RPM's suddenly climbing boat slowing down after hard impact on waves

[mtb_prodigy]

Ive got a 96 bayliner 5.7L Mercruiser and its been giving me problems the last couple times I took it out. I was in the ocean and it got a little rough. This happened on two separate occasions. After going over a wave theres a hard smack when the bow comes back down. As soon as we hit the water the rpms shot up (from 2800 to 3800) but the boat slowed down. I dropped the RPM's back down to idle, hit the throttle again and all was fine. Im worried its the coupler. Any ideas?

 

[alldodge]

As soon as we hit the water the rpms shot up (from 2800 to 3800) but the boat slowed down. I dropped the RPM's back down to idle, hit the throttle again and all was fine

The prop may have ventilated, or came out of gear, don't see a coupler doing that

 

[JackBronson]

If the stern rose up enough (which you may not have picked up on, being focused on what the bow was doing) I'd say it was cavitation.

 

[tpenfield]

Sounds like you 'caught some air' :thumb: the prop/engine goes crazy but does nothing and then finally grabs once you throttle back.

The folks running the go-fast boats through the rough stuff usually 'chop' the throttle back for a split second once they feel they are airborne.

You are just going to have to take some lessons from those guys

 

[achris]

Sounds like you 'aerated' your prop. Either trim in a bit or change to a prop that's more tolerant to a bit of air in the blades. Or slow down. Not sure where you are, but if all you have done is rivers or lakes, the ocean is a whole different world of hurt. Slow down a bit until you get an understanding of what you're up against. And getting 'a bit rough' is what the ocean is really good at...

Chris.......

 

[Scott Danforth]

I agree, you ventilated (prop came out of water, then when it re-entered, it aerated the water)

many of the go-fast boats have a rev-limiter to "snuff" the ignition for a few seconds when the motors become suddenly unloaded by getting airborne. this is to keep the drives from grenading