Correct immersion heater size for 1050x450 cylinder

[Schmill]

There are a couple of threads around with similar (or same!) titles, but none quite get to the issue of what I am trying to resolve.

I have a Glenhill stainless Envirofoam tank, 1050 x 450 (140L)

This has a TOP fitting immersion heater.
Our boiler has currently packed up so we are using the immersion for hot water, but seem to get very little from it.
The fitted immersion is a "TIH 650" 3kW, which I believe is a Tesla Incoloy 27" heater.

By my reckoning, a 27" heater, is approximate 686mm, or approximately 2/3rds the height of the tank - which surely means that only 2/3rds of the tank will be heated (heat rises after all!)
I understand that 27" is "common", but is this right for this size tank?

Would getting a 36" (914mm) heater like this one (https://www.tlc-direct.co.uk/Products/IH336CTC.html) serve us better and give us more hot water?

Will I need to be wary of the heat exchanger coil in the bottom of the tank, or are the tanks designed such that the immersion can reach to the bottom without fouling it?

Thanks in advance!

 

[ReJect]

The tanks are designed that the immersion element shouldn’t fowl the coil in cylinder. Some have the element boss on an angle.
36” element might do your cylinder.
An external Willis immersion heater is ideal for heating water near instantly and to as little or as much quantity as you want.
However, copper and stainless steel have a reaction, so I have never installed one connected to a stainless steel cylinder.