I used the term "flat" in a relative context. Next time you are strolling about a marina with lots of boats sitting on the hard, compare the fwd section hull forms and you will notice that some are less rounded- more flat, particularly toward the bow as some manufacturers use that shape form to gain interior volume.
The hull flex was most noticeable close to the design waterline. We were on a predominantly port tack pounding into a 4-6 ft sea with a stiff breeze for approximately 10 hours. Using a flashlight to cast a shadow across the hull interior, the flex was pronounced and observed by all of us, not just me. As it was close to the fwd bulkhead, I attribute the oil canning to the hull form and structure rather than lack of support evidenced by the proximity of the bulkhead.
didn't notice a list but the boat was pretty packed, the stern definitely squatted which is an inherent trait of many contemporary hull forms, sole plates felt like walking on a see-saw, and of course a leak was evident directly over my berth... inevitable I guess.
If I am not mistaken, the 400 has a very similar hull. SHe is untraditional of Catalinas, I suppose. THe 470 and 440 also have similar design. The 42 is rounded, more traditional.
You are right, K1. Walk the docks. Look at boats on the hard. There seems to be a distinct difference. You will find (my opinion) most "blue water boats" with a rounded hull. It is deep and curvatured. On the other end of the spectrum (what I feel is the newer design) is the flat bottom boats.
I will now jump into a very bad distinctio nbetween the two hull forms/shapes and some of their tradeoffs. I do this for the potential 423 buyer and others that have spent any time on both types.
Again, I find this typical of most "blue water" baots of early vintage especially. Dad's boat is a Tayana 42... a boat I have become quite knolwedgteable about for obvious reason. ANother boat of this type I have spent a lot of time around is a Valiant 42 (though the Valiant 42 is not as deep as the comparable Tayana... an interesting observation given its abilities). Another rounded bottom boat worth mentioning in the production class is the Catalina 38/380. We lived and cruised on our 380, so I know her well too.
I find that the rounded bottom boats, in general, have awesome, deep bilges. DO not discount a deep bilge and its obvious benefits. Other than considerable tank stowage, you will find that the deeper bilges allow for the inevitable water to make its way to the bilge while going to weather for long periods of time. They are also very good for lowering your weight aloft for long term sailing or cruising or just flat for performance. We kept all of our can goods (coated and bagged) in our bilge. We have also kept many, many other items, including many tools, bolt cutters, extra water cans/gallon jugs, etc. If it is something heavy and not something we will get too often, I try to find a way to get it in the bilge. Some things are not always good choices (like the tools mentioned above) without considerable caution as to keeping them dry. All boats leak... at least a little (Ooops, Bene and Catalina did not tell you that, did they!!).
ROund bottom boats handle differently then their flat bottom cousins. For one, they tend to roll more. This can be really frustrating at anchor and at sea. The heavier boats have the benefit of that roll becoming predictable. Once you get your sea legs, it is no deal. But inherently for those that are prone to becoming sea sick, I think it exhasperates the problem. Also, round bottom boats tend to be more tender. At a gust, they will lunge to leeward more than a flat bottom boat would. The difference is that the lunge, though quick, has a predictable stoping point. But it is still unerving the first many times. To be honest, I never got to where I was comfortable with teh roll on our 380. My wife especially did not like it. I found the Tayanas of the world, though still rolling and somewhwat apt to tenderness, were countered by their full/MF keel and sheer weight. The TV-42, for example, is about 38,000 lbs dry. That is a lot of inertia to get moving. A final aspect of note on the rounded bottom boats is their performance. I find them slower to weather and less prone to reach hull speed without a lot of wind behind them. However, I find them to be confortable and stable at all speeds. Even in a hard blow, dad's boat just clips along, predictably, though she gets to that "heeling" quicker then I like.
Flat Bottom Boats
Almost all of the opposite is true, with some exceptions.
I find this the newer bottom design. It is typical on most performance oriented boats. Look at Giu's, or the first series Benes, or the Catalina 400 (pre-HN 307'ish). THese boats do not round over like other boats, but instead you will find them harder chimed (if that is the word on a sailboat). In most weather conditions, you will love this boat. She is responsive, very sure footed, and pretty fast. Hull speed is easily achieved (and in many cases exceeded). Her movement is slow and predicatble to weather. You may actually find your best point of sail is to weather, versus a beam reach. It certainly is not running, where I feel my boat is the worst performing.
My boat likes to sail flat (unlike Giu's... his is a much beter performing boat than mine). It takes a lot to get her rail wet and I am not sure we relly gain that much doing so. On my particular boat (as they changed the hull in the last couple of years), it is still deep enough for drainage to the bilge and points well. She also rides an anchor well.
The negatives, as you can guss, are many of the positives of the other boats. Although I was able to get my extra batteries in the "bilge" (below the foot boards), You simply cannot get a lot down there without eating up every bit of space. The diesel tank, though below the waterline, is actually higher than the batts. Same is true for the water tanks. I would much prefer if they could have put those thanks lower and made the bilge deeper - freeing up that space for 'living'. However, because of the many stringers and the way the hull liner runs 90% of the boat, it would have instead been a large congolomerate of many tanks making up one. In essense, it would have been expensive to do (something a production manufacturer is not apt to do). On the flip side, changing any or all systems on my boat is very easy and very accessible. Still, I would have preferred the opposite. I think Giu;s boat has the opposite... a huge plus.
So there are some things to consider as you now look at your boats. Look at the hull shapes and it will give you some insite into how she will sail in good and bad weather. It will also shed some light into her performace and liveability. And just think... all this you can get from simply looking at the outside (well, maybe).
Hope that helps you in your boat search. Take care,
PS Look at a Catalina 400 if the 40 is a boat of interest. She may be comparable - but she is traditional inside. Also, you may actually end up paying more than another boat of comparable size/length. The 400's have actually appreciated in value. Unheard of, huh?