Prop and overall ratio help needed

[Redrig]

Hello all ,

I could use some help with a project that I am working on . I am converting or upgrading my drive. I am going from an old electric shift OMC stringer to a mechanical stringer. I have aquired all of the parts but now I need to get some props for the new unit and could use some help there.

My "old" electric shift drive ratios.
upper - 21/19
lower - 15/23

I dont know the overall ratio or how to calculate. I can tell you that I use a 14x14 for 'mountain lakes" and a 14x16 for lower elevations about 3700' or so and those 2 props have always worked great and put the RPMs in an acceptable range

My "new" drive ratios
upper 21/16
lower 14/26
overall .71 (from my manual)

So my main question is - How can I calculate or find out what prop(s) will be best for new drive ? basically an equivalent of what the previous props and drive ratio

any help would be appreciated so I can start buying props for the new setup .

Thank you much

 

[Faztbullet]

Make sure the lower is a 800 series not 400

 

[Redrig]

I have the complete out drive and it's an 800 with the 21/16 ratio in upper

 

[jimmbo]

Well your engines WOT range is 3800 - 4200 rpm
Your old drive, if stock, was listed a 1.79 ratio for a pre 78 175hp, your new unit is in the range of 1.4:1 which is quite a jump
What rpms were you attaining with the old unit and prop?

 

[Redrig]

Shoot , that's a pretty big difference. Thanks for figuring that out

With my 14 pitch I can hit 4000 and that puts the boat at about 35

 

[Redrig]

So knowing the large gap in overall ratio , I may have to change strategy . That may run like a dog.

That stinks , cuz that 5.7 upper is in great shape and resealed . Oh well , I can just sell it .

Now the question lies if I can use my electric shift upper with the mechanical lower. It looks identical and the vertical driveshaft is listed as the same part , so thats promising . Will have to pull it and take a better look .

 

[jimmbo]

A pre 78 upper housing has a different part#(0980804) than the post 77 upper housing(0982226).
The upper driveshafts are different too.

That doesn't mean they won't swap, but...

 

[Redrig]

Actually jimmbo , they appear to all be the same from my initial reaserch . The vertical driveshaft is the same for all low profile uppers 909120 , water pump shaft too , so that means splines areaall the same. You may have just picked a tall profile model.

Also I found this on a site that rebuilds stringers

"The ratio change is in the upper housing. All 1973 - 1985 upper housings are identical* except for the tooth count on the gears and left or right steering worm."

​​​​​​I will pull off my electric shift and verify and measure but at this point it looks somewhat promising .

I need a favor , can you tell me what my overall ratio will be assuming I can get this to work ? The ratios in the lower are different and that will change my overall ratio if I use the same upper

So it will be upper of 21/19 and lower of 14/26

That shouldn't be a huge difference on overall ratio to overcome with a prop change in pitch an inch or two from my current setup

Thanks again for the help

 

[Faztbullet]

I thought the housing where different lengths....

 

[Redrig]

I thought the housing where different lengths....

you are right , there were different lengths . The Tall vs short profile . The tall profiles though I think were only in the late 60s early 70s . Both of my units are the short profile .

 

[Redrig]

Actually , I did some digging and was able to figure out the calculation for compound gears .

My old electric shift was a total Ratio of 1.38 (21/19 upper +15/23 lower)
My "new" mechanical shift has a total ratio of 1.41 (21/16 upper + 14/26 lower)

So that is only a difference of .03 , that should be fine right ? The prop is basically spinning at the exact same RPM on old vs. new drive .

 

[Faztbullet]

Should work ratio wise.....

 

[Redrig]

So one last question on this . My electric shift used 14 inch diameter props , the 800s appear to use 15 inch diameter props .

Based on these ratios , and the change in diameter , should I stick with 14 pitch for higher elevations and 16 pitch for lower elevations ( that was basically the strategy before )

Will a 1 inch larger diameter prop change that ?

 

[Redrig]

So one last question on this . My electric shift used 14 inch diameter props , the 800s appear to use 15 inch diameter props .

Based on these ratios , and the change in diameter , should I stick with 14 pitch for higher elevations and 16 pitch for lower elevations ( that was basically the strategy before )

Will a 1 inch larger diameter prop change that ?

Can anyone take a stab at this last question ? With a 1 inch larger diameter prop , do you typically have to adjust pitch , or will pitch somewhat over ride diameter ?

Sorry to bump this thread , I just need to start getting my props lined up for "mountain" lakes vs. lower elevation.