-- Last Updated: Feb-05-14 1:19 PM EST --
Except for extremely sleek boats (especially racing boats), the maximum speed that you can go can be estimated very accurately just based on waterline length. That maximum speed is called "hull speed", and it's based on the fact that a boat can't go faster than its own wake (longer boats create a wake of longer wavelength, and waves of longer wavelength travel faster).
The 12-foot boat will have a maximum speed that's almost half a mile-per-hour faster than the 10-footer, and the speed difference when paddling "reasonably hard" will be a bit less pronounced, but noticeable just the same. If you never exert more than very light effort, or never paddle hard enough to consider yourself in a hurry to get somewhere, that won't matter and you won't end up going any slower in the shorter boat. However, if you plan to paddle even small distances offshore, it almost surely would matter (you are much more likely to find yourself in a situation where you want to get somewhere quickly).
For what it's worth, here are the theoretical maximum speeds for the two boats:
10' waterline length: 4.9 mph
12' waterline length: 5.3 mph
Chances are, your "maximum comfortable speed" will be at least 1/2 mph slower than the speeds shown, and the fastest speed that you can maintain for a fair amount of time will be a bit slower still.
Waterline length is usually a bit less than total length, but there's probably no need to try to be that accurate.
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