Go Back   Trawler Forum > Maintenance and Systems > Power Systems

Thread Tools Display Modes
Old 12-03-2012, 04:43 PM   #1
Senior Member
Baggiolini's Avatar
City: Monterey, CA
Country: USA
Vessel Name: Mahalo
Vessel Model: PT Overseas Trawler
Join Date: Jun 2011
Posts: 395
drawing from multiple fuel tanks

I'll do my best to explain this.....

Three fuel tanks two side tanks of 125 gallons each & one "saddle tank(lowest tank over the keel)" of 30 gallons. When I bought the boat the valves were such that each engine drew off of the side tank closest to it and the saddle tank was shut off.

When I filled the boat with diesel, I filled the saddle tank so full that there was diesel in the vent line. So I opened the valve to the saddle tank, shut off the side tanks and went for a little run. I got the fuel level low enough that there is no longer fuel in the vent line. There are valves on the bottom of the tank, presumable to equalize fuel levels but they don't seem to equalize anything when open, I would image the line is clogged.

So the question is: Assuming I leave the saddle open & open the side thanks, what happens when the saddle tank runs out of fuel? Will it send air bubbles into the system and shut me down or will it automatically pull from the side tanks?

Baggiolini is offline   Reply With Quote
Old 12-03-2012, 05:17 PM   #2
swampu's Avatar

City: Biloxi, MS
Country: USA
Vessel Name: Cajun Rose
Vessel Model: Biloxi Lugger
Join Date: Jul 2011
Posts: 988
Brave man just "filling up" the tank not knowing if it had a leak. Glad it didn't. To help with your question it should draw enough fuel to replace what is lost. Make sure the return line goes to the tank your pulling from.

swampu is offline   Reply With Quote
Old 12-03-2012, 06:47 PM   #3
Senior Member
Join Date: Oct 2007
Posts: 158
It sounds as I you have two 125 gallon saddle tanks (hence like saddle bags on a horse) and an auxiliary tank of 30 gallons. Either the aux tank is for a genset or left for emergency use. I doubt a 30 gallon tank would be used as a 'day tank' with twins.
El Sea is offline   Reply With Quote
Old 12-03-2012, 06:48 PM   #4
City: Palm Beach Gardens
Country: USA
Join Date: Jul 2012
Posts: 20
Having the "equalization" valves open means that the top surface of the fuel in the 3 tanks will be the same height in the boat. if one of the tanks is lower than the other two it will be fuller than the others.
CLField is offline   Reply With Quote
Old 12-03-2012, 07:05 PM   #5
City: Pensacola
Country: USA
Join Date: May 2012
Posts: 748
Great question. I also a similar problem with listing due to fuel tanks drawing off of one tank. It took some trial and error but I figured it out.
The only way to map the valves is to start at the engine fuel pump and run your hands to the valves.
Once you select the shut off valves put some tape on the sight gauges to verify the fuel flow is even.
Blue Heron is offline   Reply With Quote
Old 12-03-2012, 07:41 PM   #6
City: North Charleston, SC
Country: USA
Join Date: Sep 2010
Posts: 4,385
Originally Posted by Baggiolini View Post
............. When I filled the boat with diesel, I filled the saddle tank so full that there was diesel in the vent line........
I can't answer the rest of your question, but fuel in the vent line will go away as fuel is burned. It's not really a problem unles the vent line has a downward loop (which it shouldn't have).
rwidman is offline   Reply With Quote
Old 12-03-2012, 08:16 PM   #7
Brooksie's Avatar
City: Cape Cod, MA
Country: USA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
Join Date: Oct 2007
Posts: 863
Had a boat once with two tanks & single engine. The boat listed due to PO's removal of an old generator on one side. If both tanks were on, fuel would go from high side tank to the lower tank as it was consumed so the low side only got lower (list increased)... The water tanks did the same thing, further increasing the list. Of course it should have been ballasted back level when the generator was removed but I ran it for several years this way by using one fuel and one water tank first until level, then alternating them.
Brooksie is offline   Reply With Quote
Old 12-03-2012, 08:26 PM   #8
Master and Commander
markpierce's Avatar
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 9,115
Got two water tanks: one starboard, one port. They will equalize unless one of their valves are shut off.

Got four fuel tanks, upper and lower starboard and upper and lower port. Haven't experimented to see if the upper tanks will drain into the lowers if all valves are open. Have always treated the tanks individually.
Kar-KEEN-ez Koot
markpierce is online now   Reply With Quote
Old 12-03-2012, 09:53 PM   #9
Scraping Paint
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Very difficult to answer the OP's question without seeing a schematic of the fuel system. El Sea's correction is accurate---- saddle tanks are typically opposed-pair side tanks. We have a 60 gallon tank in the bilge in the center of the boat between the engines. This is a day tank and the engines and generator typically draw from it (and return fuel to it). We fill the day tank periodically by gravity and manual valves from the four saddle tanks which drain from their lowest points.

Whether or not your engines will pull air if all the tank valves are opened and the center tank runs dry will depend on the layout and valving of your fuel system.

On our boat I let a pair of saddle tanks drain completely into the day tank (which is what the engines were running off of) and when the starboard saddle tank went dry the engine pulled air from it instead of fuel from the day tank and the engine shut down.

Not realizing at the time why the engine had stopped I went ahead and closed the transfer valves on both saddle tanks. Fortunately I did this before the port tank had drained dry. So the port engine continued to run on the day tank as it never got a chance to pull a slug of air from its saddle tank. Not until I thought through the fuel system on the boat that evening on the drive home did I realize what I had done by letting the saddle tanks run dry during a fuel transfer.

Depending on how your fuel system is set up, it may be possible for the same sort of thing to happen on your boat, only in this case the engines would be getting their slugs of air from your just-emptied center tank. But without seeing a fuel system schematic, I think it's impossibe to determine exactly what would happen.
Marin is offline   Reply With Quote
Old 12-04-2012, 06:17 AM   #10
FF's Avatar
Join Date: Oct 2007
Posts: 14,908
An Equilization line can be big troubble with the sheen police at fill up time.

Most tanks will have their own air vent , so filling a higher tank may cause fuel to vent from a lower tank.


FF is offline   Reply With Quote

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

All times are GMT -5. The time now is 07:42 PM.

Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012