Submerged exhaust?

[fs454]

I'm just wondering what would happen if you somehow made a submerged exhaust system...like on some real boats, the exhaust exit is underwater which reduces the engine noise greatly.

With nitro engines, this would result in mass power loss or just a quieter engine?

Just an idea i was thinking about.

[CFRACR]

Actually I Believe That You Would Burn Up Your Engine. But Thats Of Course If It Would Stay Running.

[BurnZ]

Looks like it would have too much back pressure.

How likely, if it stalled, would it suck water back in?

[mjmsprt40]

Looks like it would have too much back pressure.

How likely, if it stalled, would it suck water back in?

You can bet on the back pressure. There's a huge difference between the multi-cylinder engines used in full-size craft and our single-cylinder model engines. Those big powerplants are pushing a near-constant stream of gasses with a powerstroke happening several times per engine revolution. We only get one pulse per revolution, and that not all that powerful. (You can easily stop your nitro engine by simply putting your finger over the pipe exhaust. Try that with a full-size engine. It's quite a bit harder to do, assuming it's possible at all.)

As far as pulling water back into the engine: That all depends on your setup. If your engine is below the waterline of your boat at rest, it could happen. Otherwise probably not.

[bufferoo]

I've actually given this some deep thought. I used to have a 1961 Chevy Bel-Air...ah what a great car...anyway, rather than a PCV valve it had something called a "road draft tube". This tube was angled from the top of the intake manifold and down towards the road and had a diagonal cut on the bottom. This caused a low pressure area to form at the tube's opening. Any build up of pressure in the crankcase from piston blow-by would be sucked (or pushed depending on what end of the tube you were looking at ;) ) out of the tube and onto the road. Not very friendly to the environment but effective nonetheless.

So with that in mind I would speculate that if the exhaust tube was flush with the bottom of the hull, placed in an area that would maintain contact with the water and angled "just right" you might see the same effect. The low pressure area would be created and the exhaust gasses would be pulled out of the pipe. This might be bad as well as it may work too well and reduce the back pressure rather than increase it. I would guess though that it would cause the back pressure to rise and fall and not be in any way stable. I guess the only way to know would be to build it.

Bufferoo

[rc10gtisthebest]

Submerging the exhaust under water would kill the engine almost instantly.

The backpressure from not being able to "breathe" would cause the engine to flood and cut out. Maybe when you are traveling at speed could you do this, but it would have to be a good clip all the time..

Plus, who wants unnessacary drag?

-Todd

[Chris LaPanse]

my Enforcer exhaust is at the water line, but it is still not completely submerged. Also, this is a much bigger engine.

[mjmsprt40]

RC10, the only boats I've seen that operate at speed consistantly are the outriggers. Most outriggers have to have a tossed launch, they sit so low in the water otherwise that they can't get up on plane from a standing start. I "suppose" it might be possible to run a submerged exhaust on one of these boats, but... the drag from any kind of pipe below the surface-- especially a relatively large diameter exhaust pipe-- just might kill the speed of the boat enough to cause the boat to lose the race at the least, again assuming the engine could be kept running at all.

About that '61 Chevy Bel Aire--- again, it was a big multi-cylinder engine, pushing blow-by gasses against relatively low air pressures. You could have backed that car down a boat-launch ramp until the exhaust was submerged without fear of killing the engine, or of preventing a re-start after a shut-down (say, while you're launching or retrieving your full-size Chris-Craft), because once again we're talking about the difference between a huge multi-cylinder engine and a small single-cylinder model engine.

[Hydro Junkie]

I actually tried running the exhaust out the bottom of my first boat, a Dumas 30" Pay N' Pak. The engine never quit, but it wouldn't run much over an idle, either. When the exhaust tube fell off the muffler one day, the boat picked up speed, but not enough to get on a plane. If your looking at doing this with a deep vee, you could probably get it to work running out the transom, but if you slowed the boat suddenly, water will push into the exhaust and kill the engine.

[Chris LaPanse]

Yep. I had my enforcer set a little too low on the idle one, and every time I slowed down, it killed the engine. I traced this down to the fact that when I slowed down, it would have a wave hit the back of the boat, and, even though normally this would be fine, since I had set the idle too low, it killed it almost instantly. I don't think it would survive at any settig if it was a completely submerged exhaust (instead of at the waterline)