Cutting 40mm holes in 5mm steel - best tool?

Ring drill 5mm or so holes, junior hacksaw between the holes, half round file?

Just to add to that, nothing blunts a holesaw faster than artex. I can add lots of holes to a metal CU with a cutter, but one ceiling can bugger a cutter up in minutes.

Do you mean a hole punch? I've not seen any that work with stainless over

Slow speed drill and lots of cutting paste to keep it cool.

You might do better to find a local place with a water jet cutter. Surprisingly cheap - assuming you can get the material to them.

A hole cutter will work - but use a very low speed and plenty cutting fluid. And make sure you don't overheat it.

There are hole cutters and hole cutters. Some are only for wood. You can get one for the job, they are not expensive. (Screwfix) It will need to run pretty slow, a biggish drill (ideally a pillar drill or SD type fitted with a chuck) and you need coolant on it, (water or oil ). If using a hand drill, you can rock it as will then cut faster. Cooling is the secret, without, it will overheat and go blunt in seconds.

Yes, that's why I asked! :-)

As it's a boat hull I don't think a pillar drill is going to be practical! :-) However I do have an SDS drill with a chuck accessory so I can use that.

Lots of cooling - OK, thanks.

Having done similar. If the Drill is powerful enough to make the job easy it will be strong enough to hurt you wrists if the cutter jams which it may do just before it breaks through unless you can keep it really straight. Be ready or use a drill with a clutch mechanism. Should you be drilling a hull out of water from a position below your holes remember gravity. Not many will drill such holes without eye protection I hope but you really need something to stop swarf dropping on hair /scalp and down your neck etc as well. The swarf will can be surprisingly hot and sharp as well. Thicker cutting fliuds will help by "sticking" the swarf together to an extent. G.Harman

That will be handy for making oval holes.

It won't matter too much, they're just clearance holes for two small ducts with a (supplied) stainless steel plate for the outside of the hull that terminates them. Thus a bit of 'flap' isn't really an issue because the stainless steel plate will cover any infelicities.

Plasma cutter. If not yours, ask where you got the steel

A decent hole saw will do it too, but it's a boring job. You _really_ want a drill press for this, not a handheld. Also a supply of lubricant.

Magnetically clamped drill is the usual for in-place drilling. They're hireable.

I'd ask a boatyard to do this. A bigger yard, handling steel hulls (or anywhere with much work on narrowboats), will have a magnetic drill and a plasma cutter.