My boat won't come up on plane. What's wrong?

Question

I have a 1977 Mercrusier sterndrive. The engine runs great. When I hit the gas, the motor revs up but the boat doesn't pick up much speed and it sounds like its working extra hard. It doesn't pick up fast enough to get up on plane. I changed props, I messed with the tilt, I'm not sure what to check next.

Answer 1

My first thought would be to have your outdrive or transmission checked. The 'trim" is usually what helps the boat plane up out of the water. You want the trim all the way down when starting up, then as you pick up speed, gradually bring the trim up to help push the boat forward and plane out.

Answer 2

You may have spun a hub. If the hub inside the prop is worn your prop will spin fast and the rpms will read high, but it is slipping. In this case you will not really be turning the prop as fast as needed.  

Answer 3

it may be overweighed.

Answer 4

you can have several things to check. correct prop. engine tilt all the way down, dirty hull on bottom and the engine may not be getting enough fuel. low compression on cylinders. if you have trim tabs they should be level. top rpm is about 4800. maybe a professional checkup is in order.

Answer 5

sounds like wrong prop. and trim or tilt check the power trim rotary sender  and the pump fluid level  and power trim switch  if three button  must push two button's to get trim to work on one button full speed plane adjust . get a CLYMER mercruiser stern drive shop manual TR TRS models. this book will tell you how to check everything and fix it,

Answer 6

Unstrap it from the trailer & try again.  :D

Has it been like this since you got it, or is this a recent problem?

Reason for asking is, it might just be overpropped or under powered.  

Or could have a hook in the hull.

What size motor on what size boat?

Added or redistributed any weight in the boat recently?

What maintenance has been done on the motor lately?