No speed, No power...please help!

[tygray6]

1986 Thompson Cutlass 185
Roughly 2800 lbs with fuel
18.4 length
Rebuilt carb, new fuel pump on the way because it's currently 4psi idle and 3 revved
MCM 120, compression 125-130lb
Alpha One 2.5/1 Ratio

So I hope these specs are good because I've been in and out of this engine thinking I was losing power there but it sounds amazing and compression is good, I know my fuel pressure is a little low but no indication of running lean based on looks of plugs.

I have a 14.5/19 pitch prop on currently and with two adults in the boat I get 3000 rpms and 24 mph.

I tried a 13.25/17 without even getting on plane and prop sounded like it was cavitation. I want to be able to out 4 adults in the boat and pull a tube with the kids or pull them skiing but I need help figuring out why I can't seem to get the right prop.

 

[alldodge]

MCM 120, compression 125-130lb

Your motor is tired. New motor is 140
Can also weight the boat at a truck scale to see if its within spec

 

[tygray6]

I'm sorry I was mistaken, these are the cylinder pressures. Also I did weigh the boat and it's 3400 with the trailer which I'm guessing is about 800. Production weight of that boat with no fuel or accessories is 2200.

 

[alldodge]

OK. Compression is good enough
Other things don't add up.

19 pitch can reach 25 mph at 3000 ram. Go to a 17 pitch and it won't get on plane. If props are correct the ram on a 17 should be maybe 3400 or at worst 3600, which is still below max

 

[tygray6]

I'm sorry I was mistaken, these are the cylinder pressures. Also I did weigh the boat and it's 3400 with the trailer which I'm guessing is about 800. Production weight of that boat with no fuel or accessories is 2200

View attachment 342331

OK. Compression is good enough
Other things don't add up.

19 pitch can reach 25 mph at 3000 ram. Go to a 17 pitch and it won't get on plane. If props are correct the ram on a 17 should be maybe 3400 or at worst 3600, which is still below max

I was getting 3700 rpm with 17 but wouldn't get on plane

 

[tygray6]

This prop also gets me 3800 rpm at 24mph on plane but doesn't have any power.

 

[alldodge]

Need to verify what you have, and negative numbers can not happen

19P - 2:50 ratio - 3000 rpm - 24 mph - slip = -11%
17P - 2:50 ratio - 3700 rpm - (guessing 15 to 20 mph) = slip 24 to 48%

Something wrong with trimming or props are bent up, or dinged badly
https://bblades.com/propeller-slip-calculator/

 

[tygray6]

Need to verify what you have, and negative numbers can not happen

19P - 2:50 ratio - 3000 rpm - 24 mph - slip = -11%
17P - 2:50 ratio - 3700 rpm - (guessing 15 to 20 mph) = slip 24 to 48%

Something wrong with trimming or props are bent up, or dinged badly
https://bblades.com/propeller-slip-calculator/

Well I'm thinking that 17 pitch prop is bad then because I was only going like 6 mph

 

[Scott Danforth]

check the prop hub. looks spun in the pic

 

[Faztbullet]

Prop has already been repaired twice

 

[alldodge]

Well I'm thinking that 17 pitch prop is bad then because I was only going like 6 mph

Is the vent plate even with the bottom of the keel?

 

[QBhoy]

If you are doing 3700 rpm and only 6mph...something is majorly wrong. Something is slipping or sheared in the drive train. Most likely candidate is the prop hub.