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  1. #1
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    making the switch to 4 stroke

    this maybe wrong forum for repower talk, but here goes. stats, old (70's) 15ft cobia hull @ 1300lbs (yes i weighed it), presently 87' 48spl Johnson, cruise about 20mph @4200. looking at 40, 50 or 60 merc. am surprised at weight of hull, thoughts on if a 40 would due, maybe with command thrust option. have been on the Evinrude/Johnson forum for awhile but making switch to 4 stroke, loosing faith in the old Johnson after 2 tow ins.

  2. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #2
    What does the USCG Capacity Plate show for maximum HP?

    If it's 60hp or higher, go with the 60. And command-thrust would probably work nicely on that hull.

    Moving your post to the 4-stroke forum for you.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
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  3. Moderator adchunts's Avatar
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    #3
    Might be worthwhile to check to ensure your hull doesn’t have waterlogged foam. That’s pretty heavy for a boat that size.
    Aaron Campbell
    Barling, AR
    2007 Bass Cat Sabre
    2011 Merc 175 Pro XS

  4. Member
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    #4
    well, found a place that had motors in stock, kind of shocked,(saw 4 or 5 50's and 60's), well made an impulse buy, got a 50, with 4 blade10.6 x 12 prop, got 2 hours on it now, but snow and cold forecast so no more hours for a week or two.
    during breakin went full throttle for 10 to 15 seconds and it hit 6000 with motor fully trimmed in, so spoke to dealer about props and he would like more info, needs rpm trimmed out some, I'm sure it will go over 6, is this ok? in general, hole shot is good, like 4 blades do, i guess never had one before, at 4500 runs at 21.5mph(d_m heavy boat, and yes it had water in it once, installed inspection ports and still vacuuming some water out a year later),not really concerned about hole shot(again d_m heavy boat) but would like to run 22 to 25 at a lower rpm, dealer will let me try 3 blade after more breakin hours and that full throttle trimmed out run.

    EuropeanAM wish id seen your post earlier, kind of wish i'd got the 60 now, any body need a 50 with 2 hours on it, JUST KIDDING, maybe

  5. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #5
    Before you make prop changes, make sure that those RPM's are coming from a Smartcraft Instrument or device (analog tachometers are notoriously inaccurate).


    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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  6. Member
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    #6
    yes a SmartCraft system monitor, quick question, can't seem to get trim to read anything but 0.0, PoP set to 1. and later in CAL settings Trim on.

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #7
    You'll have to turn trim off, as that engine very likely does NOT have a digital trim sensor.


    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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  8. Member
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    #8
    My Tracker Classic XL has a 50 horse Mercury. The highest I've seen the RPM is 5400ish, with a tail wind. Top speed, no wind is 33mph @ 5250ish rpm. The speed and rpm are from my vessel view mobile. According the vessel view 0 to 30 mph was 6.2 seconds. The prop is the one that came on it. I'll take a look at it tomorrow and post how many blades it has.

  9. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #9
    Quote Originally Posted by Woody b View Post
    My Tracker Classic XL has a 50 horse Mercury. The highest I've seen the RPM is 5400ish, with a tail wind. Top speed, no wind is 33mph @ 5250ish rpm. The speed and rpm are from my vessel view mobile. According the vessel view 0 to 30 mph was 6.2 seconds. The prop is the one that came on it. I'll take a look at it tomorrow and post how many blades it has.
    Just to clarify- that engine doesn't come with a propeller. Prop is added as it's rigged for the boat.


    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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  10. Member
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    #10
    yep Euro-AM, thought it was a dealers choose for the Tracker(package boat motor trailer), that hull is way lighter then mine, when i did the full throttle run. glanced at gps and i think is saw 29 mph,
    spoke with the main prop shop here in Norfolk, they too were surprised at the weight of my 15 footer, and suggested maybe a 13in.. but more testing some day

    so now that I've had a few days to start to learn more about merc's and the Command Thrust lower unit, mainly the prop size difference. I'm kind of rethinking/questioning my impulse buy, not Mercury but the horse power and lower unit

    won't be on the water anytime soon, lows going to be in the 20's for ten days maybe longer

    Euro-AM, ever taken an motor back with 2 hrs on it? well i'll just have a few more beers and ------

    two more ? can a trim sensor be added to a 50? and when I do a trimmed out speed run when does the rev limiter kick in?

  11. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #11
    Before we go any further (and I understand you started in another forum), go ahead and look through the Announcements at the top of the 4-Stroke Forum, and provide the Required Info (as well as any of the other info possible). We can then begin to work through your engine-specific questions.


    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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  12. Member
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    #12
    Serial Number: 1C676033, Model Number: 1F51413GZ, Model Desc: 50EFI ELPT Prop: 8M8026625 Current Max RPM: 6000, fully trimmed in Engine Hours: 2 Water Pressure: ?? , Oil pressure 57 all rpm's, SmartCraft 4.0, Owner installed console mount remote control, key switch, trim switch, and SmartCraft, Dealer mounted engine and first start, new 6gal fuel tank/no quick connect fittings(will be changing fittings want to have 2 tanks) and Mercury spin-on fuel filter, first gas - ethanol free will be switching to 87 octane with , usually run 100+hrs a summer and into late Dec. for striper season
    last post may have sounded like i was going to jump, had beer i'm better now

  13. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #13
    Thanks- Trim Sender is available, as is a Digital Trim KIT (8M0109720).

    Rev limiter is more a "window" of progressive action (rather than a fixed RPM like engines of old). Depending on conditions, it may activate earlier (LOAD is a big key factor). While the final number varies from one calibration to another, in general, the final limit is approximately 150 RPM's over the recommended RPM range (in other words, somewhere in the 6150 range).

    Curious about the boat- seem to recall most of those in the 70's as being a tri-hull design.

    Prop wise... sounds like you're going to need a higher-pitch prop (make sure it's really reaching 6000 with the trim ALL THE WAY DOWN (not trimmed out to best performance). Normally the RPM's are determined under a properly-trimmed condition.

    There are some prop options for heavier boats like this, even with the standard gearcase.


    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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  14. Member
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    #14
    Quote Originally Posted by EuropeanAM View Post
    Just to clarify- that engine doesn't come with a propeller. Prop is added as it's rigged for the boat.
    I wondered about that after I typed it. It's the engine, and prop that came on the boat. I'm an automotive technician. I work on autos ~50 hours a week. The last thing I want to do is work on my boat after working all day. I'm an hour from Greenville. I'll be seeing you if I need any boat engine work.

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    #15
    yes, tri hull in front to vee in back, Cobia, when I got it , cap was removed, still off, from what i gather, was gutted, new wood stringers fiber glassed, marine ply wood deck with raised bow deck all glassed over, i installed small console on right side, with 87' 48SPL johnson, hull cracked from bad trailer bunks rubbing glass thin, repaired with glass on inside and out(cut patch in deck), but now definitely water in foam, three 6 inch inspection ports by stern, still wet/dry vacuuming a little water out every week or so, any magic tricks on drying foam? kind of a P.O.S. its ruff looking, had it for 10-12 years

    yep, just reaching 6000 trimmed all the way down, dealer wanted break-in running with it down, but also now would like rpm's in the properly-trimmed condition to recommend a prop change, i know(assume) when I start trimming it out it going to reach the limiter, any concerns about that. im concerned that when a tech reads engine history and sees the limiter hit they'll be questions. I will get several more hours on it before doing that, again not any time soon cause of weather
    my history is - been around boats both large and small all my life but never owned anything bright shiny new

  16. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #16
    That engine is under-propped currently. I would suggest finding something more suitable ASAP so that you CAN trim the engine out and remain within the appropriate RPM's.

    Please keep in mind- these are merely suggestions based (solely) on the info you have provided:

    Spitfire 14P: 8M8026635

    Trophy-4 (14P): 878616A40


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
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  17. Member
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    #17
    UPDATE: got the Spitfire 10.1 x 14, rpms top out at 5500 to 5600, between 4000 and 4500 is about 21 to 25 mph, good cruise speed for me, just a little faster then old 2 stroke, much happier now,