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Old 02-08-2018, 07:40 PM   #1
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Primary Fuel Filter Heresy?

If a multi-tank fuel system feeds into a single line before heading to each engine...then why do most twin engine boats have separate primary filters for each engine?

Couldn't you just run a single dual primary for all "inbound" fuel and then head off to each engine's secondary filter(s)?

I could see why an entire fuel system is completely separated (port only to port, starboard, etc using with a "polishing" transfer circuit, etc) but for many coastal, multi-tank, twin-engine boats - all tanks lead to a common "manifold" then off to each engine.

Is this just good marketing from the fuel filter companies to get you to buy two of everything?

Here is why I ask:

I've been contemplating some fuel system improvements. Currently I have what I described above: a common copper "manifold" (of sorts) in that all three tanks (port, starboard and belly) combine into a single line on the forward bulkhead of the ER. Individual tanks can be shut off from this line using a valve at each tank.

From this common fuel line two lines then feed into respective primary systems (port and starboard systems).

In my case the primaries include the original dual spin-on filters along with a newer, single Racor 500 water separating primary installed by the PO for each engine - presumably as a work around to filter changes/priming with the older duel filter system.

I can flip ball valves to route fuel through either the Racor or the dual spin-ons independently for each engine. The on-engine secondaries take it from there (typical Ford Lehman setup) and then each returns back to that engine's respective port or starboard tank.

It all works and I don't seem to have fuel delivery issues.

But the lines are as old as the boat, the PO install is...ahem "interesting". I also started looking at adding proper supply and return manifolds to make transfers easier and yes even polishing possible.

So as I'm looking at modern, just flip to the other filter, dual filter primaries, I started wondering....do I really need complete duplication if all of the fuel comes together before the primaries? Yes, I'll go through more filters, but given the cost of dual primary setups...and what I perceive to be no significant benefit...is it worth it to buy two of everything?

OK, school me (or fire at will)!

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Old 02-08-2018, 07:48 PM   #2
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When one engine shuts down with a primary fuel filter problem, do you want the other to shut down also? If I'm going to have twins, I want to separate systems.

Ted
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Old 02-08-2018, 07:58 PM   #3
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When one engine shuts down with a primary fuel filter problem, do you want the other to shut down also? If I'm going to have twins, I want to separate systems. Ted
What he said
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Old 02-08-2018, 08:08 PM   #4
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So primaries are the weakest link in the system even if both sides are supplied fuel from a "blended" source?

I guess my question is - in this common manifold setup the systems are not in fact completely separate so why filter downstream like they are? Given the combined fuel supply wouldn't you see contamination in both systems?

PS - Don't get me wrong, I see the advantages of completely separating fuel supply per engine and adding filtered transfer capabilities but given how common this setup is in boats that aren't built for circumnavigation I wondered about the reality of true redundancy.
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Old 02-08-2018, 09:36 PM   #5
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PS - Don't get me wrong, I see the advantages of completely separating fuel supply per engine and adding filtered transfer capabilities but given how common this setup is in boats that aren't built for circumnavigation I wondered about the reality of true redundancy.
I can't speak for circumnavigation, but losing engine(s) in close quarters (inlets with current, near bridges, inside marinas) can be pretty darn exciting also.
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Old 02-08-2018, 09:51 PM   #6
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So primaries are the weakest link in the system even if both sides are supplied fuel from a "blended" source?

I guess my question is - in this common manifold setup the systems are not in fact completely separate so why filter downstream like they are? Given the combined fuel supply wouldn't you see contamination in both systems?

PS - Don't get me wrong, I see the advantages of completely separating fuel supply per engine and adding filtered transfer capabilities but given how common this setup is in boats that aren't built for circumnavigation I wondered about the reality of true redundancy.
In a perfect world, there's no gunk, water or whatever in fuel. In the world we live in, fuel problems are possibly the largest single cause of engine troubles. If you have all expensive components to have totally separate systems, why wouldn't you spend the relatively small amount to have that piece of mind?

The final thought :
Your heading down the coast after a nice two week trip. The last 40 miles lie ahead of you when the weather changes. An hour later you're in 7' seas, short and steep. In the back of your mind you know any and all crud in your fuel tanks is getting stirred up and loading up that one filter. Then you start trying to visualize changing the separator filter and cleaning the bowl in 7' beam seas in a 120 degree engine room.

Ted
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Old 02-08-2018, 10:24 PM   #7
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Not exactly sure if your question but yes, one engine, one tank. If I had three tanks I would have three filters, filters are for the tank not the engine. I have 4 tanks in my big boat, four engines too. I dont mix anything
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Old 02-08-2018, 10:43 PM   #8
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Not exactly sure if your question but yes, one engine, one tank. If I had three tanks I would have three filters, filters are for the tank not the engine. I have 4 tanks in my big boat, four engines too. I dont mix anything


Great response. Filter the tanks, not the engine. This is exactly my point. Most trawlers combine tank supply and THEN insert primary filters.
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Old 02-09-2018, 06:42 AM   #9
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I've been on a lot of diesel boats. All those I can remember had a day tank with 2 Racors (1 on line, 1 ready to go) and then the manifold, and then the individual engine filters. I've been running mostly twins since the early 60s and haven't had a fuel problem that stopped both engines. If you do proper fuel maintenance and the tanks free of sludge you should never have a problem.
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Old 02-09-2018, 06:53 AM   #10
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Great response. Filter the tanks, not the engine. This is exactly my point. Most trawlers combine tank supply and THEN insert primary filters.
If you choose to go down this road, check flow requirements for the engines and flow limits of the filters. Some engines return a fair amount of fuel to the tank. Wouldn't want the engine with the stronger lift pump, starving the other engine of fuel.

Also understand that if you develop an air leak in that one filter, you may get both engines air bound (stopped). So after you figure out and fix the problem, you'll have 2 engines to bleed.

Ted
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Old 02-09-2018, 09:04 AM   #11
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Installing separate primaries really only makes (redundancy) sense if each engine is fed independently from separate tanks/lines. Assuming this is not the case then it is better to size the primary for both engines and have duplex primaries that allow switching from the first filter to the second filter in the event of blockage. This also allows for changing the "blocked" filter while the redundant filter is doing its job for both engines.
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Old 02-09-2018, 09:36 AM   #12
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In the world we live in, fuel problems are possibly the largest single cause of engine troubles. Ted
I must live in a different world. In over half a century of boating never had fuel issues shut me down - ahem, shortages excluded. Nor do those who write in about engine issues on boatdiesel. Maybe though, good filtering setups negate fuel issues in this "enlightened" era.

Thus we see troubles from cooling systems, exhaust runs, on engine electronics, bad batteries, after coolers, fuel pumps, poor maintenance and sheer neglect.
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Old 02-09-2018, 02:45 PM   #13
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I must live in a different world. In over half a century of boating never had fuel issues shut me down - ahem, shortages excluded. Nor do those who write in about engine issues on boatdiesel. Maybe though, good filtering setups negate fuel issues in this "enlightened" era.

Thus we see troubles from cooling systems, exhaust runs, on engine electronics, bad batteries, after coolers, fuel pumps, poor maintenance and sheer neglect.
For the record "running out" is a fuel related problem. Ask a tow operator how often they do fuel drops. Most carry 5 gallon cans of gas and diesel.
A plugged filter that you can resolve yourself is a fuel related problem. Having a problem doesn't mean you were towed in or couldn't resolve it yourself, it means the engine stopped getting fuel that would allow it to run.

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Old 02-09-2018, 07:37 PM   #14
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Great response. Filter the tanks, not the engine. This is exactly my point. Most trawlers combine tank supply and THEN insert primary filters.
"Most". Well, I don't know about that, but yeah I isolate one tank to one engine as well and have done soft the last 3 boats.

Of course, I don't stop and fill one tank here, then stop somewhere else and fill the other tank, so if trouble were to strike I've probably got the same problem from the same source in both tanks and therefor engines. But....in case I have only one dirty tank, I'm ready to crossover and feed both engines from the one good tank.

The other scenario, and this one I've certainly heard play out previously is that in lively crossing the gauges register a restriction or the bowls start filling with crap. If you have separate primaries and a sufficient fuel manifold (nicely labeled as maps don't work so well when your getting tossed around with a flashlight). You can in an instant run both engines off the "other" tank, swap fuel filters on the offline tank, swap tanks, rinse/repeat and then continue to watch bowls and change filters on the fly without shutting down, cause who knows if you will get restarted with your single group 31 battery you replaced that big 8d with.

Most of the people I've heard stories of bad fuel were able to re-start and continued to replace filters as they journeyed towards their nearest safety and until they ran out of filters. Mostly they made it. Mostly.
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Old 02-14-2018, 08:22 PM   #15
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You don’t have to take on a load of contaminated fuel to experience contamination problems, many vent fittings are positioned such that really sloppy seas can push significant amounts of water into them, deck fills can develop bad O rings, under tightening, or (not me!) can be left off after refueling, allowing water to enter the tanks.
Better to be prepared with MORE than adequate filtration for each tank.
Check out the articles section at sbmar.com for the best advice I’ve found on the subject.
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