BOILS: When on a trip, fully loaded, and not looking for swim, these are really scary and something to avoid, although sometimes it is impossible to avoid them. On big water, there are almost always boils of some sort along the drop and especially at the bottom of the drop.
Sometimes it is brace and lean for all you are worth. The scary thing is that braces sometimes don't seem to work. The paddle sinks in the boils. The paddle should be in the water, prepared to pull water fore or aft, in order to brace the canoe. There are very strong within-boil currents can rush up and laterally, slamming into the side of your boat. You are essentially broadside to a wicked current that comes out of nowhere, grabbing the chine. And I mean out of nowhere. You can be pushed in any direction instantaneously in a boil, and when it is broadside, be ready with the mother of all braces which you will likely have to feather because the paddle may want to sink. Lean too far and you may also go for a swim, because unlike standard moving water, the boil current is irregular and unpredictable - as close to chaos as you are likely to get. So the powerful lean might fail as the broadside boil current suddenly decides to shut-off.
In fact it is difficult to know which direction to lean, because the out-of-no-where side current will take your boat suddenly sideways as well. The current is slamming into one side, but it will also slam into the other side as the water piles up on the opposite side to the current. You are the meat in the sandwich when this happens.
Forward power stokes are also stabilizing, so instead of bracing you may just want to pull forward and keep the boat under power. In fact the more I think about it, I think having the boat under some power in a boil is best. But not necessarily fast, because of those wicked side swiping currents.
Whirlpools. Again when tripping, fully loaded, they are not fun. Usually the big ones that are sucking down the river more than a foot are likely in section of the run where you don't want to be in an open boat anyway.
A variation on the whirlpool is the re-circulating giant flat "hole" along huge water rapids. These can hold a swimmer and boat indefinitely. They can also sweep a boat or swimmer back into the main current that might be too big for an open boat. Last summer on a solo trip on the Talston, I had to put in at the base of a falls at the end of a portage. The re-circulator was huge, and towards the direction of the waterfall base, where I definitely did not want to go (ugly). I had to seal launch from a log that I rigged, and paddle hard, counter-current to the re-circulator, and get beyond its grasp. Somewhat scary, and a must-make move. I judged it quite do-able and within my ability. But to have a screw-up on the move would have been bad. I was solo and so no throw rope would be available.
I would say leave these hydraulics for playboating, and avoid them when tripping when at all possible. But you will inevitably have to deal with them, and that is where the many hours you spend open boat playboating really pay off.
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