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Hull speed and wide sterns

8K views 24 replies 9 participants last post by  seabreeze_97 
#1 ·
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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#5 ·
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!
 
#6 ·
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!
 
#7 ·
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?
 
#8 ·
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?
 
#9 ·
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!!!!
 
#11 ·
Denr,

As for the marital aid part, I recommend that your wife douse you with it from head to toe as required. It may do nothing more than make her feel better, but isn''t that what marriage is all about?
 
#13 ·
I want to touch on a couple points that SailorMitch that was actually on the original topic, Mitch says, "But again I see those boats [Volvo Ocean Race] 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."

I think there are a number of errors in that statement. Volvo Ocean Race boats (VOR) are raced under a rule that really derived from the IMS rule and is intended to actually encourage good seaboats and close racing. As a result the VOR boats are actually not extremely beamy. They do carry their beam quite far aft. They actually have tremendous stability even without the moveable ballast water tanks. I believe that this current generation supposedly has positive stability well into the 160 degree range and minimal iverted stability. VOR boats, like most IMS rule optomized boats as well, really do not count particular heavily on form stability. If you look at their hull forms out of the water, boats like Gilmers Blue Moon and most of Shaws later boats were actually far more form stabilty dependent than the VOR boats.

With all due respect, I think that SailorMitch is confusing the Open Class boats with the VOR boats. Open class boats are their own aberation. These are constructed to no real rule and, Mitch acurately noted count heavily on form stability and moveable ballast, as Isabelle Autissier demonstrated. This is a very different and in my book far less relevant to the boats that most of us sail, breed of race boat and one that I agree is not a good model.

But it does not appear that the Open Class boats are especially influencial on production boats that we see in the US, while IMS and other VPP derived typeforms seem to be. Certainly when you look at the Farr designed Beneteaus (36.7, 42s7,440, and 45f7) and to a lesser degree the Berret designed Beneteaus, these are not high form stability boats. They tend to be low vertical center of gravity boats which is much more typical of the newer breed of boats.

I strongly disagree that this discussion is about apples and oranges. In every generation cruising boats have been pushed and shaped by influences beyond simple cruising boats. What you might consider ''traditional'' boats derive in large part from earlier generation of fast boats, be they pilot schooners, or meter boats.

I too had the chance to discuss Bruce Farr with Olin Stephens. At least in my conversation, his gripe was with the singular nature of Farr''s race boats. But Bruce Farr and his office has also designed a wide of cruising oriented designs. My own boat, a Farr 38, is one of those designs. It is a design that is still popular 20 plus years after its design, in some of the windiest and roughest sailing ventues, such as Cape Town South Africa.

It is too easy to write off what is new. It is too easy to look at the most extreme examples of a design concept that is pushing both technology and the design envelope, and condemn an idea without understanding that in each generation there have been breakthough design ideas that have become the norm and in generations to come will be looked back on as ''traditional''.

When I think about the designers that you mentioned Bill Shaw, Tom Gillmer and Bill Crealock, I can only say that Bill Shaw built his career out of taking the ideas learned in designing race boats and finding ways to use these ideas to benefit a larger boat buying public, Tom Gilmer is the guy whose fame came from a slightly stretched F.G. clone of L.F. Herreshoff''s H-28 and who also the man who brought us the original Pride of Baltimore (speaking of traditional boats that stayed quite inverted), and Bill Crealock whose fin keeled current designs reflect the current trend toward finer bows and moving the center of buoyancy aft to increase speed and comfort of motion.

Respectfully
Jeff
 
#14 ·
Jeff,

It''s late. I''m going out of town tomorrow and don''t really have the time for this. ''Tis true I only have a liberal arts degree from William and Mary, so my comments on naval architecture could have errors. But I am fully aware that Isabelle was not racing in VOR boats when she turned turtle (twice I believe). I only brought up VOR boats because someone else had in the Wide Ass series to support their argument. The first Pride of Balto was primarily designed as a dockside attraction and not really to go to sea. Did you know that? Besides, it didn''t turn turtle anyway. Pride II was designed to go to sea from day one. With all due respect, you missed the two main points of what I wrote: the Sailing World article doesn''t support the broad stern argument, and we all should stop showing off and go sailing. Please reread my last paragraph in my first post on this topic. That is what it''s all about.
 
#15 ·
Actually, you are very mistaken about the purpose of the first Pride of Baltimore. From the day she was first concieved to the day she got her CG certification, to the day she sank, she was seen a roving ambassador for the City of Baltimore. I used to have (and probably still do) a piece of the earliest fund raising literature for the first Pride and it used the term "roving ambassador" and talked about trips representing the maritiem traditions of Baltimore "overseas". She was never intended to be a dockside exhibit. She sank by being hit by a downdraft and knocked down to below 90 degrees and could not right before she down flooded.

I didn''t miss your two main points, I just disagree with them. On your first point, the paragraph that I posted clearly explained why the center of buoyancy has moved aft and sterns have gotten more voluminous on more modern designs. On you second point, I see nothing showing off here. In my mind if you can''t discuss the technical aspects of sail boat design on a sailing forum, where is it acceptable to discuss sailing. I do agree with your point that we all should buy the boats that appeal to us and go sailing. I try to follow that ''go sailing part'' of that at least 4 times a week this time a year.

Respectfully,
Jeff
 
#16 ·
Jeff,

HERE HERE!!......Thank God you are on this message board. It is so funny that a few recalcitrant fellows on here just won''t admit to ANYTHING other than what they themselves want to accept. No matter what anyone else says. Rarely do I read about anyone of them admitting to not being right, even when the evidence is weighing heavily against them. You (and other sailboat designers) show scientific & measurable evidence to the contrary to what they believe and they still argue that they are right. Lol, its laughable and I am sure as they read this they will not recognize themsleves.

Keep your head in the sand boys
 
#17 ·
All, at this point, I would like to point out that I visited the Mayflower II in Plymouth last Summer. I would say that design was very traditional, and it had a very wide stern. Perhaps the Pilgrams lashed their lawnmowers to the stern - I don''t believe that they swam off of them though. Just my 2 cents worth in this discussion.
 
#18 ·
Actually, the Mayflower (either number) was an example of the cods head and tail theory of design where the maximum beam occurred very far forward in the boat. Oddly enough this was very efficient for very heavy displacement boats that could only downwind. If all you have to do is go downwind slowly and carry a lot of weight with a minimal stability available, then the roughly teardrop shape of a 17th century Carvel was actually a pretty good shape. The compartively truncated stern occurs above the waterline by the way.

Jeff
 
#20 ·
I''ll admit that as I "sail past" the new boats in the 36-38 foot range with my little 34 footer, they do look like they are going fast. Perhaps looks can be deceiving.

Waternut, you say you missed me when you were in Chicago, were you trying to hit me? I did see a crazed skipper on a Catalina 400 this weekend that was having trouble furling his in-mast main, was that you?
 
#21 ·
Denr,
Nice try. I know how to operate a furling main. I saw that you had logged onto my computer at our Sailnet booth at sail X-po.
I was downstairs having a ccheeseborger at the time.
Glad to hear your finally going sailing.
Here in Florida we just couldn''t wait, went ahead and started without you.

The waternut
 
#25 · (Edited)
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