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post #4121 of Old 05-19-2013 Thread Starter
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Rocker and hull design

We have discussed here the shapes of modern hulls and transoms but mostly in what regards a superior view. When we look at the lateral view many surprises will appear and hulls that seemed very similar are not so similar after all. Hulls are a 3 dimensional object and it is difficult to have a complete perception of its shape.

Older sailboats had not a possibility to have what the French call a hull "tendue", meaning a hull with little rocker and for the less informed in nautical jargon, a hull where the lateral line of the boat has small angles in what regards the line that starts at the bow (at water line levell) and ends at the transom, the longitudinal profile of the boat.

They could not have that because the technology didn't allow boats light enough to do that but even so fast boats and race boats had a lot less rocker than more heavier cruising boats.

A racing boat from the 30's

and some old but more recent cruisers. Look at the angle of the entry on the bow and the angle out in the transom:

Some ahead of their time found the importance of beam and light construction in what regards controlling and diminishing rocker. The best example are Herreshoff, father and son. Look at this Herreshoff 38 a 31 old design and look at the hull profile and rocker.

Herreshoff 38

Another great example of an even older cruising boat with little rocker and way ahead of its time is the Farr 38:

Anyway I guess that Herreshoff father took the idea of more beamier boats with less rocker from some traditional boats. Look at this beauty and see how rocker is a lot less than on those heavy cruisers:

In what regards modern top race boats, rocker was diminished to a very slight one. This was to do with the boat being able to plan, going well over its hull speed. The idea is to sail the boat over the water, not inside the water like on the old days (because rocker can change a lot with the size of the boat I will post mostly boats between 35 and 40ft with some few exceptions).

A Farr 400:

A class40 racer:

A M34

Mini Racer:

Regarding cruising or day sailing these hull shapes have two problems: First they are designed not to be loaded and secondly they are not designed to be comfortable in a sea motion.

Cruising can assume many forms. Some don't cruise extensively and don't need to carry any considerable load and for all comfort is a relative issue.

Some just don't care about that and want to have the maximum sailing pleasure. Regarding loading, narrow shapes like a F400 would not be able to carry any significant load and a big load would probably make the boat unstable but the situation is different in what regards solo racers like the 40class racer. Yes the performance would be affected (still a very fast boat compared to a cruiser) but because the boat is very beamy it can take a significant load without the waterline going way out of his lines. The proof is that some days ago a Chinese circumnavigated non stop, quite fast, in one and that means necessarily a considerable initial load on the boat.

Between cruiser racers, fast cruising boats and daysailers-racers there are some very fast sailboats with lines just a bit more soft then the ones used on the top racers. These boats will not plane as easily as the top racers above, specially loaded but they can plan in ideal situations and most of all can be easily sailed on that region slightly above hull speed and that makes them very fast performance cruising boats.



Pogo 10.50

Sydney 43

Columbia 32







FinFlyer 32


Farr 42

Finflyer 34



Sydney 40

Sydney 32

Sydney 36

Sydney 38

Regarding comfort on a seaway these boats are not as uncomfortable as race boats and there are some in this forum that own several of these type of boats and none is complaining. There are even one that passed to an Opium 39 coming from a theoretically much more comfortable and heavy boat, a Dehler 41cr. The program was a fast cruising boat for the family, no racing involved and I was a bit afraid that the family would find the boat a bit uncomfortable. Well, no. They all love the boat and find that it has a lot of storage and a great interior.

This approach in what regards sailing boats and hull rocker imply not an heavy boat or a beamy boat or both but it is not an exclusive of very fast performance boats, there are mass market boats with this approach and it depends much on the designer tastes in what regards a cruising boat and hull design. For example Finot and Marc Lombard generally design cruising boats with little rocker while Philippe Briand or Farr (at least for Bavaria) design boats with more rocker.

For instance look at these boats from Finot/Conq and Marc Lombard:

First 31.7

Oceanis 323

Oceanis 331

Oceanis 37

Benetau Oceanis 41

Jeanneau 42i

Jeanneau 36i

And these ones where even in performance boats Farr or Philippe Briand use a lot more rocker:

First 40.7

First 40

SunOdyssey 409

Jeanneau 379

Having more rocker in a performance boat is not necessarily a bad thing specially in what concerns offshore conditions and going upwind with very bad weather. It is not by coincidence that the First 40 is a great boat on the Sydney-Hobart. But a lot of rocker will not be good for going fast downwind and boats with little rocker are not necessarily bad in nasty weather upwind or otherwise. The last Hobart was won precisely by one of those boats (little rocker), a Sydney 43. They are good upwind in heavy weather (probably more uncomfortable) and downwind will smoke a boat with a big rocker.

Even if the tendency seems to go into having less rocker things are not linear. Look for instance for this two Bavaria 36, the first was designed by JJ and the second one (that replaced that model) by Farr. The first one was faster, lighter and had less rocker:

And there are some surprises, for instance the new Elan 400 and new Dehler 41, view from above look to have the same kind of hull, but that is not so. The Dehler has considerably more rocker:

Curiously in the past the Dehler had less rocker:

Dehler 36

Then the hugely (and justifiably) popular Dehler 39 had a lot of rocker and I don't know to what point the choice to maintain that type of hull (in what regards rocker) was from the designer or if it was the company that wanted to maintain some of the previous boat characteristics and said so to the Designer.

Dehler 39

Dehler 38

Regarding Xp yachts it seems that the tendency is to have slightly less rocker on the new boats (but lots of rocker on the XC line):




Some more sailboats to give a general picture:

Oceanis 322

OVNI 345

IMX 40

Hanse 370

Arcona 410

Salona 41


Comet 41s

Grand Soleil 40

Hanse 40

Hanse 415

As you can see the variations regarding rocker and lateral shape of the hull are even more than in what regards the superior view of the hull, beam and shape of the transom. There are for all tastes

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Last edited by PCP; 05-19-2013 at 05:20 PM.
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