2001 glastron gx 205 over rev.

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dhollenberger

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Jun 10, 2019
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I need some insight, I have purchased a 2001 glastron gx 205 with a 5.0 mercruiser in it. The prop that came on the boat was a 14.5x19 michigan wheel 3 blade prop, the boat would over rev so I replaced it with a 14.3x21 prop. the boat still seems like it will over rev and i am only seeing about 35 mph. The boat seems to run great, any suggestions?
 
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porscheguy

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Jan 17, 2013
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That should be thunderbolt V which has a pretty conservative rev limiter. If you’re only topping out at 35 with the 21p you’re way over propped. Go back to the 19 and check your tach.
 
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dhollenberger

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From what I have read the wot should be 4800rpm. The 19p prop could push almost 6000. The 21 I did not push but it was running at 5000rpm at 35 mph with more throttle to go.
 
jimmbo

jimmbo

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Those RPMs sound awfully high.
How long have you had this boat?
Can you verify the gear ratio, of the out drive? It will be listed on the band wrapping the upper gear housing.
Also is the tach set correctly, 6 cylinders?
 
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dhollenberger

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I’ve only had the bout out twice. The stickers on the outdrive are worn off, I have all of the boat paperwork from new. It is a mercruiser 5.0 v8
 
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porscheguy

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Yes. But as I said, you may be overreving but not by much. You’ve got a rev limiter that kicks in at around 4950rpm. You’re only topping out at 35 with the 21p prop. You should be around 48-52.
 
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porscheguy

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If you were pushing 6000rpm on a stock small block, you’d be asking where to buy a new engine. You need to connect a handheld tach. Easiest way is with a timing light that shows rpm.

the standard gear ratio for that engine was usually 1.62. Engine is good for 220hp. With everything running properly it will Be overpropped with a 21p. If you’re really turning 5000 rpm with a 21p but only moving at 35mph, then something is off. Even if someone swapped on a 4 cylinder outdrive with a 1.98 ratio, you’d still be at 30% slip which isn’t normal. With a 1.62 you’re at 43% slip. You’re going too fast to have a bad coupler or hub. My guess is the data you’re getting is incorrect.
 
Texasmark

Texasmark

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Tiny tach makes a cheap stick on that works with a sticker on the crankshaft pulley and a hand held infrared (I think) digital reader...cheap. Otherwise get some numbers and go to www.go-fast.com and look up their prop slip calculator. Plug in your appropriate numbers and get your prop slip. Post. with pictures of your hull. If you don't know your gear ratio, on your stern drive, tedious somewhat, but you can rotate your engine (not running), counting the revolution plus additional partial, of the crankshaft pulley, until the prop has made one complete revolution. The ratio is the ratio....something between 1.XX and 2.00.

On the chart leave blank the information box of the unknown. One speed box is if you were turning a screw in wood, the second is turning a screw in water-in motion. Gear ration is just the number, slip depends on a lot of things but 5-15% covers a lot of boats. If worried about your tach, plug in 10% slip and solve for RPMs. Come back with what you learned.
 
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