changing gear ratio alpha

[-stig-]

hello!

On my shakespeare i got problems finding OK props, I'm on 24" now, and need more to get my rpm's down. still getting 5000rpm, and the selection of props are getting kind of thin over 24".

could i change the ratio in my drive? Got a merc 140 1986, with a alpha outdrive. i guess its a 1.98, would 1.82 do the trick? is it just to change the upper gears?

does anyone know how many teeth it is on the upper gears on the 1.98?

thanks guys!!

Stig

 

[Bondo]

Re: changing gear ratio alpha

Ayuh,.... We need more Info.... The 140 is Supposed to use the 1.98:1 gears...

My 1st guess is, you've got Other Poblems...

 

[JustJason]

Re: changing gear ratio alpha

Usually you don't run into problems with 4 bangers over reving. If I were you I would start by using a shop tach to verify that your tach is accurate on the high end.

 

[John_S]

Re: changing gear ratio alpha

and what is your wot gps speed?

 

[jerryjerry05]

Re: changing gear ratio alpha

What size boat? The drive might have a # stamped in the side.
A 140 pushing a 24" pitch just don't sound right unless your boat is 12 ft and 300# Is it the original drive? You using sterndive props or outboard props?
Check the tach.Post a pic of the prop.J

 

[-stig-]

Re: changing gear ratio alpha

the boat is a 15feet shakespeare. not shure about the accurate weight thougt..

sorry,my mistake on the subject, the prop on the boat now is a factory new mercury 14 x 23" !

with that one the boat hits 75km/t / 46.6mi/h (GPS), just a hair under 5000rpm, checked and checked again!

the boat shoots out of the water, and goes like a pro!

really happy about the boat now, but i dont like the high rpms, the last engine in this boat died due to the previous owner over -reving it daily!

about the stern, i dont know if it is the original, the boat is pretty old. i couldnt find any numbers on the drive, but it seems right with the 1.98 on the prop calcs? not a easy task to just turn the crank by hand when the engine does not have a nut/bolt on the crank pulley in front!

 

[maharg]

Re: changing gear ratio alpha

Weird, I can only hit 5 k with no gear an 2 people, thats with a 17p

 

[-stig-]

Re: changing gear ratio alpha

but do anyone know about the upper gears? can i just change?

how many tooths is it on the gears in it now, and what would work for me?

 

[Howard Sterndrive]

Re: changing gear ratio alpha

I suspect the tach is reading wrong.

either that, or you have a sorta crappy propeller on there... not the best blade design, inaccurate pitch, or maybe stamped wrong

23P, 1.98, 5000RPM
if those 3 things are accurate, the boat should be doing over 50mph with typical alum 3 blade prop efficiency

What I would do:
1. verify tach
2. if tach accurate, look for a 14X23P stainless prop (should knock WOT rpm down 100 or 200 from aluminum of same mould)

You can change gears, but there is special tooling and skill required. Basically, if you would feel comfortable setting up the gears in a 9" Ford rearend, you can do it. The manual walks you through it.

Or, you can buy a new 1.84 upper driveshaft housing from SEI for about $700 US+ ship, and sell yours

Or, you can look for a used 1.84 or 1.81 upper.

 

[-stig-]

Re: changing gear ratio alpha

thanks for a really good answear!

if i need special tools for this job its a done deal.

I'll play around a bit with more propellers then!

this one, the 14x23 is brand new, 30min of driving time on it!

i do have a 14x23 stainless steel laying around, maybee i should get it cupped a bit more! got to get that one checked, lost a bit speed with that one.