Prop issues on 1981 50 HP johnson. Help!!

[bryan65]

Have 1981 J50BELCIC. Have now tried 3 props.

Was a restore job I did on a 1981 Starcraft SS161 (16 foot)

After engine was looked at and repaired. Started with damaged 12 1/4 X 15 with converging ring. Do not remember my RPMS I was getting. But did not seem right, had to cut back on throttle at high end,, did not sound right and would cut off alittle. Suspected was due to damage on prop

Switched to 11 3/4 X 17 pitch with convering ring. Was able to go full throttle with no issues or cutting out. But could only get 4500 RPMs at engine rated for 5000. This was spare which came with boat. But had no damage so thought I would try it out. The converging ring did end up breaking and got all kinds of issues after this.

Got new prop Turning point 12 1/4 X 15 pitch. Tried it out today. Boat comes up great on plane, At first looked like it was going to hit 5000 rpm. but then bogged down and settled out at 4000 RPM.. I notice it only has a small end extending out at the end, I assumed replaced the converging ring from the older props.. It acted like at high rpm was getting alot of cavitation (which I got on my 17 pitch when the converging ring broke) To be honest, the small extension out is no where near the protrusion of the converging ring of my old props. so I figured I would test this problem.

Switched back to the old damaged 15. Again got 4700 RPM. but at high end.. had to cut back as before. I suspect the damage to the prop is causing me issues.

My question is, I am sure with a 15 pitch I can get 5000 out of it, which is where it should be. But I am looking at all the new ones, and it seems they all just have this small outer protrusion to prevent the exhaust from getting sucked into low pressure area.. casing cavitation problems. Is this the problem I am having or is it something else..

 

[steelespike]

Re: Prop issues on 1981 50 HP johnson. Help!!

Without your gps speed at max rpm we can't evaluate your setup.Some cells have a gps app.
As I understand it the converging ring only comes into play in reverse,helping to keep exhaust from fouling the prop.
I think newer props have solved the problem without the ring.Of course a damaged ring might cause issues but unllikely.
What you think you are feeling is ventilation,you don't feel cavitation.At 4000 rpm your not likely getting ventilation.
Venting lets the motor rev without gaining speed.Have you adjusted your trim for better speed?
Up trim, prop away from the boat, usually helps speed.
Is there a whale tail or hydrofoil on the motor?

 

[bryan65]

Re: Prop issues on 1981 50 HP johnson. Help!!

Without your gps speed at max rpm we can't evaluate your setup.Some cells have a gps app.
As I understand it the converging ring only comes into play in reverse,helping to keep exhaust from fouling the prop.
I think newer props have solved the problem without the ring.Of course a damaged ring might cause issues but unllikely.
What you think you are feeling is ventilation,you don't feel cavitation.At 4000 rpm your not likely getting ventilation.
Venting lets the motor rev without gaining speed.Have you adjusted your trim for better speed?
Up trim, prop away from the boat, usually helps speed.
Is there a whale tail or hydrofoil on the motor?

I will try to get GPS speed. I know I was about 22 mph with the first setup. The second setup it was about 20, but that was in heavy chop on the lake.. I forgot my GPS fishfinder yesterday, so do not have speed on new prop.

All I know is two props. both 12 1/4 by 15.

New one I could only get 4000 RPM, Engine would hesitate if I tried to give it more fuel. could not get above this.

One damage with ring.. get 4700 RPM. But still not full throttle. Had to back off throttle to find sweet spot.

Boat seemed to accelerate great on both props

with new prop, stopped at 400 RPM, with old prop, RPM went to 4700 almost immediately.

From what I am reading, we should find pitch to max rpm near rated, which is 5000 RPM.. I can not understand RPM issue with the two props.. this is my confusion, before I go buy a new prop, or try 13 pitch would rather resolve this issue. Is it prop issue, or engine issue??

I hope I am explaining it properly. I can get wordy, Any questions anyone may need to ask to make sure you understand. please ask..

 

[steelespike]

Re: Prop issues on 1981 50 HP johnson. Help!!

I think you have a motor problem, fuel or ignition or booth.
I think the 15 is a reasonable starting point for that boat.You need to be sure the motor is 100%.
In looking up a 81 ss161 it says it is a I/O. Is the boat converted? At any rate it's probably about 1,000 lbs.dry weight.

 

[bryan65]

Re: Prop issues on 1981 50 HP johnson. Help!!

I think you have a motor problem, fuel or ignition or booth.
I think the 15 is a reasonable starting point for that boat.You need to be sure the motor is 100%.
In looking up a 81 ss161 it says it is a I/O. Is the boat converted? At any rate it's probably about 1,000 lbs.dry weight.

Carbs were rebuilt by mechanic.. Just replaced all ignition parts as had issue with lower cylinder not firing. Replaced power pack and both coils. Also spark plugs and wires are new. The motor sounds the best it ever did.

As for boat. refirbished 161. Max engine is 90 HP.. I plan on getting at least 75 But can not find any here in Columbus.. And 6000 for new motor when boat is only worth about 2000-2500 is not an option..

Can find my rebuild here..http://forums.iboats.com/boat-restoration-building-hull-repair/1981-starcraft-ss-16-rebuild-568348.html

 

[steelespike]

Re: Prop issues on 1981 50 HP johnson. Help!!

A motor doesn't doesn't hesitate,stumble or bog because of a prop.
A motor in sound tune and condition will attempt to pull strongly through the entire rpm range not requiring you to select a sweet spot.
It may not make full rpm but it will try.
You have tried a number of different props all different brands and styles and condition. so you would expect different results.
The first 17 did 4500 if you went to a 15" it should make "about" 4900.It may be possible that you need a 13" to get over 5000.The motor develops its hp at 5000 and they routinely run motors about 500
over that rating.It's my feeling the motor should be close to or at 5000 with a 15" prop.

 

[bryan65]

Re: Prop issues on 1981 50 HP johnson. Help!!

A motor doesn't doesn't hesitate,stumble or bog because of a prop.
A motor in sound tune and condition will attempt to pull strongly through the entire rpm range not requiring you to select a sweet spot.
It may not make full rpm but it will try.
You have tried a number of different props all different brands and styles and condition. so you would expect different results.
The first 17 did 4500 if you went to a 15" it should make "about" 4900.It may be possible that you need a 13" to get over 5000.The motor develops its hp at 5000 and they routinely run motors about 500
over that rating.It's my feeling the motor should be close to or at 5000 with a 15" prop.

You have said prety much everything I was thinking. I am hoping to get the boat on the water Friday (weather permitting we are going camping this weekend) then will have speed on both 15 pitch props.

maybe I need to take this to the engine thread..lol