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Thread: Let's talk realistic speeds.

  1. #1

    Let's talk realistic speeds.

    Being relatively new to the world of vintage jets, what sort of speeds should I expect?

    I have two jet boats. A '73 Tahiti that is 17' with an Olds 455 and a freshly rebuilt Berk JC-A and a new PD. The Olds after fresh rebuild should be making an honest 400 hp.
    The second boat is a 21' MACH 1 with a 454 and a Berk JE-A. No diverter as of yet and Laddie could guess the HP better than me. No PD as of yet and I'm fairly certain it could use a new impeller and wear ring.
    The question is, what would you guys think these should do for top speed?
    Seeing as they both run A impellers and similar size engines they should be fairly similar and they are both slower than I'd expect. Just looking for actual experience so I know what to expect.

  2. #2
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    Re: Let's talk realistic speeds.

    From what I have heard your Tahiti will hate life over 60mph, but the torque monster Olds at 400hp should get it there quickly and easily! The cruiser boat should also hit 60mph fairly easily. 60 was the magic number back in the day for family boats. If you are not reaching that then there is extra weight somewhere (water logged floatation foam is enemy #1!!) Next step is to check the pump seals and rings. The A cut aluminum impy is rated for 500 HP and should push both boats just fine. And lastly remember, pumps like torque at low rpm or very high rpm for top speed. It takes about 100hp to pick up 10mph over stock hp ranges as well unless you have a very light tunnel hull that just go fast 'cuz....
    I am a lot happier when she is in the garage where she belongs

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    Re: Let's talk realistic speeds.

    Just for a comparison Ted's 17' Tahiti with stock Berk c pump no trim with 390 FE motor goes 59mph. Teds a little guy so weight doesn't factor.
    I am a lot happier when she is in the garage where she belongs

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  5. #4

    Re: Let's talk realistic speeds.

    So,,,in terms of RPM's? What am I looking for?
    for exammple this 454 will hit 5200 no problem. Like I said before, I know the impeller needs replacement. It cavitates bad out of the hole.

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    Re: Let's talk realistic speeds.

    Spinning aint winning!!!But how fast did she go? 52rpms means 67mph... call me I want to know
    I am a lot happier when she is in the garage where she belongs

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    Re: Let's talk realistic speeds.

    Which boat?
    I am a lot happier when she is in the garage where she belongs

  9. #7

    Re: Let's talk realistic speeds.

    Hey Bob,
    Other things to consider too are the impeller shaft could be sucking in a little bit of air as the draw from the pump shaft spins. Now before everyone goes nuts on me, hear me out. When I was in the Navy, we were off the southern tip of Vietnam, and Cambodia, and one of the water feed pumps for the boiler when it would spin up started to cavitate then settle in. I kept checking it for any abnormal shafting leakage, and nothing. My Master Chief suggested putting a sight glass on the outlet side and sure enough, air bubbles(He knew all along) what it was doing. After 43 yrs in the Navy he'd seen pretty much it all. But it was a good lesson for me too. Point being, Its not always what we think it might be. Most of the time with experience it is, but every once in a while....You go this one. Food for thought. Oh, cause for the air bubbles? Porosity hole in the top shaft seal cover! Dean.
    Last edited by AG6; 05-16-2017 at 07:20 AM. Reason: Not all things in life blow, sometimes they suck...AIR!!
    AG6= Amazing Grace VI *
    1980 BIESEMEYER 27' VIKING

  10. #8

    Re: Let's talk realistic speeds.

    The Mach 1 is the one I'm focused on currently. The Tahiti should be getting the second rebuild of the Olds by next week.
    So with this a Mach 1 @ 5000 I'm seeing 56 mph. I can't help but feel like I'm pissing away a lot of HP that simply isn't getting turned into forward motion. Now I am no expert when it comes to Big blocks but 5k seems like a lot.

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    Re: Let's talk realistic speeds.

    That's great! But keeping the pretty boat simple means I know if she is off. Gauges are for info trackers. The only false gauge I have is the speedo, its not even hooked up.
    Starting new with new motor I will call out every pissed off fish boat, offshore racer on a small lake, Roger....RAMER?
    I am a lot happier when she is in the garage where she belongs

  12. #10

    Re: Let's talk realistic speeds.

    What's a safe red line for a 454?

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