Jyak, I had no intention of implying that an extremely accurate calculation of hull speed is justified. If you read my post again you will see that I actually mentioned one reason that such accuracy doesn’t matter, and I mentioned another aspect that makes this clear by implication. So, I agree with all those comments in the discussion that you attached to your reply. As another example, I was active on this site many years back, when it was called paddling.net, and in those days it was commonly discussed that boats with the potential for the highest speed were not necessarily the ones, and often were NOT the ones, which could most easily be paddled at the speeds more practical to the average paddler. So yes, I get it.
Be that as it may, my own tests some years ago with a couple of rowboats that are shaped approximately like canoes, showed that the most basic calculation method for hull speed is really quite accurate for boats having a general-purpose design and where shaping the hull to obtain unusually fast travel speed was never a goal. I have mentioned this elsewhere, but in using those rowboats for this test, I had far more ability for “pushing against the hull-speed limit” than what many average paddlers can hope to accomplish with their boats. This, of course, illustrates one of the aspects mentioned above, which is that most paddlers really don’t need to think about the “exact” value of hull speed in the first place since they won’t ever paddle with enough effort to reach that speed anyway, but since I was using rowboats and therefore I had all that extra propulsive power at my disposal, I found it interesting to put the basic formula to the test, and it was interesting to find out how accurate it really was.
My real point, which seems to have been missed by those replying, is that reporting one’s speed using the wrong units creates a huge error in a person’s understanding of the speeds resulting from various degrees of their own paddling effort, so being aware of what the units of measurement actually mean will make the difference between understanding their performance reasonably well and having absolutely no clue. To me, that seemed worth pointing out, and quibbling about what method actually estimates the practical maximum speed for widely differing styles of boats, or whether this was even worth thinking about, was not remotely on my mind. It’s worth noting that nobody thought about nit-picking the topic of hull speed the first time it was discussed in this thread, nor did they even care about the guy who flagrantly confused mph with knots. I feel like my primary point got totally hijacked.
As to the answer to your main question, that information was already in my post, except for the exact answer in knots (which already was included for the 12.5-foot length via my comments about the reason I was posting in the first place). But to apply the basic formula specifically to a boat with a 12-foot waterline length, here’s the answer, though I think you already know since you have now have the formulas. The basic hull-speed formula for knots says that the maximum speed for such a boat would be 4.6 knots, and the same formula when tweaked to give the answer in mph, says the maximum speed would be 5.3 mph. As a quick check, note that 5.3 / 4.6 = 1.15, which is the same as the ratio of mph/knots, or miles/nautical miles.
I looked at the chart that you clipped from that rowing site, but since you plucked out a portion of the chart and left out the titles which would identify what characters are being described, along with any indication as to the units of measurement, the numbers are useless to me. However, remember that boats used for competitive rowing are very sleek so they can push through the limiting factors that apply more strongly to ordinary boats of the same effective length. Therefore, it seems likely to me that this chart would use a different calculation method for determining maximum speed than the formula that works so well on very basic, ordinary boats. Because of that, I expect it’s likely you won’t find a consistent agreement between what’s in that chart and speeds provided by the basic hull-speed formula.
