installing #4,s

[boatman747]

I busted one drive #4 small shaft with one inch spacer 1.33.1 ratio . I have found 2 new #4 big shafts with no spacer and 1.4.1 gear ratio. How much differnce in prop to compensate for gear ratio and how much difference in prop to compensate for no spacer. These drives have a #4 upper and a #5 lower. I needed to gain about 500 RPM,s anyway . I was spinning 31 inch 3 blades 4900. New motors peaked at 5600-5800. Well what do you think ?

[stirling]

You can use the spacers on the big shaft drives if you like ,the shaft thickness is the only difference.
1,41 :1 to 1,33 :1 is about 5 % prop speed difference.
I think you drop 200-300 rpm with the ratio.
So IMHO there is a 1-1,5 pitch difference.

Are you sure you still have prop bite enough without the spacer to come in plane ?

A #5 lower is a whole different style lower and will not fit a #4 or #3 upper due to different style shafts/gears.

good luck Stirling

[CIG3]

Also the #5 or Big Shaft lower is wider to accept the larger internal parts for the big shaft. You will loose a little due to that also.

[boatman747]

I was told they were #5 lowers so maybe he meant shafts only.So If I use the same pitch props and go to the 1.4.1 ratio I will increase RPMs and get to where I want to be

[boatman747]

Im going from 1.3 to 1.4 I want more RPMs

[stirling]

Going from 1,33 to 1,41 will give you +- 200-300 engine rpm,s more (with the same props off course)

Is your boat a cat ?

stirling

[boatman747]

38 fountain , old school power
572
1671 BDS low boost for longevity
941 LBs torque at 4100

[stirling]

Ok,that makes sense,I was thinking how you can swing such large props with those gears,that has to be a cat or a fast V with big torque numbers.

I have a set # 4 ssm here that is now in my garage for 4 years ,I,m still thinking to put them in a hull with a pair 572,s
or a pair all aluminium Italian V8,s with twin turbo,s ,we bought it out of an aluminium CUV here ,they stopped racing with years ago.

I still have a 31 Scarab viper that is empty,but don,t know if that hull is capable for big horsepower ?
I could only found some with twin 502,s or 454,s .

stirling

[stirling]

pic BPM twin turbo

[stirling]

with old style K jetronic