Replacement Prop

[cnadler3]

Hello Everyone,

Below are the answers to the essential questions:

1) Performance issue you are trying to correct.

No performance issue, just replacing a damaged prop. I bought the bought and one of the fins is cut/bent


2) Current prop manufacturer, model, aluminum or stainless as a minimum.

I think it is an OEM Mercruiser. the numbers on the prop are 48-78122 A40 21P. 3 FIN, Aluminum.


3) Current prop diameter and pitch (required).

From doing some research I believe the diameter is 13.75 and pitch is 21


4) Wide open throttle RPM and speed with an average load (very helpful)

50MPH@5500RPM


5) Engine/drive make, model, year, and HP

5.7 Mercruiser/Alpha one gen2, 1996, not sure on HP, I've seen 220HP? Whatever the factory HP is... if anyone knows for sure, please let me know.

6) Boat make model, year, length and weight

Bayliner 2050LS, 1996, 20.5, ~3000LBS

I'm looking at purchasing this prop as a replacement:

Solas Amita Aluminum Boat Propeller #1511-143-21 for Mercruiser Sterndrive Alpha One 5.7L 210 - 320 HP - iboats

I spoke to the local marine shop and they said the current prop I'm using is too small of a diameter for the boat/motor? They said,

"On your prop that is totaly the incorrect dia. of prop you should be a 14x19/21 or 14.5x19/21"

Is the prop I'm looking at buying look ok? It is a little bigger diameter (14 3/10) than the current..Any help is much appreciated.

Thank you,

Chris

 

[jestor68]

Re: Replacement Prop

In 1996, there are two 5.7L's that are likely candidates to be in your Bayliner. The carbed version is 210 hp; the EFI version is 250 hp. The gear ratio is listed as 1.47.

The carbed version is rated to 4600 rpm. The EFI version(s) can go to 4800 rpm.

Most 21" props I've seen for the Alpha 1 are slightly over 14" in diameter. The larger diameter equates to more blade area, which tends to load the engine more and bring the rpm down some. Yours is an old part number; been replaced by the newer 14-1/4 X 21.

I smell a bad tach reading here. The Mercury prop selector identifies your prop as a 14-1/4 X 21 pitch in aluminum. The Solas prop is an appropriate replacement.

The tach in my 1996 Bayliner(look to the left) was acting up, and it turned out to be the prongs on the back of the tach(connector) needed to be cleaned. After cleaning the pins where the connector goes on, it was reading fine again. Before cleaning, it read too high.

 

[Lyle29464]

Re: Replacement Prop

You need to reduce your WOT RPM. a larger prop will help a little but not that much. A SST will help a little also but not that much. I would check the drive ratio before I moved to a 23 inch prop.

 

[jestor68]

Re: Replacement Prop

Here is why your rpm/speed does not make sense.

If you run your numbers through the slip calculator, it comes up with a theoretical speed of 74 mph at 5500 rpm.

Now we know that's not happening.

Run the numbers again at 4300 rpm and you get a theoretical speed of 58 mph; with 14% slip at 50 mph. More realistic.

Hence, I believe your tachometer is way off. :faint2:

 

[cnadler3]

Re: Replacement Prop

Jestor,

I think you are correct, since the motor would probably overheat if it ran all day at 5500RPM. I am going to order the new prop I provided a link to and will clean the contacts. Where are the contacts you are referring to? On the back of the tach or elsewhere? Also, on the prop page it advertises a thrust washer, spacer, and prop nut. Should these be replaced with a new prop?

Also, its a carbureted motor, so it sounds like it has 210HP.

Thanks everyone for the help.

 

[jestor68]

Re: Replacement Prop

If ordering the Solas Amita prop, you'll need to order just the Solas thrust washer with it. All the other hardware is the same.

On the back of the tach, there is a plastic connector(mine is black) that pulls straight off, revealing the three prongs.