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Old 09-15-2010, 07:55 PM   #1
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Mainship 390

How do you change switch fuel tanks on a single engine with 2 tanks.
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Old 09-15-2010, 08:04 PM   #2
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RE: Mainship 390

Hiya Mr. Robkan,
**** What boat, what year, what engines????
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Old 09-15-2010, 08:25 PM   #3
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RE: Mainship 390

there will be a crossover valve, so you can in effect run from both tanks. Or you can manually close off one and open the other, whenever the trim of your boat requires a change. In all cases, you must know the routing of the returning fuel, so as to avoid overflowing the full tank while drawing from the other. All the valves should be easily accessible and quite obvious.
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Old 09-15-2010, 09:10 PM   #4
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RE: Mainship 390

where is it?
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Old 09-15-2010, 10:45 PM   #5
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RE: Mainship 390

Look in the engine compartment and see if you can find ball valves w labels that say "port return" or "stbd supply" ect. Open and close the valves to accomplish what you want to do keeping in mind that you must return fuel to the tank or tanks your'e drawing from. I never allow egress from one tank to another (crossover). I draw and dip (check fuel levels) on my 2 fuel tanks as needed one at a time. I enter in my log when I change the draw.
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Old 09-16-2010, 06:38 AM   #6
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RE: Mainship 390

If you go to this site and join you will get all of the available*knowledge there is out there on the 390 and then some.

It is breed specific to all Mainships.

mainship@yahoogroups.com
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Old 09-16-2010, 08:24 AM   #7
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RE: Mainship 390

Thanks. *That is an email link.
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Old 09-16-2010, 08:33 AM   #8
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RE: Mainship 390

Try this:******** http://groups.yahoo.com/group/mainship/

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Old 09-16-2010, 08:35 AM   #9
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RE: Mainship 390

Sorry wasn't thinking.

http://dir.groups.yahoo.com/group/ma...&sec=dir&slk=5
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Old 09-16-2010, 06:44 PM   #10
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RE: Mainship 390

The cross over lines are on the side of each tank near the forward bulkhead of the engine compartment. we run with ours open all of the time. Near the rear of the compartment on the side of the tanks are the valves that feed the fuel filter. we also run with both open all of the time. this way the tanks empty evenly.
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Old 09-17-2010, 08:25 AM   #11
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RE: Mainship 390

Another thing to consider is that if you only have one fuel guage, then it very likely only measures fuel in one tank. And if you are burning out of only one tank, you need to determine if your fuel readings are reliable....or not!....IOW, figure out which tank the fuel guage is reading out of and select your tanks(and fuel guage readings) accordingly.
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Old 09-17-2010, 11:49 AM   #12
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RE: Mainship 390

Thanks. *I checked the valves yesterday and they are open on both tanks. *So it appears that both tanks should draw evenly. *However, on the gauges on the panel at both stations it shows that the Port side tank is about 1/2 full and the Starboard tank is nearly empty. My problem is that this is a new boat and I don't have the experience with this situation, I have always had twin diesel and each had its own tank. *It appears to be drawing fuel only from the Starboard tank.

I was hoping that it would be as simple as just having a manual lever to switch tanks over.
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Old 09-17-2010, 11:54 AM   #13
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RE: Mainship 390

Hiya,
** Could one of your lines or valves be plugged or be restricted
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Old 09-17-2010, 11:58 AM   #14
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RE: Mainship 390

That is what it seems like I will have that checked. *Only thing that makes some sense.
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Old 09-17-2010, 12:06 PM   #15
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RE: Mainship 390

It does give me a reading for both tanks on the bridge and lower stations, both are the same.
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Old 09-20-2010, 06:22 PM   #16
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RE: Mainship 390

What year is your 390? my boat has only one gauge at each station with a rocker switch to turn on each gauge. maybe your gauges that read empty* are turned off.
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Old 09-21-2010, 12:24 AM   #17
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RE: Mainship 390

It's is not unusual for two tanks to NOT empty at the same rate. Unless boat trim is dead even, the lines or hoses are exactly the same length and size, the arrangement and type of fittings are duplicated exactly there will be a difference in pressure or suction drop which will alter the draw from each tank.

And of course there are also the return lines, same problem.
There is no way to guarantee that level of equality.

A balance tube can be used but must be quite large, is not perfect and presents some problems of its own.

Sometimes tanks will be close by luck and some good planning.

Most of us will need to have a few valves that can be used to alter the fuel draw and more likely, the return.

On my boat it is the fuel return line valves that I use to even out the tanks. I don't shut them off, ever, but use them to throttle the return to force more to the tank I want. Once sorted out the settings should not need to be changed. This is one area that a gate or globe valve would be preferable to a ball valve as these valve can be adjusted in small increments.
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Old 09-21-2010, 07:44 AM   #18
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RE: Mainship 390

Ain't that the truth. My boat actually makes diesel. I close off one tank and run off the other, alternating as required. This weekend, I started with 150 gallons in the port tank. Ran the generator overnight then about 5.5 hrs. with just the main running. I had 170 gallons the next morning. *These are measured using the sight tubes on the tank. I've seen this before. All I can think is that the trim is different. I can't wait to* get down there this morning with everything cool and settled and see how much fuel is in there now.
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