need help

[brogden31]

i have a 19ft sea fox with a 2002 johnson 115v4. When i run my boat it gets on plane fast, but at about 5200rpm im only doing 35, these numbers dont change with weight, IT just takes a little longer to get on plane with 800 pounds but still 35 at 5200 the motor will still rev to 7000 if i wanted it to so what kind of change do i need to get a little more speed the prop on it is a alum 17p oem prop

 

[Bondo]

Re: need help

but still 35 at 5200 the motor will still rev to 7000 if i wanted it to

Ayuh,...

The basic process of setting up the prop is,...
The motor has a manufacturer's suggested WOT, rpm.... Usually about a 500rpm spread.... I don't know what your's should be, but my I/O is 4400rpms/ 4800rpms...
When the boat is run at WOT,... the tach should be pointing to somewhere within that range....
If the motor Over-revs,... Go Up in prop Pitch......
If the motor is unable to pull those rpms,... Go Down in prop pitch...

Each Inch of prop pitch should change the WOT/ rpms about 200rpms,... Theoretically....

Soooo,... I'd think you need atleast a 19",... Probably a 21"....

 

[steelespike]

Re: need help

Sure seems odd that from 5200 to 7,000 no speed change.Is the speed by GPS pitot speedos are not accurate.It appears you may need a prop change but check the setup.The anti vent plate(just above the prop) even with the bottom of the boat is considered a starting point.Usually can go higher.
Could gain a couple of mph.Do you trim out at speed?Is it affective?Does it get up on the water or does it lay down really flat.

 

[brogden31]

Re: need help

i said it will go up to 7000 but max is 5200 it will go faster if i go more rpm but i dont so yeh i think i just need to go to about 21p from the 17

 

[steelespike]

Re: need help

i said it will go up to 7000 but max is 5200 it will go faster if i go more rpm but i dont so yeh i think i just need to go to about 21p from the 17

Are you saying it will turn 7000 if allowed to?
I'm not what I would call an expert but a 115 on a 19ft turning 7,000
doesn't sound right.The motor is surely safe to run at 5500 maybe more.
Is the prop in excellent shape? You may want to verify the tach is accurate
before making any changes.