AL Prop selection for 4.3l 175HP and 1.98 outdrive

[Chris51280]

The boat came with a SS 23P Prop but never ran it since it looks to be danged up. the prop is a mercury mirage without the vent holes. The boat weighs #2700.

I currently run a 4 blade 19P Aluminum prop Turning point. I can tell that 2 blades have a slightly different shape like being cupped. I get 40mph at 4500rpm WOT and a nice pick-up. I have a whale tale on the out drive that I will be removing and replace with trim tabs. I've seen others with up to 23P props on here. I think the prop I have now is just fine for all around like skiing and tubing. What would interest me is if I can get more speed out with probably loss of hole shot with a 3 blade. What is your take on this? Get a 14X21p 3 blade?

 

[alldodge]

19P - 1:98 - 4500 rpm - 40 mph = 2.2% slip
I think something is wrong, either the rpm or the speed, because have found no boat get that kind of effieiency

To your question 1990 4.3 WOT 4400-4800 rpm

The 4-blade 19P is the same as the 3-blade 21P for the most part, your WOT rpm I would guess to be closer to the 4650 but just a guess.

 

[Scott06]

are you sure of the 1.98 drive ratio, usually that ratio is on 4 cycle see models. 4.3 s typically have 1.81 or so. Did you swap the drive when you did the engine work?

would think if you did have a 1.98 drive and 19” pitch with a 4.3 you would have ore than 5 k rpm at 40 mph

 

[Chris51280]

I will check on the number again tomorrow. I typed in the number and I got the 1.98 ratio back. Puzzled me too. so if I have the 1.81 ratio, I should have a 9% slip. Is this still possible? Theoretical speed is 44 and actual speed is 40.

 

[achris]

How are you reading your RPM? If you're using the dash tacho, that's where your problem is. Dash tachos are notoriously inaccurate, and usually read high at the top end. Use a proper digital tacho, usually the ones in timing lights are accurate enough.

If that 4500rpm is accurate, it's too low for those engines. You need to be running as close to the top end as you can, 4800. Even a little bit over isn't necessarily a bad thing. I run either a 4 blade 18" or a 3 blade 19" and I'm just on 4900 with either. The top speed on the 4 blade is slightly lower, but it's a smoother prop and less prone to 'weeding up'...

4.3 engine came standard with either 1.84:1 or 1.62:1, depending on the era and the variant.

Post the drive serial number and we can look up the ratio.

Chris.......

 

[Chris51280]

The upper unit has this number stamped on it. "0D463089"

This is a 1.98 ratio, correct? unless it got regeared? I know the 1.98 was an option and not common

 

[alldodge]

Around 8 to 10% slip is in the normal range for a good setup

 

[achris]

The upper unit has this number stamped on it. "0D463089"

This is a 1.98 ratio, correct? unless it got regeared? I know the 1.98 was an option and not common

Yep, that comes up as a 1.98:1....... Maybe re-geared, pull the drive and do a input yoke to prop shaft turns count.

 

[Chris51280]

I just slapped it back on. I think I would rather take the top cap off and count both gears. I just had it open for the new yoke seal install.

 

[Scott06]

I just slapped it back on. I think I would rather take the top cap off and count both gears. I just had it open for the new yoke seal install.

Maybe verify the tach first ? Or did you? If tach numbers are off that could be issue as others have mentioned

 

[Chris51280]

I did check the gears today. definitely the 1.98 ratio. driven gear count is 24. I did not check the speed via GPS yet. will do so in the future. gotta wait for warmer weather here in Ohio. I guess the tach is off then?
I did check it with an external gun style and a reflective strip about 2 years ago to make sure the idle is what it says on the gauge. I was pretty spot on. Maybe it is off in the upper rpm range?

 

[Scott06]

Ok if speed was not on gps that’s why your slip works out to be 2.2%.

if tach is accurate likely you are going slower than 40. If tach is accurate at 4500 I would not go up in pitch, I’d try a 19” three blade would expect you to see 3-4 mph top speed improvement and rpms go up to 47- 4800 rpm.

 

[achris]

Here's what the numbers look like in my AoA calculator.

As you called them....



Assuming accurate tacho, and getting a good 'fit' speed.



What the engine speed might look like if the speedo is assumed accurate.



And here's what it would look like if the drive ratio is 1.84:1, and the tacho and speedo are accurate...



Chris.......

 

[Chris51280]

Thank you Chris. I will give an update since I want to know what is accurate. Speedo or Tach. Might be another month

 

[Chris51280]

So I was finally doing a GPS speed test. I got 38.5 to 39mph at 4500 to 4600 rpm. Is this in the ball park?