Avoiding the "station wagon" effect

[missinginaction]

I'm in the process of reinstalling the engine on my 1973 Silverton restoration. It's a small block Ford. The original installation used 3" diameter exhaust hoses routed through a couple of mufflers. The exhaust exited the transom at the waterline. It's pretty much a flat transom back there. Since the manifolds were original I'm replacing the entire exhaust system including the mufflers.

Here's my question for you exhaust experts out there. I ran this boat a few times before I pulled her apart and noticed quite a bit of exhaust smell in the back cockpit area. The exhaust wasn't leaking into the boat. I'm told that this is called the "station wagon effect" and being a kid in the 60's I know why. I'd like to set up this boats new exhaust to minimise the exhaust being pulled back onto the boat when she's moving forward.

Thanks in advance for any ideas or products you might be able to suggest,

Missinginaction (MIA)

Mia

thats more a aerodynamics question rather than a exhaust problem.

Boatbuiders like swimplatforms on such transom. You know why!

Regards
Richard

[missinginaction]

That's a thought Richard. There are two small platforms back there. I've thought of replacing them with a one piece solid transom platform (I can use some of my recently acquired fiberglassing skills) but that's getting a little ahead of myself. The goal for this spring is to get the boat in the water.



As always thanks and stay warm over there!

MIA

[Chris Ostlind]

You can also do what the car guys do... install very small airflow deflectors to redirect a bit of the airstream directly into the vortex area immediately aft. Anything to disrupt the collected gases and get them flowing away from the transom will work to greatly improve the swirling containment of the engine exhaust.

There are all sorts of strategies for this solution, so look around at a lot of the SUV's, or any car that has an abrupt back end, rather than a smoothly contoured body form to control the airstream over and around the vehicle.

Welcome MIA !

Some ss tube bend in shape, a few battens of black locuste (the cheapest durable) and the trick is done!
The main effect is to disturb the vortex causing the exh. gas to flow forward. It is not necessary to deflect / redirect the vortex, disturbing does it.

Regards
Richard

[Chris Ostlind]

Originally Posted by apex1

Some ss tube bend in shape, a few battens of black locuste (the cheapest durable) and the trick is done!
The main effect is to disturb the vortex causing the exh. gas to flow forward. It is not necessary to deflect / redirect the vortex, disturbing does it.

What Richard doesn't want to tell you is deflecting and redirecting air flow into a vortex at the aft end of any object that moves through the air is.... all the same thing, even though it is accomplished by different means.

Just take a look at what type of vortex disturbance mechanism will work best for you within your budget, time expenditure and design considerations and go with that initially. If it works, then you are in there fat and sassy. If it doesn't do it for you, then the search is on for what will.

I've got no dog in the fight, either way, so have at it and the best of luck to you.

[missinginaction]

Ahhhhhhh......OK, I think I've got you both!

For years I've played around with radio controlled model airplanes. The picture I have in my head is something like this:

Attach a section of stainless tubing vertically at the aft corners of the boat.

Attach some locust battens (almost like louvers) to the vertical tubing. The louvers would extend into the airstream disrupting the flow and disturbing the vortex.

Since I know how to build an airfoil I could also construct a small wing with an adjustable "flap" so that I could play around and adjust the wing to get the best result.

Thank you both very much! BTW Richard, clearly you are an engineeer. Chris, you simplified the concept just enough to get the light to go on in my head!

Regards,

MIA

[TollyWally]

Perhaps you could post pics of your prototypes

Originally Posted by missinginaction

Ahhhhhhh......OK, I think I've got you both!

I doubt that!

Attach a section of stainless tubing vertically at the aft corners of the boat.
NO! Not vertical.

Attach some locust battens (almost like louvers) to the vertical tubing. The louvers would extend into the airstream disrupting the flow and disturbing the vortex.
Not like louvres!

Thank you both very much! BTW Richard, clearly you are an engineeer. Chris, you simplified the concept just enough to get the light to go on in my head!

No, I´m not a engineer.
Regards,
MIA

Just build a swimplatform, nothing else! Of course, the exhaust should exit below the platform!
Now the vortex builds up above the platform (not at the water surface behind the transom) and does´nt suck the exh. gas in the cockpit.

Sometimes life is much easier as we assume!

Regards
Richard

[missinginaction]

Well, it was a nice thought! OK, I'll just build the platform. You sure you're not an engineer?

[Frosty]

One of the many benefits of catamarans is this is not a problem.

Originally Posted by missinginaction

Well, it was a nice thought! OK, I'll just build the platform. You sure you're not an engineer?

It is nicer, I think, to have a swimplatform serving two purposes!?

When you look at the boats we build, you can imagine, we are quite familiar with that problem. No I am not a engineer, sure.

[missinginaction]

Hi Frosty,

I can see what you mean with a catamaran funneling all that air through the center of the rig.

The more I think about Richard's and Chris's ideas the more I understand. I called Centex Industries and spoke to a rep there about mufflers. He gave me some advise regarding which mufflers to use (vernatone MKII). According to the rep these mufflers are designed to more thoroughly mix exhaust gas with the waste engine coolant, thus keeping the gas in the water for a slightly longer period of time (giving the boat time to move away). This feature along with an expanded platform will hopefully get me the results I'm looking for.

Regards,

MIA

The platform does it MIA.

We do´nt install very sophisticated muffler systems (as long as the customers do´nt ask/pay) just standard Vetus stuff, and have the result we expected.

Regards
Richard

Yes, we install a platform like this

Richard is right, it holds the smoke underneath.

Sasha