Which of these two props should I put on a 55 Johnson?

[bassboy1]

We just bought a 79 55 hp Johnson, to replace the elec shift 71 50 horse due to shifting problems. The 71 had a 13 3/4 x 15, and the 79 has a 12 1/4 x 15. We were trying to get the 50 horse going, but the elec shift caused us some problems, and we stumbled upon the 55. It is gonna be on a 15 foot deep vee Lund. Would the 55 be able to spin the extra inch and a half diameter, being as they are the same pitch?

 

[WillyBWright]

Re: Which of these two props should I put on a 55 Johnson?

The 71 uses a larger prop than the 79. It won't fit.

 

[bassboy1]

Re: Which of these two props should I put on a 55 Johnson?

Alright, I can live with that I guess. Why is it that the earlier motor spins a larger prop? You would think that with almost equal HP, the later model, with an extra 5 hp, would spin a larger one. Are the engines geared differently or something?

It seems that with the same pitch, and 1 1/2 extra inches of diameter, the 71 would actually get better speed or torque, or a combination of the two. Have I missed something here?

 

[WillyBWright]

Re: Which of these two props should I put on a 55 Johnson?

The 71 is a Hydroelectric and they used that same basic gearcase all the way up to V-4 135Hp. But that's a lot of gearcase and a lot of prop for a twin to push through the water, and aluminum costs money. I'm sure the smaller gearcase was phased in due to those two factors primarily.