Our last boat had one, I added another(3", a bit small to start), but onboard on a hot night, the worst thing that could happen was rain. I also added a small fan that helped one of the cowls in the bow.
On our present boat, we have 4, 4" cowls that do the trick in the rain even if winds are very light(at anchor, they'll funnel any breeze). Of course in a nasty blow or underway in rain and spray, they supply enormous amounts of ventilation.
I'd go with what others have said and try something that has a fan in it Craig. Maybe two.