Its astounding what some builders will do, I once owned a nice little C&C
24 where the back wall of the cockpit was parallel to the transom with only a 4" space between and a foot above the waterline and yet the builder
chose to slope the cockpit sole toward the companionway
to 2 x 1.25" drains which merged in 1 x 1.5" seacock. wtf.
I don't know the layout of your Bristol but if at all possible can you run a couple of solid tubes through the lazzarette and through the transom. They can just be in addition to the existing ones and take over if the cockpit gets filled. I have used vinyl down spout material as a form and wrapped it in fiberglass
to make a strong fiberglass tube that you can glass in, it is not important that it bond to the vinyl as its only a form but you leave it in, rectangular has more area than round.
I am going through the same thing with a customers Hans Christian 38T at present, shockingly inadequate drainage despite the reputation. It has a stupid little well which I suppose someone told the designer was required on a "bluewater boat" but they forgot to point out that having big high comfortable coamings that are as high as the trunk cabin
that remain the same height all the way around the stern would hold a huge amount of water with nowhere to get out except down the companionway. The boat has 2 x 1.5" drains at the aft end of the well but the boat sits bow down with chain rode
so someone installed a 3/4" drain at the fwd end to drain the cockpit when at the dock
. it doesn't work. The side decks are just as bad as the huge bulwarks can hold thousands of gallons with a single
1.25" drain each side, very unseaworthy. We have installed large freeing ports
through the bulwarks that are 7" x 3" approximately. Lots more to do. As someone else pointed out the best cockpit drains are an open transom, you can safely have a large comfortable cockpit at anchor
as long as you have an open transom.