Wide Transoms...A Discussion Thereon. (was Jeff-H) - SailNet Community
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Wide Transoms...A Discussion Thereon. (was Jeff-H)

Will you please explain to me how a wide transom can NOT cause a fine bow to dig in. Could you also explain the wide transom thing? (one...more...time)
I''ve always been a ''traditional'' guy and I''m really trying to understand modern hulls.
Add anything else you''d like, to educate myself and others, if you will.
I will be moving up soon and I want a boat that goes well to weather and is fast.
Thanks,

Dennis
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Old 12-21-2004
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Jeff-H

I am a little short of time but let me see if I can give a quick explanation. First lets start with a simple example. If you visualize a hull that was essentially a simple cone in shape balasted so that the point was at the surface of the water, you could spin it all day long and it would not change trim in a manner that would tend to force the pointy end down.

While the design process for a hull is obviously more complex, what allows a wider stern boat to heel with out going bow down, is that the hull is modeled so that the longitudinal center of buoyancy does not shift as the hull is heeled (at least through moderate heel anglee). Designers have simply shifted the longitudinal center of buoyancy aft but, of course, the longitudinal center of gravity has also moved aft in the hull the same amount as the longitudinal center of buoyancy. As a result the boat can heel without changing fore and aft trim at least through through small angles of heel (perhaps as much as 20-30 degrees of heel).

This of course requires careful modeling to maintain the same longitudinal center of buoyancy as the boat heels. Computer modeling becomes really critical to this process but most good modeling tools will show the center of buoyancy at various heel angles allowing the designer to move a little more buoyancy aft or forward to maintain trim with heel.

Although a very crude design compared to more modern designs, the pictures of my boat under sail demonstrates how little change in trim can occur with heel angle even on a boat with an extremely broad transom and fine bow. You can find a number of useful pictures on the Cruisers Forum Website:
http://cruisersforum.com/photopost//...er=189&cat=500

The best picture to understand this is the one labled "Synergy Bearing away". Here she is heeled roughly 30 degrees and you can easily visualize that while any individual segment of the forward portion of the hull builds increased buoyancy slower than the transom area, the bows proportionately larger area and longer lever arm offsets the comparatively rapid increase in volume in any individual segment near the transom.

The following was exerpted from an earlier discussion of this topic:

"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 (More than a year ago now) 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."

I need to get back to work!

Regards
Jeff

Last edited by Jeff_H; 05-24-2010 at 07:32 AM.
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Jeff-H

Jeff,
Thank you for slackin'' off long enough to post. I understand better and agree.

Dennis
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Old 12-29-2004
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Jeff-H

Dennis,
It won''t matter what your hull speed is when big waves/whitecaps are lifting a big azz stern. Reserve buoyancy will lift the stern, increase speed and bury the bow or start a broach type course change. Spend a few hours sailing this way in storm conditions and it won''t be fun. Big sterns are not big wave friendly when running downwind. How big are the waves is the factor.
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Jeff-H

Billpjr,
I agree and that is why I wouldnt want(though some would disagree)this type of boat for ocean voyaging but for here in Buzzards Bay, I think that I would rather have the performance to weather.
Two to four foot seas are quite common. Six footers, about three or so times a month. Eight to tens about mabey twice every six monthes and I''ve been ten to twelves once (you''d see those about once a year or so) and most of the time the seas are choppy.
What would would be you recomendations on a boat for these conditions?

Dennis
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Old 12-30-2004
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Jeff-H

Billpjr, I really disagree with your statement,"It won''t matter what your hull speed is when big waves/whitecaps are lifting a big ass stern. Reserve buoyancy will lift the stern, increase speed and bury the bow or start a broach type course change."

I think that is an antiquated opinion. I just have not found that to be the case. The fine bows and narrow water planes on the better designed IMS derived newer boats can actually actually track quite well. By moving the center of gravity aft, the tendancy to bury the bow is greatly reduced even over more traditional designs.

In my own case sailing aboard more modern designs in heavier conditions, I have not experienced the kinds of broach problems that you mention, and certainly not as badly as I have experienced in more traditional water craft, and earlier CCA derived boats with their short waterline lengths, or IOR era boats with their comparatively blunt bows and pinched ends.

If you look at the extreme conditions experienced by the Volvo race boats, or even in the most recent Sydney Hobart, these newer designs are proving to be quite seaworthy and manageable even when pushed hard downwind in very extreme conditions.

That said, anything can be taken to an extreme and an overly broad transoms and beam found in cruising boats derived from the Open Class (Around Alone) boats, can be very difficult to handle in extreme reaching and running conditions.

Jeff
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Old 12-31-2004
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Jeff-H

JeffH,
We disagree on a lot more than this. You have a major bias toward racing and appear to hold the conception that the boat is always controllable. Under normal conditions this is true. Fine bows, calcs, crew, etc only offset the effects of wide transoms to a degree. My take is your idea about the size of a big wave and mine are different. Your idea of crew size and ability for an avg sailer must also be different. Sydney Hobart mega racers are apples and oranges to yachts sailed by normal people. Thinking wide transoms are better for sea keeping isn''t something I''d endorse as easier to sail or better for the avg jack.

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Old 01-02-2005
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Jeff-H

Billpjr, I agree with you.

That''s why relatively small modern ocean going boats like modern Najads, Malos, Halberg-Rassys are less beamier and have a much more moderate transom than Bavarias, Jeaneaus and Beneteaus.

The first are heavier boats, designed mainly as passage makers, and made to withstand heavy seas. The others are boats that are used and designed mainly to the Mediterranean, taking in account the conditions you find there. Big waves are not one of those.

I am not saying that Bavarias and Beneteaus are not able to withstand heavy seas, I am only saing that the other type of boats are a lot better at it.

The Volvo ocean boats are 65f boats and comparatively to what Jeff calls "modern boats" have not wide transoms, nor have they large beams, considering the length.

Paulo
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Old 01-02-2005
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Jeff-H

I have to agree with pcp billpjr oh this one.I find alot of apple to orange comparisons.Formula 1 car and a 59 caddy.You can argue which does what better.Trying to compare a boat with another that are light years apart with different purposes goes nowhere.
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Old 01-04-2005
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Jeff-H

Billpjr

To quickly touch on your points here.....

"You have a major bias toward racing and appear to hold the conception that the boat is always controllable."

"Your idea of crew size and ability for an avg sailer must also be different. Sydney Hobart mega racers are apples and oranges to yachts sailed by normal people."

I have a major bias towards easily handled boats and boats that offer good performance. I do not have a bias towards racing. I cited the Sydney Hobart race study because unlike many other forms of racing, the Sydney Hobart fleet that was studied was marked by high winds, huge seas (12 to 20 meters), and by a mix of boats from the most up to date go fast racers to very traditional 1930''s era boats.

I do not believe that a boat is always controlable, just that IMS derived designs with comparatively wide transoms seem to be proving quite controllable as compared to many so called cruising designs.

"My take is your idea about the size of a big wave and mine are different."

I do not know what your idea of a big wave is but my idea is somewhere between 12 and 20 meters.

Respectfully,
Jeff
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