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Old 09-03-2018, 10:56 PM   #1
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Default WOT rpm

We just bought a 1999 Maxum 1800sr. It has the 4.3 Merc and a 1918p21 prop. Just curious as to what I should be seeing for rpm at WOT and what recomended cruising rpm would be? We went out today and it will hit 5700-5800 rpm fairly easy. Is this too high.
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Old 09-04-2018, 12:52 AM   #2
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Yes way to high, should be 4400-4800. The boat is under proped. WOT should not be ran for more than a few minutes at a time.
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Old 09-04-2018, 01:04 AM   #3
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Am I going to to do any damage by using it this way? Sorry for the ignorance this is my first boat.
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Old 09-04-2018, 01:12 AM   #4
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Not if you dont run WOT all the time... Maybe a 24 prop?
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Old 09-04-2018, 01:57 AM   #5
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Just donít operate for extend periods above 4000 rpm. But different props will achieve proper WOT and all around performance.
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Old 09-04-2018, 11:14 AM   #6
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Something is unusual here. You would be going 60+mph at that rpm with 1.81 outdrive. Did you check your speed with GPS during this run? If it was 60 mph then yeee-haa. Many new boaters would not be comfortable going 60 in an 18 foot boat on their first time out.
Check your outdrive and see if you can find the gear ratio stamped on it somewhere. It may be on an aluminum plate or the drive itself.
Did your tach respond normally to throttle inputs? It couldn't hurt to clean the terminals in the back of the tach. I can always tell when my tach terminals need cleaning because it indicates exceptionally high.
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Old 09-04-2018, 11:21 AM   #7
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Tach seems to work fine and respond correct. I did not check with gps but I highly doubt there were any 60mph runs.
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Old 09-04-2018, 12:06 PM   #8
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I'd verify the dash tach is reading properly using a shop tach directly connected to engine. My timing light has an RPM feature and the Faria dash gauges have been off since the boat was new.

WRT the prop, to get to the recommended 4400-4800 WOT rpm, that would need roughly 5in of additional pitch! Something doesn't sound right. Doublecheck the tach and post speeds (from a gps) and the RPM.
Could be a spun hub if the rom's are high and not having the corresponding speed.

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Old 09-04-2018, 03:57 PM   #9
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Agree with the others that something doesn't sound right (5700 rpm?, ck. that lower unit ratio). I have an '06 1800 SR with the 4.3 Merc. and when I bought the boat it had a 21" 3 blade aluminum prop and it was incredibly sluggish.
I changed to a 4 blade 19" aluminum, and have since added a Stingray hydrofoil. Now, I run up to about 4600 rpm, putting it right in between the 4400 to 4800 range.
Net, net, with only myself aboard, i'm on plane and cruising at 2500 rpm. She pops up out of the water very easily, very happy with how she runs now.
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Old 09-05-2018, 04:51 PM   #10
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After talking to a few people, I believe the tach has to be reading wrong. Someone mentioned to me the rev limiter on these engines is 5100 and I have not hit any rev limiter. Also from some reading I am not sure there is a prop out there that would add enough pitch to bring me down 1000rpm to the recommended 4800 which also makes me feel the tach isn't correct.
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