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  1. #1

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    Strange 951 seadoo questions from a new guy

    My name is Alan and i live in the Houston area. I have an idea for a project and would like to pick your collective brains.
    I would like to build an outboard motor based on the Rotax 951 motor. It will be for a small hi-performance v-bottom hull that should weigh about 850# with driver and motor. The most common motors used on these hulls are 75horse Johnson /Evinrude 3cyls and they can run near 70mph. My boat will just be a river toy that I might one day take to the drags.
    Iím interested in the 951 because of the horsepower and that itís a 2 cylinder and appears to be pretty compact. The 787/800 motor looks pretty good too but we all know theres no replacement for displacement!
    My plans are to set the motor vertical on a fabricated midsection. The mid will be 12Ē from the bottom of the motor to the top of the lower unit. Would like to keep it as compact as possible,dont want expansion chamber sticking out the back. Iím hoping it will weigh 200# or less when completed.
    Before I start this project I have a lot of questions. I should point out that I have never even laid eyes on one of these motors (I have ridden a Ď98XPÖonce). Iíve worked on my own outboards for the last 25 years but never a pwc. My strengths are in machining and fabrication. I work as a manual machinist and have pretty much unlimited access to all the machines at work. My boss is a GREAT guy !!! I also have a small machine shop in my backyard. By the way,Iím tring to do this as cheap as possible.
    I hope nobody gets their feelings hurt by me using a pwc motor this way. I just think itíll be a fun project. I appreciate any advice I can get
    With all that said, hereís some of the questions:
    1. Am I crazy or just stupid for attempting this
    2. Do the 951 motors have any durability issues if kept stock?
    3. Will running the motor vertically cause any problems as long as the crank and counterbalancer bearings are kept oiled?
    4. Is the counterbalance shaft completely necessary?
    5. Looking from the front of the ski, what direction does the motor turn?
    6. about how much does a 951 motor weigh fully dressed, minus the expansion chamber ?
    7.What is the length x width x height with the carbs and exhaust manifold?
    8. about how much horsepower will I lose if I donít use the expansion chamber and use a megaphone instead?
    9. Who would a good shop or individual in the Houston area that I could deal with for advice,parts,etc?
    10.anybody got a rebuildable 951?
    thatís all I can think of for now but I know Iíll have lots more questions.Thanks for the help


  2. #2
    KrunchovXPL-GTX-RX's Avatar
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    A 951 without the exhaust manifold, carbs, flywheels, and mag weighs 88lbs (I know this from shipping them) so I would think a dressed one without the pipe is going to be over a hundred, perhaps as much as 110lbs.

    I would think that you would need the CB shaft and that might be the problem with trying to run it vertical. There is an oil cavity that holds the lube for the drive gears. You may not be able to get enough oil in there to keep the gears in oil.

    You are also going to have to figure how to deal with the water injection since you are not going to have a water box you will not have a control valve (though one could be rigged up, or an AM water injection system used).

    I have started mine without the pipe on it but that is all and I have never heard of anyone trying to run one like that. It may work fine with the megaphone, or it may eat a piston.

    The engine rotates counter-clockwise so if you are looking at it from the front it spins clockwise.

    Even with the mech issues cleared up, that is going to be one big, awkward mess sticking on top of a motor foot. I am not saying not to do it. If you must, have at it. This is how stuff gets perfected/created, but doing stuff that others say is nutty. I do not think it a good idea with any Rotax marine engine. Now, Rotax does make some high output, light weight aviation engines. One of those might do the trick.

  3. #3

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    thnx for the info Krunchov

    since youve shipped them,can you give me some approximate physical dimensions. Like basically what size box did you have to build to ship.

    counterbalace shaft: is the oil cavity a place where unburned fuel mix pools or is it a reservoir that actually has to be filled with its own oil? I believe that many outboards use the intake pulses with small check valves to feed the upper and lower crank bearings some fuel mix for lube. Could rig up something similar. I wouldnt rule out sealed bearings for the counterbalaner
    Durability is not a main concern. A typical day for this boat would be:cruise a couple of miles upriver,pull over on the beach to tell lies and drink a beer make a few fast passes then back to the trailer

    water injection: not familiar with that. Is it for cooling the exhaust? one thing to keep in mind is that the motor will not be enclosed in the hull. It will have a cowling of some sort but built up heat shouldnt be much of an issue. With that said,I wont need a flame arrestor either

    Exhaust: I wuld love to keep the pipe but it looks like it would just be too bulky for what I want to do. I am willing to sacrifice SOME horsepower to do away with the pipe and run a more compact but less efficient megaphone. 10-20 horsepower loss would be acceptable. Production outboards sacrifice a good bit of horsepower so they can keep everything in a neat package. However the benfit of NOT running a tuned pipie is aless"peaky"powerband.
    I do know that anytime you start modding the motor or messing with the exhaust you'll have to re-jet. I plan on being careful but wouldnt be shocked to burn a piston or 3

    You mention that you wouldnt use a rotax marine motor for this. Do you feel they're not durable or is it a weight issue. I dont know what a dressed 75horse evirude is but I'll bet its close to that. as far as an aviation motor,that $ure $ounds expen$ive. Power to weight ratio is one of the main reasons I'm looking this direction. The next best alternative for me is a Yamaha 90 3cy but they weigh about 250#. If I can keepit under that and be at about 110-120 hp I'll be pretty happy. after all this will still be a stock motor.
    I really do appreciate your input. I think the best thing for me to do would be buy a running 951 powered Seadoo and learn as much as I can about it without having to bother yall with these basic questions.
    Last edited by loop63; 01-29-2012 at 10:32 PM.

  4. #4
    KrunchovXPL-GTX-RX's Avatar
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    Quote Originally Posted by loop63 View Post
    since youve shipped them,can you give me some approximate physical dimensions. Like basically what size box did you have to build to ship. I always used a box from a certain remanufacturer that I will not name (cough, SBT, cough). It was pretty much the size of a large UHaul box say about 3'Hx2.5Wx2.5L

    counterbalace shaft: is the oil cavity a place where unburned fuel mix pools or is it a reservoir that actually has to be filled with its own oil? I believe that many outboards use the intake pulses with small check valves to feed the upper and lower crank bearings some fuel mix for lube. Could rig up something similar. I wouldnt rule out sealed bearings for the counterbalaner It is a sealed resivoir that has its own oil. The 787 CB shaft is lubed by the fuel/oil mix but not the 951.


    Durability is not a main concern. A typical day for this boat would be:cruise a couple of miles upriver,pull over on the beach to tell lies and drink a beer make a few fast passes then back to the trailer

    water injection: not familiar with that. Is it for cooling the exhaust? one thing to keep in mind is that the motor will not be enclosed in the hull. It will have a cowling of some sort but built up heat shouldnt be much of an issue. With that said,I wont need a flame arrestor either The water injection is a component of the "tuned" part of the tuned pipe. It does cool the rubber coupler at the end of the pipe, but the pipe itself is cooled by its water jacket. Without the injection, you loose a lot of the pipes effect.

    Exhaust: I wuld love to keep the pipe but it looks like it would just be too bulky for what I want to do. I am willing to sacrifice SOME horsepower to do away with the pipe and run a more compact but less efficient megaphone. 10-20 horsepower loss would be acceptable. Production outboards sacrifice a good bit of horsepower so they can keep everything in a neat package. However the benfit of NOT running a tuned pipie is aless"peaky"powerband.
    I do know that anytime you start modding the motor or messing with the exhaust you'll have to re-jet. I plan on being careful but wouldnt be shocked to burn a piston or 3 Well, at least you know what you may be in for. The question is, if you lost that much power, how is it then better than a unit built for the task (other than the WTF looks you would get at the lake, which would have a certain value, I admit).

    You mention that you wouldnt use a rotax marine motor for this. Do you feel they're not durable or is it a weight issue. It is not durability, they are all as durable as the operator makes them. The problem I see is both the weight and the fact that the layout of the engines was designed to mouted with the crank horizontal. You will have issues with a vertical mount not even considering if you can use the tuned pipe and then how can you properly mount it in that orienation (they are sensitive to mount integrity as it is).

    I dont know what a dressed 75horse evirude is but I'll bet its close to that. as far as an aviation motor,that $ure $ounds expen$ive. The reason I mentioned the aviation engines is that they can be mounted vertically.

    Power to weight ratio is one of the main reasons I'm looking this direction. The next best alternative for me is a Yamaha 90 3cy but they weigh over 250#. If I can keepit under that and be at about 110-120 hp I'll be pretty happy. after all this will still be a stock motor. Even without the tuned pipe you would probably beat that ratio. You also gain a lot by hooking it to a screw as these boats lose some 70% of their power in the pump.


    I really do appreciate your input. I think the best thing for me to do would be buy a running 951 powered Seadoo and learn as much as I can about it without having to bother yall with these basic questions.
    I am not sure what equipment you have available to you or skills you possess but there was a guy who put a 717 (pump and all) into an aluminum boat. There is a build thread about it on PWCToday. He basically made it a jet I/O. You do lose efficiency with a jet pump but it is safer for swimmers and other aquatic life .

    I think the challanges you face can be resolved (even the CB shaft thing) but it comes down to how much money, and time, is "worth it." It would be quite a sight, I must say.

    Have you considered using a smaller Rotax? The 717 is more compact both in the pipe and the engine itself. It does not have the CB shaft issue and is rated at 85hp. You could likely keep that power by being able to use the pipe and you would be a lot lighter than 250lbs.

  5. #5

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    good info
    Is the 787 motor much more compact than the 951? I think theyre rated around 110hp right? less power wouldnt be too bad if theres less weight too. The 717 @ 85hp probably wouldnt be worth the effort unless I was racing some sort of class with cc limits. A 90 horse outboard is pretty easy to come by but anyone can do that. The fact that 30% of the horsepower is lost thru the pump always botherd me. I know the motors in pwc's are more are more advanced than the outboard so I guess I just want to see how they do with a more efficient drive system. I wont be pulling any skier and not planning on getting thrown out so safety isnt an issue. No manatees over here to worry about either
    The WTF looks as you so well put it are a definite factor in this project. I always like to do things at least a little different. Plus being a machinist I cant help wanting to build it myself. You dont EVEN want to know about some of my other ideas. This idea actually seems straightfoward by comparison. I dont however JUST like to be different. My hopes are that it may be a little better than what you can buy. I could go out and find a used SST60 Johnson motor (3cyl 100hp),write a $4000 check for it and be on the water next weekend but that aint my style.I've always enjoyed hotrodding of all kinds. But I get bored when its always the same ole thing over and over. I get no enjoyment from seeing someones hotrod that I know all they did was just write out a big check for it.
    I've attatched a picture of a midsection a guy built. His project was a much larger scale than mine his is a Mercury powerhead with nitrous and a Lenco transmission. Thats the kinda stuff I like. The other pic is of a v6 3.4 liter that was being built by Wayne Taylor from Alabama. He unfortunately passed away before he finshed this project. You may recognize some of the parts on his motor.
    thanks again for all the info I've learned alot already.
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    Last edited by loop63; 01-29-2012 at 10:32 PM.

  6. #6
    dghal's Avatar
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    let us know how this project progress's

  7. #7

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    local guy

    dghal
    I see your in league city
    I'm just down the road in Santa Fe /Hitchcock
    If I ever make this happen I'll meet up w/ you on the San Jac
    maybe next summer

  8. #8
    KrunchovXPL-GTX-RX's Avatar
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    The 787 also has a pipe that wraps around the engine. The 717 has the carbs on one side and the pipe on the other. The water injection is also self contained with no control valve to worry with.

    You still have a much greater HP to weight ratio with the 717 but that is just my .02

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