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  1. #1
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    Mercury to SEI Lower Unit speed changes

    I just replaced my lower unit to an SEI, and while the mechanic was at it, polished my 25 Fury prop. I noticed I'm not getting the top end like I used to. I used to be able to do 68mph @ 5800 rpms, but now the most I could get is 63.

    Any one experience a change when switching LUs? Or is it more that the mechanic messed my prop up?
    2000 Bass Cat Pantera Classic
    Mercury 200 EFI (0G374590)
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  2. Member
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    #2
    If the gear ratio is the same it shouldn’t really cause a speed loss, if it’s turning the same rpm it should be spinning the prop the same speed. My guess is prop, but not sure if polishing would cause such a large loss.
    Mercury 2002 200hp Carbed Saltwater
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  3. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #3
    You're probably going to end up biting the bullet and buying an OEM gearcase in the end. That and the 25P Fury is probably steep for that engine/boat combo.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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  4. Member
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    #4
    Quote Originally Posted by EuropeanAM View Post
    .... the 25P Fury is probably steep for that engine/boat combo.
    Respectfully, I don't know why people say that. I have no problem with reaching max RPMs and have plenty of trim left to go past max rpms with enough water pressure. Hole shot is good. I'd even venture to say that I could even turn a 26P.
    2000 Bass Cat Pantera Classic
    Mercury 200 EFI (0G374590)
    26P Trophy Plus / Garmin Force / Garmin EchoMAP / Dual Power Pole Pro Series II

    "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

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  5. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #5
    What RPM's are you seeing with the SEI and the 25P Fury?


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  6. Member
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    #6
    63 mph @5800 rpms. I accidentally went up to around 67 @ 6100 rpms once.
    2000 Bass Cat Pantera Classic
    Mercury 200 EFI (0G374590)
    26P Trophy Plus / Garmin Force / Garmin EchoMAP / Dual Power Pole Pro Series II

    "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

    - Benjamin Franklin

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #7
    Ok... so it's not that 5800 RPM's is all you can reach, you're simply comparing speed previous to speed currently at said RPM's. That's not quite how I read your question, sorry for the confusion.

    That's going to be a prop or gearcase problem- your prop slip is on the North side of 16% (which is just not acceptable). Drop the gearcase deeper in the water and see if that eliminates the excessive slip (3/8" should tell you).


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  8. Member
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    #8
    Quote Originally Posted by EuropeanAM View Post
    Ok... so it's not that 5800 RPM's is all you can reach, you're simply comparing speed previous to speed currently at said RPM's. That's not quite how I read your question, sorry for the confusion.
    He left out that very important information,, I read the question the same way you did.