Fuel draw with twin tanks on single diesel

[J.T kearney]

Hi. Trying to balance my fuel drawdown on our mainship 350 with twin 150 gl fuel tanks and a single 315 yanmar. Tried to adjust fuel return line to reduce fuel flow to starboard tank but still drawing down port side tank.
Another question is if one tank becomes empty will and both tank lines are
open will the engine draw from only the tank with fuel ?

Cheers
J.T.

 

[Peter B]

Hi. Trying to balance my fuel drawdown on our mainship 350 with twin 150 gl fuel tanks and a single 315 yanmar. Tried to adjust fuel return line to reduce fuel flow to starboard tank but still drawing down port side tank.
Another question is if one tank becomes empty will and both tank lines are
open will the engine draw from only the tank with fuel ?

Cheers
J.T.

Probably not, as it sounds like you have a fuel manifold that allows you to choose which tank the engine draws from. Find the taps on the manifold and you can control that. Personally, I prefer a sustem where there is a T-junction that allows both tanks to automatically level themselves, so you don't ever risk drawing air and stalling at a bad moment - and which often then takes some time to sort out and re-start the motor(s). But maybe that's just me...

 

[mvweebles]

....Personally, I prefer a sustem where there is a T-junction that allows both tanks to automatically level themselves, so you don't ever risk drawing air and stalling at a bad moment....

Unbalanced pair of saddle tanks is a common thread on TF. Problem is that if the boat is out of trim in the first place, having an open crossover valve will make the list worse as fuel transfers to the lower side.

Either trim the boat properly with ballast to compensate for load, or manage your fuel and water tanks and sacrifice capacity to maintain trim. Diesel is running close to $1/lb - lead ballast pigs might cost $1.50 and they are forever.

Peter

 

[Choices]

With a single you can use your tanks to balance the boat daily pretty easy.

If both valves are open, fuel should balance by gravity, and if one tank is lower back fill it.

Otherwise you'll suck air.

 

[Cold Duck]

I've come to believe all MS350/390s list to port even if both tanks were full, both water tanks full. So an equaling line probably will not solve the problem. After a long run (12 hours) with tanks full, I found a significant list to port. My system has cut off valves at both tanks so you can manage the tanks. Problem is you have to watch it or you will end up with one tank full and one empty. Probably because of list, the return line fills port tank faster than Stbd. I have played with plugging the return line on Port but again you have to watch it very closely. Plus pain to lift hatches to play with these valves. Short answer, it's a pain.

 

[mvweebles]

Is there a reason the boat cannot be trimmed with ballast pigs? Beneath the side decks?

https://www.rotometals.com/whole-lead-pig-ingot-57-62-pounds-99-9/

Peter

 

[Cold Duck]

Absolutely. I have heard there was 250# placed on stbd side of the 350s. When I was working on a leaky stbd tank, it was empty and only kept port tank half full, port water empty, stbd water full and I used 4 15 gal water tanks on stbd side to level out boat for about 4 months while figuring out how to fix tank leak ( that's a whole other story). Lead obviously a much better choice for more permanent installation. My Willard had a stbd list but fuel was not the issue.

 

[DrissZougari]

I had similar issue when I purchased my MS390 4 years ago however, I found out that my balancing hose valves were closed. That made a major improvement especially after 6 to 8 hrs cruise as far as the port listing issue is concerned. you can also adjust the port draw valve to force starboard fuel usage!

 

[wkearney99]

With other boats I've had in the past that had twin tanks it was never consistent how it'd draw from them if they had a cross-connect tee. Various factors, including any vent line into the tank, come into play. The only time it was even close to 'even' was if there was a separate, additional, cross-connecting line directly between the tanks and not part of the suction line pulling to the engines. But that was a gasoline setup. I've since never expected tanks to balance themselves 'automagically'.

 

[backinblue]

I have a single engine and twin saddle tanks feeding a T. I do notice some list at times, but usually it's when I have a very full water tank and very empty holding tank. I never try to manually balance the tanks or ballast, I figure the T connection does that well enough. I know it's not perfect, and I understand that a list can/will worsen because of this, but only to a certain degree. It's not going to continue to list till the boat lays over because 1 tank is full and the other is empty. I never considered it a big enough issue to worry about. Underway, trim tabs will level the boat and the fuel should level itself.

 

[Comodave]

I have a single engine and twin saddle tanks feeding a T. I do notice some list at times, but usually it's when I have a very full water tank and very empty holding tank. I never try to manually balance the tanks or ballast, I figure the T connection does that well enough. I know it's not perfect, and I understand that a list can/will worsen because of this, but only to a certain degree. It's not going to continue to list till the boat lays over because 1 tank is full and the other is empty. I never considered it a big enough issue to worry about. Underway, trim tabs will level the boat and the fuel should level itself.

You just need to poop more to keep the holding tank full…

 

[backinblue]

You just need to poop more to keep the holding tank full…

Or maybe drink more beer, especially if it's stored on the low side.

 

[OldDan1943]

Here's thought, return to the tank from which you draw even if you are cross connected on the supply.

 

[J.T kearney]

Thanks all for your insight. I've been scratching my head for 5 years trying to figure out why someone would but 300 lbs of lead in the port side of the engine room!!!. That caused a severe list to port side when the tanks were topped off. Now their downrigger balls. I think this winter I'll try to reconfigure my fuel lines to meet at a cross over in the centerline of the boat and then to the racor filter which is on the port side. Hopefully this would even out fuel line distance and fuel draw to the racor and then to the engine Are there any ABYC rules that prohibit having 2 fuel valves on 1 line? Both tank valves are directly above their perspective tank? This would allow me to remove only one hatch to access the valves
Thanks again
Cheers
J.T.

 

[Snug Harbor]

The only foolproof way to is to run the boat for so many hours on one tank, then switch to the other. Don't forget that the fuel return lines need to be managed as well or you could be drawing from one tank while returning to the other. We spend many hours in engineering at a builder trying to come up with a system and never did. We tried to be sure all lines and fittings were equal and of the same length so that one tank didn't present more resistance to flow than the other, that seemed to make a difference. You'd have the boat drawing really well on one trip then badly on the next for no apparent reason though...

 

[Symphony]

My MS 34HT with the Yanmar370 had no problems drawing fuel evenly. Both tanks drew from the same spot and met in the middle of the boat at a T.. The fuel return went only to starboard tank. That engine returns a huge percentage of fuel, it’s practically like having a polishing system.

The tanks were always level, magically if you like that explanation.

One suggestion is to be sure your tank air vents are good and clear. A small obstruction could change flow rates between tanks.

One of the best and most practical boats I ever sailed.

 

[Csellars]

I must be old school. I always draw and return to the same tank. No way to accidentally have a spill. Switch tanks half way to keep them balanced.

 

[OldDan1943]

I must be old school. I always draw and return to the same tank. No way to accidentally have a spill. Switch tanks half way to keep them balanced.

If you draw and return to the same tank, you are missing the benefits of cross connecting the supply. I have not check but the cross connect line should be larger than than the supply line.

Am I correct in my thinking?

 

[mvweebles]

If you draw and return to the same tank, you are missing the benefits of cross connecting the supply. I have not check but the cross connect line should be larger than than the supply line.

Am I correct in my thinking?

What are the benefits? I don't have a picture of the final install, but attached shows a fairly standard manifold - top gallery is return, bottom is supply (engine (via the Racors to right), port tank, starboard tank, and a polishing/transfer circuit). The supply/return valves are aligned - supply and return for stbd tank are above/below each other. Very intuitive to see what the fuel routing will be.

For me, I can think of no compelling reason to run a pair of saddle tanks as a giant single tank. I 'get' that folks want gravity to level the tanks. In practice, doesn't always work out. And with a transfer pump, you can suck one side dry - likely providing a good 15-20 gallons on the other side.

Peter

 

[catalinajack]

What are the benefits? I don't have a picture of the final install, but attached shows a fairly standard manifold - top gallery is return, bottom is supply (engine (via the Racors to right), port tank, starboard tank, and a polishing/transfer circuit). The supply/return valves are aligned - supply and return for stbd tank are above/below each other. Very intuitive to see what the fuel routing will be.

For me, I can think of no compelling reason to run a pair of saddle tanks as a giant single tank. I 'get' that folks want gravity to level the tanks. In practice, doesn't always work out. And with a transfer pump, you can suck one side dry - likely providing a good 15-20 gallons on the other side.

Peter View attachment 130415

What are the benefits? I don't have a picture of the final install, but attached shows a fairly standard manifold - top gallery is return, bottom is supply (engine (via the Racors to right), port tank, starboard tank, and a polishing/transfer circuit). The supply/return valves are aligned - supply and return for stbd tank are above/below each other. Very intuitive to see what the fuel routing will be.

For me, I can think of no compelling reason to run a pair of saddle tanks as a giant single tank. I 'get' that folks want gravity to level the tanks. In practice, doesn't always work out. And with a transfer pump, you can suck one side dry - likely providing a good 15-20 gallons on the other side.

Peter View attachment 130415

Peter, a couple of questions on your manifold. Six hoses are clamped, one is not. Why? Perhaps because your fittings appear to be push-on fittings not needing clamps? And that last valve on the return with no hose attached, why is it not plugged, for obvious reasons?

 

[mvweebles]

Peter, a couple of questions on your manifold. Six hoses are clamped, one is not. Why? Perhaps because your fittings appear to be push-on fittings not needing clamps? And that last valve on the return with no hose attached, why is it not plugged, for obvious reasons?

I should have been more clear. In this picture, install has not been completed. I thought i had enough valves (Grocos, top row) but turns out one was 1/4" instead of 3/8." It took aeveral weeks to source a replavement Groco valve. The empty valve top right is the engine return - engine has not been installed yet. Those are push-on fittings that do not require clamps. I had trouble sourcing the right hose so decided to go the belt and suspenders route.

There's a mildly interesting back story on this. I designed an elaborate series of 3-way valves and such to control flow. I made it as simple as possible (the Groco 3-way fuel valves are indeed nice). But the guys who originally installed the system were hacks - the hoses weren't supported well, the valves and junctions weren't placed optimally, etc. So I asked the new guys go clean it up. Orlando, the lead electrician and a helluva mechanic in his own right, says in very broken English "a manifold would be better." What a great idea! A few sketches later and the old design was gone. My fuel manifold was of course custom fabbed, but i believe Reverso makes a pretty slick manifold block.

Peter

 

[backinblue]

I don't disagree with anything said here, but trying to understand why it's not a good idea to treat twin tanks that are onnected to a T to feed a single engine fuel line like one big tank. It may not be an absolutely perfect and equal draw from both tanks at all times, and the return is only to one, I get all that. What I fail to see is why they would not self-level if all lines are open. Again, maybe not perfect to the exact gallon, but generally I can't imagine how 1 tank could have substatially more than the other. In fact sometime when I am buying a small abount of fuel, I will only fill one tank and let gravity take over. Seems to work fine for me. I never let my total fuel supply to get too low, maybe 1/4 tank.

 

[mvweebles]

I don't disagree with anything said here, but trying to understand why it's not a good idea to treat twin tanks that are onnected to a T to feed a single engine fuel line like one big tank. It may not be an absolutely perfect and equal draw from both tanks at all times, and the return is only to one, I get all that. What I fail to see is why they would not self-level if all lines are open. Again, maybe not perfect to the exact gallon, but generally I can't imagine how 1 tank could have substatially more than the other. In fact sometime when I am buying a small abount of fuel, I will only fill one tank and let gravity take over. Seems to work fine for me. I never let my total fuel supply to get too low, maybe 1/4 tank.

In my thinking, there is nothing inherently wrong with combining two saddle tanks into a single large tank via an open crossover. However having ability to run them independently is highly desirable. There are many reasons you might want to be able to isolate one tank from the other. I personally prefer to be a bit more deliberate in managing fuel by using one tank at a time, which is why I was willing to spend considerable money on redoing the fuel plumbing with a central manifold.

That said, tank imbalance and associated list is a fairly common topic on this forum. In my opinion, the root cause is the boat likely wasn't trimmed in the first place, and gravity combined with an open crossover will exacerbate the list - fuel will run to the lower tank until the list equilibrates. If you ever need the extra range of your tanks, it won't be there because some amount of usable fuel will remain on one side when the other is empty and sucking.

Trim issues can be temporary due to trip loading, or more permanent due to changes in equipment such as batteries, etc. Temporary changes can be mitigated by running on one tank, permanent list should ideally be balanced via moveable ballast, almost always lead pigs placed below side decks. It's easy to do - usually, builders trimmed the boats before it left the factory, but they are not always re-trimmed after commissioning when additional equipment was installed thus the persistent list.

Peter

 

[backinblue]

Thanks Peter. To be clear I have the ability to isolate tanks, but just don't see the need or desire. When underway, I can trim out list if there is any, and the tanks should stay pretty balanced. In that way I think I could use up most of the fuel capacity, but I never run close to empty anyway. I also don't want to add extra ballast even if I experience some list at times. I attribute that more to how things are stored and also the level of the water and holding tanks. Maybe if I removed all gear and supplies and emptied or filled all tanks, my boat would be in perfect balance, but I'll never know. Being that I run at SD speeds a lot, adding extra weight is not something I'd favor.

 

[mvweebles]

I suspect on many boats, Step 1 is moving existing ballast, not adding more (though thar may be needed too). Someone up-thread mentioned they were perplexed by 300 lbs of ballast under a side deck.

Before redoing the plumbing on my fuel system, I had valves at the bottom of each tank. Because of other equipment, they were a pain to access and the engine return only went to one tank. My boat needed to be re-trimmed and consequently had a permanent list to port that inexplicably got worse as the water tanks drained (they too are saddle tanks so theoretically should not effect side to side trim). Once my refit is complete I will definitely need to spend some quality time with several hundred pounds of lead ballast both fore and aft and side to side. I suspect the majority of owners who open TF threads about balancing fuel tanks or some other list-related topic really need to re-trim their boat, not tinker with their fuel/tank system. BackinBlue - sounds like you're not one of those boats.

Peter

 

[backinblue]

I suspect on many boats, Step 1 is moving existing ballast, not adding more (though thar may be needed too). Someone up-thread mentioned they were perplexed by 300 lbs of ballast under a side deck.

Before redoing the plumbing on my fuel system, I had valves at the bottom of each tank. Because of other equipment, they were a pain to access and the engine return only went to one tank. My boat needed to be re-trimmed and consequently had a permanent list to port that inexplicably got worse as the water tanks drained (they too are saddle tanks so theoretically should not effect side to side trim). Once my refit is complete I will definitely need to spend some quality time with several hundred pounds of lead ballast both fore and aft and side to side. I suspect the majority of owners who open TF threads about balancing fuel tanks or some other list-related topic really need to re-trim their boat, not tinker with their fuel/tank system. BackinBlue - sounds like you're not one of those boats.

Peter

I prefer as little tinkering as possible, thanks!

 

[C lectric]

Peter, a couple of questions on your manifold. Six hoses are clamped, one is not. Why? Perhaps because your fittings appear to be push-on fittings not needing clamps? And that last valve on the return with no hose attached, why is it not plugged, for obvious reasons?

I had the same questions. I have had Push Lock hose loosen as the hose ages. Granted it was not in a fuel supply application but lose its grip it did.
I know it is good hose, if actual Push Lock, but still it could cause an air leak.

I too would use a plug on the unused valve. Good you have a spare. Tie the hose nipple close by with a Tyrap so it's handy and suggest use a plug.

 

[C lectric]

Just a note to the O.P. If the faster draw down tank exposes the fuel supply fitting as the level drops the engine will suck air and shut you down.
Rough conditions makes that more likely as the boat pitches and rolls as the fuel level constantly changes at the intake.
For that reason I never let my tanks draw down past a third capacity.

As far as ballasting to correct/reduce a persistent list some times it is needed, liked or not.

I was faced with the same problem. Single engine, twin tanks. The crossover was not well done. It is also the feed from the two tanks.

There is equipment placed so there is more weight on one side than the other and to change it would require a tear apart and redesign of the boat eng.compartment.

I have trim tabs and use them which helps reduce listing while running.
Yes, it is a slow, 7 to 8 knot boat and the tabs help and DO have an effect.

I have moved all my heavy tools, supplies and such to the other side which helped but was not enough.

THe only option was some ballasting of the opposite side. To estimate the need weight I used 5 gal pails and filled them with water. A table will tell you what a full 5 gal pail should weigh or use a bath scale for a good check. Left them overnight so the crossover would allow the tank levels to even out and examined the next day.

From that I was able to get close. I will add one more lead ingot sometime over the next year as it is still listing somewhat. And yes they need to be secured into place.

 

[jbbmillar]

I once closed the cross-over line, filled both tanks, left it closed and and closed the starboard tank supply valve. I thought I could take the slight port list out of the boat. Shortly after leaving the fuel dock diesel starting coming out of the starboard tank vent - and just as I was passing the USCG station! I caught it quickly, opened the cross over and starboard tank supply valve, stopping the problem.



My return line has a T and no valves, so return fuel was overfilling the starboard tank. I've never gotten around to changing anything, I only close the cross over when fueling, then run with all the valves open.

 

[backinblue]

I also run with all lines open and it seems to work relatively well. I don't bother closing the crossover for filling. I get why you do, but I assume that in the short time for fueling, minimal transfer will take place and eventually everything will be back in balance. BTW, I only have a fuel gauge in one tank, so unless I added another gauge, it's another good reason to keep the crossover open.