Glass new stringers afloat or on the hard

[Concerto]

I'd say it depends on where strengthening is needed. as I said, the keel area is thick and rigid, so is unlikely to change in shape in or out of the water, whereas thinner parts of the hull may indeed deform out of the water.

The real question should be why the OP has decided he wants to to this work. Was it because a surveyor suggested it was necessary or is the OP not trusting the designed hull strength that is in this section of the hull.

 

[Focused1]

Scanmar 33’s can suffer with pendulum keel.
Haven't witnessed it in my own boat but still want to add the strengtheners for my own peace of mind.
This is a common modification in scanmar 33’s and 35’s.

 

[Bajansailor]

Will you be fitting limber holes in way of the base of each transverse frame / floor?

 

[anoccasionalyachtsman]

Scanmar 33’s can suffer with pendulum keel.
Haven't witnessed it in my own boat but still want to add the strengtheners for my own peace of mind.
This is a common modification in scanmar 33’s and 35’s.

I'd make them as wide as possible - little extra cost, major benefit. If I'm right in thinking that the cabin sole support is part of a liner moulding then you're limited on access, but it looks like you could get them an inch higher, if not a bit more (and hence significantly wider). As someone else said, do round the edges because laminating over sharp corners is a nightmare.

Edit - I reckon 2" higher is more than achievable, if you can get them as far as what looks like a longitudinal stringer then even better.

 

[Ceirwan]

Do it afloat, the hull will be supported nicely. It might be OK out the water, but it might not, why take the chance.

As already stated make sure to round the foam off before laminating, and a fillet around where it meets the hull would be a good idea as well, the glass will conform much more readily.