Join Date: Feb 2000
Location: Annapolis, Md
Thanked 211 Times in 163 Posts
Rep Power: 10
I think that this has turned into a good discussion of this complex issue. I did want to touch on a couple of Keelhaulin's discussion points.
He is correct in saying that many classes such as the Volvo Series boats, and open class boats are LOA driven, boats racing under the old IMS and IRC rule, where we first saw race boats with nearly plumb bows, are measured in a way that that essentially digitizes their lines above and below the waterline and attempts to predict the performance of that hull at varying heel angles.
In the case of these IMS/IRC designs, their near plumb bows had little or nothing to do with downwind performance. At the time that the IMS became popular, there was a feeling that there was little that a designer could do to beat the rule in terms of rig or hull design. Consequently, designers began to look a motion as an under penalized area of design.
It had been known that large roll and pitch angles and sudden motion changes interrupted the flow over sails and underwater foils hurting performance upwind and close reaching. A lot of effort went into studying motion dampening and the results of those studies really changed the hull forms of race boats. The biggest impact of the research was on the cross sectional shapes employed, movement of the longitudinal center of buoyancy further aft, the use of finer entries and near plumb bows, lowering of the VCG, and careful design of foils to increase dampening roll dampening. The real gain on the race course with these fine bows occurs upwind and when power reaching rather than downwind and when sailing in a shorter length wave, or in vertical seas.
As these near plumb bows are rendered in the current generation of race boats, they have U shaped sections that are moderately flat on the bottom and so the more extreme versions of the current crop of IRC race boats will pound a bit on a hard beat in short seas.
Cruising boats eventually began to pick up on the potential for improved motion comfort that came out of these newer hull forms. But unlike the shapes employed on purpose designed race boats, as these motion controlling principles have been carried into cruising boats with some moderation, particularly at the bow, where cruising versions tend to have more flare in the forward sections, tend to have Vee'd sections below the waterline, and tend to have a deeper forefoot. All of which means cruise versions have a drier ride and a more comfortable motion than would be found in the full race versions, where wetted surface mitigates against vee shaped sections.
I do think that Keelhaulin may have missed the point that I was raising about displacement of new vs. older boats. The point that I was bringing up was that there is a perception based on L/D numbers that modern performance cruisers are substantially lighter than the boats that preceded them.
As I was trying to point out a modern 35 footer is not all that much lighter than a CCA era 35 footer (perhaps 5%-10% at the most, and often the newer design is actually heavier) but they tend to have substantially longer waterline lengths and substantially lower VCG's, both of which would tend to make the boat more seaworthy, have motions that are easier on crews and increase carrying capacity as compared to an earlier era equal displacement/shorter waterline boat.
The point that I was making is that there is a problem that arises in comparing equal weight boats modern boats to older designs using these older formulas. The problem is with the expected values which were based on older style short waterline boats. When you apply these standard range of values to modern designs with their long waterlines, modern boats appear to be much lighter than they are and so also appear to be less seaworthy.
I gave a similar example above, but just for the sake of clarity if we compare say a modern 35 footer to a CCA era 35 footer, both would weight roughly 12,000 to 13,000 lbs empty and would have similar weights fully loaded, but the modern boat could easily have 5 or more feet of waterline, both static and heeled. But since the LD, CSF, and MCI use waterline length, the equal weight modern boat would appear to be extremely lighter and less seaworthy, when in fact it is the same weight, and if properly designed, more seaworthy.