Lee's water heat exchanger:
  
>     J tube \ ___
>             /               water tight tube
>       out__|_____________ /
>             \_|__________|___
>               |__________|_ 
>                             in
>     

You can buy these from Wizard. They're not cheap, I think about 40 quid, but 
it would save a lot of work making your own. They definitely work. A friend 
uses one to heat water for camping and it gets a 10 gallon tank really hot 
after a 10 miles or so.

Andy E.

A.C.Erskine@lut.ac.uk