Join Date: Feb 2000
Location: Annapolis, Md
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How heavy is too heavy II ?
You keep focusing on the weight aspect of the Righting Moment (RM) without looking at the generally negative changes in the righting arm (GZ) that is implicit with changes in weight. It is way too simplistic to assume that more weight is a good thing for stability.
Focusing simply on weight ignores that relationship between adding weight and the GZ of a boat. Generally speaking when you add weight to a boat the Vertical Center of Bouyancy (VCB) moves lower which reduces the RM of the boat by reducing the GZ. This can be mitigated a little but it comes at the price of a wider waterline beam which results in a quicker motion.
Also you do not seem to be focusing on the vertical location or typical sources of that additional weight. If the weight is added in a manner that maintains the same relationship between the Vertical center of Gravity (VCG) and the VCB so that the GZ remains constant, the ratio of ballast to displacement, as well as, the draft of the boat would need to increase. This would be the case because the vertical center of gravity would need to be lowered by the same amount that the vertical center of buoyancy was being lowered.
What generally happens in cruising boats is that the weight increases disproportinately to the increase in ballast and draft (in fact mosy cruising boats usually have less daft than higher performance boats.) And most of that weight increase occurs well above the VCB in heavier interior appointments, heavier deck hardware and heftier rigs to deal with the higher stresses that come from added weight, increased storage above the CB, larger engines and tankage and so on, all of which generally raise the VCG.
The Swan 46 to Swan 45 comparison is a good one. In the case of the Swan 45 (in contrast to the 46) all of the interior case work and components are very light, veneered cored material so as to reduce weight and the rig is proportioned to be comparatively light so as to reduce weight aloft. Deck hardware is proportioned to the lighter loads. Storage above the waterline is comparatively minimal. Freeboard is compartively low. All of the weight saved by these measures were placed in the bulb located very deeply at the bottom of the keel so in comparason the 45 has a much lower VCG and with its lighter weight a higher VCB. In other words you would expect the Swan 45 to have a similar or larger area under its righting curve. With its lower VCG and its better dampening relative to momentum, you would also expect it to have a more comfortable motion on five of the six degrees of motion, with heave being the exception.
When you look at the average cruising boat the VCG is generally at or slightly above the VCB in the static position. Small changes in the vertical height of either the VCG, or the VCB, or both can dramatically change the GZ so that owering the VCB even as little as several inches (50mm) can often result in a doubling or tripling of the GZ and consequently the RM. It is hard to get that kind of increase in the RM by increasing weight because of the attendant consequences to the VCB mentioned above.
The other side of this is the overturning moment, which also generallty decreases proportionate to weight so that the lighter boat will also have a significantly lower overturning moment than the heavier boat and therefore typically requires proportionately less RM.
One ofter point here, modern high performance boats have an amazingly comfortable motion compared to older performance boats of the IOR and CCA eras dispite being considerably lighter in weight. One of the advances of the early IMS was a careful study of motion. Early on in the IMS era it was determined that the kinds of motion that are uncomfortable for crew, is also really bad for performance. Quick or large amounts of motion tend to disrupt hydrodynamic and aerodynamic flows and so slow the boat and increase leeway. Designers quickly discovered that improved motion was an un-handicapped element within the rule that could add a lot to performance and so modeled boats to minimize both the amount of motion and the acceleration/ deacceleration during impact with waves, and during rotational motion. This has resulted in race boats that offer an amazingly comfortable motion as compared to earlier performance boats. There was nothing worse than slamming to windward in short chop in an IOR era boat despite their substanially greater weight.
(I need to finish my sandwich and get back to work.)