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Old 08-13-2018, 04:03 PM   #1
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Default Unable to get over 20MPH

Hi everyone,
Want to start by saying new to this forum and second year boating.

I have a 2005 Maxum 2400SC3 5.0L. Boat ran great in June. Was able to plain and get up to 35MPH. Was away for a while in July but he next time I was able to use boat was late July.
Now I am unable to get boat to plane, and max mph is 19. RPM's seem to stall around 3100.
I did start season with a 21 pitch prop, but I changed it last week to a 19. Did notice RPM's go to 3500 and MPH to 22.

I do have marine growth as it is unpainted. Could marine growth cause this type of problem?

Engine has 480 Hours. Boat was very well maintained by previous owner.

Thanks!
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Old 08-13-2018, 07:00 PM   #2
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Growth will definitely keep you from planning out..
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Old 08-13-2018, 07:16 PM   #3
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Wow, I had no idea. Right at waterline the growth is about 2-2 1/2" long, further down much less.

Is it possible to test engine by seeing if rpm's can reach 4000? I tried but the warning alert buzzer started sounding.

Thanks again.
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Old 08-13-2018, 07:20 PM   #4
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Is it a carberated engine? If so it sounds like the secondaries are not opening.
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Old 08-13-2018, 07:23 PM   #5
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No, its a MPI 5.0L
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Old 08-13-2018, 08:45 PM   #6
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I would start with a bottom cleaning. This has a bigger impact than you would think.
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Old 08-13-2018, 09:06 PM   #7
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If you are going to leave your boat in the water without antifouling paint rent a slip with a boat lift...

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Old 08-14-2018, 09:20 AM   #8
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I wonder if your gas is bad/ old after setting. Also depending on where you are boating the water is hotter in July which could effect your water & fuel temps. I can't imagine 30-40 days of marine growth would slow the boat that much. Was your U/W configuration the same each time ie; fuel, water, people, and gear?
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Old 08-14-2018, 01:24 PM   #9
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I wonder if your gas is bad/ old after setting. Also depending on where you are boating the water is hotter in July which could effect your water & fuel temps. I can't imagine 30-40 days of marine growth would slow the boat that much. Was your U/W configuration the same each time ie; fuel, water, people, and gear?
Last year filled tank and added proper amount of stabilizer.
As for U/W config, pretty much the same.I have been out about three times since, including the last time with prop change.

Should I test the rpm by racing engine in neutral? Or could this damage the engine? I just ant to see if it will reach 4K.
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Old 08-14-2018, 01:28 PM   #10
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Wow, I had no idea. Right at waterline the growth is about 2-2 1/2" long, further down much less.
This is too much growth. Haul the boat and clean the bottom. Max RPM should be between 4400-4800.
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Old 08-14-2018, 01:41 PM   #11
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Reving the engine in neutral proves nothing and can do damage. Without a load even an engine with a missing cylinder will rev easily.
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Old 08-14-2018, 02:36 PM   #12
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Something is wrong other than the growth. When was the last complete tune up? Plugs, cap, rotor etc...
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Old 08-14-2018, 04:35 PM   #13
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I would haul it out clean the bottom, replace that old gas and fuel filter, give it a basic tune-up (plugs, wires, rotor, points, and cap) maybe even a new coil while your there and put the 20" prop back on.
Basically try to go back to your 35mph baseline and check your rpms again. Tach's are not always accurate.
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Old 08-14-2018, 08:30 PM   #14
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I had a similar problem earlier this year when I had a couple of extra people on board. I couldn't get up on plane and was stuck at 3100 RPM pushing water.

Based on the advice of the fine gentlepeople on this board, all I had to do was trim the drive all the way down, then trim it back up slightly when up on plane. Worked like a charm. Prior to that, I was getting one of the passengers to go sit right up forward in the cabin, which also worked, but was not at all convenient...

I understand from my reading here that using the trim tabs can also help to get up on plane, though I'm not entirely sure how they should be used.

FYI, I get marine growth very quickly and lose about 200-300 RPM as a result.
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Old 08-15-2018, 02:06 AM   #15
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I would do one thing at a time otherwise youíll never know what the real issue was plus you can spend all kinds of money unnecessarily. IMO start with cleaning the bottom.
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Old 08-15-2018, 03:52 AM   #16
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It's a pretty major event for me to haul my 2700scr back and forth from the lake to the house so I tend to do a couple extra things while she's out of the water. But I do agree with Mike "one thing at a time" until you find the cause.

Please follow-up with us so we can all learn from this situation. I'm curious to know if the marine growth was the culprit.
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Old 08-15-2018, 01:01 PM   #17
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Take it to a boatyard or marina and pay for a short-haul. Call around and get prices. It is very common to arrange a powerwash with the short haul. They usually give a time limit (1-4 hours) or you end up paying by the hour. They can leave it in the slings and powerwash it right there. If you need to do any other minor work, like zincs, or clean intakes, that is a good time. Many places will charge extra for blocking, which isn't necessary for what you're doing. Unless they are hauling a lot of boats that day, you can usually sit in the slings.
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Old 09-08-2018, 07:05 PM   #18
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Well I took the boat out, and holy crap. Here are some photos. Started on the cleanup and will be putting on bottom paint. I contacted interlux for primer and paint for my region.


We will see how the spring goes. Thanks to everyone!
Attached Thumbnails
boat1.jpg   boat2.jpg  
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Old 09-08-2018, 09:36 PM   #19
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Quote:
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Well I took the boat out, and holy crap. Here are some photos. Started on the cleanup and will be putting on bottom paint. I contacted interlux for primer and paint for my region.


We will see how the spring goes. Thanks to everyone!
I'm surprise you make 20 M/H with that jungle on your boat....LOL.
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Old 09-09-2018, 01:35 AM   #20
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ik right.

Last couple of week could only get 10 mph. lol

Started cleaning process with pressure washer. Got a lot off. Need to get between the bunks and scrape off barnacles.

Ordered set of Brownell Trailer lifts. Should help!

Will keep you all posted on the progress and results....
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