Jackplate or Prop troubles

[katbird]

I have a 1986 Ranger 18 ft. with a 120 Johnson same year. this boat is new to me. all of the weight is in in back, the motor probably weighs 300 lbs and three batteries and 2 built in gas tanks which is 14 gals. each plus the live well plus 2 storage compartments and me and my son which I weigh 250 and my son weighs 250.the motor has a 21 ballistic prop and a 6" jackplate. we can't get the boat to plane off, at full throttle while taking off the motor will only do 2700 rpms. if my son sits in the front it still takes a long time to plane. the jackplate is adjusted with the cavitation plate is about 2" below the bottom of the boat.will it help to raise it some or lower it more or is the prop too big. I thought of trying a 19 pitch prop to see if it would help. another thing is the boat will run 45 mph at wot but only doing 4600 rpms .any help will be appreciated.

 

[Ron G]

Re: Jackplate or Prop troubles

the cavation plate needs to above the bottom of the hull i would try raising it up you can way on up.just watch your water preassure.and you are a little over proped i think but try raising the motor and keep us posted on your rpms.and that is a heavy boat to start with.

 

[J BLANK]

Re: Jackplate or Prop troubles

what is the purpose of a jack plate? is it more for speed or something else.

 

[KCLOST]

Re: Jackplate or Prop troubles

Is your boat a 361,371V????

If so you are over propped no doubt... Your motor is way too low also. My cavitation plate is about 3" above the very bottom of the hull (371V Ranger). But I have a 175hp Merc on the back not a 120hp...

With a 120hp, which would be on the low end of hp rating for that boat (slightly underpowered), you will need to drop down in pitch to get the rpms up and improve hole shot, raising the motor will also raise rpms at WOT. Test at 1 inch increments upward....

 

[LubeDude]

Re: Jackplate or Prop troubles

Everyone here is being very kind and the replies are all correct. But, the truth is that you are "VERY" underpowered for that heavy of a boat and your fuel consumption is going to be worse than if you had a 150 on it due to you having to run the living H*ll out of that 120 just to get around. My thought would be to find a used 150 to replace your 120 with. You will be much happier in the long run.

 

[katbird]

Re: Jackplate or Prop troubles

Jblank, the jackpost is there for raising or to lower your motor if needed.

Kclost, my ranger is a 350v with max hp at 150 hp. the boat alone weighs 1250 lbs. the guy that I bought it from said that he had just adjusted the jackplate but this guy only weighs 120 lbs. I will try the upward adjustment on the jackplate and see if it changes. If it doesn't help I will try a 19 pitch prop.
thanks guys

 

[KCLOST]

Re: Jackplate or Prop troubles

I think you are going to have to do both... The hole shot will not be improved much by raising the motor, but a lower pitch prop will. The WOT rpms will increase however with a higher mounted motor...