"Ventilation"

[Don't Wake Me]

Hello To All - Recently purchased 1999 Sea Ray 180 Bowrider (Mercruiser I/O 135HP) and just joined this forum. Very pleased with the boat thus far... and seems like a great forum to join. I ran into an issue yesterday that I'm hoping you can provide assistance.

We were running at high speed, then all of a sudden the engine got louder and sounds like the RPM's went way up (wasn't driving at the time, so I wasn't able to glance at the gauges). From searching the internet, I believe I ran into something called "Ventilation" per the description below:

"Air from the water surface or exhaust exiting from the exhaust outlet being drawn into the prop blades causes Ventilation.
When this situation occurs, boat speed is lost and engine RPM climbs rapidly(seems to rev up or breaks traction).
This is often the result of trimming the engine too far up or out, sharp turns, and outboards mounted too high."

I think we mistakenly had the engine trimmed too far up. The good news is that we ran the boat for a half hour to get home and it still moved! The bad news is that it sounds louder than before, seems to vibrate occassionally and just overall doesn't sound right (something's just a little off - hard to explain). I could be imaging this, but I don't think so.

Any one run into this before, have any suggestions? I like to have an idea to try and fix myself before taking to boat mechanic.

Thanks to all!

Don't Wake Me

 

[fwebster]

I believe you have diagnosed your own problem........i.e. trimmed the drive out too far and caused ventilation. Typically, this doesn't hurt anything unless you over revved the engine while the prop was sucking air.

Before doing anything, I think I'd run the boat again with the drive trimmed down and see what happens.

 

[jg300da]

Sounds like the exhaust bellows must have come off due to the excessive trim and a loose stainless worm clamp (acccounting for that louder sound). Pop it on the trailer and trim it all the way up to the "trailer" position and have a look. It's a little tricky reattaching. Not a big deal if it's left disconnected as most newer I/O's come without an exhaust bellows from the factory (mine included).

 

[tobnpr]

When you say it "still moved", are you saying you remained or got back on plane, or did you need to come off plane?
Could very well be ventilation as you say, but if the engine over revs with the drive trimmed in, you might have spun the hub on your prop.

 

[Don't Wake Me]

Thanks for the responses.

When I said "still moves", I was referring to after we shut down and looked around a bit to make sure nothing was visibly wrong, we started up again and headed home (as soon as the engine got louder and the RPM's spiked, we brought it back to neutral and cut the engine). On the way home, it ran at all speeds ("still moves"), just seems louder than before. Took it out for a quick spin yesterday, same thing. Sounds louder, idles a little rough, and a little rough at low speeds. I think it sounds okay at high speeds, but it's tough to tell since your going fast and everything is louder at that point. After looking at the engine again yesterday, looks like the power steering belt needs to be tighter, but that's all I could tell.

How do I check to see if the hub spun on the prop?

I found the exhaust bellows, but not certain why I would need to trailer it as I can get to it from the inside... unless there's something else on the outside I'm not familiar with??

 

[Weave]

Dont think you can get the exhaust bellows from the inside...The guys are saying trim the drive to the trailer position so you can see if they are still attached.
When fully trimmed in trailer position,the bellows are stretched to the furthest point. This will also expose more of them to inspect.

 

[tobnpr]

How do I check to see if the hub spun on the prop?

If the boat runs properly at speed ( prop is not slipping), it's not a spun hub.
The propeller has a rubber hub that is pressed in place; and absorbs normal torque/ loads without slipping. If the hub fails (such as striking an object), the prop may "hold" at low speeds, but the rubber hub will slip at higher rpms under load. Clearly this is not your problem in this instance.

 

[jg300da]

The exhaust bellows is a 4" oval shaped accordian type piece of rubber that expands and contracts as the lower unit is trimmed in and out. It cannot be seen from inside the engine room, only outside when the boat is out of the water. Like I said before no big deal if it's off but the exhaust will be noticably louder, especially at idle.

 

[Kurt]

I have had this happen several times with my "go fast" boats I used to have. My situation both times was a tree limb that got hung across the outdrive and created that water starvation effect you described. Same symptoms, high RPM and I didn't go anywhere. Just to be safe I would check around the Coupler and make sure you have not spun it. If it is bad there will be rubber shit thrown all over the back of the boat near the transom.