New gear ratio; What Prop pitch??

[America's Finest]

I have an 88' Mercury xr4 150. It had 1:78 gears, and I had a three blade stainless, 24 pitch prop on it. It was a little sluggish getting off the line, but had great top end....no over revving. I replaced lower unit with bigger gear case, with a 1:87 gear ratio. My prop no longer fits, due to the fact that the old gear case was the smaller bullet casing. I tried a 19' pitch prop on it and it launched like a mo'fo, but no top end! It was an aluminum prop. If I wanted to stick to a smiliar launch and top end that I had with the stainless and old gear ratio, maybe with a just a touch more get up, and less top end, what pitch prop should I go with? Does it make a difference if it's stainless again or aluminum??

 

[Bondo]

Re: New gear ratio; What Prop pitch??

I tried a 19' pitch prop on it and it launched like a mo'fo, but no top end!

Ayuh,... I'd think a 21",... Maybe a 23"...

 

[steelespike]

Re: New gear ratio; What Prop pitch??

Tell us about the boat you were running with the old gears; rpm(no over reving is a little vague) and speed,dry weight,style etc.
Based on your old prop size I'm guessing a fairly light/fast boat.
The new ratio should use a similar prop.Would depend on your old performance actual numbers.

 

[45Auto]

Re: New gear ratio; What Prop pitch??

In the first place you went the wrong way with your prop if you were looking for the same performance. Going from a 1.78 to a 1.87 means that you're going to need a BIGGER pitch prop to get the same performance. You lowered (higher numerically) your gear ratio by about 5%, so you'll need a 5% bigger prop to deliver the same performance. 10% of 24" is 2.4", so 5% is about a 1" bigger prop to perform exactly like your 24" on the 1.87 ratio drive.

It's easy to figure what a theoretically equivalent prop would be.

New prop pitch / old prop pitch = new gear ratio / old gear ratio.

So new prop pitch = new gear ratio / old gear ratio * old prop pitch.

New prop pitch = 1.87 / 1.78 * 24" = 25.21"

Your change from a small to a large gearcase could possibly also affect your prop results, but I doubt if it would be enough to notice. You made a very minor gear ratio change, so you probably wouldn't be able to tell the difference if you stayed with a 24" prop on your new lower.

 

[America's Finest]

Re: New gear ratio; What Prop pitch??

Thanks for the help, sad to say, but I use to be a master mechanic, but for some reason I was thinking that it would have to be a smaller pitch to be equivalant. MY BAD! I used the 19' pitch prop because that's what I had to use. If I used my old prop, it would just aerate like crazy and would not get up and out of the water because of the smaller hub diameter, all the exhaust was coming out around the hub and directly into the blades. I borrowed the 19' which had the bigger hub size and worked fantastic for the bachelor weekend cruise, but I obviously knew it wasn't right, but needed to test things out. I think I will just try to trade the stainless 24' prop with the smaller hub, for one with the bigger hub. Thanks.