Exhaust fitting info needed please..

[scot]

I need some input from the sand box on exhaust discharge fittings;

First question is about the placement on a new installation. In most installations I have noticed that the fittings are about 50% submerged at rest. Would you guys consider this a fairly normal set up?

Second, I also have noticed that many do not use the Salsbury type flappers...but rather just let the water come on in at will. I have always throught that backing down hard or slow trolling in a following sea could load the exhaust and cause excessive back pressure or worst...find a turbo? Thoughts please.

Last, Thoughts on the glassed in fittings vs bronze or SS units?

Thanks in advance,
Scot

[nic]

Scot,

Just going through the very same thing.

We are glassing a f/g pipe through the transom and fitting a flap as well as fitting a riser to the exhaust by the engine. Not so much the momentary back pressure but to avoid any chance of a gutful of water coming back into the engine.

Maybe over-cautious but backing up in a swell, one engine hard and the other at idle I could see it happening. A friend's 43 semi-displacement hydraulic'd both engines on sea-trials backing up hard, then idling to go forward. Those engine were closer to the water line but why take a chance.

Nic

[Harry Babb]

Hi Scot,
I would certainly use the "Flap type". Its been 20 years now but one of my buddies anchored up on a snapper spot in the Gulf and started fishing. one of his buddies came along and tied up STERN TO STERN and started fishing with him. The second guy stayed for a while then started up his engines, untied the lines and throttled up and went back to the dock. When the first guy tried to start his Detroits they were both locked up. We always belived that the first guy took off and blew water up thru the exhaust and into the engines. I think that the Flaps are a good thing.

Recently I installed a genset in my boat and I put the 2" exhaust below the water line and constant bubbling is anoying to me. I am looking for some kind of noise suppresser to go over the genset exhaust. BTW I have an "Inline Flapper" on the genset exhaust to prevent back flow.

Harry Babb

[Bruce]

placement of the exhaust out the transom is due in part to what size exhaust you are running and whether it is outboard or inboard of the outer stringer.

What you do want to do is always have a down hill run to the outlet otherwise you create the effect where no water comes out for a few seconds and then a guss comes out. This is due to water backing up and finaly being pushed out by the exhaust pressure.

Glassing a fiberglass pipe in just looks much cleaner but is a personal preference.

Using a guide line of 1 foot to water line below turbo outlet is where to start. Lower and a riser should be used.

Also some guys don't use flappers cause they have surge tubes or use internal flapper mufflers or check valves.

I have seen where backing down hard with outlet flappers that were new and fit tight stalled the engine with excess back pressure.

A big wall a water is tough to push against. But there was no water and engine started right up.

[scot]

Bruce...carify please, on a 6" exhaust outlet....would you use 1' below turbo outlet centerline.... or 1' below the lowest point (6 o'clock position). If the later, then that would mean the 12 o'clock position on the turbo outlet would be 18" above water line?....and still need a riser?

[Bruce]

It is measured from the bottom of turbo outlet.

The turbo outlets are not 6" diameter. Most all turbo outlets are 3" in the 300HP range.
It only gets bigger because of the water and the backpressure that and the piping will cause.

Always check to see what the manufacturer of your engine recommends for the warrante to be valid.

Yanmar, Cummins and Volvo are all different.

[Brewster Minton]

I use the flapper type, my exaust is outside the stringer. It is about half submurged when loaded for an overnight to the trench. At rest full of fuel it is just in the water maybe a quater inch, empty it is out of the water.