Not sure what pitch to use

[Nova_Scotian_996]

I recently purchased my first boat. Its a 1987 Peterborough Cosmos with the Mercruiser 470 (170hp 4cyl)

The guy that sold it to me was using a 21 pitch. He told me this was the wrong pitch for this boat.

It is damaged (chunk gone out of prop) so I cant do an RPM test on open water as the result will not be accurate.

If anyone out there has suggestions Id really appreciate it.

Andrew in Nova Scotia

 

[Dhadley]

Re: Not sure what pitch to use

If there was a "chuck gone" we might assume it's an aluminum prop. If so, you might get it repaired and try it. You'll need a spare anyway.

The scary part is that it seems you're saying you bought a boat without a sea trial.

 

[Nova_Scotian_996]

Re: Not sure what pitch to use

I bought it from a neighbour who wanted to get rid of it at a very low price....i wanted a project...

I went to the local boating supply store and picked up a 19P Mercury Prop....tried it ou and the max rpms seem to be around 4800....the recommended max rpm for the motor is 4200-4600

ir got up out of the hole good and assumed plane fine...so until I wreck this one...ill keep it..

 

[steelespike]

Re: Not sure what pitch to use

Looks likeyou are close.You might want to confirm your prop is accurate.Then run the 19 but keep under the max.(NOT PROP; TACH see what happens when you don't proof read.

 

[Larson123]

Re: Not sure what pitch to use

I have a 2007 Larson LX 180Sei w/ 4.3L MPI 220HP and a 19" prop.
My top end RPM is around 5600 and cruise around 50mph.
To drop the RPM and keep the speed or increase the speed, what size prop would be best for that/

 

[MikDee]

Re: Not sure what pitch to use

I have a 2007 Larson LX 180Sei w/ 4.3L MPI 220HP and a 19" prop.
My top end RPM is around 5600 and cruise around 50mph.
To drop the RPM and keep the speed or increase the speed, what size prop would be best for that/

Welcome to Iboats, But you really need to post your thread to be seen, rather then tag onto someone else's

As to your question, If your Max rpm is that high, it really doesn't sound right?
Unless your boat is really light, & hull design is optimum, then you need a 23" pitch prop (theoretically,,, if you stay with the same prop material), because you don't say if your prop is Stainless, or Aluminum? There would be a difference! As it stands now:

A 21" prop = 5200rpm

A 23" prop = 4800rpm - probably your Max recommended rpm

 

[Larson123]

Re: Not sure what pitch to use

I thought something was funny. I have verified its a cast 19" prop standard on the Alpha outdrive from the factory. I just know that with the boat wide open, Im hitting around 50MPH and tach is in the 5200 - 5400 RPM range.
I know from the engine specs, 4400 - 4800 is the max RPM range.