Fuel Filters

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PhilPB

Guru
Joined
Oct 5, 2021
Messages
676
Location
Palm Beach County
Vessel Name
Sun Dog
Vessel Make
Mainship 34
So I ran my tanks down to 1/4 full out of laziness and when I went for a cruise I ended up in rather turbulent water, which in turn seriously agitated the tanks to where the fuel gauges went violently from empty to 1/4...back and forth...back and forth. The next time I went out the engine was surging about 200 rpms every 15 minutes or so. The surge interval was for about 20 seconds every 15 minutes but that was all. I decided it was time to change my fuel filters. I have a Racor 500fg primary filter and then 2 engine mounted filters on my Perkins T6.354. In the past I've had the maintenance done by mechanics and decided that I should learn and do it myself. So I change the primary Racor and it had a 2 micron filter (it was black at this point) and the engine filters were 10 micron. I ran the boat for a bit and it was still surging. I called an on-call mechanic I've used before and asked him to drive 20 miles to fix whatever is wrong. He asked for all the details of what happened and what I did. He laughed first and asked if I thought about what I just told him (2 micron primary, 10 micron secondary (2 of them)). He said that a lot of people go with the 2 micron primary to avoid needing to change the PITA secondaries. Basically, polishing the fuel before it gets to the on engine filters. He then told me to go with a 10 micron primary and take it for a good run, which I did. After changing to the 10 micron primary the engine ran perfect for me and had a couple beer cruise for a couple hours. With the 2 micron primary, which it has always had that setup at WOT would run about 2250 rpm. After going to 10 micron WOT is now 2400 rpm and I picked up 1 knot! I guess I've been starving my poor engine. During this exercise I also filled up the tanks with an additional 150 gallons of fresh and clean diesel.
 
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I have my doubts that there isn't more that's being missed.

If the boat wasn't surging before you ran the fuel low, why was it surging when you replaced the filter with the same micron rating?

If you check Racors website, the 30, 10, and 2 micron fuel filters all have the same flow rating (when new). While it's easy to accept that the old filter was fouled and reduced the flow rate, a clean filter shouldn't have been an issue.

I agree with the mechanic that the 10 or 30 micron filter would be a better choice in your setup. I'm doubtful that your engine was never returning fuel to the tank when running WOT. Not sure how fast your boat cruises, but gaining one knot for an increase of 150 RPM seems incredible.

Ted
 
I have my doubts that there isn't more that's being missed.

If the boat wasn't surging before you ran the fuel low, why was it surging when you replaced the filter with the same micron rating?

If you check Racors website, the 30, 10, and 2 micron fuel filters all have the same flow rating (when new). While it's easy to accept that the old filter was fouled and reduced the flow rate, a clean filter shouldn't have been an issue.

I agree with the mechanic that the 10 or 30 micron filter would be a better choice in your setup. I'm doubtful that your engine was never returning fuel to the tank when running WOT. Not sure how fast your boat cruises, but gaining one knot for an increase of 150 RPM seems incredible.

Ted


After initially changing the filters, still using a 2 micron , I ran at low rpms after leaving the slip and turned around after about 10 minutes due to the continued surging. I suspect I didn't run it long enough. I'm going to change back the filters shortly to the initial configuration and see what happens.
 
Ted, what do you mean by this? Please explain.

If this is a single engine displacement hull boat, I would be very surprised to see an increase from say 7 to 8 knots with an increase of 150 RPM. If cruising at 7 knots takes 2250 RPM, I would expect it would take atleast 2,600 RPM to go 8 knots.

Ted
 
If this is a single engine displacement hull boat, I would be very surprised to see an increase from say 7 to 8 knots with an increase of 150 RPM. If cruising at 7 knots takes 2250 RPM, I would expect it would take atleast 2,600 RPM to go 8 knots.

Ted


Actually, at 2250 rpm I can hit ~11knots. At 2400 I was going 12 knots (plus a little) Maybe the wind, maybe knot (haha) but the boat never hit 2400 rpm or 12 knots prior to the 10 micron filter install. Just me and the gps are the only witnesses.
 
If this is a single engine displacement hull boat, I would be very surprised to see an increase from say 7 to 8 knots with an increase of 150 RPM. If cruising at 7 knots takes 2250 RPM, I would expect it would take atleast 2,600 RPM to go 8 knots.

Ted

Gotcha. But the boat in question is an old 34 Mainship. They had a semi planing hull.
Those boats can be "on plane" at 7.5 knots with big trim tabs.
I know that after I repowered mine with the 6BTA 270 I would get 1 knot every 100 rpm once it was "on plane" (at approx 7.5/8 knots with the trim tabs down)
 
To add additional info. My normal cruise is at 1900 rpm and depending on conditions I am at 8-9 knots at that rpm. I do have trim tabs, which can have an impact of +/- .5 knot.

I also mentioned it was a 2 beer cruise. The beers are NA, so that wasn't a factor.
 
This post couldn’t have come at a better time, I am currently having similar issues with filters and fuel. Thanks for the post
 
As mentioned, I doubt with clean filters your filter's micron level has anything to do with it. I run 2 micron in my racor 1000s with a 30 gal per hr cruise burn rate. WOT it's probably 40+. With clean filters my vacuum gages show low vacuum indicating low restriction. 10 and 30 micron show no difference. I'm battling a fuel contamination problem and don't want bad stuff getting to the engines. The vacuum gages tell me when to change the filters. Tony Athens sells the vacuum gages with the max indicator if you're interested.
 
Before deciding on what micron primary fuel filter you need, it's helpful to know what mic the secondary OEM filter is. If 10, then a 10 microwave primary makes sense. On my engine, the OEM filters are two microns, so a two micron primary would be best.

Just FYI, but even "fresh" fuel can have particulates across the micron spectrum, so if it's heavy with junk in the 2 - 10 mic range, and your engine wants fuel filtered to 2 microns, you'll clog the OEM filters pretty quickly if you put a 10 microwave filter upstream.
 
Before deciding on what micron primary fuel filter you need, it's helpful to know what mic the secondary OEM filter is. If 10, then a 10 microwave primary makes sense. On my engine, the OEM filters are two microns, so a two micron primary would be best.

Just FYI, but even "fresh" fuel can have particulates across the micron spectrum, so if it's heavy with junk in the 2 - 10 mic range, and your engine wants fuel filtered to 2 microns, you'll clog the OEM filters pretty quickly if you put a 10 microwave filter upstream.


I have looked for quite a while and cannot find what micron rating the engine mounted filters should be,per Perkins. With so many engines produced I know the information is out there, I just haven't come upon it yet.
 
I have looked for quite a while and cannot find what micron rating the engine mounted filters should be,per Perkins. With so many engines produced I know the information is out there, I just haven't come upon it yet.
I had to call CAT to find out what mine was.
 
Easy way to know all the time what the condition of your racor filters is is to invest in the AcrossOceanSystems.com fuel vacuum nmea2000 sensors.
They are specifically designed to fit Racor filters single or dual.
You can see installation video here:

https://youtu.be/2_t354gV3ao?si=X1Q4USNruU60DJW_

You can setup alarm on your mfd and do preventative maintenance when required, rather than getting surprised.
 
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Gotcha. But the boat in question is an old 34 Mainship. They had a semi planing hull.
Those boats can be "on plane" at 7.5 knots with big trim tabs.
I know that after I repowered mine with the 6BTA 270 I would get 1 knot every 100 rpm once it was "on plane" (at approx 7.5/8 knots with the trim tabs down)


Jay, you have me curious. With the 6BTA 270, which is >105hp more than mine @ 165hp, how much of a speed increase did you experience?
 
Jay, you have me curious. With the 6BTA 270, which is >105hp more than mine @ 165hp, how much of a speed increase did you experience?

I originally had the 160 hp Perkins and. 2.1:1 gear ratio. Boat would get to about 10 knots at WOT and i could cruise at 8 knots, better at 7.5.
After the repower with the Cummins 6 BTA at 270 hp and a 1.56:1 gear ratio, the boat would run at 18.4 knots at WOT. I would normally cruise at 12 knots but could cruise at up to 15.5 knots and be within Cummins’ guidelines for continuous operation.
These hulls cant really run over 16 knots because the get squirrelly as they “ chine walk” at that speed. ( roll from side to side).
 

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