johnson 50s "slipping" whats the deal?

[tonyscj5]

On my 19-6 aquasport fishing boat I have a pair of 50hp johnsons that seem to have a slipping problem. The problem seems to be related to load on the engines which would make sense, when over loaded the motor slips. The engine revs, but the boat dont go. Whats up? I thought that it was simply a prop hub, but I dont think so after inspection. I pulled both props checked them out. One prop hub looked spun(the ruber had a twisted look) so I replaced the prop. Problem is better but it still seems to happen on occasion. Normally when under heavy load. Boat load of gear, people, or fish, or often when I turn the boat hard to one side then power it up. The damn engines rev, but no response from the boat?<br /><br />WTF? Do these motors have a tranny that could be slipping? I am no outboard expert but would like to fix this problem. Whats going on?<br /><br />Thanks<br />Tony

 

[R.Johnson]

Re: johnson 50s "slipping" whats the deal?

We need more information here! Is this a new problem, or something that has gone on for awhile? You are running twin engines, one prop replacement seem'd to help. It is hard to say if you have a prop problem, or a set up problem.

 

[JB]

Re: johnson 50s "slipping" whats the deal?

"Slipping" could be prop hub trouble, but with twin installations it is more likely to be ventilation or even cavitation.<br /><br />That, as RJ suggests, is likely to be a set-up problem.

 

[rodbolt]

Re: johnson 50s "slipping" whats the deal?

a year model and prop model may even help us help you.

 

[tonyscj5]

Re: johnson 50s "slipping" whats the deal?

the engines are 1989 johnson 50hp, originally VRO, but it has been removed, the boat is a late 70s dual console 19'6" aquasport cuddy, the props are the stock props, sorry I dont have a model number in front of me.<br /><br />Problem started a couple of years back when one motor was down and we were trying to run the boat at slow speeds on one engine. I guess we over worked her. I pulled the prop off the slipping engine, and the hub definetly looked spun, so it was replaced. The the problem went away, but everyonce in a while it will slip again. Either motor will do it. Like for example when we are running in with 3 guys, gear and some fish, beating up one side of a swell, so the motor is working extra hard. It will just slip, the engine will suddenly rev up super high, but no boat response.<br /><br />What is ventilation or cavitation?<br /><br />Thanks for the help<br />Tony

 

[Silvertip]

Re: johnson 50s "slipping" whats the deal?

Think of cavitation and ventilation in the same way you would your car when it is sitting on glare ice. If you mash the gas pedal, the engine revs, the wheels spin like crazy and the car doesn't move. The lesson here? You got no traction mate! Place mark on the hub and another in the same location but on the outer hub of the prop. Do the same for both engines. Then go out and try to duplicate the problem. When you return, if the hubs are slipping, the two marks will no longer be in-line. If they are still in line you are experiencing cavitation/ventilation (the prop is trying to move through air instead of water). If that's the case, you need to check the location of the cavitation plates in relation to the bottom of the boat. Engines mounted too high will cause the problem you are experiencing. When you turn sharply or get into rough water, the props begin getting air. In your application you might consider four-blade props to get a better bite although it might slow you down a tad.

 

[JB]

Re: johnson 50s "slipping" whats the deal?

Ventilation is when the prop grabs air, makes foam and then spins in the foam. It happens when trimmed out too far, prop is lifted too high by the boat posture on a wave or by hard turns. On twin engine installations it doesn't take much tilt of the hull to ventilate one of the props.<br /><br />Cavitation is when the pressure on the trailing face of the blades gets so low that water vaporizes, causing the same effect as ventilation. It is commonly caused by damaged blade edges.<br /><br />Compare ventilation to getting a tire on ice and having it lose traction and spin.<br /><br />Compare cavitation to applying too much torque to a tire and spinning it on dry pavement.<br /><br />Good luck.

 

[tonyscj5]

Re: johnson 50s "slipping" whats the deal?

Alright I will try the two mark idea on the prop hubs. The ventilation theory seems to be a good one, with the twin engines each motor is off to the side so when the boat tilts it could easily be grabbing air. But it seems to happen when going straight also?<br /><br />i do have some nicks in one of my props, I will also get those taken care of.<br /><br />thanks for the help all.<br /><br />Tony