Midship sections for fast cruising yacht.

G'Day All,
I'm trying to get some opinions in the pros and cons of a midship section with considerable deadrise vs one that is basically an elongated semicircle.

Any opinions would be welcome.

I can't be the only person wondering why the midship shape of boats has changed so much in the last 30 years!

http://www.boatdesign.net/forums/attachment.php?attachmentid=72722&stc=1&d=1343216443

http://www.boatdesign.net/forums/attachment.php?attachmentid=72721&stc=1&d=1343216443

Thanks in advance.

 

Lighter displacement, greater initial stability, and a bigger cabin. The flat area at the bottom to compensated for the displacement of the keel, so as to keep the flow clean . It is,I think a general move towards a more dinghy
type.

 

A cruising yacht's sections can have it's cake and eat it too. The canoe body midship section is to decrease wetted surface for speed, though this does decrease the hull volume and consequently the floor heights and storage space available aboard. The built down hull form, disregards the wetted surface increase to offer more hull volume, which is a good thing on a cruiser, that's not as interested in speed as it might be stores capacity.

In my opinion, a solid cruiser will have relatively flat, squarish midship sections, with a good "U" shaped entry and more of a "V" shaped exit. This keeps wetted surface down and makes for a comfortable riding boat, which is also important to the cruiser.

The question you pose about mid ship sections can get fairly complicated, as you sort through the variables and compromises necessary, for any particular design. The section ultimately employed, will reflect the decisions and compromises made, to meet the priorities established in the SOR. Therefore each yacht's section will be different, because no 2 SOR's are the same.

 

Builders want more interior volume (sales) for less material. At a given displacement the flatter bottom will be wider and roomier. Plus initial stability is better, this reassures the nubes........ The wider boat with flatter bottom will have more wetted surface and thus be slower in light air, but that's okay as cruisers mostly power unless the wind is 10-15 in the right (downwind) direction, and even then......

 

What you really want are sections that are close to arcs of circles through the heeled waterlines.

That way as you heel the volume distribution doesn't change much. It results in a nicely balanced shape. With this type of shape you can have a nice, wide stern for storage and fast sailing.

Here is a set of lines from Perry's latest book. While not the best at this type, it does show the general idea.

 

Thanks so much for the replies so far.
I was onto the wetted-surface issue, surely a semicircular cross-section will develop a"death roll" downwind?
At 6'3" I'm more interested in headroom, than cabin sole width.
Is anybody else concerned about wide cabin soles at sea? It's a long way to fall in a heeled boat......

 

Nope.

 

As par points out, there are many trade offs a designer must make in hull design, inteiror volumn vs. behavior at sea, etc. So a lot depends on the designer's priorities.

A narrow deeper hull of the same displacement will be faster, cut through heavy seas, but also heel further over and have less interior room. A wider flatter hull will not heel as far, making for a more comfortable ride in light weather, but can pound hard in rough conditions. A slim bow tends to mitigate it somewhat, but will sacrifice room in the v-berth.

There is no right answer, but rather what is perceived as what it is that current buyers of yachts want to see, some of it is fashion, some of it is practical. But what a buyer wants to see in a costly purchase today is very different than what they wanted 40 years ago. Designers and builders do what they have to to stay in business.

 

I would say that it was made possible by introduction of computers. Computers and numerical analysis allowed to approximate certain conditions and based on that to modify hull shape in order to achieve most desired results.

Water, wind and gravity did not change -- just new tools appeared and allowed to run numerical analysis and simulations, literally hundreds of them for various hull shapes, heel angles, etc. It have been proven by numerous real time physical experiments and that is why many people could say something like that what was written above.

So, kudos to Naval Architects, computer engineers, mathematicians and programmers -- just my small opinion.

 

The last 30 years? You really think hull shapes have changed that much since 1982? Race boats, sure. Cruising boats, except those where the hull shape is dictated by whatever shape wraps around a given amount of interior, I doubt it.

 

In real speed boats have often gone backwards. If we consider the advances in materials, older designs were often faster. For example, it took a bare bones high-tech trimaran to beat the times of a fully laden tea clipper with flax sails.

 

Shapes have changed considerably in the last few decades. Gone are the pinched stern rule beaters, material advances have required shallower forms, with some hulls 50% lighter then distant predecessors. Speeds have dramatically increased as a result of the shape and material advances made. Three month ocean passages made by clippers where more dependent on sea states and luck on the wind, more then hull and speed advances. White Cloud and Thursday's Child where becalmed for weeks in their record breaking attempts. This isn't testament to design, but the obnoxious nature of mother nature. Sail and powerboat speeds are substantially higher. Take a good look at the world speed records for both in sail and power 30 years ago and again for today and tell me there haven't been monstrous advances. Crossbow managed about 38 knots in 1982, but we're considerably faster then this now. The Spirit of Australia still holds it ultimate speed record of 35 years ago, but the average high speed pleasure craft in that era thought 50 knots was quick, while now 100 is a requirement of those who can afford these types of craft. Hell a $20,000 Bayliner can run 50 now.

 

I just looked at the Cruising World Show issue from 1982. A typical new cruiser of the time was the Perry designed Norseman 447, 44'7" by 13'0". Displacement was listed at 28,000 and ballast 12,000. A 2012 Beneteau Oceanis 45 (Finot-Conq design) is 45'5" by 14'9", displacement is 21,000 and ballast 6000 pounds, she carries 200 sq ft more sail and draft is a foot deeper, oh and the waterline is 5'4" longer........

Cruising boat design has changed in the past 30 years. And certainly wrapping a given interior is part of it, but these new boats do sail rather quickly.......

 

I'm certainly appreciating everyone putting in "their two bob's worth" on this topic!
Regarding tad's post above -
Speed is great, but does the reduction in ballast ratio mean less sail-carrying power, or the necessity to have a team of tame gorillas on the windward rail in order to generate enough power to push to windward in a nasty sea?
Form stability is all very well, but ballast becomes more effective with greater heel angle - yes?
This does relate back to my original question about midship section shape.
Relying on form stability rather than ballast is likely to produce a "whippier" motion at sea which will be more tiring surely?
Having sailed quite a few Southern Ocean miles in boats with sections like the Adams (and Duncanson), I am wary of going to a section like the Dufour as used by Van de Stadt (whose designs I really respect).
Am I being "stuck-in-the-mud"?
To me, it seems that if you REALLY want to go fast, you get yourself a tri, hang on, and get there first!

 

I think some of the trend we see can be attributed to the increase in size of the boats. Tough to get a 34' canoe body with 6'6 headroom that looks decent with out using a built down design. Rather easier at 44'. The motors and the expected use of them has changed quite a bit as well; which would account for a lot of that 5'4" WL increase Tad mentioned. If you just assume that the motor will be running if you are going less than five knots, that changes how much emphasis a designer puts on wetted surface. Performance at three knots used to be considered important for a cruiser. Now it is often nearly irrelevant.

I found the OP's question a bit puzzling. Who goes shopping for a midship section? I have a casual interest in the 'nature of boats', to borrow Gerr's phrase, but I don't go shopping for a lines plan. I find the boat I like at the price I like; and if I recognise the designer's name, I'm pretty confident it will have the lines it needs to be the boat I like. I really am not interested in questioning folks like Stephens, Crealock, Lapworth, and Cherubini. There are a lot more important things to be concerned with when buying a boat IMO- things that can actually go wrong.