Did I over prop?

[Sheehandler24]

Hi there, I just ordered an aluminum 21 pitch prop for my 80s StarCraft holiday with the 3.0. The guy that owned it before me tossed a 165 stringer on in place of the 140 stringer, I’m not sure what the difference is but that’s beside the point. My WOT as it sits with a 17 pitch prop is around 3200 rpm. From what I understand every pitch up adds 150-200 rpm’s, so 21 pitch will lift me up near 4000 rpm’s. is that too much for my little 3.0?

 

[Scott Danforth]

You have that backwards. Every 1" of pitch you go up, your RPM goes down 200.

Even with the correct 2:1 drive you don't have enough motor to spin the 21p prop

However with a 1.65:1 drive you need about a 9-13 pitch which they do not make (smallest is 15p)

So you need to get the correct drive, then try the 17p

 

[Scott06]

Hi there, I just ordered an aluminum 21 pitch prop for my 80s StarCraft holiday with the 3.0. The guy that owned it before me tossed a 165 stringer on in place of the 140 stringer, I’m not sure what the difference is but that’s beside the point. My WOT as it sits with a 17 pitch prop is around 3200 rpm. From what I understand every pitch up adds 150-200 rpm’s, so 21 pitch will lift me up near 4000 rpm’s. is that too much for my little 3.0?

You have it backwards increasing pitch reduces rpm at wot. I would expect 800 rpm less with this. You either have an incorrect drive ratio or engine that needs maintenance attention , or both. You need to go down in pitch not up

by 165 stringer you mean engine or drive. If he used a drive meant for a 165 the ratio is too high for a 3.0 engine need correct lower drive ratio.

Would not invest a lot of money in a stringer powered boat. That drive system has been obsolete for 30 years

 

[Scott Danforth]

Would not invest a lot of money in a stringer powered boat. That drive system has been obsolete for 30 years

Actually 40 years. The cobra drive was supposed to come out in 1983, bit was pushed back to 1984 (model year 1985)

Most of the stringer parts are NLA

 

[Sheehandler24]

Shoot okay, I’ll cancel the order. It’s a165 stringer on a 140 horse engine. Not sure why he changed it but he gave me the old 140 stringer when he sold it to me

 

[Sheehandler24]

Would it be possible to swap the lower gearing from the 140 lower unit into the lower gearing in the 165? I just realized The 140 has a damaged fin on the bottom of the unit. I’d imagine the upper gearing would have to be swapped out as well

 

[Scott06]

Actually 40 years. The cobra drive was supposed to come out in 1983, bit was pushed back to 1984 (model year 1985)

Most of the stringer parts are NLA

Yeah but i figured it wasnt obsolete for a few years after it was no longer sold new.. either way its old .

 

[Scott Danforth]

Would it be possible to swap the lower gearing from the 140 lower unit into the lower gearing in the 165? I just realized The 140 has a damaged fin on the bottom of the unit. I’d imagine the upper gearing would have to be swapped out as well

so get it repaired. its a simple job for any marina that does props and skegs. usually about a half hour job.