The degree to which you track with a keel is far higher than what you get with a counterweight (even as large as a second person.) It really feels like a keeled boat… a kayak or whatever. It also depends on the size of the keel-pole. A thicker pole provides more resistance to turning. The first one we tried was definitely too thick, and made it difficult to turn.
But then maybe there are benefits to an oversized keel. If one found a nice log on a remote beach you could tie on top and have a truly gigantic keel that could be milled when it made its way back to civilization beneath your packraft and tired arms. And any time you have a keel that approaches or exceeds the mass of the people in the packraft(s), I expect you’d be very unlikely to capsize.
But overall we found that we plateaued on absolute speed very early in improving tracking… once you put 25 lbs or so on the bow, you’re going to get about as much of a bump in speed as is possible, even though tracking can be improved much more. We were disappointed to find that you don’t raise the hull-speed by tying the boats together.
Practical speed is a different matter. It’s very cool to take a nap while your partner paddles, or take photos while they keep you positioned. On a recent trip we considered implementing a longboat so that the person with the sore shoulder could trade off with the person who might need to comfort a baby now and then… When I’m in good paddling shape I can push to paddle the longboat as fast as my steady pace in a single boat without Erin’s help. Erin can’t quite get there, but she can get close… well over half her solo speed.
Also when fighting wind the long-boat is preferable… If you’re straight into it you have two paddlers with one bow. If you’re at an angle, you have the keel to make it much easier to keep a heading, though more difficult to spin into large breakers at the last second as is possible with a single packraft. In dangerous winds, the longboat also prevents separations.
The added stability of the larger boat is quite substantial too. I haven’t tried it yet, but I think you could comfortably stand in a longboat (if the other person was sitting.)
And if you let the pole stick out a bit you have a ram-bow for ice-bergs.
Maybe this October when we’re in Anchorage we can build a 6-raft longboat (perhaps a cata-longboat?) and paddle across Kink Arm for a picnic.