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Modern Hull forms and Motion Comfort
To kick off this series of discussion topics, I am posting the following article:
Modern Hull Forms and Motion Comfort
By Jeff Halpern
(This is a comparatively long discussion)
Over the past 10 -15 years hull forms have changed dramatically. In the case of the IMS (IRC) typeform that is influencing many of the new breed coastal and offshore cruisers, the longitudinal center of buoyancy has moved aftward. Cross sectional shapes have changed from the semi-circular, or hard-chined MORC inspired hull shapes, or IOR three-part cross sections to vee’d sections forward that merge further aft into elliptical shaped cross sections with softer bilges than had been popular. Keels have gone from the IOR era delta to shorter chord length keels with large bulbs. Entries have become much finer and plumb or nearly plumb bows have become increasingly popular. These changes from came rather quickly, resulting in a question about the relative merits and liabilities of these design features.
To begin with it may be helpful to look at a bit of history and more specifically, the source of this rapid change. Many of these same features were Although plumb bows, soft bilges, and centers of buoyancy located proportionately far aft in the boat have been with us nearly from the beginning of yacht design history (for example, look at a set of line drawings for the yacht America or a Friendship sloop, ignoring their trailboards and bowsprit made necessary to support their 19th century sail plans) and were the norm for cruising yachts and working water craft in the early 20th century, finer entries, soft chines, and plumb bows began to show up again in large numbers in the late 1980’s and early 1990’s. The primary impetus for this current trend was the result of research leading up to boats designed around IMS rule.
Unlike earlier measurement rules which measured boats at a limited number of identifiable measurement points, and whose formulas were widely known and so could be ‘beaten’, the IMS was a velocity prediction program derived formula that measured the boat and created in effect a prediction of performance based on its actual shape. The formula that produced the rating was kept secret so that designers were unable to push specific parameters to beat the rule.
Unlike earlier rules where a slower boat with a big rating advantage could win, in the early days of the IMS, there was no advantage to designing a slow boat because you could not cheat the rule. Long overhangs offered no advantage under the IMS in terms of un-rated speed. In fact, the IMS rule precisely understood the extent to which a short waterline was a liability since much of the prediction formula testing had used contemporary short waterline hull forms.
Which is not to say that individual designers did not seek ways to produce faster boats than the rule could predict. One reality of racing is that a faster boat generally has a strategic advantage out on the racecourse over a slower boat in its class because the faster boat can sail a little further and therefore take strategic advantage of being sailing on the fastest portion of the course (i.e. more wind or more favorable current). Longer waterlines, for a given boat length, results in less induced drag and higher speeds through the water. But the strategic advantage of that extra speed is only small part of why plumb bows became popular.
In looking at other un-rated advantages, designers began to look at motion as an area that no rating rule had previously considered. Studies in the wake of the Fastnet Disaster had lead designers to understand that IOR era race boats (and to a lesser extent earlier CCA era boats) had serious motion issues that were beating their crews to death in any kind of rough going. But beyond the comfort issues of this motion, designers also came to understand that large ranges of motion and rapid changes in motion direction had a real negative impact on the air flow over the sail and water flow over the keel. As sails and keels became more efficient but easier to stall, this issue of motion control offered real opportunities for improved performance for the sails and keel; performance that would be un-rated at that.
Designers and research scientists quickly identified three forms of motion that could easily be addressed through the careful weight distribution and hull modeling, namely: rolling, pitching and wave collision. It was always understood that the rate of motion could be slowed by adding inertia. In the case of angular motion, roll and pitch, inertia is actually a moment meaning that the weight is multiplied by the length of the lever arm (squared or cubed depending on the type of calculation) from the axis of rotation. The further from the instantaneous axis (I use the term ‘instantaneous’ because the roll and pitch axis changes as the shape of the hull in the water changes with movement), or the larger the weights involved, the greater inertia that the vessel will have. With greater inertia comes a slower acceleration of the motion.
But like everything else in yacht design, high moments of inertia come with a price. A greater moment of inertia also stores more kinetic energy. So while a vessel with a high moment of inertia has a tendency to change direction more slowly, it also has a tendency to move through a larger range of motion. From a performance design standpoint, (or from the standpoint of crew comfort for that matter) this larger range of motion is also undesirable.
To some extent, the source of the inertia is important as well. For example, we can compare two otherwise exactly identical boats, each with equal roll moments of inertia, but one has a heavy mast but less ballast by the same amount added to the rig while the other has a lighter mast and heavier ballast. The high vertical center of gravity of the mast would tend to cause the first boat to roll further than the second boat for an equal roll loading, because the weight in the mast would be on the motion side of the roll axis while the weight in the keel would be on the opposing side of the roll axis.
Speed of motion and range of motion can be offset by what designers call dampening. Dampening is the ability to absorb and resist undesirable motion. Dampening is creating a resistance against motion that reduces the amount of kinetic energy that can be stored by reducing the amount of motion in any one direction. From either a performance standpoint, or motion comfort standpoint, dampening ideally needs to reduce the rate of change as well. To do so, dampening needs to be progressive, rather than sudden. If dampening occurs too suddenly too effective the boat will jerk as forces attempt to produce motion and the boat fetches up against the entity providing the resistance. (Pounding is a good example of that) In concept, controlled progressive dampening is exactly how shock absorbers are supposed to work on an automobile.
With regards to individual types of motion, designers of IMS type boats began to address roll motion in a number of ways. First of all, the typical IMS typeform reduced form stability from the excessively high levels popularized in IOR boats, ‘Open Class’ boats and to a lesser extent in MORC derived designs. But beyond reducing the initial stability of the IMS designs, they also focused on a progressive increase in form stability. In other words, as a boat healed, it would initially increase form stability progressively rather than all at once as had been the case with the harder chined light weight boats of earlier generations. Early IMS type form boats don’t heel until they suddenly fetch up on chine and stop heeling all at once. Instead they slowly and progressively generate form stability over a moderately wide angle.
As a result of the reduced form stability, when IMS type form encounter a wave there is less roll forces exerted on the hull and similarly, because the form stability comes on progressively over a wider angle, these roll forces do not exert as sudden a loading on the boat.
These reduced roll loadings are further reduced by careful dampening. In the case of IMS type forms, their tall rigs and deep keels provide high dampening lever arms and as a result, despite their smaller sail and keel areas, proportionately large roll resistance moments of inertia.
Which brings us to fine bows, plumb stems and the afterward re-location of the center of buoyancy popularized by the IMS typeform. If you have ever beat or motored to windward in a short chop, try to remember the motion that you felt as the boat encountered each wave. Initially, there was a moment as the boat collided with the wave that to one degree or another you could feel your body thrown forward as the boat de-accelerated. And while you were feeling that force, you could feel the bow being be jerked upward, the whole boat rotating about the pitch axis of rotation. Then you could feel the boat heave (moving vertically) as the wave passed under the center of buoyancy. There is a brief moment during which the boat seems to hang in the air, before the bow begins to rotate downward. And lastly, you felt the bow jerk to a halt or even upward as it hits the back of the wave below.
In that description, are three of the six forms of motion; heave, negative surge, and pitch. Heave is heavily dependent on the size of the waves relative to the length of the waterline of the boat. There is little that can be done about heave once the wave spacing approaches the length of the boat. A lighter boat will rise and fall with the wave with very little lag time, essentially moving vertically at the speed and height of the wave. A heavier boat will also rise and fall with the wave with a slight delay between the time that the wave hits the boat and the time that the boat starts to rise. This delay can actually cause a heavier boat to move through a larger range of motion (its momentum carrying it higher out of the wave crest and deeper into the trough).
This increased range of travel may also result in less harsh changes in vertical direction at the top and bottom, but not always. Depending on the size, frequency, and configuration of the wave, and the delay in movement of the heavier boat means that it is coming down farther back in the wave, falling further before being fully supported by the wave again. That increased distance means a greater speed and momentum at that point in the cycle and so the heavier boat may actually have a greater de-acceleration when it bottoms out.
What can be designed around is the amount of impact force imparted into the hull by the collision with each wave, and the amount of change in speed with which the boat pitches. To begin with, visualize two boats with equal displacement, equal longitudinal centers of gravity, the same deck shape when viewed from above and in profile, same depth of canoe body, same mid-ship cross section, and the same reserve buoyancy (meaning volume of the hull above the waterline forward of the center of buoyancy), but one has a plumb stem and the other has four feet of overhang at the bow. In this example, by its very nature, the boat with the overhang will be more blunt, less knife like, than its plumb stemmed sister. Instead of cleanly slicing through the wave, the boat with the overhang, actually collides with the wave with a greater impact force, and that impact force, both slows the boat down, and also is wearing on the crew.
Because both boats have equal reserve buoyancy, the deck will stay equally dry, but because the waves act more suddenly upward on the blunter ends of the boat with the longer overhangs, there is more concentrated rotational force imparted and applied more rapidly (i.e. more of a collision than a gradual application of force) into the forward end of the boat with the longer overhangs. All other factors being equal, greater concentrated force applied further outboard forward means a greater rotation angle; more rapid application of force means a more rapid change in direction. Greater rotation angle and greater speed of motion means a larger flow interruption over the sails and foils and less comfort for the crew.
On the boat with the modern hull form, as the bow moves upward, the fuller stern sections build reserve quicker than the overhanging sterns of more older hull forms and that earlier progressive building of buoyancy serves to further dampen the speed and amount of rotation.
At this point the boat with the longer ends is moving upward at a greater speed and will have greater kinetic energy causing it to rise higher than the shorter overhang boat. Because it has farther to fall, and the acceleration of gravity is a constant, it will hit the water later on the wave, and with a greater impact force.
Another factor that further improves the motion of the IMS type form is that the center of buoyancy is located further aft. If you visualize two boats having their bows lifted an equal distance by a wave, but one boat rotates about a point that is further forward than the other boat’s axis of rotation, the angle of rotation on the boat with the longer distance to the axis of rotation will rotate through a smaller angle than the boat with the axis of rotation located further forward. And since the boat with the center of buoyancy located further aft ends up with not only small rotation angle, but with a shorter distance from the axis of rotation to the cockpit, there is less vertical distance experienced by the crew in the cockpit.
Last edited by Jeff_H; 10-18-2007 at 10:09 AM.