Here's the short of it;

Not a good idea!

There are plenty of reasons, search the archives for things like 'marinise
engine' etc but the basics of it come down to the fact that most marine
engines are rated to do their hp continuously while most road engines are
rated for periodic (maybe not the right term?) use. Apart from all the
normal issues that you might have (cooling, salt water corrosion etc etc)
you also have to worry about mating the engines up to the jet drive unit,
but then the bike engines are designed to have their torque curve throughout
the rev range while marine engines usually have most of it at the top.

Jet drive vessels are pretty bad for fuel efficiency just the same. When i
was in the coastguard in new zealand we had a 6 litre V8 jet boat as our
first response vessel and it really did chew through the petrol. Running
two 1200cc engines at WOT constantly you're going to be sucking gas no
matter how you slice it.

My guess is that even if you already had the two engines, or could get them
for free, with what you would spend on doing all the conversions for heat
exchangers etc, custom fittings, machining, etc etc you could probably sell
your existing ride and buy a better jet ski. On top of that, I dont think
the beemer engines would have a long life living in the ocean. Plus,
without going too far into it you just dont get anything for free. Four
strokes are heavier for the same HP as two strokes, so you would either end
up with less power or more weight, and if you had the same power then you
are probably going to use the same amount of fuel anyway. If you end up
with more power then you would almost definitely end up with more weight,
then you have to start looking at the centre of ballance for your ride,
which would probably move. Worse case scenario; you may not have enough
reserve bouyancy to carry the extra weight so you may lose the capacity to
carry a passenger.

But if you are going to do it, go with twin hayabusa's, then turbocharge
them ;-)

Shaun

wrote in message
oups.com...
I have a seadoo 1800 jet boat it has 2, 85 HP Rotax 2 stroke motors in
it and runs about 45 Mph
only problem is it uses so much fuel you can drain the 150 lt tank in
2 hours at WOT.
I would love to replace thes 2 strokes with 4 strokes I was thinking
of BMW K1200 motors.
I think they would fit the only problem I see is a cooling system
because I use it in salt water.
also the K1200 is rated at around 150 HP so the extra speed would be
nice.

Does anyone have any ideas of has anyone tried this sort of thing ?