I'm thinking of getting a non-inflatable life vest for the less hairy days racing, i'm at a dilema in regards to the fitment.
How effective are they likely to be under a jacket? Obviously firm fitting is going to be key and that is going to be a very different size in summer with tshirt to with waterproof winter gear on so the only way i can think of wearing the same one is wearing it under the jacket.
Obviously inflatable is a bad idea but does that also apply to non-inflatables?
In my opinion, its a really bad idea.
I can't really understand your logic as to the fitment.
Why do you think it will be different if you're wearing winter or summer garb??
.??? Life jackets & PFDs are worn over the top of your clothing. Any special reason for trying to conceal it ?
It's not about concealment, i don't care if i'm the only one out there in a PFD.
It's more about making sure it fits well and when i measure the difference between thinnest and thickest clothing there's a bit over an inch in circumference difference.
I'm probably over-reacting to that part and i'll just go with a good middle ground between the two.
Volvo,
You're right in being concerned about the fitment, in actual fact its really important, a loose lifejacket is dangerous if you go overboard. You tighten them so you can get your fingers inside the waistband, but no less than that.
But all lifejackets have adjustable straps, you simply cinch them tight after you've put it on.
Incidental info: make sure its got a crotch strap. And cinch it tight enough too to the point where its not uncomfortable when moving about the boat.
Being a Volvo driver may I recommend a cardigan and a cor blimey hat as being suitable attire. 
I wear mine (for dinghy and sportsboat sailing) over everything for a few reasons....
Firstly if it's wet and cold I'm wearing my smock which is too small to wear over my PFD and if it was big enough to wear over my PFD it wouldn't fit very well when I'm not wearing a PFD.
Secondly, when I get off the water the first thing I want to take off is my PFD but I want to stay warm and keep the rest of my layers on.
Some dinghy sailors will wear oversize rashies over everything to stop tiller extensions getting caught in your PFD and for windage in high performance boats like moths.
Look at the kayak side zip style for comfort. Burke, Zhik, Musto, Ronstan and Spinlock all make good ones depending on $$ and fit.
This thread in a way deals with some issues raised in a recent one on Sailing Anarchy about an offshore sailor washed overboard drowned when his inflatable failed to inflate (even as he was seen manually blowing into it as a last desperate measure) while boats around him were trying to maneuver into position for a rescue.
So one of the issues was whether a foam PFD might be better, and that inevitable leads you down the path of whether a PFD 50 might at least offer some protection/ additional time, where an inflatable has initially failed on you. In the cold waters where this event happened, some doubts were raised about that would be enough to give you positive buoyancy with all the heavy gear. But in many Australian waters a PFD 50 might be better than nothing where, as volvo says, its for "less hairy" and presumably daylight sailing conditions. It might even be a backup to an inflatable PFD 150 on hairy days.
if that is what we're talking about a snug fitting one under wet weather gear would I think be better and I'm thinking here of an offshore yacht - where the first thing you want off is the dripping wet jacket as you come below at sea - not a sports boat where your undressing after being safely ashore.
