there is no absolute answer on this....it kind of depends on your objective and how you like to balance the protection of the extra layer vs the added weight.

if your main goal is to give the keel some extra protection anda bit more strength in the bottom, a typical 'football' pattern will track somewhere around 50 to 60% of the distance between the centerline and the chines (sides).

if for some reason i wanted to do the entire bottom, i would wrap it up the  side  an inch so that i also protected the chines (edge between bottom and sides) which is also a high wear area.  if i have already given up the weight of doing the full bottom, i actually have not bought myself a lot of protection if i don't go around the chines....which is only a little farther than strictly the bottom (and hence why i only need to go up the side an inch).

my personal bias is i like to keep my boats light so they are easy to handle and if i do any football...its typically been a partial bottom (50 to 60%).  but in many cases i simply go with a single layer and just add an additional layer or two of glass on the centerline just at the bow and stern areas (bow or stern knuckle and about two to three feet towards the center).

adding addiitonal layers of glass quickly translates to extra weight....so that is what i am balancing as i sort out my reinforcement strategy on each boat,.  what am i picking up in protection for the incremental weight of the decision?....and then i try to decide appropriately.

on methods, my preference is do one layer at a time.  so the football would be my second layer on top of the initial glassing.  i consider doing it in two steps as easier to control and better control leads to better results, less epoxy usage and clearer wetouts..  if you are using mas epoxy, you should be able to do the second layer as soon as the first layer is hard/not tacky ....which is typically a day or less.   no special prep is needed for the second layer if you do it within 48 hours of the initial layer.

all that said, if the build guide says you need a second layer of glass on the bottom, i respect the build guides.  glass is important structurally as well as providing protection from abraison.  interestingly, however, it is much more effective in adding structural strength than abraison protection.  so if my build is structurally sound i deal with abraison concerns with things like rubstrips that are more tactically applied to where it is likely to have an abraison problem and to handle it with material that is better able to handle abraison (like delrin which is used for rubstrips).

h

As a general rule, smaller glass on larger. Otherwise you can get voids where the larger piece overlaps the edges of the smaller one, leading to microcracks, leaks and delamination.

Wetting 2 layers at once can work, but can also lead to excessive epoxy in the layup unless you know what you're doing and do it well. You also have to watch out for air bubbles between the layers.

Wet on wet is OK, especially if you wet out the second sheet before laying it on top of the first one, but it's messy and takes a lot of room.

So the way Howard suggests is probably the way you want to go.

Laszlo