please help a newbie out

[mr roach]

i have a merc 115 thunderbolt i believe it to be an early to mid 70"s it is on a boat that was given to me i know the previos owner well and have been out in the boat many times. In the past the motor was always difficult to impossible to start once it was in the water i pretty much resolved that issue and put the boat in the water. Once i had done a couple of laps around the basin at an idle to check everythng out i gave her some throttle & we picked up speed ok we give her a little more & it looks good so i give her enough throttle to come up and start to come up on plane so i can play with the trim and see where i will run the best. Ok just as we are starting to raise the front end of the boat it flattens out and won't go any faster but the motor sounds like a car with a bad clutch the motor is running well but the boat is not moving anywhere near the speed of the motor i figure the woodruff key is sheared and shut it down and use my trolling motor to get back to the launch ramp and load my boat up and go home. I get home and i notice the prop has some minor damage and know that the damage was not there the last time we took the boat out but it had been out once without me just before it was given to me so i put the boat in gear and turn the prop and am met with a lot of resistance enough that it feels like i could turn the motor over i do the same thing with it in reverse with the same results this was the only thing i could think of to check it out without pulling the prop. Sorry for being so long winded my question is this do you think it is the woodruff key,prop or something else that is causing my problem also what would the other problem require as far as experienc,time and money to fix. I am pretty good with mechanics but most of my experience is with cars. In advance i would like to thank you for your patience and help towards a newbie boater i really want to keep this boat but have to weigh the cost of the repair as i just put a new floor in and rebuilt the hydraulics on the lift/trim and i have learned when i need to cut my losses anyway thanks again and look forward to hearing from everyone

 

[usskydvr]

Re: please help a newbie out

My first impression is that the prop has a spun hub. This causes the motor to rev up but the boat itself doesn't increase its speed. It happens as the load increases. There is no woodruff key. The propshaft is splined as is the prop hub. There is a hard rubber layer between the prop hub and the prop. This layer can breakdown and allow slippage under load. Minor damage will effect
top speed first and as the damage increases so will vibration and overall ability to push the boat. More severe damage 'could' prevent the boat from getting on plane, but it would have to be pretty significant.