First of all, I want to say that I completely agree with my fellow moderator, Ron (Faster), when he says, "that's one cute little girl!" Bob, she's a charmer. I can only imagine that Violet must bring you a vast amount of joy.
Originally Posted by outbound
Still, have a question. Seems a given to assume ultra modern designs sail faster in more circumstances but is there a limit imposed by the design parameters as regards comfort motion. I see the light hulls, beamy aft hulls flying in semi displacement mode and wonder if a wave slaps against the usually unwetted portion of the aft canoe body ( except when running) will it produce a snap heave or corkscrew motion that many find most unpleasant. I also wonder about the sensitivity of the ultra modern hulls to the weight cruisers impose on their vessels. To what extent does the skipper need to attend to "trimming ship" when loading and is there a risk that overloading with stores or that 300' chain rode will screw the whole thing up. Lastly even when operated within design displacement ( with weight central/aft and low)given the semi planning behavior, decreased wetted surface with the boat on the water instead of in it -are these designs evolving to approximate the ride of a cigarette boat with a testosterone drunk driver.
To start my reply, for the sake of clarity, I want to make it clear that when I was commenting that current 'modern hull forms' have gone in an unexpected direction, I want to note, that I was referring to the general direction being taken in some of the more extreme grand prix level race boats, by which I mean the boats like the TP 52's, Open Class Boats, or the Volvo Boats. I believe that these represent much of the leading edge thinking in race boats, and that some of the thinking that created those boats is filtering into production coastal cruisers, if not into offshore cruisers as well.
To put my comments into a fair perspective, I suggest that it is important to also understand that I have never sailed these latest boats, and so much of my commentary is conjectural at best. My opinions are mostly shaped by studying at footage of these newer boats under way, observing a few examples on the water and also talking with people who have designed and sailed them.
With that in mind, here are my initial conclusions. I have watched a lot of footage of these boats in a range of conditions, and was especially interested in how they behave in transitional conditions, either being hot by a gust or in waves. I initially thought those conditions would be the Achilles’ heels of boats with this kind of extreme beam. It is amazing how much footage is out there, and with the ability to stop frame these images there is a lot that can be learned, albeit at the price of spending way too much time staring at a computer monitor rather than actually sailing.
I came away with a range of impressions. First of all, to get the most performance out of these boats, for any given wind and sea state, and for any given course, there appears to be comparatively narrow range of idea hel angles. On courses where the boat cannot plane, that angle appears to be roughly 10 to 15 degrees, just steep enough to allow the boat to sail on the turn of its bilge and lift one rudder blade mostly out of the water. Looking at shots from astern, it appears that at that heel angle, these boats actually have a comparatively narrow and symmetrical submerged hull form. I would expect that ignoring other factors for a moment, in that mode, these boats could behave like a long narrow hull form; meaning easily transitioning into semi-displacement (small wave making) mode, being balanced, producing minimal drag and perhaps even generating a fair amount of directional stability.
That conjecture seems to be borne out in shots from astern where you can see the helmsman’s hand, the elevated rudder and/or wheel, and see that there does not appear to be a lot of course corrections taking place, and that the helm appears surprisingly neutral, even as heel angles, wave impact, and sail plan forces cycle up and down. But also long narrow hull forms are prone to less heave and pitch, albeit with a more sway and roll. (The roll seems to get dampened out by their muscular sail plans and their ability to stand up to a huge amount of sail area relative to their displacement.)
That behavior is striking to me. Before these boats, the prevailing wisdom was that beamy boats were subject a lot of ‘roll steer’, meaning as a beamy boat heeled, she would change fore and aft trim, and the plan sections of the hull in the water would get very asymmetrical, and that combination would cause the boat to develop weather helm and wander, requiring a lot of course corrections, sail trim changes, and so on to maintain a straight course in waves or changeable conditions.
The lack of fore and aft trim change is truly amazing to me, and in some ways this is the seeming miracle that the science, coupled with advances in computer analysis, allows so effectively. To explain, when almost any boat heels, there is some amount of static change in fore and aft trim. Even on the majority of traditional designs, that trim change may be imperceptible, but its still there. As speed increases, trim also changes due to dynamic rig, hull and keel forces as well.
But if you study pictures of these boats in wind and waves, the way that their hulls are shaped, they do not seem to go through the kinds of major bow down stern up changes that you might expect meaning that the point of entry remains about the same height on the stem, and the corner of transom at the waterline remains at about the vertical height at the water. That should not be mistaken for ‘rolling up’ or ‘rolling down’. These boats clearly roll up, meaning that their vertical center of gravity rise vertically with heel angle. (Many traditional designs roll down, which historically was seen as better for tracking and motion.)
That kind of behavior would have been extremely difficult to predict in the days when boats were drawn by hand and calculations were manual, since the iterative process to analyze at trim change, let alone dynamic force distribution, with heel angle would have been wildly time consuming drudgery and would not represent real behavior when completed.
Where it gets harder to predict without sailing the boat, is what happens when meter or so wave hits one of the flats (topsides or bottom) when heeled. My sense is that these boats would behave about as well as any boat of their displacement. The impact would be felt, but for a boat of their weight nothing better or worse would happen.
But that begins to drift into the discussion of weight carrying. What tends to happen in many of these discussions is to say something like, “How can a 9,500 lb, 40 footer carry enough ‘stuff’ to be a good cruising boat?” But the answer really lies in the 9,500 lb part. Regardless of length, excess weight carrying capacity is somewhat proportional to design displacement, and to a lesser extent to the waterline plane of the vessel. So if you ask about carrying capacity in the abstract, I would say that a 9,500 lb boat of the current modern hull form can tolerate as much excess weight as any 9,500 lb boat, including a traditional design. And in theory, the long water plane should be reasonably tolerant of loading fore and aft.
But when you start asking about carrying enough weight for a couple to go off and do some serious voyaging, I would expect that the boat would minimally need something like 16,000- 20,000 lbs of displacement, designing around the types of hull forms in question, that gets to be a pretty long boat, probably a 50 or more footer. And also the current design trends really are dependent on efficient foils and bulbs carried very low meaning a lot of draft, or the complexity of a lift keel.
And that gets to some of my reservations about the wider range applicability of the current design trend.
To answer Ppvajko’s Question: "Would you mind giving us a few of these ideas (Whitbread 60-IMS style hull forms) filtered into cruising boat designs, that resulted in some wonderful cruising boats? Boats that are fast and forgiving, with comfortable motions and so on."
A few example of these designs to which I was referring that come immediately to mind are the Morris Ocean Series, or Coast Guard Training Boat, the current Hallberg-Rassey 40, and to a lesser extent the Malo 37 and as a coastal cruiser, perhaps something like the Xp 38.
Re: JameWilson’s comments: When you look at these videos, you are mostly seeing these boats racing and being pushed to the limits. Much of the “hit by a fire hose blast of water every 30 seconds, where water runs over the deck and the cockpit is constantly awash, water draining out of the open transom, where every gust translates into an additional 10 degree heel angle, where I am forced to concentrate on minute adjustments continually in order to stay out of trouble.” is the result of being over canvassed for the conditions and pushing to get the absolute max out of the boat. What is more revealing in those videos are the moments when they are not being pushed into the high drama shots that you describe.
But by the same token, my concern with this model translating into production cruisers, is that much of the advance in performance on these boats come from being able to carry enough sail to overpower their potential for generating very high drag for their weight. And I worry that like the old CCA boats which needed to heeled in a very narrow range to perform, requiring constant trimming to stay at speed, as these designs translate into cruising boats, they will also need care in sail trim to keep them loaded up enough to perform anywhere near as advertised.