25mph on 3.0 Merc?

[Scoop88Cheetah]

Hey guys i got a question which i think i might know the answer on already lol. I have a 88 Cheetah 195es (dare you to find any info on it) open bow ski boat w/ 3.0 merc and getting it ready for the season. I had this boat for a year and the first time i took it out it was WOT 25mph and around 6K RPM, trimmed out. (dont worry, i didnt let it hit 6K but for a second). My manager at the boaters world store told me it might be slipping on the shaft under weight or high RPM. I highly doubt that because i took the prop off and see NO signs of wear. Its a 19 pitch and not sure the diameter but need to know how much bigger i should go. And if it helps, its 19ft V hull and about 2100 to 2200lbs.

 

[junior1113]

Re: 25mph on 3.0 Merc?

im thinking its more than the prop at 6000 a 19 is spining at like 55 mph at least

 

[Scoop88Cheetah]

Re: 25mph on 3.0 Merc?

could it be the lower unit might need to be serviced or at least looked at? It does click pretty loud when im running it in the yard.

 

[rrhodes]

Re: 25mph on 3.0 Merc?

I think your prop is slipping

 

[bear_69cuda]

Re: 25mph on 3.0 Merc?

I think your tach is off... (or it's slipping like aforementioned post) Should be 4400-4800 rpm at WOT

If your boat has a 19"P and you were spinning 6000 rpm then your boat would be traveling 108 mph in theory.

6000 rpm x 19" = 1.8 miles per min. = 108 miles per hour. I think that's correct???

True rpms at WOT are needed for pitch determination.

 

[Tacklewasher]

Re: 25mph on 3.0 Merc?

I think your tach is off... (or it's slipping like aforementioned post) Should be 4400-4800 rpm at WOT

If your boat has a 19"P and you were spinning 6000 rpm then your boat would be traveling 108 mph in theory.

6000 rpm x 19" = 1.8 miles per min. = 108 miles per hour. I think that's correct???

True rpms at WOT are needed for pitch determination.

Somewhere in your calc you missed the gear ratio. Have no idea what it is for this motor, but it will cut the speed down. I.e. if it is a 2:1 gear ration, your calc would come out at 54 mph (which is what jr. posted).

Either way, it sounds like a spun hub, a tach issue or something.

 

[Rocky_Road]

Re: 25mph on 3.0 Merc?

I'd eliminate the possibility of the prop hub slipping first.

Either try a different (known good) prop...or you can take a magic marker, and draw a line across the nut and hub, run the boat, and see if the line has moved.

 

[steelespike]

Re: 25mph on 3.0 Merc?

88 Cheetah 20',7'8" beam,2,020 lbs.says 180hp are you sure its a 3,0?Perhaps it was changed? Which may account for the inaccurate tach reading.
Usually a switch on the back of the tach.
Props don't slip to the point of 1200 rpm difference..A spun hub doesn't spin a little.I think your tach is off perhaps not set for your 3.0.The 19" pitch
should put you around your rated rpm.may be a little to much for a 3.0.

 

[bear_69cuda]

Re: 25mph on 3.0 Merc?



oh yeah... forgot gear ratio.... ooops...

108/2 = 54mph

 

[Scoop88Cheetah]

Re: 25mph on 3.0 Merc?

88 Cheetah 20',7'8" beam,2,020 lbs.says 180hp are you sure its a 3,0?Perhaps it was changed? Which may account for the inaccurate tach reading.
Usually a switch on the back of the tach.
Props don't slip to the point of 1200 rpm difference..A spun hub doesn't spin a little.I think your tach is off perhaps not set for your 3.0.The 19" pitch
should put you around your rated rpm.may be a little to much for a 3.0.

The boat only had one other owner and i talked to him and the boat came with a 3.0 brand new. But thats when i got it and now i dont know how to get in contact with him to ask him about his top speed.

SO me and a friend installed his tach off his wrecked boat (3.0 merc) it seems to work fine but im going to test this weekend to see updated speed/RPM.
i will post results.

 

[hwsiii]

Re: 25mph on 3.0 Merc?

Scoop I found the information on your boat here.

http://www.nadaguides.com/default.a...&m=0338&y=1987&ml=C&gc=mr&gtc=MR&d=1600039088

This is the information I found for your motor.

http://www.mercurymarine.com/engines/inboards/sterndrives/tks3l_specs.php

The boat weighs 2,200 pounds and the engine is a 135 Horsepower. It should weigh about about 2800 pounds with 20 gallons of gas 2 people and normal ice chests and stuff.

Her are my numbers on what this boat ought to have as a prop and the speed length calculations on the motor

This is my backup calculations for the speed you ought to attain.




You ought to be doing somewhere in the neighborhood of about 38 to 40 MPH top speed. Hope this helps you.


H

 

[junior1113]

Re: 25mph on 3.0 Merc?

a bad hub wouldnt get on a plane though it would spin as soon as you went to take off thinking its more than the prop