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Old 04-09-2013
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Re: Modern Hull forms and Motion Comfort

(Bob, Please make that four fans, since you can count me in as a fan….)

But back to the topic of this tread, with regards to the rest of the thread, it is interesting to see an ancient thread like this one come back to life, and to look back at things which I wrote perhaps a decade or so ago, from the present with the luxury of hindsight. I cringe when I look at the title “Modern Hull Forms and motion comfort” seeing as “modern hull forms” are not modern hull forms now.

It does not surprise me that symptomatic of so many internet discussions, reading some of this thread, I see science driven discussions seemingly vying on an equal footing with emotional driven responses, with overly broad brush comments being made on both sides, and refuted with snap shot anecdotal evidence taken sometimes from the exceptions.

One thing that struck me in reading my own comments is that some of what I wrote has become dated in that the world of ‘modern hull forms’ that I referred to in 2001, has moved on in a different direction than I would have expected at the time. In 2001, I thought the science was pushing performance yacht designers towards hull forms and rigs which produced well rounded designs, boats that were easy to handle, had comfortable motions, and sailed well in a very broad range of conditions. But as it turned out the world of fast boats took a turn toward a very different direction, and that new direction is affecting production boats in ways that I am not fully convinced will produce wholesome designs.

At the time that I wrote some of those comments, I had attended a number of lectures on what was happening in the science of yacht design. It was a time when VPP driven racing rules were more popular than they are today, and so well balanced designs seemed to be emerging from the racing world and the ideas filtering out of the racing world were working their way into mainstream yacht design in a very positive way for cruisers.

At that time, the oft repeated claim in this and similar threads, that that race boat designers do not care about motion was patently false. Designers had come to the realization that the motion of a boat was an 'unrated' aspect of speed. Designers had known for a long time that the various motions of a boat underway interrupted the flow of air around the keel and sails, and that the force of waves striking the hull, and the deceleration forces of rapid pitch, yaw and roll, sapped speed from the boat.

Around the time that I had written some the items which are quoted above, there was very serious investigations into ways of minimizing motion and reducing acceleration and decelerations. While these investigations were predominantly about improving unrated speed, the lessons about how to improve a boat’s motion had great relevancy to the motion comfort of cruising yachts.

And while much of the ‘lessons’ of this research was well known “Doh” moments, the ability to study dynamics of boat passing through waves as seen over time, with precise measurement of the forces involved as compared to the rotational and linear motions, allowed the understanding of the science of motion comfort to advance very quickly.

The key lessons that came out of the research began with some simple core points:
• It is important to minimize the forces which are causing unwanted motion.
• It is important that the forces that cannot be reduced are handled in a way that they occur progressively rather than suddenly.
• It is important that as the boat moves through a reoccurring cycle, that the amount that the vessel gets out of phase with reoccurring be minimized as a strategy to minimize the amount of the force imparted as the boat tries to get back into phase.
• That as much as possible it is important to dampen motion in a way that does not work to get the boat out of phase.

The tactics to address these strategies had to consider the way that forces entered and left the system over time, whether these were linear or rotational, and whether these were one time events or cyclical events. When I read some of the comments above about bow shape and the collision with waves, these are presented as two dimensional concepts, by which I mean, that the ignore the transitional properties of waves passing a boat in motion over time.

With this better under standing of these strategies, the shape of boats and their weight distribution changed. Plumb bows with finer entries were not only about speed, but also about reducing the force of the collision with each wave and allowing that force to build progressively so that less force was felt by the boat and crew. The moving of the center of buoyancy aft also moved the momentary longitudinal axis of rotation aft, and with the greater distance between the point of immersion and the axis of rotation, through simple geometry, the angle of rotation of the boat was reduced and so the occupants free less rotational motion as well as vertical motion since they are also closer to the axis of rotation and so rise a shorter distance for any given pitch angle.

Similarly, out of the science the cross sections of the hull forms had changed as well. Hull forms went from the large flat areas of the earlier designs to more elliptic sections which progressively increased in form stability with roll angle and so progressively dampened roll as well as ‘sensing’ the shape of the wave face and helping the boat to remain in phase with the waves and minimizing the kind of snap rolling that was more common on earlier lighter displacement for their length designs.

The roll moment of inertia was increased by placing larger bulbs on deeper draft keels. That slowed the natural roll rate, while the weight carried low developed leverage working counter to the roll and so further reducing roll angles. Deeper length, shorter chord foils also provide greater dampening as their sideward motion more effectively generated moments resisting rolling with out adding as much drag inducing wetted surface as would be the case with a shorter aspect keel would need to generate the same roll dampening.

And the net result was that as these ideas filtered into cruising boat designs, it resulted in some wonderful cruising boats. Boats that are fast and forgiving, with comfortable motions and so on.

But what I had not counted on was the ability to designers to employ computer design, to produce wildly beamier boats, which ride on the turn of their bilge in semi-displacement mode, effectively behave closer to a multihull than a monohull, and which easily plane when there is enough wind. Without computer modeling, I think it would be next to impossible to create tame versions of these boats, but with the use of twin rudders and sophisticated dynamic trim modeling, designers have been able to produce successful versions of this concept.

Still and all, I have very mixed emotions about these designs. Clearly they are very fast, and properly designed can be moderately easy to handle, but I am not convinced that these are well rounded designs, or that they make sense as cruising boats as least at the skill levels of most sailors out there.

But I also want to touch on a couple of the anecdotal items that the thread accumulated. SloopJB mentioned a quarter tonner that he owned that had a very comfortable motion. I, like many people, toss around the term IOR design, like the term describes a single hull form and rig. But in fact, the IOR evolved over a relatively long period of time, so that it is kind of like the term Victorian Architecture, which when really considered carefully really was a broad collection of styles. And in the many variants of the IOR era, there was a surprisingly wide range of hull forms. What SloopJB seems to be describing is what I called the ‘clamshell period’ in which the boats had vee’d sections from bow to stern. Good examples of that might be the Seidelmann 25 or the Tanton Quarter tonners of that era. Whatever their other attributes, going upwind in a chop these boats did display less pitching and a gentler start and stop to the pitch than might be expected for their weight and timeframe. To some extent the basis of that gentler motion is that when seen in section they progressively built buoyancy in much the same way that early IMS era designs progressively built buoyancy with their fine bows when seen in plan.

As to Wolf’s physics defying Atkin’s, I would say this, Atkins was a master at modeling heavy displacement hull forms in a way that went far beyond what the numbers might predict. But even Atkins was not a miracle worker. The laws of physics do eventually kick in. So while it may be true that Wolf’s boat performs better than might be expected, and it may be true that it performs better than some modern designs, especially if they are not skillfully handled, I would respectfully suggest that Wolf’s Atkins will not out perform the better designs of the past 30 years. I can tell you that as the wind builds even a design as dated as my own boat more quickly out distances more traditional designs as the wind builds, and newer designs clean my clock. When it comes to winds of 20 knots or above, its pretty easy to sustain close to 8 knots upwind at a much higher pointing angle than a similar length traditional design, let alone a similar displacement traditional design. I have a pretty easy time reaching at a sustained 8-10 knots in wind speeds of that range with brief surfing well above that. You just cannot do that with a traditional boat of the same displacement or length.

And similarly, having owned traditional boats of the same displacement as my boat, from a motion comfort standpoint, a well drawn more modern design will offer far superior motion to a traditional design of the same displacement.

The fact is that boat design has evolved not through marketing per se, but through a better understanding of the science which allows a well drawn modern design to offer greater speed, motion comfort and a lot more space for a given displacement and or pricetag (since displacement almost as much as anything else controls cost).

That said, I would rather look at Wolf’s boat rather than mine any day, but if I wanted to sail anywhere, I’d still take mine…..

Respectfully,
Jeff
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Last edited by Jeff_H; 04-09-2013 at 03:49 PM.
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