Join Date: Feb 2000
Location: Annapolis, Md
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more beam aft, or amidships?
There seems to be a lot of discussion about why newer boats have wider sterns. There are a lot of reasons that modern boats tend to have wider sterns but increased accomodations is not necessarily one of them. More on that later. To begin with, modern boat design seems to have forked in two distinctive directions; one roughly based on the IMS typeform (and I would include the Volvo 60''s in that catagory) and the other based on Open Class boats. The Open class boats are the extreme beam racers that race in various distance races including around the world single-handed. The Open Class model produces very extremely wide beams compared to the IMS Typeform models (Look at a Beneteau First 40.7, an IMS typeform, vs the Beneteau 393 which is more of an open class model) Of the two, the IMS typeform produces a much more well rounded set of sailing characteristics with better motion comfort. I personally think that most cruising boat adaptations of the Open Class boats are not very appealing in terms of sailing ability.
If we look a little bit of history, after the Fastnet disaster a lot of attention was focused on what makes a good seaworthy boat. Motion at sea became a popular research topic. Hull forms and weight distribution was studied in great detail. One of the trends that came out of all of that study was boats with longer waterlines and finer bows. Moving the waterline forward reduced pitching and making the bow finer reduced the impact with waves in a chop.
As bows became finer the center of bouyancy moved aft as well. At first this produced boats that developed a lot of weather helm as they heeled and which tended to jack their rudders out of the water and wipe out easily. As designers got better at modeling these hull forms and were able to shape the hulls to minimize assymetry when heeled this became far less of a problem.
This combination of fine bow and powerful stern sections were found to offer exceptional upwind performance and reaching speeds that are substantially higher than theoretical hull speeds. So this fine bow, more powerful stern hull forms were really a win-win design trend that offered greater speed, coupled with better motion comfort and seaworthiness.
In a past issue of Sailing World there was an interesting couple paragraphs dealing with theoretical hull speed which touched on the issue of theoretical hull speed as it relates to these new hull forms. I am quoting here:
"Waterline''s affect on hull speed is theoretical and not absolute. As a hull goes faster, the bow wave stretches to the point where the bow and stern wave become on wave cycle, whose wavelength is equal to the waterline length. This brings us to wave theory. "
"The speed of a wave (in knots) is equal to the square root of the wavelength (in feet) multiplied by 1.34. If your boat has a waterline length of 32 feet, the theoretical hull speed is 7.6 knots. The waterline length is thought to limit the hull speed because if the boat goes any faster the stern waves has to move further back taking the trough between it and the bow wave along with it. As the trough moves aft, it causes the stern to drop, making the boat sail uphill."
"Except for planning designs, sailboats typically can''t generate enough power to go any faster and climb their own bow wave. But a boat with extra volume in the stern can exceed its theoretical hull speed because the extra bouyancy prevents the stern from dropping into the trough. By the same token, a fine-ended design might not achieve its theoretical hull speed if buoyancy in the stern is insufficient." (Written by Steve Killing and Doug Hunter).
I do think that it is a bit of a stretch to say that these broader sterns resulted from trying to stuff in additional accommodations. I say this because as the stern gets broader, displacement is removed from the bow thereby reducing usable accomodations volume in the bow. If anything the accomodations are just shifted aft a bit. That is not necesarily a bad thing as the stern is generally a quieter area with less motion than the bow.
The broader stern offers the ability to get a better cockpit design where the helmsman is actually able to get far enough outboard to see up the slot of the jib and that seats are wide enough for to be comfortable when lounging, something that was harder to achieve on earlier pinched end boats. This of course can be carried to the extreme.
Again like so many things stern beam of course can get too extreme but properly modeled it produces a much better boat on all counts.