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Thread: 21 superboat slowed down
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07-08-2020, 10:17 PM #1Junior Member
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21 superboat slowed down
So, I have a 1987 21" Superboat. It has a 12" setback and It is rigged with a 2004 mercury 250 3.0 with 520 hours. The boat used to run 72-74 mph all day at 5,200-5,400rpm. Over the past few months, It has been going slower and slower. It currently only does 63-64mph at 5,000-5,150rpm. I brought it to my mercury mechanic and he went over the entire entire top to bottom. Compression was strong, plenty of spark, and a clean fuel system. I run a trophy 26P propellor and have since i got the boat. About 100 hours ago I replaced the propellor with a brand new propellor, same one. The boat was still running 72-74mph after getting the new propellor.
If it isn't the engine and it isn't the propellor, what could be wrong with the hull? The boat does not sit crooked/lower in the water, so I do not believe the core is wet, But I have absolutely no clue what else could be slowing me down 10mph.
Also, Could this mean the engine is due for a rebuild? Even with solid compression numbers? I have ran the boat in between 4,000-4600RPM since I got it with 262 hours on it.
Im sure many of you guys recognize this boat.
Any help is appreciated, Located in South Florida
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07-09-2020, 12:21 PM #2
What is your WOT? Should be 5750 I believe.
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07-09-2020, 12:56 PM #3Junior Member
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07-09-2020, 03:30 PM #45000 RPM
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injectors are clogged probably
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07-09-2020, 07:07 PM #5Junior Member
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07-09-2020, 08:12 PM #65000 RPM
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Might be dropping a coil.
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07-09-2020, 08:28 PM #7
is that on gps ?
most overnite successes usually take at least 10 years
Certified turd polisher,
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07-09-2020, 09:05 PM #8
Shouldn't loose 10mph with only 200-300rpm decrease. Have a scanner to verify RPM with tach?
Checkmate 2002 Convincor 270 496 MAG-HO
2003 Cougar 22MTR w/300xs SOLD
90 21Skater w/300xs - sold
98 STV Euroski w/280 - sold
2006 Trailblazer SS 6.0l 395hp
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KIRCHNER thanked for this post
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07-09-2020, 10:17 PM #9Junior Member
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07-09-2020, 10:18 PM #10Junior Member
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I have a Tac and the mercury smartcraft to verify
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07-10-2020, 01:30 PM #11
Check to see if the boat is wet
21 Intimidator 300x Sold
http://www.youtube.com/watch?v=KBoYUJ58sLE
1/2 21 Skater 300x Sold
http://www.youtube.com/watch?v=tKNFL8b6kWo
30 Activator 850 HP Sold
https://youtu.be/NZgOTNGKzqg
30 Concept Twin 300Rs
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07-10-2020, 05:26 PM #12Junior Member
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powerabout liked this post
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07-10-2020, 07:42 PM #1321 Intimidator 300x Sold
http://www.youtube.com/watch?v=KBoYUJ58sLE
1/2 21 Skater 300x Sold
http://www.youtube.com/watch?v=tKNFL8b6kWo
30 Activator 850 HP Sold
https://youtu.be/NZgOTNGKzqg
30 Concept Twin 300Rs
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suerboatsuperboat liked this post
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07-11-2020, 01:17 AM #14
Probably a coil. It won't show up on a scanner. You will lose 10 mph instantly. Either that or you are eating too many donuts?
" IT'S NOT HOW FAST YOU ARE, IT'S HOW COOL YOU LOOK!!! "
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Things owned. NOTHING!!
Ain't got no toyz, Ain't got no money! I is jus a poor Florida boy!
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07-11-2020, 09:25 PM #15
1982 12' Eli with 7.5 Evinrude (sold)
1963 16' Glasspar with 40 hp Evinrude (sold)
1969 13' Boston Whaler with 40 hp Evinrude (sold)
1990 20' Searay with Merc i/o (sold)
1990 22' Activator with 2.5 EFI (sold)
1998 24' Superboat Step Hull with 300 Pro Max (sold)
1983 21’ Shadow with 300 promax
2003 13’ Boston Whaler with 40 hp Merc
1976 16' Superboat with 75 Yamaha
2010 30' Superboat Center Console with twin 300 Mercs
One good hour on the water will make you forget 24 bad hours on land
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suerboatsuperboat, bayman96 liked this post
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