Prop Selection - Johnson 50

[XoDiViAoX]

I've got a Johnson 50 on a 15' Polar Kraft Bass America. I've been trying different props, and it gets kind of expensive I was wondering if someone could tell me what would be a good size prop I need to use.

The motor calls for a 11 3/4 x 17 - I've tried an aluminum one and the max WOT is 5500 RPM and I went to 6,000 with plenty of throttle left

I bought a 11 3/4 x 17 stainless steel prop, same thing it'll do 6,000 RPM and I backed off .. I'm guessing it would do at least 6500 or more but I dont want to blow the engine up or anything so I kept it under 5500 or around that

Before I bought the stainless steel I bought a 12 1/4 x 15 aluminum and it wont go as fast and the RPMS would be way too high on this one also if i gave it full throttle

I want to get a prop that will do 4500 - 5500 RPM when i give it all the throttle and not hurt the engine, if someone could recommend a good size and pitch prop that will fit this engine and be a good combination for this small boat to give me max MPH in that rpm range let me know

 

[cshyde]

Re: Prop Selection - Johnson 50

Have you used any of the prop calculator programs on any of the manufacturers web sites or here on iBoats? The Michigan Wheel web site works good. It sounds like you could push a 19 inch pitch but you may want to check a prop calculator first. The prop diameter really doesnt matter that much.

 

[steelespike]

Re: Prop Selection - Johnson 50

What year boat and motor?
I think first I would check that the tach is accurate.It may be set wrong.
While its possible it might over rev with a 17 I don't think it would do it so easily.
Also a 19" while possible seems extreme.

 

[XoDiViAoX]

Re: Prop Selection - Johnson 50

The boat is a 1999 polar kraft bass america 15' aluminum boat, it's very light, and the motor is a 1993 johnson 50 2 stroke. the tach im using is the one that came from the factory from polarkraft in the dash, i dont know how to check if it's accurate, i was just assuming it was because when i go over 6000 rpm it sounds like the motor is realy really winding up

 

[steelespike]

Re: Prop Selection - Johnson 50

assuming the boat is truly light then a 19" pitch would be a good selection and in the same brand and model should reduce your rpm "about" 400. my 88 Erude 50 runs a 19 on my 18 ft Crestliner utility about 375 dry weight.Don't have a tach but weith 60+ years of boating it sounds right and makes sense compared to the typical
17" prop selection on these motors.

 

[Haffiman]

Re: Prop Selection - Johnson 50

A 15' light Alu? boat.
21" and most likely dubbel-cupped and if possible even raked.
I do not see year of motor?
This might have implication as the 'newer' gearcase is not the ones I like best.
The anti-ventilation plate is a bit too short, and makes the props spin out too easy.
Whatever you choose, SST might be the only option going this high in pitch.

 

[eavega]

Re: Prop Selection - Johnson 50

I do not see year of motor?
This might have implication as the 'newer' gearcase is not the ones I like best.
The anti-ventilation plate is a bit too short, and makes the props spin out too easy.

The OP mentions the prop on there is a 11 3/4 x 17. That diameter corresponds to the small gearcase. The large gearcase props are in the 13" diameter range.

 

[Haffiman]

Re: Prop Selection - Johnson 50

The OP mentions the prop on there is a 11 3/4 x 17. That diameter corresponds to the small gearcase. The large gearcase props are in the 13" diameter range.

They have had a 'small' 13 splines case since late 70's.
The design of the casing however changed late 80, and not to the better!!!

 

[XoDiViAoX]

Re: Prop Selection - Johnson 50

I'm now trying the 12 1/4 x 15 aluminum prop on my boat again, with the throttle as far as it will go, the RPM gauge is showing around 6100 or 6200 RPM ... Does that sound about right, or would you think my RPM gauge is off a little? Will that blow my motor up or do any damage running it that high for very long?
... At idle the gauge shows around 1000 or 1100

My top speed is 33MPH (gps) with that prop, with the engine trimmed all the way down as far as it will go, i'm pretty new to boating and am afraid if i trim my engine up it will mess something up with the water pump or something, so i havent really messed with it

 

[Haffiman]

Re: Prop Selection - Johnson 50

You can not go down in pitch if you want to reduce the top RPM!
Try 19 and/or 21" pitch.
Trim out and do not be afraid of loosing water providing the installation on transom is too high. A std alu/SST prop will probably loose the grip before you loose water to the pump.

 

[steelespike]

Re: Prop Selection - Johnson 50

The boat is a 1999 polar kraft bass america 15' aluminum boat, it's very light, and the motor is a 1993 johnson 50 2 stroke. the tach im using is the one that came from the factory from polarkraft in the dash, i dont know how to check if it's accurate, i was just assuming it was because when i go over 6000 rpm it sounds like the motor is realy really winding up

Unfortunately "very light " doesn't really help much.If its a full dress 15' bassboat,
floors,fishing platform,live well,pedestal seats trolling batteries steering console etc. I wouldn't call it light certainly heavier than a 18 ft bare bones utility with tiller steering.
Is it such a bass boat?

 

[XoDiViAoX]

Re: Prop Selection - Johnson 50

it has all that, but i'm pretty sure it's fairly light compared to a fiberglass boat, it has no storage compartments except under the seats.

 

[steelespike]

Re: Prop Selection - Johnson 50

I would guess your boat at 600 to 700 dry weight.Yes lighter than a 15 fiberglass at 800 to 900.But not light compared to my utility at 375 dry and a 19" pitch.
I think 6000 with a 15" pitch is reasonable depending on the year the 50 is rated for 6,000.33 is a reasonable speed.the same brand,same style 16" prop would put you at "ABOUT" 5900-6,000. may add a smidge of speed lose a smidge of hole shot.
Typically idle speed is about 700/750 I don't know if that is in neutral I would set in neutral and see how it acts in gear.If it seems too slow set it around 700 in gear
till you get the correct info on idle.For the life of the shift dog slower the better at idle
while keeping a strong idle.