Unfortunately, it’s a lot more complicated than that, and I also believe that the OP’s plans to put himself in a 14- or 14.5-foot boat and his wife in a 13.5-foot boat will not create a situation where his wife is noticeably slower. But if she has the size and paddling strength, the T. 140 might well be better than the T 135. Anyway, I think it’s important to realize that those “facts of basic fluid physics” only come into play at speeds that are faster than average paddlers travel anyway, except in cases where rather long and slender boats are being pushed fairly hard (as people who own such boats are more likely to do) and someone in a substantially shorter boat is trying to keep up. At the speeds most people paddle, this is much less of an issue, and in actual fact, at anything that can be called a relaxed pace, boats that we’d call “short” actually require less effort, especially if they are decent boats to start with and not just crappy barges.
I’ve used this example before, but I have two rather similar rowboats, one that’s 12 feet long and another that’s 15 feet long. The 15-footer is faster, topping out at 6.0 mph in a sprint (the same speed that is indicated by wave theory), and speeds from 5.0 to 5.5 mph are “practical” to sustain, thought I wouldn’t call 5.5 mph all that easy. How “slow” is the 12-footer in comparison? It tops out at 5.3 mph according to wave theory, but I seem to remember sprinting for three miles at 5.5 according to the GPS (I could be wrong. Maybe it was 5.3). Cruising along at about 4.5 mph is pretty easy. And here’s the thing: At speeds of 3 to 4 mph, the 12-footer requires noticeably less effort than the 15-footer, and there’s no question about it. Also related to ease of effort in that speed range, the 12-footer can accelerate from a dead stop to 5.0 mph in a single stroke of the oars (confirmed many times by GPS), and even though the 15-footer is ultimately faster and weighs just 20 pounds more, I can’t begin to make it accelerate that suddenly (I haven’t checked, but I think hitting 4 mph in a single stroke would be about the best it could do without risking a broken oar). At a speed of 4 mph, I’d say both boats are about equal in terms of effort required, though if that’s not exactly right, I’d still assign the efficiency advantage to the 12-footer. The 15-footer really shines when substantially greater speed is needed, but the effort required is in a completely different realm than what’s used for slower speeds like 3 to 4 mph. Yes, these aren’t kayaks, but the fact that they are so similar in overall shape, and the fact that they max-out at almost exactly the speed that wave theory says that they should, gives pretty good reason, I think, to give credence to the observation that at speeds of 3 to 4 mph, the faster boat is NOT easier to propel. This is something numerous other people have pointed out in discussions here over the years. I would add one other qualifying factor, though, which is that for very skinny boats, how “abrupt” it feels to hit the maximum speed will be less pronounced, and for such boats, the range of practical cruising speeds will range closer to the maximum than for fatter boats, and the maximum will more easily be exceeded (it’s not a “true” maximum anyway, but in practical terms, for average boats, it’s pretty close).
Oh, and what of the boats proposed by the OP? Well, in the case of my two rowboats, total length is equal to waterline length but that won’t be the case for the kayaks in question. So, not knowing the actual waterline length, I’ll use a figure that’s one-half foot shorter than the total length, as that will be close enough for this purpose. Using those lengths, the theoretical maximum speeds of the 13.5-footer, the 14.0-footer and the 14.5-footer will be 5.6 mph, 5.7 mph, and 5.8 mph, respectively. Those figures are so close that I would expect that for each boat, the range of practical speeds that are well below the maximum would be mostly overlapping each other, and I’ve never seen anything on group paddles that would lead me to doubt that this would be true. All I’ve seen are slow paddlers, not slow boats (obvious exceptions exist of course, but I’m thinking in terms of relatively similar designs), unless the boats are shorter by a bigger amount than we are talking about in this case.
I’ll finish up by pointing out that a friend of mine paddles a 13.5 foot kayak, and can keep up on any group paddle we’ve been on. I have another friend, who’s a very small and somewhat elderly woman, has a 16-or 17-foot kayak in which she’s slower than molasses, and a 10-foot “beach toy” in which she’s definitely faster. When she got into the sport, she followed very bad advice from “experts” to get a long boat so she could keep up with other paddlers, but she lacks the strength to overcome all that extra skin friction of the longer boat, so she can’t take advantage of the faster speed that the boat is ultimately capable of.
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