1968 Johnson 9.5 short shaft only hits 6.4mph??

[bansheerider84]

I have a 14ft Northwood semi jon that i used mostly for small electric lakes but recently started fishing some tail waters of a much larger lake. On flat water with two guys and gear (650lbs) I can hit 6.4 mph on flat water. Whats very odd, the boat hits top speed in about 4 seconds. Its very punchy and the outboard seems to be running great. It starts in one pull and other then the speed I have no complaints. The prop looks like its in good shape, the edges are worn but no chips or bends. My questions, would a better prop help out with top speed? If so what would be the best prop for this boat? I'm not looking to put the boat on a plane but would like to able to do 10mph, should I ditch this motor or is 10mph doable? Thanks

 

[shane@]

Have you done a compression test?

 

[bansheerider84]

Compression was 85psi when I checked it in the spring. I'm not outboard mechanic but im very familiar with two stroke ATV motors. Whenever you dropped compression there was always a rough idle, popping at high rpm, some type of compensation

 

[shane@]

It could be running off one cylinder.

 

[Zum]

You "might" be right about needing a different prop but could be alot of differrent factors...motor setup, weight distribution, hull in good shape,etc...
I think before you get to 10mph,your boat will have to be on plane.
With the weight and size of your boat, your going to have things just right to acheive this.
Boats just push water until they get up on plane...did you try different tilt positons,move weight around or go solo to see if it would plane then?

 

[bansheerider84]

I pulled one plug and noticed a big difference in the way it was running. The last time I was out I moved a friend up to the front that thats when i hit 6.4mph. The hull is in good shape. When I put the motor all the way down I lost 1/2 mph. For some reason it likes to be in the 3rd position.

 

[shane@]

If it is not compression and running on both cylinders then the prop would be the only other thing I could think of. Like Zum said go solo and see how she runs.

 

[bansheerider84]

I'm thinking its the prop as well. I tried to find some numbers on my prop but couldn't locate any. Whats the best pitch to get for a 9.5hp with a decent amount of top end? Oddly enough it will only do 6mph if I am alone. It gets there really fast too 4 seconds tops and I am at max speed.

 

[SumDumGuy]

To properly prop the motor for that boat you would need a tach to ensure you were at correct RPM.

The 9.5 motors are nice motors, but they are not known for power.

 

[rickybobbybend]

From the photo, the motor appears to be a short shaft; make sure it is compatible with the transom. And before you do much else, the suggestion to verify RPM at WOT is a good one.

 

[Y_J]

bansheerider84 » 08 Jun 2014, 08:38[/url]"]I'm thinking its the prop as well. I tried to find some numbers on my prop but couldn't locate any. Whats the best pitch to get for a 9.5hp with a decent amount of top end? Oddly enough it will only do 6mph if I am alone. It gets there really fast too 4 seconds tops and I am at max speed.

Try taking the prop off and look from the back inside if possible. There are numbers on that prop somewhere. BTW that's where I found them on mine.

 

[acabtp]

without knowing the pitch of the prop you have now and what RPM the motor is turning at wide open throttle, this is just a guessing game. you need to install a tachometer and see what it reads. good news is you can get them pretty cheap these days. i just got this one
https://www.amazon.com/gp/product/B00AR2R090/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1
off of amazon for $30 the other day. tinytach is also available for $40 or $60 if you want more of a 'name brand'

once you have the tach installed (takes 2 minutes, you just wrap a single lead around a spark plug wire) report back what the motor is turning when you are running it wide open with your boat loaded like it normally would. with that info and the pitch of your current prop, it's easy to calculate what kind of prop you should run to get the best performance.

 

[Dark3]

Its a really old 9.5. Its not gonna plane with 2. It should hit around 12-15 mph with one if on plane. It should rev up pretty high once on plane. You wont get full power untill then