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  #1  
Old 10-31-2003
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more beam aft, or amidships?

I''ve noticed alot of modern boats carrying most of their beam aft. I know this helps push you along in a following sea, and helps bite in when then wind picks up, not to mention putting more interior space where it''s easily usable.

What are the advantages of carrying most of the beam towards midship though?

Thanks.

-- James the everlearning newbie
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Old 10-31-2003
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more beam aft, or amidships?

I''m surely just scratching the surface but here are a few admittedly generalized answers:

Far less asymetric waterline as the boat heels (since the waterplane of a more balanced hull is more symetrical), requiring less weather helm.

Less tendency to stuff way too much ''stuff'' w-a-y back aft (due to the volume), unless the boat is designed to be loaded in that fashion.

''Living Space'' more centrally located, where it''s more comfortable at sea (vs. if the same functional areas are squished further forward because of that big aft cabin). Thus, the space is where the crew want to spend most of their waking moments, vs. tucked under the cockpit.

With the narrower stern and the lessened tendency to make a cabin back aft, there will also be a lessened tendency to drive freeboard up in order to accommodate that aft cabin.

As discussed in another thread, lessened tendency to put the head of a berth 1/4" from the transom and occasional wave slap.

Narrower stern could, all things considered, offer a more functional cockpit with better protection, better foot bracing, etc.

That ''following sea'' that''s ''pushing along'' the big wide butt may in fact be pushing the boat sideways, skewing the stern more than with a narrower stern simply because there''s more surface area (for both wind & sea).

Jack
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Old 10-31-2003
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more beam aft, or amidships?

Big aft ends can provide space for a big aft cabin and perhaps room for good seating, if not a a couch in the cockpit, but contribute nothing positive to any aspect of sailing performance and tend to make a boat even less suitable for offshore use. Buyers of these boats want to cruise the gang in comfort for their afternoon sail, and for that, such designs make sense.
In terms of useable space, it''s all an illusion anyway, because the stern of a sailboat is generally unliveable, other than perhaps being a good place to kennel a pet. The average quarterberth is the least habititable spot in a boat: make the area real wide, tucked under the cockpit and put a door on it, and what you have is a nicely finished maximum security cell.
All boats are an accumulation of tradeoffs - big sterns provide daysailing comfort at the cost of sea-kindliness and offshore capability.
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more beam aft, or amidships?

Ok. I''ll let Jeff H give a more detailed reason. But the wholesale statement that "Big aft ends ......contribute nothing positive to any aspect of sailing performance and tend to make a boat even less suitable for offshore use" is not completely accurate. There are many Ocean sailing yachts with very wide afts where the boats stay just as wide aft all the way back from midships. Just look at some "Around the world Alone" or "Volvo Ocean race" boats.

(example http://2002.volvooceanrace.org/gallery/photo/leg_9/L9download/images/prevs/prev19.jpg )

I could also gather many more examples, but you get the idea. Most of the best wide aft ends racing boats also have a fine entry. It seems that these boats can really get some good speed while "power reaching" It really matter''s how the aft end is done and the "bullock" lines and how cleanly the water breaks from the stern. There are some poorly designed pinched end sailing vessels that needs more energy from the sails just to get the quarter wave off the stern. Take a look at Nigel Calder''s new Cruising Handbook, there is a section that talks about this phenomenom with detailed engineering analysis and pictures of examples.

Now I don''t disagree with you that a lot of "dock side condo''s" with very wide sterns aren''t necesarily going to be a better sailing boat. Probably worse _Especially_ when the aft section also comes with a lot of freeboard.

But to discount all wide afts of having no sailing benefits is not accurate and misleading.
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Old 11-01-2003
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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.

Respectfully,
Jeff

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more beam aft, or amidships?

Valid point - "contribute nothing positive to any aspect.." did not consider the examples you cite. However, these boats are specialized race machines designed to optimize downwind performance surfing in the high wind conditions of the southern ocean. I don''t much relate to what''s useable for you or me or anyone we know, whether daysailing or circumnavigating. The wide-stern designed being build for us schmoes only intend to sqeeze in an extra couch.
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more beam aft, or amidships?

I don''t know if you read my post but the point is that the design principles that are pushing the more powerful stern sections and finer bow sections are not derived from "race machines designed to optimize downwind performance surfing in the high wind conditions of the southern ocean." The IMS type form boats are intended to perform well in a wide range of conditions. The same is true of the Volvo 60''s which are heavily optimized for conditions where the wind is forward of abeam, and the big gains in the Volvo race boats are on the upwind legs and not in the Southern Ocean.

The production offshore and coastal cruising boats derived from IMS type form have demonstrated tremendous seaworthiness and motion comfort in a wide range of conditions. Walking around boat shows these days one sees trickle down of these principles being applied in all kinds of cruising boats from companies representing the elite to the mundane and as far flung as Beneteau First series, C&C, Dehler, Elan, Hanse, Morris Yachts, Swan, Tartan, Wauquiez just to name a few. This is not about craming in more berths, it is about creating better sailing boats, boats that are easier to handle, and boats which are more seaworthy and seakindly through a wide range of conditions.

Jeff
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Old 11-02-2003
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more beam aft, or amidships?

Jeff, would this apply to a Beneteau First 285 or 305 for example - Boats built in the eighties?
Thanks,
Mark L.
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more beam aft, or amidships?

By an large these boats were more influenced by IOR designs although the not extremely so.

Jeff
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more beam aft, or amidships?

Take a look at the Challenge series of boats "optimized for upwind sailing in the Southern Ocean".

http://www.challengebusiness.com/about/yachtsboats.htm

You''ll see moderate sections compared to many current designs and the Open series racers. The BT Challenge racers are going around the world the other way (generally to windward).
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