Bigger prop.. over revving is worse??

[camarokidz28]

Boat: 17' 1980 Ranger 235v
​Motor: 1983 Mercury 70 HP
​Old prop: 11" 12p
​Old speed and rpm: 32MPH at 5500 RPM
New prop: Mercury 10-3/8" 13p

​Put the motor on my boat with a bent and chipped prop for testing. Motor ran 32MPH but would hit 6000 RPM with any amount of trim. So I went to replace this prop and the guy at the boat shop sold me the above listed prop. He said it would bring my rpm down to a good range with the trim. I installed this new prop after cutting the old prop off the output shaft. Went to the river and to my surprise it was much much worse. Boat sky rockets to 5000 RPM fails to plane unless my passenger is sitting on the nose of the boat. It reaches a max speed of about 14MPH now and basically revs to wherever it feels like. So what do I do from here?

 

[82rude]

Sounds like a classic spun hub to me.

 

[alldodge]

Agree, the guy sold your a bad prop

 

[camarokidz28]

Took the prop off and see no indication that the hub spun

 

[Sea Rider]

A spun hub on a brand new prop would be the last straw. Is 70 HP wot range 5000-5500 rpm ? Some tech issues to consider :

With boating mate sitting at bow does prop has aeration issues when at full hammer down or at close turns on flat water cond ?
Does OB has out/over transom water splashes when OB sits at 90? ? need to pull head over transom at plane to check that one.
Is OB 101% healthy, revs even throughtout the whole throttle range ?

If with 12" old prop achieved 32MPH at 5500 RPM why didn't you replaced exact same pitch prop which performed top as loaded.

Happy Boating

 

[camarokidz28]

A spun hub on a brand new prop would be the last straw. Is 70 HP wot range 5000-5500 rpm ? Some tech issues to consider :

With boating mate sitting at bow does prop has aeration issues when at full hammer down or at close turns on flat water cond ?
Does OB has out/over transom water splashes when OB sits at 90? ? need to pull head over transom at plane to check that one.
Is OB 101% healthy, revs even throughtout the whole throttle range ?

If with 12" old prop achieved 32MPH at 5500 RPM why didn't you replaced exact same pitch prop which performed top as loaded.

Happy Boating

They told me that prop was aftermarket. So I bought the next pitch higher, which was mercury brand. The guy at the boat shop said they only use odd #s

 

[Sea Rider]

Most times aftermarket props does not match same wot rpm if compared to same OEM pitch props. Is it possible to find same aftermarket 12 pitch prop ?

Happy Boating

 

[camarokidz28]

Most times aftermarket props does not match same wot rpm if compared to same OEM pitch props. Is it possible to find same aftermarket 12 pitch prop ?

Happy Boating

The only marking on the prop said 12p. So I'm not sure how to identify it

 

[ondarvr]

You said you cut the old prop off, but can you compare it to the new one to see if there's only about 1" difference in pitch? It's possible that number wasn't correct, or was hard to read and maybe it said 17P. It may have also been modified.

 

[Fed]

Do you really think that changing the pitch by 1" would change the speed from 32MPH to 14MPH?
Test it for a spun hub, more often than not a spun hub isn't obvious by simply looking at it unless you know exactly what you're looking for.

 

[Fed]

Good point ondarvr, a 12" prop is way too small for a 70HP pushing a 17' boat.
Makes me wonder what the boat shop were thinking when they sold it unless camaro didn't give them all the details like boat & motor size.

 

[b.gagnon]

Make a mark on the hub and prop in a straight line...Take it out for a hard run.
if the marks don't line up when you get back you have a bad hub!

 

[camarokidz28]

I just looked for scrape marks on the plastic hub and didn't see any... being plastic I expected there to be scrape marks on it. I'm also not exactly sure how to compare the two when looking for a 1" pitch difference

 

[82rude]

You've lost approx 18 mph or so just by a pitch change and you say it revs all over its either a spun hub or a worn prop shaft thats slipping ?Its possible its a defective prop and the insert is slipping.My friend owns a prop shop and though rare it has happened with the new hubs.Keep us updated this is a strange one theres no way possible to lose 18mph by a 1 degree pitch change it just doesn't happen.

 

[Fed]

I ran it through the Mercury Prop Selector (had to use a 75HP due to year model).
It came back with 5 different props... each one was a 19" pitch!

Allowing for a 5HP reduction down to 70HP you should be looking for around a 17" prop pitch.

Don't get confused, props are measured by pitch & diameter.
The diameter is hopefully obvious to you.
The pitch is a measurement in INCHES which is the theoretical (no slip) distance a prop would travel forward with 1 revolution.

 

[jimmbo]

I ran it through the Mercury Prop Selector (had to use a 75HP due to year model).
It came back with 5 different props... each one was a 19" pitch!

Since there was no 75hp in 1983, What year did you use? In 1984 the was a 75 introduced(it was an 80hp in 83), but it is a much larger engine( 4 cylinder 66.6 cu in) vs the 1983 70(3 cylinder 49.6 cu in). An 84 75 would also have a bigger gearcase with a larger dia prop and a different gear ratio(2.3:1 vs 1.83:1), so the results are not applicable. If you were using a later 3 cylinder 75, again it is an apples to oranges comparision

 

[Fed]

I used the oldest available 75HP jimmbo, I think it was 2000.
Eased back on the prop pitch to allow for the HP reduction.
I know off the top of my head a 17" prop is OK but I was trying to qualify it a little by using the prop selector.
I agree not quite apples for apples but hopefully enough of an indication to drag the googleboys away from the 12" props.

 

[82rude]

Why not go on here and use a 70 hp which shows max 17p. up to 1983.70 johnsons of that vintage usually used 15p for that size boat even 13p if really heavy, my 93? 60 mariner had an 11p which i still have on the shelf.P.S. I hate not being able to sleep:blue:

 

[jimmbo]

Why not go on here and use a 70 hp which shows max 17p. up to 1983.70 johnsons of that vintage usually used 15p for that size boat even 13p if really heavy, my 93? 60 mariner had an 11p which i still have on the shelf.P.S. I hate not being able to sleep:blue:

The Merc 3 cylinder 49.8 70hp used a 1.83:1 gear ratio with props approx. 10" - 11" dia, while the OMC 3 cylinder 49.7 70hp used a 2.41 gear ratio and props approx 13" - 14" dia.

 

[jimmbo]

I used the oldest available 75HP jimmbo, I think it was 2000.
Eased back on the prop pitch to allow for the HP reduction.
I know off the top of my head a 17" prop is OK but I was trying to qualify it a little by using the prop selector.
I agree not quite apples for apples but hopefully enough of an indication to drag the googleboys away from the 12" props.

There is a big difference between a 1983 70hp and a 2000 75hp. Displacement, loop charging vs. Direct Charging with power porting. crank vs. prop rating, to name a few. I haven't looked up the gear ratio for a 2000 75hp, but I think it is 2.3:1, the 83 70hp is 1.83:1. The 2000 would also be swinging 13" - 14" dia props, while the 83 70hp uses 10" - 11" dia props