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Boat Forum / Building / August 2007BMW K1200 POWERED JET BOAT
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 >I have a seadoo 1800 jet boat it has 2, 85 HP Rotax 2 stroke motors in > it and runs about 45 Mph [quoted text clipped - 8 lines] > > Does anyone have any ideas of has anyone tried this sort of thing ? On Aug 11, 6:41 pm, "Shaun Van Poecke" <shaunvanpoe...@bigpond.com> wrote: > Here's the short of it; > [quoted text clipped - 51 lines] > > > 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 > On Aug 11, 6:41 pm, "Shaun Van Poecke" <shaunvanpoe...@bigpond.com> > wrote: [quoted text clipped - 72 lines] > > 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 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. >I have a seadoo 1800 jet boat it has 2, 85 HP Rotax 2 stroke motors in > it and runs about 45 Mph [quoted text clipped - 8 lines] > > Does anyone have any ideas of has anyone tried this sort of thing ? > 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. > > <tobydej...@connect.com.fj> 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. 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! >> Why not act a little grown up and run at 3/4 throttle for most of your >> ride. [quoted text clipped - 9 lines] > 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. > Thank you for your small minded response! > [quoted text clipped - 25 lines] > > - 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 ,,|,, >> Thank you for your small minded response! >> [quoted text clipped - 35 lines] > 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! > <justwaitafrekinmin...@gmail.com> wrote in message > [quoted text clipped - 51 lines] > > - Show quoted text - Boy, you are a real hero! <plonk>... On Wed, 15 Aug 2007 13:12:28 -0000, justwaitafrekinminute@gmail.com Please don't stop. Only a couple more layers and we get to Nazi and shithead...D'OH! >Boy, you are a real hero! <plonk>... Human bevaviour: Bestiality with a brain > On Wed, 15 Aug 2007 13:12:28 -0000, justwaitafrekinmin...@gmail.com > [quoted text clipped - 4 lines] > > Human bevaviour: Bestiality with a brain Nice try troll. On Aug 16, 2:17 am, justwaitafrekinmin...@gmail.com wrote: > > On Wed, 15 Aug 2007 13:12:28 -0000, justwaitafrekinmin...@gmail.com > [quoted text clipped - 6 lines] > > Nice try troll. I will not be doing the motor change just now It could be done im sure but it will take time and lots of money and R&D thanks for all your advice I will let you know when I start. Regards |
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