Hey Brian,
You will not really see much benefit in vacuum bagging unless you’re making a composite board and attaching higher density, reinforced panels over a lightweight core. Vacuum bagging over a lightweight core with a reinforcing fabric (either glass or carbon) can help you reduce weight by removing excess resin, and it can be helpful in keeping the glass tight around otherwise difficult curves (think hard edges), but those are really the only two benefits. You might save MAX 1/2# on a shortie like the fish you describe by using this technique, but it will certainly be more costly as to do this effectively, you’ll need to have a wet-out table and likely have to resort to consumeables
-peel ply; a perforated film that will allow excess resin to pass yet still release nicely from the part,
-bleeder ply; a bulky, absorbent material that will sit right on top of the peel ply and suck up any excess resin,
-breather; a bulky material that allows the gases to flow around the part
-the bag itself
Furthermore, you’re going to need to determine a vac/cure cycles that will allow these layers to work properly; too much vacuum too soon and you can end up with a very dry laminate that is not saturated; too little too late and you end up not taking advantage of the technique, other than for keeping your laminate in the right place as it cures.
I would highly recommend shaping the blank and glassing by hand if you need the board quickly. If the blank is designed for regular shaping and glassing (i.e. 2#/cubic foot), all you need to do is glass it and vacuuming will only complicate things for you. If it’s a low density blank (<1.5# cubic foot), you’re going to have to add a sandwich layer (for which you WILL need to vacuum) or resort to alot of glass (2 x 6oz bottom, 3 x 6oz deck), which will effectively cancel any weight gains. Glassing by hand will give you alot of insight as to how much epoxy you need for a given lam, the working times and will allow you to practice good wetout and lapping technique. Save the vac for once you’ve done a board traditionally, then pony up to the task and make a sandwich board (even if it’s just the deck) and you’ll really see the benefits of the bag. As a start, I would suggest getting a large piece of poly or mylar that can be laid on the ground as an ad hoc wet-out table, to lay your glass down after you’ve trimmed it on the blank and weighed it. Mix a batch of resin such that the total weight (resin and hardener) is equal to the weight of the glass for the lam; use a foam roller to saturate the glass on the poly rather than a squeegee, then transfer the whole thing; plastic too, to the board. Peel off the plastic carefully, then use your roller to smooth out the glass and take care of the laps. That’s about the best introduction to the technique that you can get.
I’ve used the vac to lam a board, and I’ve done it by hand; honestly, if it’s just glass and a bit of carbon, there’s very little benefit and lots of opportunities for heartache. If it’s sandwich, or pre-preg, then the vac is a must.
HTH