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  1. #1

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    Guys, we need to re-think our approach.

    As some of the Yami guys pointed out, the GPR's have won overall top speed at Mudbug every year. They are at a power disadvantage, but still put up high speeds. The RXP and GPR are 2 of the fastest ski's made, but get their speed using 2 totally different approaches. One uses brute strength and the other an efficient hull design. Hp for hp, it looks like the GPR hull is about 5 mph faster than an RXP.

    There were 13 GPR's over 80 mph and only 9 RXP's over 80.

    We need to figure this out.

    Lets start by looking at what could be causing the problem. Pretty much, you can split it up into 4 components that effect speed.

    Power- Clearly, the RXP has the advantage here.

    Weight- The GPR weighs in at 655 lb, whereas the RXP is 795 lb. Is this where all our speed is going? 150 lb is about a 15% difference when you factor in a 150 lb rider. However, I can come within 2 mph of my normal best even when I have a passenger. This is probably part of it, but not all of it. What can we do to shed 150 lb?

    Hull- It's pretty tough to compare 2 hulls and tell which one is faster. Is the GPR hull flatter at the rear? I think this would make it faster.

    Pump- This is an interesting one. Hydrojunkie put an RXP pump on a single-pipe GPR and it LOST 4 mph. Could the Yamaha pump just be that much more efficent than the RXP pump?

    Just looking to get some discussion going here. . .

    Mike


  2. #2
    BLACK MAGIC HEADERS hydrojunkie's Avatar
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    well it depends on how you look at it mike. i hit 88.1 @ 8200 rpm's this weekend on my conversion with a seadoo pump on my conversion. the power curves a totaly different from the yami to the seadoo. i am sure if i had more time we could have narrowed the gap from 4 mph loss to 2.

  3. #3
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    Hull has some effect to...

    The GPR hull has 4 chines, and they are practically at a 90 degree angle...While the RXP has just 2 and they are rounded over...You can see the GPR gets more lift than the RXP, just look at where the spray first starts to exit the hull...More lift equals less drag...less drag is more speed...

  4. #4

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    Quote Originally Posted by Danny View Post
    The GPR hull has 4 chines, and they are practically at a 90 degree angle...While the RXP has just 2 and they are rounded over...You can see the GPR gets more lift than the RXP, just look at where the spray first starts to exit the hull...More lift equals less drag...less drag is more speed...
    You may be onto something. We need to find some pic's to compare.

  5. #5
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    Quote Originally Posted by engineermike View Post
    You may be onto something. We need to find some pic's to compare.

    Here is a pic of a GPR hull, and my RXP hull I had flipped over to true...
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  6. #6
    Moderator The Bandit's Avatar
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    It is all in the ride plate, tabs and how far the pump is set back on a gpr hull. The chines are not even in the water when the hull is on plane. An RXP still has too much hull in the water on plane, and that is why it is significantly slower. IMO

  7. #7
    BLACK MAGIC HEADERS hydrojunkie's Avatar
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    Quote Originally Posted by GPRXP Steve View Post
    It is all in the ride plate, tabs and how far the pump is set back on a gpr hull. The chines are not even in the water when the hull is on plane. An RXP still has too much hull in the water on plane, and that is why it is significantly slower. IMO

    yes i agree thats why jim's plate get's 4 mph on a rxp, due to lift

  8. #8
    I am by no means an expert nor do I claim to know the least bit about hulls and pumps. Here is a little I think I understand from listening to you fellas.

    That said just judging from some pics in the photo gallery it's obvious that the RXP hull has a lot more wetted surface than the GPR. To me this screams out DRAG! The RXP hull is eating up more HP to go through the water.

    When it comes to pumps all I know is that the amount of veins in the stator affect speed. The more veins the better the hookup, but lower top speed. The less veins the worse the hookup but the higher top speed. This is generally speaking. How many vein stator does the RXP have? How many vein stator does the GPR have? I thought the RXP was 12 and the GPR was 6?


    To me it seems there are currently 2 ways to make the RXP hull faster........JD1's pump extension and Jim's open loop rideplate. VERY costly though.

  9. #9
    Moderator The Bandit's Avatar
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    Quote Originally Posted by RXP-REX View Post
    That said just judging from some pics in the photo gallery it's obvious that the RXP hull has a lot more wetted surface than the GPR. To me this screams out DRAG! The RXP hull is eating up more HP to go through the water.

    .
    Bingo!

  10. #10
    BLACK MAGIC HEADERS hydrojunkie's Avatar
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    perfect example of a rxp hull out of the water with jim's plate

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