'79 Merc 260 -- No prop with boat, what to start with?

[jmorgan87]

I used the prop finder, and believe I found the correct spline/gearcase information.


* Manufacturer: Mercruiser
* Horsepower: 210 - 320 HP
* Years: All Years

* Model: Alpha One 5.7L Sterndrive
* Specs: Thru Hub Exhaust & 15 Tooth Spline, 4-? Gearcase

I don't know if it's Alpha or Bravo one, but they both listed the same specs for my year/size.

The boat did not come with a prop, and I am completely new to props. I understand it's trial and error and increasing/decreasing pitch etc will affect your top speed RPM and to find the 'sweet spot'. Most sites say to adjust +/- from what you currently run, but I don't even know what's ok to start with.

Is there a place I can find out what my boat would of came with? Or anyone know? It's a 350 c.i, Merc 260hp on a 79 cobalt.

I plan to look for a used prop, so any advice on what's an acceptable range of pitch/size/blades would be appreciated. I assume the spline and gearcase specs are set in stone, and indicate that it will 'fit' the boat. I have no clue though if any pitch will work, or if there is a certain range of pitch I should start with. Ideally, i'll go hunting local shops, ebay, craigslist with an ideal range of 'ok' props and settle on the best bang for the buck to get started with.

 

[craze1cars]

Re: '79 Merc 260 -- No prop with boat, what to start with?

Props that "fit" will be a dime a dozen....very common outdrive. But which pitch to purchase? That's the big question. We'd need to know approximate size/weight of boat, and your outdrive gear ratio, in order to offer a possible suggestion.

Even at that, it will be nothing but an educated guess.

Finding a local propeller shop or marina that will let you test drive a prop (many have used props all over their shops) and return it/exchange it if incorrect will be your best option. You really need to just put on a known good prop of a given pitch, do a full speed run to make note of speed and RPM, and then adjust from there as needed

 

[Bondo]

Re: '79 Merc 260 -- No prop with boat, what to start with?

Ayuh,... I'd start with a 19" pitch prop,...

That's about in the middle of what's available...

 

[jmorgan87]

Re: '79 Merc 260 -- No prop with boat, what to start with?

Thanks for the advice.

Is the gear ratio stamped anywhere? The only thing i've found so far is:


WOT throttle 4200-4600 rpm
Engine rotation CCW
Model is 'MCM 260'

 

[craze1cars]

Re: '79 Merc 260 -- No prop with boat, what to start with?

Sometimes it's stamped. And sometimes they're regeared from what the stamp says. I'm not honestly real knowledgeable of the markings on these older drives.

IMO the best way to know for sure is to manually check it yourself by manually turning the engine:
http://www.mercstuff.com/drivegearratio.htm

Or, as Bondo says, just get ahold of a 19-ish pitch to borrow, and take 'er for a ride to see how it works....as I stated, even if we know your ratio, we'll still be guessing....

 

[Mike Zee]

Re: '79 Merc 260 -- No prop with boat, what to start with?

14 x 19 3 blade or a 14 x 18 4 blade will get you up and running with respectable results. Make sure it's 4 3/4 dia.

 

[jmorgan87]

Re: '79 Merc 260 -- No prop with boat, what to start with?

Great, thanks guys. I'll use that as my baseline and go from there.