Avoiding the "station wagon" effect

It's going to come down to the specific dynamics of your boat. I have a full width swim step on my boat. Depending on wind conditions and course traveled the station wagon effect can still rear it's ugly head.

The suggestion that Chris and Apex made about some sort of device to disrupt the exhaust flow bears consideration. Swim steps are a useful feature for many boats but are not an automatic solution to the problem you are trying to solve for all boats and all conditions.

 

You miss the point Wally.

The vortex has nothing to do with wind conditions. The fast moving boat produces it. And disrupting the exhaust flow is wrong.
You have to disrupt the vortex of the moving boat at the transom area. The part that goes up from the water surface, and then in the same direction the boat moves.

Swim steps are not the solution, that is nonsense. A swim platform as apex / Richard recommended (and built on thousands of boats) is the solution. The vortex build up on the platform instead of the water.

The deflector / redirection brabble from Mr. Ostlind was not worth to mention.

Sasha

 

I may very well have missed the point. I speak from years in the commercial fleet as well as the experience of my own boat moving at speeds of 5 to 20 knots. I have a full width swimstep which does not keep exhaust from clinging to the transom under some conditions. Variables that do appear to have an effect are wind and course heading.

I thought Chris's idea had merit. The little spoilers that are often placed on station wagons do make a difference in that application. It seems reasonable that something similar might help a boat.

Commercial boats the world over run drystacks to get the exhaust away from the working crew. If it was as easy as adding a swimstep you think think you might see that a bit more often instead of a more expensive drystack.

 

Hmm,

I am not sure if we are talking the same topic.

Cars, especially station wagons, have that problem easily fixed with a deflector at the right position.
The same is valid for power boats with a transom. The swimplatform does the trick.
Of course, when the wind blows the fumes in your cockpit, the swimplatform does not help. (and when it rains you still get wet)
I am not aware of boats in that class with dry stacks. And I´m not aware that anyone here was talking commercial ships.
Not every customer likes to have a swim platform (not swimstep as you stubbornly like to repeat), not every yacht has a benefit from it.
But although you know all that, you like to contradict.
So, contradict `til the cows come home, who cares.
Mia has what Mia needs, that should be enough.

Regards
Richard

 

I can't thank you all enough for the discussion. Thanks for helping me understand and solve this problem.

Sure, you're going to get some fumes backing into the cockpit under some conditions. I can imagine traveling say, east, at 7 knots with a 7 1/2 knot wind blowing from the west. That combination is going to cause a problem no matter what you do unless you go a bit faster!

Regards,

MIA

 

Different areas use different terminology. What we call a swimstep here is a horizontal shelf like platform a few inches above the waterline usually a foot or two wide (.3-.6 M) generally running the width of the transom, normally supported by brackets below. I assume that is what you call a swim platform.

There has been a trend in the last few years to greatly extend this area and instead of using brackets to support it, extend the hull several feet aft beyond the transom location.

On my boat the existing swimstep or platform doesn't keep the exhaust from clinging to the area just aft of the transom. As you mentioned with the right wind conditions it can blow into the cockpit.

I wasn't speaking of commercial ships rather boats 30-150 ft or so.
I brought up the subject of drystacks as an illustration of a common method used to prevent exhaust from being a problem on deck used on working boats comparable in size to many pleasure yachts.

It is a more expensive option than a wet exhaust. Despite the cost and often cumbersome inconveniece of running a vertical stack, around here it is the method of choice when a boat is designed around working on deck.

Lastly Richard, the use of the term swimstep is not a matter of stubborness but of habit and local custom.

 

Sorry did´nt know that, although selling our boats worldwide.

 

Sasha,

I don't mind at all if you wish to prove my suggestions incorrect, but it would be decently courteous of you to provide some measure of proof. Failing to provide such support, as in: you have wind tunnel data and flow analysis imagery, showing the ineffectiveness of deflectors. This would do it for me and I'd gladly retract my suggestions.

In lieu of that kind of data support, I direct you to the veritable mountain of real world proof in the form of deflectors that are mounted, either after-market, or specifically as OEM, directly from the automobile manufacturers and the relentless fact that they work and continue to do so on all bluff body vehicles moving through the air. A hard transomed vessel qualifies as a bluff body object for purposes of generating terminal vortices which, in this case, contain spent exhaust gasses.

As in most things in life, there is usually more than one way to solve an issue. Remaining open to that potential allows you to serve your customers much more effectively.

I refer you to this Patent violation lawsuit finding from the US Court of Appeals in 1962. http://bulk.resource.org/courts.gov/c/F2/311/311.F2d.273.13706_1.html and I quote, "... He then conceived the idea of eliminating the vacuum by directing the slip stream of air inwardly across the rear window area... He also found that the deflectors kept exhaust gases and road dust from entering the vehicle if the rear window were kept open..."

 

Interesting Chris. I understand Richards point about the swim platform but it also looks like my original thinking was not far off the mark from what you were suggesting. Sometimes there is more than one way to attack a problem.

Thanks for posting!

MIA