How can you tell if a prop has had work done?

[Offthepath]

We use our Crownline 240ex between 3600 and 6200 feet above sea level (with occasional use at one lake being a over 7k).

Boat currently has a 22p bravo 3 and runs around 4850 rpm at 4600 ASL. I wanted to get a second prop for the higher lakes. I found a used mercury 20P prop and ran it for the first time this weekend. Much to my surprise, the rpm stayed the same. I'm assuming either the 22p or 20p prop has had some kind of work done. Is there any way to tell what the actual pitch of the prop is?

I'm considering getting a hill marine 4x4 and would like to figure out if I should go with a 20p or 18p prop. Right now, I have no idea if I'm actually running a 20p or 22p prop based on the boat turning the same RPM with both props.

Thanks

 

[dingbat]

Props can vary widely between manufacturers and models.

Example, model X and model Y props are advertised as having the same pitch and diameter, but have very different performance characteristics based on design.

Unless both comparing props are the same make and model, you may not be comparing apples to oranges.

 

[Bondo]

Ayuh,..... Welcome Aboard,.... Any prop shop worth their salt, will restamp a reworked prop,.....

 

[Texasmark]

I don't know if it applies to your rig or not but my current engine has an overrev limiter which starts removing trigger pulses as the engine reaches the limit. I was testing 2 props of similar design and did not get the change expected from "Prop Slip Calculators". What it told me was that I was in the limit with one, meaning that it needed more pitch for the operating conditions at the time of the test, and that the other prop had more pitch than the conditions warranted.

On answering your question, back in 1972 I bought a new Johnson 125 and it had the OEM aluminum prop. On Sunday we would go boating and on Monday, during my lunch break, I'd drop the prop off at the prop shop for repair. I'd get it back with a fresh coat of white paint looking like new. First few minutes of operation the paint would wear off and you could see the welding marks where material was added to the blade to bring it back in spec.

Unrelated comment, but in 1973 OMC introduced (became available to me) the Stainless Steel Teflon (coated) prop and the trips to the prop shop ceased. No idea as to what a repaired SS prop would look like, regardless of the repair.

 

[Offthepath]

Both props are stock mercury bravo 3 props, so I'm assuming there should not be any manufacture variance. The engine is a 350mag MPI rated for 4800 to 5200 rpm so I'm not on any kind of Rev limiter.

​​​​​​I'll double check the props for any new markings. Would it be apparant if the props had been cupped?

 

[QBhoy]

Both props are stock mercury bravo 3 props, so I'm assuming there should not be any manufacture variance. The engine is a 350mag MPI rated for 4800 to 5200 rpm so I'm not on any kind of Rev limiter.

​​​​​​I'll double check the props for any new markings. Would it be apparant if the props had been cupped?

Pretty sure you might be a little off with the rpm range of your engine.
As far as I know your engine should be 4600-5000 rpm and limit at 5150.
I have a similar engine and can certainly tell you that there is a huge difference between what the dash tachometer says and what the actual smart craft accurate rpm is. Especially at the top of the rpm.

I can also tell you that I’m often on a similar sized maxum 2400 with a 350 mpi and bravo 3. His props are 26”. It will run to right on 5000 rpm on the smart craft data.
That said...he may have a different gear ratio to you. Also I’d say that if you were on the limiter...the dash buzzer beeps at you when you hit it for more than a few seconds for sure. Mine will for certain.

 

[Offthepath]

Wot range is listed on the spark arrestor as 4800-5200.

So I looked closer and the 20p prop has an "L" stamped behind the cast mercury part number. It looks like it was added with a hammer and die. What would this indicate?

 

[Texasmark]

Wot range is listed on the spark arrestor as 4800-5200.

So I looked closer and the 20p prop has an "L" stamped behind the cast mercury part number. It looks like it was added with a hammer and die. What would this indicate?

SWAG (S is for Scientific): Prop shop has reworked the prop and the L is their mark for "the record".

 

[QBhoy]

Wot range is listed on the spark arrestor as 4800-5200.

So I looked closer and the 20p prop has an "L" stamped behind the cast mercury part number. It looks like it was added with a hammer and die. What would this indicate?

Ah. Ok. You may be right about the rpm. I remember thinking this before and it turned out the 350 was a fresh water cooled or seawater version or something. For some reason they have a 200rpm difference. Could never work that one out. Is it one of these maybe ?