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Old 10-23-2019, 09:07 AM   #39
Portage_Bay
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City: Coupeville Wa.
Vessel Name: Pacific Myst
Vessel Model: West Bay 4500
Join Date: Oct 2015
Posts: 2,415
When running a portable genny on your swim step at anchor consider the air flow around an anchored or stationary boat. If I were a half way decent artist I’d provide sketches so illustrate the point, but I’m not. You’ll have to suffer with my written description.

Consider the cross sectional shape of a sail boat mast or many airplane wings. Kind of tear drop shaped with the ‘fat’ section leading and the ‘thin’ section trailing. This creates a nice smooth air flow off the trailing section. Turn the tear drop around with the fat section trailing and now the flow off the trailing section is turbulent and can be at a lower relative air pressure. This can lead to flow back towards the trailing edge. Kayakers and white water runners know this phenomenon, drop into the down river side of a rock and get a back eddy for a bit of a rest.

Ok, I’m getting long winded. How does this apply to running a genny on the swim step? A boat is a very rough tear drop shape hull and deck when viewed from above. Pointy end into the wind, fat end down wind. Under the right conditions air mass at the back can actually move towards the stern and sometimes flow onto the boat and forward.

No, this isn’t fear mongering. It’s real life experience and having to figure out why exhaust was entering the living spaces from the stern with a gentle wind blowing on the bow. It doesn’t always happen. Some boats tend to suffer from this more than others. So, please, do keep your CO detectors in good working order. Don’t assume because it’s on the stern all the bad stuff automatically disappears.
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