Ratio Question

[KRH1326]

Hi Guys,

I am working with HWSIII on a prop issue. Does anyone know what the sterndrive ratio is for my rig?

1976 235 HP OMC Electric shift.

I tried the aftermarket manual ( still trying to get factory manual ) and got this data, but I don't think its what I am looking for:

Upper 21.16 Lower 15.23 Total .86 Overall ? 1.16

Thanks Fellas

 

[KRH1326]

Re: Ratio Question

OK I think it was a typical seloc/ clymer typo does this look right?

Upper 21 : 16 Lower 15 : 23 Total : 86 Overall 1 : 16

So I think my ratio would be 1 : 16 So is that crank shaft to prop shaft or prop shaft to crank shaft?

Crank shaft to prop doesn't sound right, if my crank is doin' 4000 rpm then my prop would be doin' 64000 rpm? I don't think so.

4000 rpm / 16 = 250 prop rpm does that make more sense?

 

[Howard Sterndrive]

Re: Ratio Question

15/23=.652173913

(16 / 21) / .652173913 = 1.16825397

engine goes around 1.168 times for each propeller revolution

or another way to visualize it:
Engine going 1168 rpm
Prop going 1000 rpm

At 4000 engine rpm, your prop is going 3425 rpm

Those drives must run very low pitch props. A 16P would be for a light fast boat.
I dunno what heavy cabin cruisers would use.... 12P?

What's HSWIII?

 

[KRH1326]

Re: Ratio Question

Thanks HS,

HWSIII is a forum member ( that's his user name), that REALLY seams to have the prop stuff down pat.