Exhaust fumes

[tmac]

I've had an annoying issue when motoring where the exhaust gets sucked back into the cockpit to the point where I get headaches from the fumes. I think what's happening is my dodger is causing a vortex that draws the air up from the transom - kind of like in cars, where the rear spoiler is designed to prevent this same air vortex action.  Opening the front window on the dodger changes the airflow enough to alleviate this, but there are many days where it is uncomfortably chilly and we don't want to open the dodger window, so I'm looking for alternatives. Has anyone else ever had this issue, and if so, what did you do to correct it?  (And no - I don't want to add a spoiler on my stern rail  )   1988 C34. 

[Ron Hill]

Tom : I've never heard of your problem before!! Do you have an enclosure? or some side curtains??

A thought

[tmac]

Ron - I don't have a Bimini (yet...).  Just a typical dodger. I might experiment with some kind of deflector on the top of the dodger to change the airflow. The air must be coming off the top of the dodger and curling down into some kind of a vortex as it flows over the transom, pulling exhaust fumes up around the steering wheel. I've had about 10 episodes where it has gotten quite noxious at the wheel.

[Jim Hardesty]

Tom,
I don't have any good suggestions for you.  I would suggest that you make doubly sure that there isn't an exhaust leak.
Jim

[Jon W]

Hi Tom, It's sometimes referred to as the "station wagon effect". Depending on weather conditions, the engine exhaust can roll up over the transom into the cockpit.

I know you said you don't have a bimini, but I first noticed the exhaust fumes after motoring a long time one day after installing my bimini and connector. Never noticed it before. I removed the connector and vast improvement, but still a hint of it. I checked my exhaust system and it seems fine. I haven't been able to get to the boat in months for various reasons. When I do, next steps are to:

recheck my exhaust system.
add fuel injector cleaner (maybe poor combustion).
if still noticeable install my aft weather cloths to see if that helps.
If still happening call a diesel mechanic to have the system checked out.

[Ron Hill]

Tom : I wouldn't do anything until you get the Bimini on - because that will change the air flow and may solve the problem?!?

Might be worth rechecking that you are not getting exhaust from INSIDE the engine compartment!!

A few thoughts

[scgunner]

Tom,

Something not right here, you shouldn't need a Bimini to correct the problem. In fact if your boat is still in original configuration you shouldn't have a problem in the first place. I've got a dodger and 1500+ hours motoring and I've never gotten a wiff. Has the exhaust out on your boat been relocated? Are you getting a good exhaust water flow? Is it warm but not hot to the touch?

[tmac]

Like Jon said, it's the "station wagon effect".  I don't have any exhaust leaks inside the boat - I have CO detectors in the cabin for that eventuality. The exhaust is the same as the factory installation and is in good condition. Engine runs perfectly, and has good water flow out the exhaust. 
It isn't a consistent problem, just occasional, and most typically when motoring into the wind.  Like I said in the original post, if I open the dodger windows, it clears up. I'm seriously thinking of doing a test and mocking up a spoiler to mount onto the aft support of the dodger to see if that changes the airflow. Maybe I should get a smoke generator and watch the airflow...

[rjabara]

Tom, I am wondering if you have solved this as I have the EXACT same problem on my 1990 MK1.5.  I have a Bimini and Dodger.  Opening the front window alleviates nearly all of it.  It is bad when sailing into the wind.  It is not coming from the engine room but can't figure out how it could be trapped in dodger when going upwind?

[KWKloeber]

Spoilers are installed (not the nascar wannabees who bolt quilt racks onto their trunk lids) to create down pressure due to the foil shape and increase rear wheel traction. 

You have another issue, fumes are not being sucked into the cockpit due to a dodger.

[Ron Hill]

Tom : I always have the "windshield" in the center of the dodger pulled up underneath! I only zip it down when it rains.
If you do that maybe the exhaust fumes will not go into the cockpit/cabin!!

A few thoughts

[tmac]

I've played around with this issue all summer.  To all those who suggested that I have exhaust leaks, I've had the boat checked over by a professional marine mechanic. The exhaust system is original and unmodified. There are no exhaust leaks, and I don't have exhaust fumes anywhere in the cabin. (I also have CO monitors deployed in the cabins, which register zero.)  I know a bit about fluid flow dynamics from my engineering education, and this is definitely the "station wagon" effect happening.  My dodger is probably shaped different from what the other respondents have, and even a slight difference can have a dramatic effect on air flow. As I mentioned, I DON'T have a Bimini, and wouldn't be surprised if adding one would improve the situation.
Ron - yes - I've started unzipping the center window on my bimini and it eliminates the problem - even just partially opening it works.
I may eventually have my dodger replaced, modifying the shape a bit, but until then I'll just open the window. My initial inquiry was just to see if there were other factors I wasn't considering.

[rjabara]

Today I motored for a long time.  The exhaust smell was fairly strong, so I opened the dodger window, better but still there.  Opened the side window to dodger and better but still minimally there.  Finally, I folded the bimini back and Voila, 100% clean air. 

Unfortunately this a terrible solution on most days; for lack of sun protection  from the bimini on sunny days and on colder days will have to keep the window closed.

Must be a way to divert the fumes elsewhere.

[Jon W]

FWIW I never noticed an exhaust smell until I added my Bimini, so not sure adding a Bimini to your 1988 is going to improve your situation. The exhaust smell is not always present. When it is present it's barely noticeable, but don't want it at all. When it gets cooler I plan to install my aft weather cloths across the push pit to see if that solves the problem. If you don't have weather cloths, you can try draping something over the push pit to block off from the toe rail to the top of the push pit to see if that helps. Let us know what you find out.

[Ed Shankle]

Tom, do you do a lot of downwind motoring? That's the only time I notice exhaust smell.
Good luck solving,
Ed