What happens if these don't work?

[Deleted User YDKXO]

I have underwater exhausts on my boat

which have flaps in them

What would happen if these flaps got stuck open or closed? Should I be maintaining them in some way? If so, what should I be doing?

 

[Cymrogwyllt]

stuck closed may be a boat full of exhaust.


stuck open no big deal as long as at some point the exhaust kine is above the the waterline.

 

[Bandit]

They are like that so if a wave or tidal flow comes the wrong way up the Jacksi it stops the water getting in to the pots which can be expensive.

If they dont open cooling water could flood back in to the pots.

As far as maintenance keep them free , either scrape them clean or use brick cleaner and wash them off.

 

[Deleted User YDKXO]

They are like that so if a wave or tidal flow comes the wrong way up the Jacksi it stops the water getting in to the pots which can be expensive.

How can a wave force water up the exhausts? They're underwater all the time. So you're saying that the health of my engines are dependent on a couple of twopenny halfpenny flappy things?

 

[Bandit]

On my boat it has a similar thing but a rubber flap that acts as a one way valve as yours does.

Boats moored stern out on a pontoon can be exposed to waves and a tidal stream and boats at anchor rolling can siphon water up the exhaust which is why some have a bleed line above the water line to break the siphon.

 

[Spi D]

Same principle inside the bullhorns on sterndrives (I've seen them on at least Merc V8). Meant to prevent sea water from being pushed all the way up to the engine.

Might happen only when reversing and/or when engine is below waterline, but Merc apparently find them important.

Design have changed over time but principple is two hinged flaps. Tend to get noisy (rattle) at idle or low speed with just a little wear. May come off and make it down the bullhorn in an attempt to block it but that obviously isn't relevant for for your setup.

 

[vas]

mike,

do these flaps operate on exhaust power alone, or do they open/close somehow mechanically?
I guess #1, so is it not a matter of checking them annually that they do actually move freely?

V.

 

[DAKA]

How can a wave force water up the exhausts? They're underwater all the time. So you're saying that the health of my engines are dependent on a couple of twopenny halfpenny flappy things?

if you ever hook a pot line at speed you will stretch the line, engine stalls and then you get tugged backwards , that or a tidal race will force water up your jacksi and hydraulic your engine as soon as you try to restart (it will look like a hand grenade has gone off inside your engine)

running on one engine in a large following sea could also have the same effect on the dormant engine.

Mooring stern too on a fast tidal river may also be enough.

 

[[2068]]

As usual, Daka exaggerates!

But yes, backing down hard on that Tuna at 6kts in a choppy sea with one engine stalled and a flap stuck open could be bad news. The height of the exhaust loop inside is also part of the equation.

 

[Deleted User YDKXO]

mike,

do these flaps operate on exhaust power alone, or do they open/close somehow mechanically?
I guess #1, so is it not a matter of checking them annually that they do actually move freely?

V.

No they're not mechanically powered, they just operate on exhaust/water pressure and thats what worries me. Frankly, I've never even checked them in 3 yrs ownership of the boat and it sounds as if, should they fail, it could be catastrophic for the engines in certain circumstances. When the boat comes out of the water shortly, I will certainly be checking that they are in order

 

[Deleted User YDKXO]

. The height of the exhaust loop inside is also part of the equation.

I would say that the loop inside the engine room is at least 3ft higher than the exhaust outlet

 

[DAKA]

As usual, Daka exaggerates!

under normal conditions with a 3ft header Mike hasnt got a problem.

He asked what could cause a problem.

I named 3 incidents that I am 100% sure will have happened to some boats .

Exaggerate ?

If all the boats based in the solent went across the the CI through the alderney race just twice a year then those obscure examples would be common.

Now thats the exaggeration as few will manage Yarmouth twice in the year

 

[Dave_Seager]

How would the Alderney Race cause water to pass up through the exhaust system? Although there are strong currents, the speed of the boat through the water would normally still be forwards. You would have to anchor back to front before the race could possibly cause what you suggest.

 

[DAKA]

How would the Alderney Race cause water to pass up through the exhaust system? Although there are strong currents, the speed of the boat through the water would normally still be forwards. You would have to anchor back to front before the race could possibly cause what you suggest.

Yes Dave.
The response was following two posts above talking about hooking a pot line which effectively anchors you astern.

 

[benjenbav]

Mike, there's a lot of interesting stuff about exhaust design posted by latestarter1 on this thread:

http://www.ybw.com/forums/showthread.php?342510-Winter-rebuild&highlight=exhaust

 

[burgundyben]

I have underwater exhausts on my boat

More to the point, what is that bloody great wedge doing in front of it? I suppose something to do with backpressure underway, but even so, its a chronic interuption to flow.

Fine example of where unsilenced side exit exhaust would be best. Or perhaps, as Daka would have it, route the exhaust to exit at the bow, just in case you find yourself anchored by the stern in the Alderney Race.