Cuuting a 115mm hole for Wema tank hatch

Halo

Halo

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My fuel tank is thermoplastic - possibly nylon or similar. Access is pretty good from above now I have fitted a 375x375mm access hatch above it via the quarter locker. I am fitting a Wema hatch to allow me to clean out the tank for the first time since commissioning the boat 20 years ago. The method is to cut a 115 mm hole in the tank. Has anyone done this in a plastic tank ? if so how did you do it ? Did you manage to hang on to a 115mm hole cutter for the job ? How did you minimise fine plastic debris geting into the tank
Thanks for any thoughts
 
If you don't want any swarf in the tank, but I cant see it mattering too much if the purpose is to clean it.
Use a 115mm hole cutter that incorporates a pilot centre bit.
I would drill the pilot hole and measure the thickness of the tank, then mark the cutter a mil or two shy of the thickness of the tank, use the hole cutter and finish off cutting through the last mil with a knife.
Or you could have a vacuum cleaner sucking up the swarf as you cut.
 
If your tank is slightly flexible moulded plastic I'd VERY VERY strongly suggest not to use a WEMA hatch. I did and it leaked horribly as soon as the boat heeled and the fuel swished about.

I used a big hole cutter, then pumped out and filtered what fuel was in there before getting arm in and thoroughly cleaning interior. But after the first lively sail I had maybe a cupful or so of diesel slopping all round the shallow bilge including into virtually hidden areas .....

Refitted WEMA with sealant, no better. Threw the WEMA away and made a 4mm alloy hatch with an interior piece with tapped holes and about 20 machine screws bedded on cork/nitrile washer and Hylotyte. That fixed the leak. Took years to get the smell of diesel out of the bilge though, and there is someone who says they can still smell it. Ended up changing the berth cushion above the tank.......

The WEMA may work well on a stiff metal tank, or a very rigid Tektanks type plastic.
 
Most (all?) hole saws have a central drill bit but the OP already has a hole and thus cannot conventionally, or simply, use a 115mm hole saw. He needs to provide 'something' through which the central drill bit can operate - Piece of wood screwed in place, inside the tank, with screws will work (then fill the screw holes).

A 115mm hole will allow you to get your hand and complete arm (unless you frequent a gym too much) inside the tank and cleaning the debris is a minor issue. If you fear the inside of an empty tank - apparently this is why vacuum cleaners have a hose and nozzle :)

Our tanks were similar and were sealed with circular lids, same material as the tank, sealed with sealant and screwed down. Leave sealant to set then tighten each screw has a turn. Repeat each time you clean the tank.

Because you are not using some swish device to seal the access point - you can and size hole saw you want, but it has to be bigger, obviously, than the existing access point.

We have Meercats in Australia, despite our strict environmental rules

Simples

Jonathan
 
Neeves said:
Most (all?) hole saws have a central drill bit but the OP already has a hole and thus cannot conventionally, or simply, use a 115mm hole saw. He needs to provide 'something' through which the central drill bit can operate - Piece of wood screwed in place, inside the tank, with screws will work (then fill the screw holes).
Click to expand...

If he’s got a hole already Starrat make an arbour that uses another hole saw as the pilot to enlarge existing holes.

https://www.screwfix.com/p/starrett...MIgM252ueykQMVmnNHAR0hgjPeEAQYASABEgLMMvD_BwE
 
If the hole saw methods mentioned above prove tricky, I cut a hole for a Wera inspection hatch in my (stainless) tank with a jigsaw. Undoubtedly not as perfectly round as a hole saw but the tolerances were adequate, it worked fine and has not leaked.
 
Stemar said:
If the screws can be fitted within the 115mm, there won't be any holes to fill. In the kind of plastic my tanks are made of, holes are a nightmare to fill because stuff just doesn't stick.
Click to expand...
To add to this - most modern production boat tanks seem to be some sort of polyethylene, and NOTHING sticks to it, hence the need for a top plate on the smooth top surface held down on a gasket with a lot of pressure. and also some Hylotyte in the screw threads When they build boats they tend to purt tank fittings into the top of the tank with self-tappers and a nitrile gasket, which works fine when new, but maybe the 2nd time the self-tappers are undone and replaced, they can't seal tight enough and you get leaks.

The other problem is that some of the "fuel safe" nitrile gaskets actually degrade and go soft and slimy with biodiesel. Cork/nitrile seems to work well though.

The fix I made that actually works is similar in principle to https://fueltankparts.com/cdn/shop/collections/IMG_6265.jpg?v=1658924360 except mine was quite crudely made out of two rectangles of 4mm alloy plate with holes cut in centre. Top plate drilled and the bottom plate tapped for about 20 machine screws.
 
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https://flic.kr/p/WYNKEa

This is one of my workmen welding on to a rectangular tank lid from steel or even stainless steel and the hold to suit can be cut in a plastic tank eith a jig saw. You can see the hoes around the side to allow the cover to be bolted down,

The hold can be made as big as you wish for easy access and big to get your head inside
 
If you have enough access to the top of the tank a multitool will easily cut a plastic tank with very little swarf and would be very controllable. You can get narrow blades and angle the tool slightly to help with the radius, it would likely need a trim around the inside edge after with a knife to tidy up the hole.
 
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