exhaust flapper requirement??

[centerline]

what is the determining factor in whether an exhaust outlet needs a flapper installed.

my boat uses a vernalift muffler and the wet-exhaust outlet on the transom is about 12" above the waterline on the overhang, with a 14" raised loop in the exhaust tube before it leaves the boat.
the outlet tube would never be underwater except in heavy seas. should it have a flapper installed?

 

[Tom J]

I've only seen them on powerboats with the exhaust outlet at or below the waterline. Have you seen any on sailboats?

 

[centerline]

my neighboring boat has a flapper on the exhaust, but his outlet is closer to the water also... but I dont know if its required on his boat or if he just installed it because he could...

 

[Sam Salter]

I've got an exhaust flapper on my CnC 26.
It was on there when I bought it.
There is a raised loop under the cockpit before it exits the flapper. The flapper is close to the waterline. Actually, when she picks up speed, she squats at the rear end and the flapper is partially under water.
Having said all that, I lake sail and I'm sure it's not necessary. But it's on there and I've never considered taking it off.

 

[Tally Ho]

The exhaust outlet on my Hunter 280 is underwater, and it does not have a flapper.

Greg

 

[Ralph Johnstone]

my boat uses a vernalift muffler and the wet-exhaust outlet on the transom is about 12" above the waterline on the overhang, with a 14" raised loop in the exhaust tube before it leaves the boat.

Yes, you should a flapper for the occasional rogue wave that comes by .

Barring that, I think you're well protected even with a direct impact from a survivable wave. I would think for sea water to cause any problems:

1. The incoming slug would have to have enough velocity to lift 14" while overcoming the internal friction of the exhaust hose.

2. The incoming slug would have to overcome the internal air compression within in the hose as the water quickly enters.

3. The incoming slug would have to be of sufficient volume to supply enough water to overcome the volume of the hose and muffler.

The best defence is to have the internal loop as high as possible and then forget about it.

Our exhaust outlet is below the water line and the internal loop is 30" above the water line.

 

[JK_Boston_Catalina310]

I've only seen them on powerboats with the exhaust outlet at or below the waterline. Have you seen any on sailboats?

Catalina 30 MK II's have flappers on their exhaust ports.

As has already been noted, the concern is water going back up the exhaust and hydrolocking the engine. A large loop just prior to the exhaust thru-hull could eliminate the need for the flapper if you have room.