Join Date: Feb 2000
Location: Annapolis, Md
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Fat Ass Sterns II
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. 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 hull forms 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).
That said, as with anything in yacht design all things need to be done in moderation and no matter how theoretically good any design idea might be, it can obviously be taken beyond a reasonable moderation to the point that it becomes a bad idea. Unfortunately from my perspective that is occurring in the case of many of the newer performance cruisers. If you look at the Volvo 60, IRC, or IMS race boat derived designs the transoms are not all that wide and the waterlines and flare are quite moderate. While these designs are fastest when sailed flat, they still are very easy to sail at reasonably large heel angles. Denr and I appear to be in agreement on the Open Class derived designs which tend to push transom widths to an extreme resulting in extremely large wetted surface, and poor handling without such remedial devices as trim daggerboards and dual rudder.
It is important to understand that it is much harder to design a good boat with a wide transom and fine bow than a more traditional hull form. These designs require more careful weight distribution and buoyancy distribution studies than more traditional designs. Sectional properties need to be more carefully configured as well. When done right, a properly designed fine bow, wider transom boat has no more tendancy to go down in the bow when heeled than a pinched stern boat (remember IOR boats had very pinched sterns and yet they were very prone to going bow down and wiping out with heel angle) and also has no more tendancy to wipe out.
But there is the rub. Proper design of fine bow, wide transom boats requires careful modeling at all heel angles and all pitch angles. It requires reasonable assumptions about loading and trim angles. Boats like the new Tartans seem to take on the wide transom look as a kind of fashion statement rather than as a carefully balanced design concept and as such seem to produce designs that are not all that great sailors in many of the repects mentioned above.
Beyond all of that, many new sailors seem to be unaware of the importance of weight distribution and trim angles. Traditional boats often carried 10% of their ballast as trim ballast that could be located as necessary to adjust the trim of the boat due to its state of loading. This is unheard of today. People tend to load boats almost haphazzardly thinking if the designer has created a locker then it is there to be filled with what ever fits regardless of its weight and position.
But all of that said, I do think that it is a bit of a stretch to say that these broader sterns solely 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 generally has less motion than the bow.