Maybe its just me, but I find this thread very curious.
If I get the question and issue right: Tripping scenario, not playboating, and running big water, with a lot of water coming over the side, purposely with no spraydecks, and so much so that it will fill the boat, but with a continued desire to paddle without having to bail and sponge, and relying on a bilge pump?????
Hi Ayates - Good question, but I think this is asking for big trouble IMO. When running big water like that that is so big that significant volumes are coming over the decks, then you are not in perpetual running mode. You are inevitably eddying out to scout the next section, or with running scouts (not exiting the boat, and standing up and eyeing up the lines if it is relatively straight forward stuff). Otherwise you are setting yourself up for some long swims. So there is time to quickly bail and sponge.
I think the fundamental problem here is not using a spraydeck. There are limits to open canoes, which I think you are crossing with the original proposal into a wilderness tripping danger zone. I think spray decks are essential for running the conditions being proposed here (constant over-the-decks water). To me, it is illogical on trips to want to be shipping huge amounts of water without wanting to keep it out in the first place. (Playboatting is different of course). Being on a long remote wilderness trip and having a bilge pump malfunction is not where you want to be. You want a deck. Long swims in wilderness conditions are not benign events. People get hurt or die sometimes.
Open lake crossing in big chop in an open boat, relying on a bilge pump instead of a deck, is asking for a serious accident. On big cold lakes, if you have to "rely" on a deck, its probably a stay on shore day. It can be a life or death issue for hypothermia, and possible related drowning.
Pool and drop rivers, in summer in warm water where you go without a deck, and occasionally run something big and knowingly will fill up by the time you hit the recovery pool, is no big deal, and not a bilge pump issue, since then it is an easy bail and sponge and go, or dump and go. To carry the extra weight of a bilge pump on such a trip with few filling event areas is illogical to me.
Spraydecks still let in water of course. But you will never get it all out with a bilge pump with all the packs in there, and the uneven sloshing, as others have mentioned. Big boat sponges are very good at quickly mopping up the bilge that is too shallow to bail. The packs will ooze water for a while too. You can eddy out without getting out of the boat, sponge out, and get going again. Then later sponge again as the water sloshes into new areas.