50 HP johnson Prop ?

[kskiller]

I have a 2000 johnson 50 hp on a 16' lowe bass catcher aluminum boat.The boat does not have a tach so i don't know the RPM's i'm running.Right now i'm using a stainless 15 pitch 3 blade prop on it and running around 33mph.I called the local prop shop and he told me to drop back down to a 13 pitch aluminum prop and the boat would run faster because i'm not in the right rpm range.Is he right? I'm wanting more speed but don't know weather to believe him or not.I'm also running a 4'' jack plate on the boat but don't think it helped much at all.Thanks for any help guys!!

 

[Tig]

Re: 50 HP johnson Prop ?

The real answer is that you need a tach, but you can roughly esimate the RPM using this website
You'll need to enter;
Pitch: 15
Speed: 33. If this is not GPS verified then the margin of error is huge and this method will not give a reasonable estimate.
Gear ratio: From your owners manual or the manufacturer website.
Prop slip: 10% This is a reasonable guess. It is also the biggest source of error since we are not only guessing, we are assuming that slip is not part of your problem.
Someone else may have a better guess at what slip you should be experiencing.
For rough comparison sake I have a 850lb boat with a 200lb 50hp motor. Using a 13 pitch aluminum prop, my max RPM exceeds reline by 300 RPM at 33.7 mph. This would seem to be the right prop for me. that being said I know my motor need to be raised a notch or two and that may change things.
So, to do this right you really need a tach, a GPS and then give us all the information listed HERE

 

[steelespike]

Re: 50 HP johnson Prop ?

First in my opinion you shouldn't make any prop changes without a tach.A tinytach is reasonable,easy to install and yours would fit any 2 cylinder 2 stroke motor.
Your motor could end up performing poorly and could have premature repair problems.
From my limited research if your motor is healthy a 15 should be very close to what you need.Your speed seems very good.As far as the jack plate have you carefully raised the motor to a point just before venting (cavitation)becomes a problem?
Without a rpm and speed reading you can't tell how well your setup is working.

 

[Silvertip]

Re: 50 HP johnson Prop ?

I can guarantee that you do not need a 13P prop. Been there and done that on a similar boat. Your engine on nearly any 16 - 17 foot aluminum boat will be perfect with a 12-1/4 x 15 which is what you have and the engine will be spot on for RPM. Go to a 13 and the engine will over rev by a bunch. In fact, depending on how light a load you carry, you may be able to spin a 17P but without a tach, stick with the 15. It will put the engine at 5500 - 5800 RPM which is perfect.

 

[kskiller]

Re: 50 HP johnson Prop ?

O.K. i finally got a tach on the boat and got it on the water.GPS is showing 33MPH at 6100 RPM at full throtle and trimmed right to the point where its not cavitating.That seems like a little to many rpm's to me.Am i right? I'm still running the 15 pitch prop right now.What would you guys recomend for a prop? I am thinking a 17, but will it be enough to get the rpm's down? Would a 19 be to much? With the 15 i have NO prblems coming out of the hole either.Thanks for helping me guys!!!

 

[steelespike]

Re: 50 HP johnson Prop ?

I would say your speed is spot on.Slightly heigher than typical. While the rpm is a little over the top certainly not excessive.Perhaps a good prop shop could add a little cup or pitch.
The cup might allow a little more trim and or height.I'd be inclined to leave well enough alone.Especially if you usually run with the family or a partner and some gear.

 

[kskiller]

Re: 50 HP johnson Prop ?

Sounds good to me.THANK YOU!!!