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  #1  
Old 05-22-2002
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Hull speed and wide sterns

There was a recent discussion on this Forum about why newer boats have wider sterns. In the most recent issue of Sailing World there was an interesting couple paragraphs dealing with theoretical hull speed which touched on this very issue. I thought it might be an interesting topic for further discussion since both topics (wide sterns and hullspeed are frequent forum topics). 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).
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Old 05-22-2002
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Hull speed and wide sterns

Fascinating minutiae, I doubt that this is applicable to the production boats sold to the general public. Thanks for the information, very powerful!
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Old 05-22-2002
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Hull speed and wide sterns

"Convince a man against his will; he''s of the same opinion still."
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Hull speed and wide sterns

I resemble that remark!
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Old 05-22-2002
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Hull speed and wide sterns

Let me begin by complementing the restraint of my main man Denr! I was looking forward to some pyrotechnics, but maybe your brain is more engaged in thinking about a big sail this holiday weekend????

Jeff, I, too, read that article in Sailing World and have to admit I chuckled a bit when I read it, thinking of the "big ass stern" topic. Unfortunately, no where do the authors state that "excessive" (my term) volume in the stern is a good thing. The authors speak of "extra volume" and "extra buoyancy" without defining what that is. Extra compared to what?

Obviously, sailboats need SUFFICIENT volume in the stern to achieve the performance goals intended by the designer. Working from memory here, but someone wrote on here that you can even look at a Bill Crealock canoe stern and see how beefy it really is. They are most correct because Crealock makes sure to design enough reserve buoyancy into the stern. As he puts it, most times when in a storm out at sea the stern effectively becomes your bow while running before a storm. He criticizes the modern wide sterns with sugar scoop transoms because of what is likely to happen if the stern is swamped by a big wave.

Of course, Bill Crealock designs all his boats for blue water. the modern designs with the huge cockpits and scooped transoms are primarily on coastal cruisers -- a different animal to me.

Someone else pointed out in the Fat Ass topic that the new Volvo Ocean Race boats have wide sterns, and they must know something about speed. Yes, they do. But again I see those boats as entirely different animals from the kind of boats most everyone else on this list will ever need. The VOR boats use extreme beam for form stability to save overall weight. They also use water ballast to keep the boats upright. As Isabelle Autisier (no doubt misspelled) has demonstrated on a couple of occasions, turn these modern, wide ocean racing boats over, and they will stay there. Bill Crealock designs his boats, hopefully, to stay upright to begin with, but if they do turn turtle, they will bounce back up in a hurry.

And to repeat several other posts in the Fat Ass series, we really are talking a preference for "traditional" design vs. "modern" when addressing this wide stern phenomenon. But we also are mixing apples and oranges in my humble opinion concerning the intended uses of these boats and their design elements. While lots of people cross oceans in Beneteaus (not to pick on them obviously), give me a Crealock or Tom Gillmer design anyday for a blue water cruise.

In my short career as a part time freelance writer, I''ve had the good fortune to interview Olin Stephens, Bill Shaw, Tom Gillmer and Bill Crealock. When asked who is the best modern designer, Stephens told me "Bruce Farr, but I don''t like his designs very much." Shaw, Gillmer and Crealock all stressed moderation in their designs. Also, Shaw and Crealock are in their 70''s, Gillmer is in his 90''s. I expect that naval architecture has discovered a few things since they all went to school -- although the Sailing World article goes back to wave theory we all studied in high school physics. (Well, I did anyway.)

Admittedly, Bill Shaw probably would be cranking out fat assed stern boats if the old Pearson Yachts was still around because the company was there to sell boats, and people like those big cockpits and the swim platforms. Heck, if I can ever buy that new 38 footer it will have that, too. But I don''t intend to sail to Europe, either.

Bottom line after all this: yes, big ass sterns have their place in sailboat design, and we all have our own opinion of exactly where that place is. Buy the boat you like, for the purpose you have in mind. Then go sailing. I wish I was on the Chesapeake right now!
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Hull speed and wide sterns

Very well said SailorMitch, you took the words right out of my fingers. I would like to suggest another possible advantage of the "powerful" sterns. They can be used to carry your mower. You are however wrong regarding my resraint, tonight I''m sailing SW 15-20 perdicted....YEEEEEEEEEEESSSSSSSSS!
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Hull speed and wide sterns

Denr,

Yep, I forgot about a place to put the mower. Just add another 3 feet to the width of the stern and all is well. But that is probably overkill for that 18 inch reel type you bought for the house trailer, right? Nothing like going with the traditional design. I meant to ask before if you''re going to use Cetol or varnish on the handle -- another fav topic on here. I also suggest spraying Sailkote on the blades for faster mowing.

Where are you sailing to tonight? It''s Wednesday. Racing are we?
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Hull speed and wide sterns

We purchased the 18” low maintenance reel type model with the SS handle. We went nuts and purchased the Martha Stuart patio set someone suggested as well. I do use Cetol on the handhold of the mobile home next to the entrance door. My neighbors have commented to me, how nautical it looks! No racing tonight, just cruising, I consider my vessel a family station wagon rather than a sprinting machine. The Sailkote (I hate cutesy word spelled phonetically) is a good idea, would I apply this after each passage or only once a season?
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Hull speed and wide sterns

Denr,

I''m a big fan of Sailkote. I use it for nearly everything, so I have done away with all sorts of other containers onboard. It works better than WD-40 in many applications, and is a passable substitute for it in others. Sailkote works better on my fair skin than SPF-15 sunblock. It''s slightly less effective as an insect repellant but try it and see if it works for you. I spray it on the knotmeter paddle wheel to keep crud from fouling it. In a pinch, Sailkote even can be substituted for Pam or butter when frying eggs for breakfast. It also shrinks painful, swollen tissues, but won''t go into details on that one.

As for you, I''d spray a little on the mower before each use. Can''t hurt.

Oh....did I tell you I own stock in the company? So please buy the gallon containers of Sailkote, and use it LIBERALLY!!!!
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Hull speed and wide sterns

I’ll pick up a few gallons of the stuff on my way to the harbor tonight. Speaking of the “pain swollen tissue”, could the Sailkote be used as a marital aid? If so, would I apply it to the unit in motion or the stationary receptor?
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