water maker?

Hi Folks,

On my trawler they put a 200 gal gas tank next to a 175 gal water tank on each side of the boat. The total gas is 400 gal. with about a 600 mile range at 8 knots. I would like to get over 1000 mile range. I can use the water tanks for additional gas, which should give me well over 1000 miles at 8 kts. That leaves me with no water. After confirming with the owner of the company that I can do this he suggested using a 40 gal blatter in the lazaret for water. I was thinking about getting a water maker to supplement the 40 gal water blatter but I hate to fill up the lazaret with a 40 gal blatter. If I used a 20 gal blatter with a water maker will that work for showers? Am I wasting my time with all of the above? Has anyone done something similar? Any thoughts or comments would be appreciated.

Thanks!
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Old 07-27-2017, 01:25 AM   #2
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Will your normal cruising habits require more than 600 miles of range ? If not, perhaps on the occaisions that it does you could supplement your fuel supply with a temporary fuel bladder?
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Old 07-27-2017, 01:32 AM   #3
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Will your normal cruising habits require more than 600 miles of range ? If not, perhaps on the occaisions that it does you could supplement your fuel supply with a temporary fuel bladder?
A simple but elegant solution.

Thanks Benthic2!
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Old 07-27-2017, 03:16 AM   #4
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There is nowhere in North America where you need a 600 mile range.

Not even close.

I would keep your tanks as configured and go boating.
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Old 07-27-2017, 04:50 AM   #5
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"The total gas is 400 gal. with about a 600 mile range at 8 knots. I would like to get over 1000 mile range."

Slow to 7K
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Old 07-27-2017, 04:53 AM   #6
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Its a one way trip, once the tank is used for gas it can never be used for water.
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Old 07-27-2017, 06:37 AM   #7
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Watermakers do not work well generally in coastal waters or the ICW, for example. They require relatively clean seawater. They work great in the Bahamas and offshore where the particulate load is much lower. We enjoyed ours in the Bahamas but shut it down as soon as we got back to the ICW because filters plugged so fast. Plus, any hiccup with power supply, RO membrane(s), or running out of prefilters, would mean running out of water. I would never rely on a WM as my only source of fresh water unless I only drank beer and did not shower!!
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Old 07-27-2017, 08:37 AM   #8
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Ding ding ding, we have a winner!
Quote:
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"The total gas is 400 gal. with about a 600 mile range at 8 knots. I would like to get over 1000 mile range."

Slow to 7K
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Old 07-27-2017, 10:07 AM   #9
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You haven't said whether your water tanks are metal or plastic, but if they're polyethylene don't even think of trying to convert one to fuel holding. PE water and waste tanks are linear low density PE...petroleum will dissolve it. PE fuel tanks must be high density cross-linked PE, which is a different material from linear...plus, plastic fuel tanks must be USCG certified that they meet strict safety standards-- all fittings on the top of the tank, constructed to prevent liquid from pooling on any surface and a couple of others I can't recall without looking 'em up.

Even if it is metal, it's a still a very bad idea 'cuz some of those requirements apply to all fuel tanks and as swampu said, once a tank has held fuel, it can't be used for anything else.
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Old 07-27-2017, 10:24 AM   #10
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I agree with others above.

- Where do you need to go that is 1000nm between fuel stops?
- How often will you have this need?

If you slow down to 7 knots you may have a fuel burn of about 2 g/h. That is 200 hours of operation with 400 gallons. At 7 knots that is 1,400 nm. Even if my numbers are off by a bit, I would bet that you could find a cruising speed that would easily give you a 1,000nm range with a 20% reserve.

Even if that seems like too little fuel for you, then consider converting one of your water tanks to fuel to give you 575 gallons. This is way more than you would likely need but would still give you 175 gallon water capacity.
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Old 07-27-2017, 05:13 PM   #11
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My question would be gas or diesel and type of engines? You say "gas" above so curious...
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Old 07-27-2017, 08:32 PM   #12
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I am not sure how big a water tank you would need. I would figure out how much water you would use in two days. Consider cooking, washing, showers etc.... I would want to have a couple days supply so that if I was in a location where I didn't want to run the water maker I would still be OK.
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Old 07-27-2017, 09:22 PM   #13
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Quote:
Originally Posted by dhays View Post
I am not sure how big a water tank you would need. I would figure out how much water you would use in two days. Consider cooking, washing, showers etc.... I would want to have a couple days supply so that if I was in a location where I didn't want to run the water maker I would still be OK.
I agree. I think that's why 40 gal was recommended. I need to look at some water makers and figure out how long it takes to make 20 gal and how efficiently the water heater could keep it warm for a shower. I keep coming back to 40 as the answer but a 40 gal tank is big! That's what I have on my Carver and it's good for one for about 4-5 days. If the water maker could keep it full even during use I think a 20 gal tank might work. But if the water maker went down I would have to depend on bottled water until I pulled anchor and got into port.

I think 40 gallons is the answer. Unfortunately.

Do you have a water maker?

Thanks,
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Old 07-27-2017, 09:49 PM   #14
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Quote:
Originally Posted by FF View Post
"The total gas is 400 gal. with about a 600 mile range at 8 knots. I would like to get over 1000 mile range."

Slow to 7K
One of the reasons that I want more diesel is so I can go faster and not worry about losing my range. I'm coming from a planing boat so I'm a bit worried that 8 knots will feel too slow. I've been driving my Carver at different speeds to see what it's going to feel like running at 8 knots. 10 knots is OK but 8 feels really slow to me. I've been told that it feels different in a trawler.

I guess I'll find out in September when I take possession of the boat.

Cheers.
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Old 07-27-2017, 09:59 PM   #15
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Quote:
Originally Posted by HeadMistress View Post
You haven't said whether your water tanks are metal or plastic, but if they're polyethylene don't even think of trying to convert one to fuel holding. PE water and waste tanks are linear low density PE...petroleum will dissolve it. PE fuel tanks must be high density cross-linked PE, which is a different material from linear...plus, plastic fuel tanks must be USCG certified that they meet strict safety standards-- all fittings on the top of the tank, constructed to prevent liquid from pooling on any surface and a couple of others I can't recall without looking 'em up.

Even if it is metal, it's a still a very bad idea 'cuz some of those requirements apply to all fuel tanks and as swampu said, once a tank has held fuel, it can't be used for anything else.
That's why I asked the owner of the company if it was possible with the current tanks. He said it wouldn't be a problem but I will double check on the material of the tanks. This would be done at the factory so I'm sure that it will be done to USCG safety regulations. I will be sure to have an in depth conversation with them. If I go through with this.

Thanks for your input!
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Old 07-27-2017, 11:06 PM   #16
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Just one comment on the bladder style tanks as I knocked this idea around for a while (before going with poly replacements)... bladders have a fixed 2-D shape. They don't actually "fit" in a space better than a poly tank, in fact, you will find the number of available dimensions of poly tanks far far greater than bladders. Once I figured that out, I saw zero benefit to having a variable height dimension.
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Old 07-27-2017, 11:12 PM   #17
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IMHO you would ruin a perfectly acceptable setup for 99% of us by screwing up the tankage specs. Think resale.
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Old 07-27-2017, 11:14 PM   #18
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A watermaker is handy in many ways. And you can make water almost anywhere with proper filters. I make all of my water. Last dock water was 2011. I use washable 20 and 5 micron in front of my regular watermaker filters. In rivers or bays I wash them about every 1-2000 gallons of final water. No membrane problems.
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Old 07-27-2017, 11:30 PM   #19
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How much fuel are you going to burn running the genset, so you can run the water maker?
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Old 07-27-2017, 11:43 PM   #20
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IMHO you would ruin a perfectly acceptable setup for 99% of us by screwing up the tankage specs. Think resale.
Good point. Although a 1000+ mile range with a water maker might change those numbers. Still, definitely worth considering. I do intend to work my way up to a true blue water trawler one day. My dream is to motor to Palau and live on the boat. I spent a couple of weeks there on business and fell in love with the people first then the island. What a wonderful place.

Cheers
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