Emptying the bath quickly - thoughts please!

I'd like my bath to empty faster. The waste pipe runs the length of the bath, through the wall, and bends straight into the hopper.
Someone recently compared the flows with the waste pipe sloping 1:40, versus horizontal, and it made next to no difference.

Thinking - if I lowered the hopper, and took the waste pipe down the wall, the water could be "pulled" out of the waste by gravity, right?
Obviously could pull the trap, so use a Hepone.

But the vertical part of the waste, which is outside and goes into the hopper, would get air up it, breaking the slight siphon effect I want.
So how about adding another hep trap on the end of the waste, inside the hopper. I reckon that might stop air going back as the water comes out??

A non return valve would be smaller and might work, as might reducing the end of the waste pipe to 32mm.

Thoughts, ideas?

Surely the more traps and reducers you have on the pipe, the more the flow would be impeded.

That's true, but what I'm hoping to achieve is a negative head (suck!) on the pipe which is greater than the extra resistance to flow.

The principle of what you're proposing is sound. It is an accepted method of draining large roof areas with smaller rainwater pipes than would otherwise be required. I recently came across a tech. article on this but can't remember where. Maybe a Google on syphonic rainwater drainage might find something?

I know from experience that such syphonic waste pipe systems are very noisy when used with water traps, but as you say the Hep thingy might go some way to reduce this. I would expect a powerful vortex in the bath anyway - could be interesting (I'd definitely want to keep my dangly bits well away from the plug hole)! I expect some of your advanced maths might be called for - it might even be that the whole waste run in 32mm was optimal.

what you need is 2 s traps inline on the vertical drop these will act like a auto flush in a urinal cistern ........and discharge a fullbore plug of water impractical because they`d be outside .......and also you want a separate overflow to the tub because the conventional/modern one will let air into the waste via the banjo ..just don`t ask me for the maths also your drop would need to be 2.1/2 feet to generate the pull so the hopper would be replaced by a plain RWP with the waste pipe in top .......like a tundish

Or ... I've got a few used but working ch pumps... now what could I use for a flow/level switch in the waste pipe