RPM & Speed sound right??

[rolrandy]

1975 17' Hewes flats boat 1994 120 johnson
13-7/8" x 19 p 3 blade viper stainless prop
I'm turning 5600 RPM wot at 43 mph GPS speed

Does this sound about right??

 

[Dhadley]

Re: RPM & Speed sound right??

That's right at 15% slip. Not so good. Is that with a normal load? Are you looking for only top speed or are you running with a tournament load?

 

[rolrandy]

Re: RPM & Speed sound right??

No load empty boat 1 person
Just got my Tach working never knew what I was turning
15% slip ?? do I need more pitch to reduce slip

 

[Dhadley]

Re: RPM & Speed sound right??

There's lots of ways to reduce slip (increase efficiency). I'm a little surprised the Viper is slipping that much. Is it in good shape or worn some?

 

[rolrandy]

Re: RPM & Speed sound right??

age?? came with the boat looks alittle worn

 

[Dhadley]

Re: RPM & Speed sound right??

If it's worn that would explain what's happening.

 

[rolrandy]

Re: RPM & Speed sound right??

can it be reworked to add more cup and or a hair more pitch??

 

[rolrandy]

Re: RPM & Speed sound right??

Another thing I just noticed id that the 13-7/8 dia prop is less than 1/4" from hitting the cavitation plate.Does that sound correct I'd be afraid to go
to a 14" dia prop

 

[Dhadley]

Re: RPM & Speed sound right??

That's probably about right. Going from a 13 7/8" diameter to a 14" means the blade tip would be 1/16" closer. That is, if the 14" prop has the same rake angle as the 13 7/8 you have now. A 14 should be fine.

 

[rolrandy]

Re: RPM & Speed sound right??

do I want to go smaller dia more pitch or just a larger dia more cup ?? want to bring my RPM some

 

[Dhadley]

Re: RPM & Speed sound right??

You first need to determine how worn the Viper is. But you don't want to decrease rpm. 5600 with a V4 looper is border line lugging. 5800 - 6000 is the target rpm for that motor.

Is the motor mounted all the way down? Do you have a jackplate?

 

[rolrandy]

Re: RPM & Speed sound right??

No jackplate at normal trimmed position the cavitation plate is about 2" above the bottom of the hill in the center abourt ast my max speed location.

the more I trimmed the motor I couldn't get it to rev above 5600 unless I really caught alot of air and the speed really dropped.

so I need to rehub or recup to reduce slip?

 

[Dhadley]

Re: RPM & Speed sound right??

Measure the location of the AV plate (plate right above the prop) when it's parallel to the keel. I'd still bet the prop is worn or has been rebuilt using an SST pitch block. I'd stick with the same pitch and test a different prop. A Raker or Turbo Lightning Jr or a Pro Tour Titan Jr. would be a good choice.