Wide Transom

[mikehoyt]

OK. I know we have had a thread before about the wide sterns on modern boats. I recently looked at pictures of the new C&C99 and noticed it too follows this design trend. Looking further I noticed many other boats of similar aged design and size also have teh wide transom. They are almost Wedge shaped!

Then I looked further to find examples of boats that looked aestheically pleasing to me. The look I was after did not take into account actually living on the boat but was more concerned with curves and lines from the exterior. Most of the boats I liked were built in the late 80''s or were 60 feet or longer.

So here is the question:

Are the wide transoms in todays boats a compromise of performance for comfort or do they actually improve the sailing characteristics of the boats?

Thanks,

Mike

[tsenator]

If you look at a lot of the around the world racing boats (e.g. Around Alone, etc) they have exactly the same trend ("almost Wedge shaped"). There is a definite sailing performance enhancement to that. Not necessarily a "better" more comfortable ride though...that is where the details are important in the design.

BTW, I agree with you when it comes to the aesthetics of some boats, but you can still get very nice designs that are being built today, but the original designs were probably in the 80''s. My boat for instance (''99 build) has the original underbody and design from the 80''s, but has been enhanced "slightly" over the years to have a slightly wider stern. But not so much so , that it looks like a wedge of pie.

[Jeff_H]

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 recent 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.

Regards
Jeff