Mercury 500 wot bogging no propulsion

We took our Fisher john boat out with just bought used with a mercury 500 thunderbolt, ran fine for 3 hours then all the sudden prop bogged down, would move with low rpms, but when give it more gas on the throttle it would act like no movement in prop. It would drive fine low speed. But when giving it more gas would be fine then no movement and motor would rev-up, I don't know much about outboards, if it was a car I would say transmission slipping,,could someone help me, I am hoping that it is something simple..

Re: Mercury 500 wot bogging no propulsion

Sometimes the motor will catch some seaweed, a branch, or a piece of plastic or such around the motor part in the water in front of the propeller. This will effect the way the water flows to the propeller causing something caused cavitation.

When this happens on my boat I pop it in neutral and lift up the motor to see if some branch of leaves doesn't come floating out of the motor area. Sometimes I can just back up slowly while turning back and forth to dislodge the stuff, pop it in gear and continue.

If this is not the case with your situation you may need a propeller. There is a part between the shaft and the propeller itself called a hub. If the propeller hits something while underway this hub can break(rather than that a gear) leaving the propeller to only engage at way less than power speed.

That can be checked by putting a dab of nail polish on one side of the propeller nut, and another on the rim of the propeller adjacent to the first, let it dry then go run it. If the prop hub slips, the marks won't be lined up when you get back.

PS if debris finds it's way wrapped around the lower unit, it can block off the cooling water intake possibly leading to catastrophic failure.


And for future reference a motor that bogs or is bogging is not able to make rpm and may exhibit a bee-augg (bog)sound when the throttle is applied.

Re: Mercury 500 wot bogging no propulsion

Thank you, we believe it is the prop hub. So we are going to send it off. Thank you for help.