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 ?

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 ?

On Aug 11, 6:41 pm, "Shaun Van Poecke"
wrote:
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 ?

Thanks Shaun for your thaughts I agree with most of what you say.
The weight factor is not a problem the BMW motor is about 30 KG more
but the boat is a large volume and can handle it.
I wonder if I can replace them with the new 4 stroke Jet boat motors?
I am an Australian living in Fiji and just selling the boat and
replacing it is much harder than it sounds.
I think their are only 2 or 3 of these boats in Fiji.
and bringing in a new or second hand boat is a big cost, duity is 27%
+ big freight costs.
I could just pay the huge fuel bill each time I use it $1,98 per lt
here in Fiji thats around $300 to go boating for the day.
It would cost less in the long run but their is also the fact that the
enjoyment factor of 2 strokes is low for me they stink their dirty and
they are loud.
I only wish I had a single Jet it would make any mod alot easer.
If anyone has any ideas please feel free to let me know.

Thanks

Toby

Why not act a little grown up and run at 3/4 throttle for most of your ride.
Save's fuel, wear and tear on the engines drives and you. The people around
you may also like you a little more.

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 ?

On Aug 12, 9:58 am, "John Cassara" wrote:
Why not act a little grown up and run at 3/4 throttle for most of your ride.
Save's fuel, wear and tear on the engines drives and you. The people around
you may also like you a little more.

wrote in message

I did not see anything there in his post that would suggest he has
problems with others. What's your point? You don't like fast boats? We
get that. Do what I do, I go to places where there are speed limits,
that's all. Sharing resources is a big part of boating, if you were
nicer to folks, maybe someone would take you on a fast boat too.

Maybe a fuel flow meter would help you decide how hard you want to
push your engines. Most engines have an enrichment circuit that comes
on at full throttle backing off even a bit can save a lot of gas.

wrote:
On Aug 11, 6:41 pm, "Shaun Van Poecke"
wrote:
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 ?

Thanks Shaun for your thaughts I agree with most of what you say.
The weight factor is not a problem the BMW motor is about 30 KG more
but the boat is a large volume and can handle it.
I wonder if I can replace them with the new 4 stroke Jet boat motors?
I am an Australian living in Fiji and just selling the boat and
replacing it is much harder than it sounds.
I think their are only 2 or 3 of these boats in Fiji.
and bringing in a new or second hand boat is a big cost, duity is 27%
+ big freight costs.
I could just pay the huge fuel bill each time I use it $1,98 per lt
here in Fiji thats around $300 to go boating for the day.
It would cost less in the long run but their is also the fact that the
enjoyment factor of 2 strokes is low for me they stink their dirty and
they are loud.
I only wish I had a single Jet it would make any mod alot easer.
If anyone has any ideas please feel free to let me know.

Thanks

Toby

What is the pay-back time (in hours) of this engine adaptation?
(Assume it uses 1/2 the fuel of the Rotax - Wrong - maybe 3/4 at the
same power level and these can crank out more.)

My bet is it won't pay out this decade. If the new engines were free,
by the time you get the parts made (heat exchangers, water cooled
exhaust manifolds, engine mounts and drive adapters - just for
starters), you will find you could have had a new boat freighted in.

You have two problems. 2-strokes are problematic when it comes to part
throttle fuel consumption - many use almost as much fuel at 1/2 as they
do at WOT. The other thing is that the jet drive efficiency is
typically in the single digits. A very good small boat propeller is
maybe near to 50%.

Good Luck Guy

Matt Colie - Naval Architect Marine Engineer with alphabet soup
Lifelong Waterman and Licensed Mariner

Thank you for your small minded response!

I have owned several fast boats, 80+mph boats. I was a marine mechanic for
more years than I care to explain and have run most every kind of boat from
grossly overpowered 18 footers to twin engine floating hotels. It is common
knowledge the jet boat and jet ski community carry negative appeal with
other boaters. Many jurisdictions are imposing greater operating restriction
on theses vessels. These negatives have been brought on by the operators of
theses vessels. I won't go into it any further!


wrote in message
ups.com...
On Aug 12, 9:58 am, "John Cassara" wrote:
Why not act a little grown up and run at 3/4 throttle for most of your
ride.
Save's fuel, wear and tear on the engines drives and you. The people
around
you may also like you a little more.

wrote in message

I did not see anything there in his post that would suggest he has
problems with others. What's your point? You don't like fast boats? We
get that. Do what I do, I go to places where there are speed limits,
that's all. Sharing resources is a big part of boating, if you were
nicer to folks, maybe someone would take you on a fast boat too.

On Aug 14, 7:48 am, "John Cassara" wrote:
Thank you for your small minded response!

I have owned several fast boats, 80+mph boats. I was a marine mechanic for
more years than I care to explain and have run most every kind of boat from
grossly overpowered 18 footers to twin engine floating hotels. It is common
knowledge the jet boat and jet ski community carry negative appeal with
other boaters. Many jurisdictions are imposing greater operating restriction
on theses vessels. These negatives have been brought on by the operators of
theses vessels. I won't go into it any further!

wrote in message

ups.com...



On Aug 12, 9:58 am, "John Cassara" wrote:
Why not act a little grown up and run at 3/4 throttle for most of your
ride.
Save's fuel, wear and tear on the engines drives and you. The people
around
you may also like you a little more.

wrote in message

I did not see anything there in his post that would suggest he has
problems with others. What's your point? You don't like fast boats? We
get that. Do what I do, I go to places where there are speed limits,
that's all. Sharing resources is a big part of boating, if you were
nicer to folks, maybe someone would take you on a fast boat too.- Hide quoted text -

- Show quoted text -

You ranted on a guy asking a question. You threw a couple of lame
insults. It was uncalled for, period. I won't go into it any
further ,,|,,

wrote in message
oups.com...
On Aug 14, 7:48 am, "John Cassara" wrote:
Thank you for your small minded response!

I have owned several fast boats, 80+mph boats. I was a marine mechanic
for
more years than I care to explain and have run most every kind of boat
from
grossly overpowered 18 footers to twin engine floating hotels. It is
common
knowledge the jet boat and jet ski community carry negative appeal with
other boaters. Many jurisdictions are imposing greater operating
restriction
on theses vessels. These negatives have been brought on by the operators
of
theses vessels. I won't go into it any further!

wrote in message

ups.com...



On Aug 12, 9:58 am, "John Cassara" wrote:
Why not act a little grown up and run at 3/4 throttle for most of your
ride.
Save's fuel, wear and tear on the engines drives and you. The people
around
you may also like you a little more.

wrote in message

I did not see anything there in his post that would suggest he has
problems with others. What's your point? You don't like fast boats? We
get that. Do what I do, I go to places where there are speed limits,
that's all. Sharing resources is a big part of boating, if you were
nicer to folks, maybe someone would take you on a fast boat too.- Hide
quoted text -

- Show quoted text -

You ranted on a guy asking a question. You threw a couple of lame
insults. It was uncalled for, period. I won't go into it any
further ,,|,,


I do believe it was only you who found insult. I quote the direct email
response I received from the original poster:

That's exactly what I do I could not afford to run WOT but she still
likes to drink.

He responds with no resentment and in fact agrees with what I have said. So
take off your internet police cap and return to your lurking position!