20hp increases rpm but looses power

Hi. i've got a 2009 4stroke 20hp honda that has a strange problem. When I give it full throttle it is building up speed but before the boat reaches plane, the motor suddenly gets max rpm but I'm loosing all speed. It seems as it is to much resistance for the engine and that it in some way disconnects from the driveline/propella. What shall I do? Anyone else experienced the same problem?

Does it happened suddenly or is it same from the beginning when you use first time ? , It can be ventilation. when the boat is going to plain the propeller lift up and turn free or If over load the boat and engine cant push, in both the case it is ventilating, propeller doesn't get enough water from bottom of the boat, check engine installation height (anti ventilation plate line with boat bottom )

wajira

Hmmm... When I think about it, it happens pretty much at the same "stage" every time... Guess i have to lift up the boat to check the installation height. How is the propeller able to lift up and turn free? Does it have something to do with the function of going into neutral if it hits an obstacle?

It sounds to me like the prop bushing has snapped causing the prop to slip when under a higher load. If you can borrow a prop from somewhere to try it first then it`d be worthwhile.

Matt

when you start go forward by accelerating, boat back push down and propeller dip more down in the water as soon as boat try to plain boat back comes up and propeller lift up from the water, And i forgot mention, better to check the trim settings,is the engine in full down position? If hits something the propeller can slip but i am not sure with this 20hp prop.If the prop slip or gear damage it will give prob at all the time. going in to neutral cant guess because if it goes to neutral the gear lever in the handle also should move,that you can make out, while using.

wajira

Put the outboard in gear. Pull the kill switch lanyard so that the boat CANNOT start. Attempt to turn the propeller by hand. If you can turn the prop against engine resistance, even if only a very small amount, then the prop hub is coming apart and your prop is slipping under load. They can be damaged from hitting something but they also go bad from usage and just sitting around in the sun.

Agree with Matt - sounds like a classic case of prop spin due to busted hub. That's the round rubber piece that connects the blade section to the drive shaft. Remove the prop and carefully inspect the rubber surface. You are likely to see raised pieces of rubber all around the outer ring where the rubber attaches to the blade hub. Sometimes, that raise rubber will be where it connects to the metal jacket that fits over the drive shaft. In either case I don't try to repair these myself, but send them in to a "prop shop." Get a replacement prop while it's in the shop. It's always good to have a spare prop anyway.