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  1. #1
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    Changed motors and now getting massive slip

    I have a Seebold Ski Eagle modvp hull. When I bought it it had a jack plate and 6 inch extension. I ran a 200XS on it first. OK, but sort of light on power. Then ran a Mariner Super Mag 225. Nice motor, should have kept it. The boat ran about 88-89 with the Super Mag and a sporty LU. About 80 with the XS. Both of those were run with a 28 Hill Signature 4 blade and 1.87 gear. I ran across a great deal on a 300 promax so gave it a try. I removed the bracket/extension to help relieve transom stress and help the hull float a little higher with the heavier engine. Saw about 92-93 running a 28 Merc Pro ET with a 1.62 sporty. That works out to be a normal 8% slip at 6100 rpm. I marked the jack plate so I knew where level with the pad was and generally ran all of these motors about 1 inch above the pad.

    The 300 absolutely drank gas. Worse than I could have imagined. So I ran across a 280 and decided to give a whirl. I mounted it up straight to jack plate same as the 300. Ran the same Merc 28 Pro ET prop and it won't get traction. 7000 rpm at about 72 mph with the same 1.87 sporty I ran on the Mariner. 40mph was over 4k rpm. Tried raising and lowering the jack plate pretty far in either direction. It would get a little better with it down further but not much. The motor would spin 6-7k rpm without any effort at all. But I just couldn't get her moving. I understand the motor is lighter but thought dropping the jack plate some would compensate. I couldn't get it to run 70 without turning at least 6500 rpm with the motor so low it felt bad. 25% slip or more.

    I reinstalled the extension and hope to run it this weekend. Hope to get back to normal slip. Just looking for knowledge on all of this. I was completely perplexed by it on my first test run with the 280. The mid section is drilled if it matters. Almost like it's cavitating the prop?

    Anyhow, thanks for any input.

  2. #2
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    Has the gearcase been modified by someone that thinks they know more then the engineers at mercury racing? damaged and repaired? check it over- It often a mystery that only set up can solve when we make changes but with your two other motors working fine at that height and the 3L working okay w/o the setback as well, it leads me toward the case shape causing cavitation.

    Joe

  3. #3
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    Thanks Joe. The gear case is absoutely stock. I bought it new from Merc last summer when the cle that came on the mariner self destructed. Its the same gearcase that ran 88 on Mariner. Not just the same model, the same exact case.

  4. #4
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    Very strange-

    Joe

  5. #5
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    Next time you run see how far you can turn the steering wheel left or right without turning. The change might be giving you paddle wheel causing extra steering input and skeg blowout or burn.

    Basickly youll be in slow left circle but going faster. And then when you steer right to go straight drop speed.

  6. #6
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    Alright. I'm hoping the additional set back is going to help. Every 2.5 i ran on it was fine with the bracket installed. 3L was the only motor that ran right without the bracket. Makes no sense as to why. But its the main variable that changed. I also was running a Hill 28 4 blade thats a slightly larger diameter than the merc ET the last time i ran a 2.5. But the ET ran fine on 3L sporty. Makes no sense to me but im only 18 months into Hi Perf boating...

  7. #7
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    Yep, props can bite you. Some will change things so much you have to adjust a jack plate an “ or more up or down.

  8. #8
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    I reinstalled the setback bracket and its better but still not great. My buddy with 280 on stv is running 100 at 7300 or 7400 rpm with 28 3 blade chopper of some sort. Im seeing low 80s at 7k rpm with 4 blade labbed 28 ET.

    Do the ETs carry weight and grip well or are they more of a lightweight hull race prop? My boat weighs about 1550 with motor. The et props im running are 14.5 diameter. The Hill i used to have was closer to 15. The ET feels like wheel spin in a car. Lots of revs (beyond initial plane) then boat sort of catches up. But still lots of slip.

    I plan to try his 3 blade chopper soon to see how it runs on my hull. Thinking his LU is a taller gear. Are there different gears for 2.5 sporty? Mine is 1.87. His slip number is like 3 or 4 if his is a 1.87 and prop is truly a 28 as stamped.

  9. #9
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    HEY KIRT. HAVE U checked your prop hub. they do fail with a heavy load

    after reading your post, sounds strange. everything the same, but all the slip.? u can test your hub for slip. sounds like a strong possibility, assuming your tach is ok. ART in fl.

  10. #10
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    Just coming back around to this. I was able to get my 28 Hill 4 blade (labeled at 14.75 diameter but measures close to 15) back from a buddy and run it last night. It hooks up far better than the Merc ET props 14.5 diameter. Back to a more normal 70mph around 6k rpm. 80 at about 6300 or 6400. Interesting that the Merc props didn't seem to work well on my boat with a stock sportmaster LU. Doesn't make a lot of sense to me, but results are results. I'll be selling 3 Merc ET's soon. I was running 70 at 7k rpm with the ET's.

  11. #11
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    Might have water pressure but jacked the prop out. Never though to see if you dropped a 1/4” or more and got grip. The things we forget.

    Itd also possible to get props off listed pitch.
    '06 Tracker All-Fish/Tournament V-18 90 Optimax, 46.8 gps Goal one complete reach 45 mph.

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