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Prop question

K

keithm

Contributing Member
I have a 76 johnson 85. I was thinking of replacing the prop. It is a stock prop, whatever that means. I can find no pitch or diameter marks on it. So did johnson just put a generic pitch on them or did the boat make a difference. How can I determine what I have so I can replace with same setting.

thanks
keith
 
Dia and pitch should be imprinted upon the prop right in back of the prop nut, also the part number which would reveal the size.
 
Yes, a 13-1/4 x 17 prop is quite common... works fine on many boats.

And a 85hp lower unit is a pretty good size lower unit... wouldn't want to drop it on my foot. :)
 
It is 4 1/4 prop had one nick, it will rev to 5500. The manual list wot as 4500 to 5500. So if I put a 12 3/4 21 would that drop it about 800 rpm, I had read that an inch of pitch was roughly 200 rpms. If that is true is 4700 rpm to low?
i don't really want to turn a 38 year old motor 5500 constantly
 

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I managed to get some more solid info. 18ft ski boat baja. Tried the 12 3/4 21 prop, 3 adults 1 child, the best I saw was 4000, tried it with a tuber 3500 for a half mile finally would come on plane. Tried moving weight didn't help much. Put the 13 1/4 17 back on, same conditions, 4600 rpm came out of the hole much faster, got on plane quicker. Checked the ventilation plate it running about an inch down in water. It is also about a 1/2" higher then keel at the transom. If the ventilation plate were running on the surface the tip of the prop at the high point would be only 3/8" below the surface. I did attach a hydrofoil and noted that the loss of traction feel just as the boat goes on plane went away. There seems to be a disagreement on top rpm, some say 5000 some 5500. Am I making to much out of this.
suggestion please?
Thanks
 
I understand that to turn more rpm I need to take load off the prop and that can be done by decreasing pitch or diameter or both. I just happened to have had the 12 3/4 thought I'd try it. It did exactly what I thought it would lug the motor down. What I'm having trouble understanding is the motor heigth. Looking at the lower unit it appears that if the ventilation plate is on the surface then the tip of the prop will be sticking out of the water causing cavitation and this would account for the feeling of a loss of traction. If that is the case wouldn't the smaller diameter prop with a pitch that would allow the engine to turn 5000 alleviate this?
 
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