Fuel tank access port

[malyea]

Lots of crud in my ‘87 Irwin 43 fuel tank and only access is three 1/2” threaded holes under a 12” square cabin sole lift panel. Polishing fuel seems silly (putting clean fuel back in a dirty tank). I’m reluctant to disassemble the cabin interior to remove the cabin sole to cut multiple access ports into the molded fiberglass tank...
I’m considering cutting/installing one 6”-8” access port under the 12” lift panel for tank cleaning - not perfect but significantly better access than the 1/2” holes.
Any better ideas would be appreciated! Thanks

[Bill O]

I assume the fuel in the pan is from your fuel filter. How long did you run that filter to accumulate the crud?
Ideally you are right, adding clean out ports to clean the tank would be the best to get the most crud out, but not fun trying to rebuild the cabin sole and fiberglass cabin insert.
Could try make your own polisher w/an automotive 12V electric pump, a hard metal pick up tube to get to the bottom of the tank and a filter, then pump out your fuel into 55 gal drums (depending on your tank size). Not perfect but did this before we moved our boat a few years ago and worked well. Barely even see a blip on the vacuum gauge when motoring in really sloppy conditions. This is something simple to try to see how it works vs. cutting up your cabin sole.
Also if you are using a biocide, this is a double edge sword and can create loads of black crud in your tank.


Bill O.

[tkeithlu]

Several approaches to try before you start cutting the tank in that very awkward space. Other than suctioning it with a thin "straw," consider draining it with your fuel pump and flushing it with fresh diesel, and draining again. Messy, but every thing can be filtered in the process. If you have to have better access, consider making one of the 1/2" ports into a 3/4 or 1 inch port, while sizing down to remount the pickup tube or fuel return.

[malyea]

I’ve decided...

I’ll soon be cutting a 6”x6” access port thru the top of my 32 year old molded in fiberglass diesel fuel tank on my ‘87 Irwin 43 sailboat.

I’ll have the local machine shop fashion a 1/8” + thick aluminum cover plate (unless there’s a better material...?).

Question - how do I attach the cover plate to the tank top in a leakproof, seaworthy and shipshape manner?

What’s best ... 12 self tapping screws, glass in a “aluminum rim” tapped for machine screws, imbedded metal threaded inserts into the fiberglass to receive machine screws, drill and tap the surrounding fiberglass for course thread machine screws...or...?

Here’s a photo of the 12”x12” cabin sole opening under which I’ll be cutting the access port into the tank top.

The question is - how best to attach a cover plate to a fiberglass fuel tank?

Thanks

[wingssail]

Originally Posted by malyea

What’s best ... 12 self tapping screws, glass in a “aluminum rim” tapped for machine screws, imbedded metal threaded inserts into the fiberglass to receive machine screws, drill and tap the surrounding fiberglass for course thread machine screws...or...?

Assuming that the fiberglass is at least 5mm thick then I would use #12 (or 1/4" x 20) machine screws spaced about 25mm apart around the hole. The top of the tank where the hole is must be flat. The screw holes must be tapped with the proper sized tap. You can then use a rubber gasket and/or sealant. Be careful not to over tighten the screws which could strip the threads.

[sailormed]

When I installed an inspection hatch in my fuel tank I inserted bolts in the pre drilled holes, from inside the tank to outside. The end of the threaded bolt was flattened on 2 opposing sides, enabling me to put a nut on the bolt and tighten the nut while holding the bolt with a big plier. It works, you just need 2 hands.

[malyea]

Originally Posted by wingssail

Assuming that the fiberglass is at least 5mm thick then I would use #12 (or 1/4" x 20) machine screws spaced about 25mm apart around the hole. The top of the tank where the hole is must be flat. The screw holes must be tapped with the proper sized tap. You can then use a rubber gasket and/or sealant. Be careful not to over tighten the screws which could strip the threads.

Drill and tap directly into the fiberglass?

[scherzoja]

You can make a fiberglass (epoxy?) ring the has an ID = the OD of the hole. OD of the ring = a hair bigger than OD of the metal plate. Make sure the top of the ring is perfectly flat (plate/gasket will seal to the top of this ring), glass that ring to the top of the tank with epoxy putty. Make sure you use enough epoxy putty to seal around the ring/tank interface to prevent leaks.

Now tap into the epoxy ring and install your metal plate with gasket. If you make ring about 3/8" thick, that would give you enough thread depth to tap and you won't penetrate the tank with the bolts (potential leaks).

[mabowers]

I am considering something similar. There was an existing soft patch on top of my 50 gal fuel tank. I just had a mechanic pull it, to power wash, solvent rinse, & dry my tank, after I plugged 2 Racors during a storm 6 miles off Sarasota FL a couple weeks ago. I could not believe the sludge & slime we pulled out.

Leaning toward replacing the soft patch with a G-10 plate and cover, with some threaded inserts to hold the (gasketed) cover on rather than trust the existing tapped holes in the fiberglass tank top.

[gamayun]

Following! I'm having to do the same soon (but with an aluminum tank) so any discussion about fuel polishing and access ports kinda perks me up

[malyea]

Originally Posted by sailormed

When I installed an inspection hatch in my fuel tank I inserted bolts in the pre drilled holes, from inside the tank to outside. The end of the threaded bolt was flattened on 2 opposing sides, enabling me to put a nut on the bolt and tighten the nut while holding the bolt with a big plier. It works, you just need 2 hands.

Interesting...let’s say I’m using twelve bolts, all sticking up out of the tank from inside to outside - how do I get all twelve bolts to stay in place while I lower the cover plate in place to then add a washer/nut to each of the bolts? Thanks

[malyea]

Originally Posted by scherzoja

You can make a fiberglass (epoxy?) ring the has an ID = the OD of the hole. OD of the ring = a hair bigger than OD of the metal plate. Make sure the top of the ring is perfectly flat (plate/gasket will seal to the top of this ring), glass that ring to the top of the tank with epoxy putty. Make sure you use enough epoxy putty to seal around the ring/tank interface to prevent leaks.

Now tap into the epoxy ring and install your metal plate with gasket. If you make ring about 3/8" thick, that would give you enough thread depth to tap and you won't penetrate the tank with the bolts (potential leaks).

I think your main point/advantage is to avoid drilling thru tank top in order to negate the risk of leakage - correct.

I’d like to gain a higher level of confidence in “drilling and tapping” threads directly into either fiberglass or cured epoxy - have you (who has...) actually drilled and tapped threads into fiberglass and/or epoxy? (Obviously I haven’t - have only drilled and tapped aluminum).

Thanks

[malyea]

Originally Posted by mabowers

I am considering something similar....

Leaning toward replacing the soft patch with a G-10 plate and cover, with some threaded inserts to hold the (gasketed) cover on rather than trust the existing tapped holes in the fiberglass tank top.

What type of “threaded inserts” are you considering? I’ve also thought of something similar. Thanks

[FL Dan]

Take a look at this site. They have an interesting tank access "answer":
http://www.seabuilt.com/

[scherzoja]

Malyea,

I have tapped both epoxy and fiberglass. Polyester-based fiberglass matrix does not tap well or hold machine screws well. I did this as a test and will never tap into poly-based fiberglass (other than temporary holding of something). I will and have tapped epoxy, both straight epoxy and mixed with powered glass filler. I don't trust it for heavy loads like grab rails, cleats, or winches (those get through bolts), but I will use tapped epoxy for things like securing deck fills, the storm hood over the companionway hatch, securing deck hatches (as long as there is a 'breakwater/dam" around the hatch to deflect green water), water tank lids under settees, and interior shelf cleats where there is a possibility of moisture causing wood cleats to rot.

Water Tanks - New Custom Access Ports - LA Gallery
My water tank lids are slightly different for your project in that the epoxy 'ring' is on the inside and they are blocks, not a continuous ring.


Epoxy is better for tapping and holding the screws. Somewhere on this forum, Thin Water posted posted a thread discussing whether epoxy with no filler or epoxy with filler (and what type of filler) is stronger for holding machine screws.

For your application, making a ring of epoxy (or better, buy G10, https://www.mcmaster.com/g-10) might be a good solution. G10 will have a perfectly flat top surface for the gasket to seal and when you use epoxy putty to adhere this G10 ring to the tank top, you will not have to worry about making the top of the tank perfectly flat/smooth. You also will not have any bolts penetrating the tank.

I agree with another poster that you should use a round hole, not a square hole (cut with a 6" or 8" hole saw).

Originally Posted by malyea

I think your main point/advantage is to avoid drilling thru tank top in order to negate the risk of leakage - correct.

I’d like to gain a higher level of confidence in “drilling and tapping” threads directly into either fiberglass or cured epoxy - have you (who has...) actually drilled and tapped threads into fiberglass and/or epoxy? (Obviously I haven’t - have only drilled and tapped aluminum).

Thanks