mercruiser alpha gearing for 23 tsunami

[hedgehog]

Just had my Mercruiser alpha outdrive serviced with new bearings/seals and pressure tested. The mechanic informed me that somewhere down the line a previous owner had switched the gearing on the drive to 24/24...making it a 1:65 or 1:62 ratio. I am running a mid eighties 260 mercruiser engine in a 1973 23 Tsunami.

Last year I was running a 15x17 prop and the boat was handling ok about 200-400 rmp shy of max wot (4600).


Iam wondering if this conversion was done to gain low end on the boat or if I should switch the gearing back to the 20/24 1:50 ratio.

Any and all help or suggestions appreciated

[Bushwacker]

In the mid-70's a good friend of mine repowered his SeaCraft 21 with a 250 HP Mercruiser, but Frank Brown of Brown & Hauptner Marine, who occasionally used to crew for Moesly in his race boats, changed the upper gear case from a 1.5 to the 1.65 ratio because he thought it would be an improvement. A couple of years later, that engine rating was changed to 260. The only obvious change was that it had the 1.65 ratio instead of the 1.5! I think I'd stay with the 1.65 ratio!

[1bayouboy]

And which prop are you running......just changing your prop could get you to WOT. I gave up some top end and a little efficiency to get my merc/bravo to the right WOT range....which crazily of you ask me....on the 350 MAG MPI Horizon is rated to 5200RPMs!!

[hedgehog]

Thanks for the input....I was trying to do the math in my head and was assuming, maybe incorrectly, that the 1.65 ratio would give me a little more low end which is much needed for the weight of the boat.

I am running a 15x17 alum prop which seems to perform well...just keeps me a few hundred rpm shy of max wot. I am more interested in overall performance rather than speed so looks like I will stay with the same dia/pitch this season.

Thank you both for your reassurance and input

[1bayouboy]

The 1.65 is engine rev to 1 prop rev. So the lower the ratio the less prop pitch you can usually turn. Bravo 3 drive with 22 pitch props are more like 2 or 2.2.