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  1. #1
    Phil's Avatar
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    Question Sea Doo 215hp vs. Kawai 250hp

    Does anybody know the thrust an RXP generates? Kawai has published their thrust at over 1,540 ft. lbs. Also, I have read that the Kawai 250 has 250hp @ 7,750 rpm. Some people on the Ultra 250 thread have stated that the Kawai will turn 8,900 rpms! I think we had better get our facts straight here!

    For acceleration and top speed as well, THRUST and hull efficiency is the only thing that matters, not horsepower and not torque. You know Newton's theory, "Action and Reaction!" We are not turning wheels on pavement here and jet skis (PWCs) are not boats with propellers. There are specific formulas to calculate speed with boats and cars. I am not aware of any formula directly related to jetskis performance.

    I have heard theoretical speeds for the Ultra 250 as low as 62mph and as high as 82mph. My dealer told me that a stock 15F on our lake runs 61-62mph. If Kawai adds 5mph to that figure with the Ultra 250 based on a 60% thrust improvement, I wouldn't be a bit surprised. But a 10mph increase to 72mph may be a little too optimistic.

    I have purposefully posted this thread on both the Sea Doo and Kawai forums in order to observe the reaction of both factions.


  2. #2
    Quote Originally Posted by Phil View Post
    Does anybody know the thrust an RXP generates? Kawai has published their thrust at over 1,540 ft. lbs. Also, I have read that the Kawai 250 has 250hp @ 7,750 rpm. Some people on the Ultra 250 thread have stated that the Kawai will turn 8,900 rpms! I think we had better get our facts straight here!

    For acceleration and top speed as well, THRUST and hull efficiency is the only thing that matters, not horsepower and not torque. You know Newton's theory, "Action and Reaction!" We are not turning wheels on pavement here and jet skis (PWCs) are not boats with propellers. There are specific formulas to calculate speed with boats and cars. I am not aware of any formula directly related to jetskis performance.

    I have heard theoretical speeds for the Ultra 250 as low as 62mph and as high as 82mph. My dealer told me that a stock 15F on our lake runs 61-62mph. If Kawai adds 5mph to that figure with the Ultra 250 based on a 60% thrust improvement, I wouldn't be a bit surprised. But a 10mph increase to 72mph may be a little too optimistic.

    I have purposefully posted this thread on both the Sea Doo and Kawai forums in order to observe the reaction of both factions.
    Momentom>>>

    Yes most motors have a peak HP at a RPM that is not its peak RPM.

    HP is measured at its peak performance. Anything beyond that starts to loose HP even though RPM is rising.

    Normally we dont feel it. Takes less energy to move a moving object then to get an object moving. So if you hit your max HP sooner then you have a stronger pulling motor.

    Thats the basics but others here will tell you the detialed dynamics and probably loose all of us....

  3. #3
    way2fast's Avatar
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    Phil; The Ultra 250 has a redline of 8,900 rpm, as per Kawi literature. This is where the ECU is programed to stop additional revs. The maximum power output is at 7,750 rpm. Reving it over where the max power output is may or may not add additional speed and acceleration. It all depends on how the boat is proped. Remember how outboards perform...the Merc 2.5 Drag motor might have max HP at the 8,000 range but it can safely rev to 10,000 rpm. If you have any hope of winning a race, the motor would have to be proped as near the 10K redline as possible.

    Thrust only determines speed if read from identical pump outlets on identical hulls. It can not be compared to different spec pumps and hulls or those from different manufacturers. There are too many variables, design, nozzle size, boat weight, hull design, etc., involved. Thrust is determined by torque and HP, so you can not say that it is the only thing that matters. On identical boats, the one generating the most thrust would be the one with the more powerful engine and the one that would win the race. different boats = different results

    Richard

  4. #4
    Hydrotoys's Avatar
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    There will be no facts to get straight, until the ski hits the water. Why the constant comparison to the RXP? Why don't you just compare the RXP to a 1996 XP? They both are two seaters. I had a dealer the other day tell me the RXP goes 80mph out of the box. I think we need to get our facts straight here!

    Im not going to compare it to the RXP... It WILL be a direct competitor the RXT. Size, speed, comfort, overall ergonomics. This is a cruiser. Pure and simple. Camping, rough water, long distance cruising, Long Beach to Catalina, etc... Accelleration will be great, and top speed will be around 68 low on gas.

    If it doesn't suffer from blowing up, delamination, quirky handling, or some other strange "personality" trait, I'll be happy to just ride it for what it was built.

  5. #5
    ph2ocraft's Avatar
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    Quote Originally Posted by hydrotoys View Post
    There will be no facts to get straight, until the ski hits the water. Why the constant comparison to the RXP? Why don't you just compare the RXP to a 1996 XP? They both are two seaters. I had a dealer the other day tell me the RXP goes 80mph out of the box. I think we need to get our facts straight here!

    Im not going to compare it to the RXP... It WILL be a direct competitor the RXT. Size, speed, comfort, overall ergonomics. This is a cruiser. Pure and simple. Camping, rough water, long distance cruising, Long Beach to Catalina, etc... Accelleration will be great, and top speed will be around 68 low on gas.

    If it doesn't suffer from blowing up, delamination, quirky handling, or some other strange "personality" trait, I'll be happy to just ride it for what it was built.
    AMEN!!

  6. #6
    Quote Originally Posted by hydrotoys View Post
    There will be no facts to get straight, until the ski hits the water. Why the constant comparison to the RXP? Why don't you just compare the RXP to a 1996 XP? They both are two seaters. I had a dealer the other day tell me the RXP goes 80mph out of the box. I think we need to get our facts straight here!

    Im not going to compare it to the RXP... It WILL be a direct competitor the RXT. Size, speed, comfort, overall ergonomics. This is a cruiser. Pure and simple. Camping, rough water, long distance cruising, Long Beach to Catalina, etc... Accelleration will be great, and top speed will be around 68 low on gas.

    If it doesn't suffer from blowing up, delamination, quirky handling, or some other strange "personality" trait, I'll be happy to just ride it for what it was built.
    AMEN from me too!

    This BOAT is a beast. It is freaking awesome in the articles and the engine specs and such but I just want to see it perform. I have no opinion on it other than it looks good and sounds good on paper.

  7. #7
    I hate to say it but Im shallow. To me anything stock just doesnt tickle me. Kawi could make a 90mph monster and I would still be at the part counter looking for something to bolt on.

    66 is 2 under my low expectation but I know this can be a monster. From what my 15F motor put out. The air intake and exuast mods are already developed. Those two alone are good for 3 to 5 mph gains. Prop and plate 1 to 2 mph and cams could produce up to 5 mph. ECU might complement with timing, squeezing out 1 to 2 mph

    So an off shore kit would probably be around...... 76 to 79 mph. Just thinking of anything I can ponder up.......

  8. #8
    Hydrotoys's Avatar
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    66 is 2 under my low expectation but I know this can be a monster. From what my 15F motor put out. The air intake and exuast mods are already developed. Those two alone are good for 3 to 5 mph gains. Prop and plate 1 to 2 mph and cams could produce up to 5 mph. ECU might complement with timing, squeezing out 1 to 2 mph
    Hmmmm.
    15F vs Ultra:
    -Air intake totally different, but you read it here first... the HULL AIR INTAKE is a very interesting place to look. See my WF pictures. This baby may have been purposefully STRANGLED at the hull inlets. Crazy needs to send me some 4" flow-rites!
    -Exhaust is the same or did they tune it for the Ultra?
    -Plate is flat on 15f. Plate looks like a R/d unit on Ultra. I'd be surprised if there is much left here. I'm sure it can be milled on a touch, unless you ride in rough water. No, not the boat chop that the flat water guys call rough water, I mean 1-2ft cross hatch whipped up mess.
    -Prop is always a great place to look. Wonder who will have the first one out?

    -Cams? ECU? Okay, now we just crossed over into mods that might have a huge effect on reliability. I'll wait on that for a while! Let me know how it works out for you.

  9. #9
    1FastRXP's Avatar
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    Thumbs up

    Quote Originally Posted by hydrotoys View Post
    There will be no facts to get straight, until the ski hits the water. Why the constant comparison to the RXP? Why don't you just compare the RXP to a 1996 XP? They both are two seaters. I had a dealer the other day tell me the RXP goes 80mph out of the box. I think we need to get our facts straight here!

    Im not going to compare it to the RXP... It WILL be a direct competitor the RXT. Size, speed, comfort, overall ergonomics. This is a cruiser. Pure and simple. Camping, rough water, long distance cruising, Long Beach to Catalina, etc... Accelleration will be great, and top speed will be around 68 low on gas.

    If it doesn't suffer from blowing up, delamination, quirky handling, or some other strange "personality" trait, I'll be happy to just ride it for what it was built.
    I second that. My Ultra is on order and I intend to enjoy it for what it is.

  10. #10
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by hydrotoys View Post
    Accelleration will be great, and top speed will be around 68 low on gas.
    I agree 100% with this statement Hydro. I think when these things hit the consumers hands, and they load 'em down with 20gal. of fuel, towels, a 12 pack on ice, frisbee, they're going to see 65mph or so.

    Maybe it's my age, but remember back in the day Mark, when we would buy brand new runabouts and stuggle to mod them to 50mph... and have broken backs to show for it, due to the poor ride quality?
    Nowadays, if someone can't buy a 3 seater that does 70mph out of the crate, it's a turd!!!

    And I'll say it again, anyone who thinks this thing is maxed out from the factory.... wait till you good 'ole boys have them in your hands for a while!!! I have all the faith in the world, and in you guys, that there's probably 3-5mph hiding there somewhere... and the little Hulksters running around on this site will find them all, just wait and see!!!

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