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Old 09-14-2003
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Confused about overhangs...

(Warning! This is a long one)

I respectfully disagree with the implication of Paul''s conclusion that racers go for speed, not comfort and that the current crop of race boats offer uncomfortable motions because they have short ends. In fact, much study has gone into minimizing pitch and roll angles, AS WELL AS, harsh accelerations in the current crop of IMS type form boats. It may be that the prime interest is in improving the continuity of flow over sails, keels, rudders but the result is a very comfortable motion compared to race boats and race boat infuenced cruisers of the past. In the case of the better modern designs, great effort has been extended to decrease the amounts of inertia within the boat (your tight rope example is a high inertia model) and then use smaller dampening forces that progessively develop so that the accelerations can be much smaller. The fine ends on modern boats means a much smaller impact with waves and a more unformly progressive dampening of pitch than would occur with a long ended boat.

To address the questions contained in the original post, I do not know who told you that an Alberg hull (like the Pearson/Alberg 35) has good sea going motion attributable to their long overhangs but they are mistaken. I cannot emphasize too strongly that that long ends were an unhealthy, and excessive design tend that came out of efforts to beat the racing rules of the era (in the case of Alberg''s designs the CCA rule). In its day, this was a rule that was seen as producing slow, un-seakindly and un-seaworthy designs especially during the era when this rule had reached its most extreme. That said, these designs were seen as being seaworthy and offering a comfortable motion only as viewed in light of the excesses of the IOR rule that followed. On any objective these old short waterline CCA boats had a miserable motion.

Having sailed much of my life in long overhang boats in a wide range of conditions, as well as, on more traditional short ended craft and on the current crop of more modern offshore designs, both offer considerably more comfortable motions from the standpoint of rate of acceleration and from the amount of motion, present. Comparatively speaking these long ended boats have pretty poor motion comfort and their motion discomfort problems to a great extent result from their long overhangs and short waterline length.

Alberg 35''s on particular tend to hobbyhorse (pitch) miserably with a quick jerk at either end of each rotation that occurs when the fullness in their bow and the flat of their counter collides with each wave. They also tend to be real rollers, which comes from having a low center of buoyancy and comparatively high vertical center of gravity (exactly backward of what is desirable for motion comfort.) While the height of vertical center of gravity results solely from the construction of the boat, the low center of buoyancy is the result of trying to cram the necessary displacement into a very short waterline. This means a deep canoe body and that means a rolly motion.

I think it is a misnomer to call these long overhang boats ''Traditional''. The really are not when seen from a longer point of view. Long overhangs strictly result from loopholes in racing rules (and before that, shipping measurement rules) and only linger as aesthetic affectations. If you look at traditional working watercraft that worked in areas where they had to deal with rough sea conditions they almost never had long overhangs. Similarly if you look at modern offshore racing craft, where they also are no longer shaped by a racing rule, they never have long overhangs.

In a more general sense, long overhangs do nothing good for the motion comfort of a boat. To explain the physics of motion comfort of long overhangs, there are two aspects of motion that affect the perception of comfort of a boat''s motion. These basically are the change in speed of motion (acceleration and deacceleration) and the amount of motion. In US Navy studies of motion fatigue and motion sickness, there was a near equal split between those that can tolerate large amounts of movement but cannot tolerate rapid accelerations, those who cannot tolerate large amounts of movement but can tolerate rapid accelerations and those who can''t tolerate either.

When you talk about rotational motion (pitching, rolling and yawing) which are the primary causes of discomfort, short waterline boats tend to swing through wider angles than longer waterline boats of the same displacement. There are a lot of reasons for this but the basics deal with moments of inertia (momentum) and dampening (the forces quelling that momentum).

To explain moment of inertia in rotational terms, it is the combination of the each weight involved and that weight''s distance from the center of rotation. The bigger the weight and the further away from the center of rotation, the higher will be the rotational moment of inertia (momentum) of the object. So placing and anchor on the bow of a boat will generate a lot of inertia. Placing that same anchor on an extension of that bow will create even more inertia. Using a heavier anchor will also increase the inertia as well.

An object (like a boat) with a large moment of inertia will tend to accelerate more slowly but develop greater kinetic energy causing it to require a greater force to stop that rotation and therefore swing through a wider angle. A boat with a small moment of inertia will tend to accelerate more quickly but typically though a smaller angle.

In both pitch and roll directions, a centralized and low vertical center of gravity really helps in the kinetic energy department. When there is a lot of weight in the ends or high on the boat, this weight develops a lot of kinetic energy in the direction of the motion. Objects that are above the axis of rotation, the force of gravity works to dramatically increases the tendency to rotate further. Keeping weight low and out of the ends means that is a lot less energy trying to make the boat rotate in the direction it is moving, and the force of gravity works against the force of rotation on those items that are low in the boat.

The reason that the angles of rotation vary has to do with the forces that arrest the rotational motion. A boat would spin in a circle if there were no other forces opposing this rotation. Gravity, in much the same manner as a pendulum, comes into play to arrest the swing in one direction, but in doing so the kinetic energy is stored at the end of the swing, and without dampening of some kind the pitching and rolling would continue indefinitely. To one degree or another, dampening comes in the form of frictional resistance of the water against the hull, the aerodynamic dampening of the mast and rigging arcing through the air, the hydrodynamic dampening of the keel and rudder as they rotate through the water, and the shifting of the buoyancy in the hull (both athwartship, and fore and aft).

When you talk about motion comfort, the ideal damping increases progressively, but at a uniform rate of increase so as to not create a jerky motion yet arrest the amount of movement quickly. This is where long overhangs come into play. In terms of pitching, the weight of the bow and stern overhangs hang past the waterline and create a long lever arm for the weight of the hull, hardware and anything else contained in these regions. Comparatively speaking, there is a higher pitching moment of inertia for a boat with longer overhangs as compared to a boat of equal weight and waterline, but shorter ends. The primary means of dampening pitch comes in the form of a shifting of the center of buoyancy in relation to the center of gravity. (This is also the usual definition of form stability, which is more normally associated with athwartship stability.) In a long ended boat, the shift in buoyancy tends to be abrupt as overhanging portions of the hull are submerged in the water. Alberg''s designs are particularly notorious in this regard, as they tend to have a lot of fullness in the ends just above the waterline.

In terms of rolling, obviously one way to arrest a rolling motion is with form stability. An equal weight, ballasted plank laid flat would roll through a narrower angle than an equal weight, ballasted cylinder. But arresting motion with high form stability comes with a price; that price being a much quicker, jerkier motion. From a motion comfort viewpoint, you do not want to round a bilge because you would roll through a wide angle or too flat a bottom because of too jerky a motion. (BTW Multihulls are extreme form stability vessels which is why they hardly roll but their motion tends to snap back and forth through these small angles.) Traditional working watercraft (with some notable exceptions), CCA designs, and the current crop of IMS type form offshore racers and the designs influenced by the IMS type form tend to have a comparatively low form stability.

Fortunately there are other ways to dampen roll. One of the best is rotating a tall rig through the air and the comparatively flat sides of a deep keel through the water sideward. Here again long overhang boats do not do that well. If you take two boats of equal length and displacement but one has a shorter waterline, the boat with the shorter waterline would have to have a deeper canoe body and/or fuller submerged ends to carry the weight. That means a hull that is rounder in section (more prone to a wider roll angle) and with less dampening surface than a boat with a shallower canoe body, and equal draft.

The reality of long ends is that, while they are beautiful to look at they are not a great idea for an offshore cruiser or even a coastal cruiser for a venue where you are likely to encounter a lot of chop.

Respectfully,
Jeff


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