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  1. #1
    Jarrett's Avatar
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    What is it with the stock Solas 15/20R and the RR ECU?

    I make it a habit to keep all my mods and results in a spreadsheet so I can go back and review periodically what's working and what's not. I was just looking back through tonight and something dawned one me. There is some Black Magic Voodoo going on with the (unbent) Solas 15/20R and the Rotax Racing ECU.

    Last year, I personally rode Frank's Riva Stage II and hit 75.4 on it with low gas and cool air. I don't remember the exact RPM, but it was somewhere in the 7950-8050 range it seemed like. (Frank do you remember?) Regardless, it was the stock ECU, so it definitely was not over 8200 RPM.

    Now flash forward to this year and I've got the reflashed Rotax Racing ECU on my ski now. With some cool temps and an Solas 14/19 repitched to 14/20 by Impros, I've hit a best of 76.3 with me on it at 8400 RPM.

    So hearing all the good results with the prop, I thought I would give Frank's Solas 15/20R a try on my ski. Bear in mind, its the exact same one that I went 75.4 with at 7950-8050 on Frank's ski just last year. I put it on my ski and my results were like 72.4 @ 8150 RPM. I immediately slap the 14/20 back on and average 75.6 at 8350.

    So my question is, with the same rider:

    Why on Frank's Riva Stage II does 15/20R @ ~8000 RPM = 75.4
    and
    On my RR Stage I does same 15/20R @ 8150 RPM = 72.4?

    I mean I understand the two different setups make power at different RPM ranges, but how can two different RPM readings with the same prop yield such substantially different speeds?


  2. #2
    Gotta love this sport! 05Limited's Avatar
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    Re: What is it with the stock Solas 15/20R and the RR ECU?

    Quote Originally Posted by Jarrett
    ...but how can two different RPM readings with the same prop yield such substantially different speeds?
    Jarrett... I feel ya buddy!

    I've always enjoyed friendly banter with my boating buds, and have always stood proud when I win a discussion (arguement) while flaunting my self-proclaimed physics/hydrodynamic expertise ... OK, maybe my somewhat limited knowledge

    But, along comes these 14/19, 15/20 issues and it blows my limited (no pun intended) expertise right out of the water

    Ya gotta love this sport

  3. #3
    EZ Dock of Long Island Shibby1485's Avatar
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    dont even get me started... these props and ECUs and setups are so darn inconsistent.... i wish there was a scientific explanation.... i think anyone would be hardpressed to give you a factual answer about why some skis love certain props and why others hate em...

    similarly, a riva stage 2 performs comparable to a rotax stage 1 which has an ECU and more rpms... that don't make sense!

  4. #4
    bttek1's Avatar
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    This is a very good question why one prop would yield X MPH and another boat with the exact same prop and mods yields a different MPH.

    There are so many variables but lets look at the simplest:

    1: Just because a prop says 15/20 does NOT mean it is that exact pitch. Leading edge could be off as well as trailing edge. Variations between blades..etc..etc...

    2: Water being tested in. Riding a boat that turns 8550 going 79.5 in fresh water testing in salt water will yield 8400 RPM @ 77 MPH so the water itself is a factor.

    3: hulls are not exactly equal and can have a great affect on RPM's and speeds.

    4: Time of the day, barometric pressure could have been higher on one day yeilding more power than another day. Daytime temps and water temps play a large role too.

    5: Are the speeds reported by others the TRUE speeds without any fabrication? Unless I see for myself or from someone you explicity trust, I would consider such info as heresay.

    The bottom line is that you tune your boat for exactly what IT needs and adjust accordingly.

    Brian
    BT Tech

  5. #5
    Jarrett's Avatar
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    1: Yep, same prop
    2: Yep, same water
    3: Diff hull, but still a RXP
    4: Diff day, but very similar temps
    5: Same GPS

  6. #6
    Vern's Avatar
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    I think the point was that the exact same prop, with MORE rpms, runs SLOWER on Jarrett's boat ... we always have thought that the same prop on similar boats, will run similar mph, IF it pulls the same rpm. What we have seen with the 15/20 / RR ECU is that quite often the prop x rpm formula just does not work, which is the real puzzler. With RXPs, GPRs, etc, two boats that run the exact same prop at the same rpm are usually pretty close in mph, IF the hulls are similar. With the RXPs, there is very little magic or secrets in the hull/ride plate, so typically they run very similar mph numbers if the prop and rpm is the same. There is something strange to this combo ...

  7. #7
    jk's Avatar
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    This weekend's rides:

    Did runs with 3-5 bars of gas in the ski and air temp at 87, water temp at 82 and RH at 72%. Ran with stock ECU and brand new 15/20 R out of box, best run was 8050 RPM and 76.3 mph. Then switched to reflashed RR ECU and best run was 8100 RPM and 77.6 mph.

    Mods to ski (so far)

    RR Stage 1
    RR Factory Water Box
    Thru Hull Exhaust
    Riva 2 degree Wedge & Intake Grate
    Ride Plate holes filled
    Custom 4-inch Forced Air Intake.

    Still waiting for RR SC Impeller and install of RR Cam, Sprocket, Shims and Lightweight Retainers.

    Hopefully will see 80mph then.

  8. #8
    jk's Avatar
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    (RG's got bad typing today) that was 76.3 mph on the first test for best run and 77.6 mph on the second test for best run.

  9. #9
    bttek1's Avatar
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    Based on what is being said, I would have to guess that if everything is the same then there can only be signfificant differences in either the hull or the pump assembly or both.

    If you really want to dig deep into this I would pull the pump from the faster boat and bolt it in to the other and then see what you get. Don't forget that even the pump can have minor differences that add up to losses in speed such as excessive clearance in the liner, larger or smaller venturi...etc..etc..

    The ecu should not matter if both boats are turning similar RPM's.



    Thanks!!

    Brian
    BT Tech



    Quote Originally Posted by Vern
    I think the point was that the exact same prop, with MORE rpms, runs SLOWER on Jarrett's boat ... we always have thought that the same prop on similar boats, will run similar mph, IF it pulls the same rpm. What we have seen with the 15/20 / RR ECU is that quite often the prop x rpm formula just does not work, which is the real puzzler. With RXPs, GPRs, etc, two boats that run the exact same prop at the same rpm are usually pretty close in mph, IF the hulls are similar. With the RXPs, there is very little magic or secrets in the hull/ride plate, so typically they run very similar mph numbers if the prop and rpm is the same. There is something strange to this combo ...

  10. #10
    Jarrett's Avatar
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    Quote Originally Posted by jk
    Custom 4-inch Forced Air Intake.
    Well, there goes that theory This prop/ecu combo makes no sense to me. Maybe I just need to add more air to my setup.

    jk, can you post pics or some details on what you did to get your air intake system setup? Thanks.

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