Using Toni V's Freeship 'Iggy' model, I ran KAPER numbers on the hull in its original configuration and scaled up to make the waterline length 2' longer. No changes were made to other hull dimensions, and displacement was the same for both hulls. Here are the results:

Original: 16.56' OAL, 13.68' WLL, 17.36 sq-ft wetted surface area
Modified: 18.96' OAL, 15.67' WLL, 19.87 sq-ft wetted surface area

Knots - lbs. resistance original - lbs. resistance modified - % change

1.5 - 0.447 - 0.494 - 10.5%
2.0 - 0.760 - 0.838 - 10.3%
2.5 - 1.139 - 1.259 - 10.5%
3.0 - 1.587 - 1.752 - 10.4%
3.5 - 2.175 - 2.348 - 8.9%
4.0 - 3.064 - 3.155 - 3.0%
4.5 - 4.779 - 4.437 - (7.2%)
5.0 - 7.252 - 6.609 - (8.9%)

As you can see in the graph, the longer hull does not have a resistance advantage until you get to 4-4.5 knots, which is considered to be the upper end of "average" paddling speed. The resistance is 10% greater up to 3 knots, which is going to be a more common paddling speed for most. Now 10% isn't a lot, but over a long paddle it will mean you wear out faster since you are using 10% more effort to maintain your speed up to 3 knots. Now, if you are a strong paddler and can continually push the longer hull fast enough to see the advantage, then you can use the lower resistance to paddle further on the same amount of energy expended. BUT - its not as easy as just paddling faster; the effort required to paddle 4 knots is almost DOUBLE the effort required to paddle 3 knots in both cases, and MUCH MORE than double to get into the speeds where the longer hull shows noticeable efficiency. If you can paddle those speeds all day long, you might want to sign up with an olympic kayak team...

: What PRACTICAL difference is there in a long boat compared to a shorter boat?
: Is there really any noticeable difference for an average paddler at
: average speeds? I understand longer water line means a higher maximum (or
: maybe potential) top speed. But if you had two identical boats other than
: one being 2' longer is there really any noticeable difference at normal
: speeds? What advatages of disadvantages do boats have?