Do flappers on a thru hull exhaust really keep the water out?

I have a 25ft Donzi with a 5.7 OMC Cobra with thru prop exhaust.

I am thinkng about installing some sort of thru hull exhaust this winter.

I have the center style straight back riser, and I have a swim deck.

I have saw boats that have the thru hull in kinda the same setup as me. I am gonna take some pics and start a new thread on the best way to do all this but I have a question.


On the exhaust tips that have the flappers on the end, do they really stop the water form coming back in?

I cant see how they would stop the water if they are open any from the engine running. If the engine is running the flappers are open, so if there is water, the water will come into the flappers.

I dont have any room in the boat to make any kind of signifigant drop from the risers to the thru hull. It would be angled down slightly but not a bunch. Maybe 2"

Re: Do flappers on a thru hull exhaust really keep the water out?

Those flaps are effective when you back up. Then they will close to stop the major rush of water from going in the pipes.

Re: Do flappers on a thru hull exhaust really keep the water out?

When running, the engine exhaust pressure will keep water out. When the engine is idling, low RPM or off, the flappers are going to keep out water that comes rushing at the transom.

Think like this: The flappers are pretty much closed when the engine is low RPM, so a surge of water can't really get past them... the water rush might actually close them... but the engine has enough power to keep them open if it has to. But serious water surge against the transom might momentarily shut them... and that is the point. Close out water, protect engine... yet not stall it.

They are critical pieces -- absolutely critical. With the engine off, I cannot even imaging a no flapper setup. You need them.

They do make tips with internal flappers. They are much more popular now than externals. Check out EddieMarine.com or CCperformance.com

A major note on your downward slope: Please, look up what the minimum slope requirement is. This is also CRITICAL, as your own exhaust cooling water is likely to be ingested back into the engine at idle and low-rpm puttering if the slope is not proper. It's called water reversion, and it's common on Thru-Hulls that have the wrong slope (or an aggressive cam... or both). At idle to very low-RPM running, your engine has a moment during the exhaust valve duration that sucks back into the engine, so if your exhaust water isn't far enough away or down slope enough, you are going to ingest your own water. The proper slope helps ensure that water droplets don't walk their way back towards the exhaust valve/port piece by piece when you idle or putter. As a matter of fact, on a thru-hull setup, prior to shutting down some engines, you should run the engine at 2k RPMs for up to a minute to blow the droplets of water away from the engine.

Both issues, use of flappers AND proper slope of the exhaust pipes, are going to save your engine from ingesting water.

Re: Do flappers on a thru hull exhaust really keep the water out?

Thanks Philster

That was what I didnt understand. I didnt know that at low RPM or idle the flappers would be closed or almost closed. I thought they would be open.


I plan on taking some pics of my engine, risers, swim platform, and compartment and seeing what everyone thinks.


thanks again

Re: Do flappers on a thru hull exhaust really keep the water out?

I was thinking about doing the same thing soon. And to keep the tone nice and mellow, I was going to use corvette Z06 stock mufflers. They're titanium and folded so there isn't a long length from "in" to "out" and they've got a nice tone to them. (I have a modded '04 and I kept mine on the car)

Take pics of your project and post them. I'd like to keep a watch on it.

Re: Do flappers on a thru hull exhaust really keep the water out?

Even your through drive exhaust should have internal flappers. Mine does right at the top of the 'Y". I have friends with the baloney-cut style through hull exhaust on their boats and theirs have internal flappers as well.

Re: Do flappers on a thru hull exhaust really keep the water out?

It should have internal OR external flappers, which are not to be confused with shutters.

Flappers are found on setups where the exhaust pipe is coming right off and down from the riser/elbow and straight out the transom, to a tip. It's the tip that will have the flapper.

Re: Do flappers on a thru hull exhaust really keep the water out?

shrew said:

Even your through drive exhaust should have internal flappers. Mine does right at the top of the 'Y". I have friends with the baloney-cut style through hull exhaust on their boats and theirs have internal flappers as well.

Click to expand...

You might be thinking of shutters. Flappers sit on or in the exhaust tips, not up in the exhaust system. Those are shutters.

Re: Do flappers on a thru hull exhaust really keep the water out?

Philster said:

You might be thinking of shutters. Flappers sit on or in the exhaust tips, not up in the exhaust system. Those are shutters.

Click to expand...

I happily stand corrected.

Re: Do flappers on a thru hull exhaust really keep the water out?

shrew said:

I happily stand corrected.

Click to expand...

Sorry to bring this thread back from the dead....

Do you think Internal shutters will be needed along with the flappers? Or will the flappers on the tips be enough?

Re: Do flappers on a thru hull exhaust really keep the water out?

**deleted **

Re: Do flappers on a thru hull exhaust really keep the water out?

Flappers, Shutters or otherwise, Reduce the amount of water that surges into the exhaust.
They prevent huge backflow surges. They were never meant to be waterproof.

Even when they are closed with the engine off.
Water will leak past them up to the point of the water level outside the boat.
A big wave or passing boat wake will only produce a small, momentary increase in the water level inside the exhaust.

They were never meant for use on a Submarine!

Re: Do flappers on a thru hull exhaust really keep the water out?

Bubba1235 said:

Umm, why?

Click to expand...

Compensation syndrome.

Re: Do flappers on a thru hull exhaust really keep the water out?

Old thread!