Join Date: Feb 2000
Location: Annapolis, Md
Thanked 95 Times in 71 Posts
Rep Power: 10
This is a bit long and a bit of the subject but from the way that you are asking this question, I am assuming that you are not familar with rating rules. So to start with a crash course in rating rules, there are a number of ways that sailboats can race against each other. The most fundamental test of the individual sailor is racing in a One-Design Class. In one design, all of the boats are theoretically of the same design. (Example, Lasers, Sailfish, J-24''s) Individual one-design classes have rules that more or less restrict what can be changed on the boat and still conform to the one design class rules.
Then there are ''box'' or ''Development'' classes in which there is a rule that allows some variation between the boats and which encourages development of faster boats. (Example: International 14''s, 12 meters, America Cup Class, or Volvo Round the World 60''s.)
In order to race dissimilar boats against each other there has to be a way to adjust for the inherent relative speed of the boat. There are a variety of ways this is done. First of all there are VPP (velocity prediction program) based systems. (IMS is the best known of these) These rating systems measure as many of the speed controlling factors as are currently understood and then tries to predict the relative speed of the boat. Because some boats excel in strong winds and others are optimized for light air, boats racing under a VPP based rating rule have a number of different ratings that attempt to correct for different abilities in different wind speeds and different points of sail. These are by far the most accurate of the rating systems but they are also the most complex to measure and administer.
As good as VPP based rating rules are at handicapping similar boats, they cannot fairly adjust for a fast boat''s speed advantage in changeable conditions. VPP based ratings assume a pretty constant wind speed and direction. In those conditions the relative speed of the boats are easy to rate. But in changeable conditions a faster boat can afford to sail a little extra distance to get into more favorable sailing conditions. Because of that a faster boat can leverage its speed advantage in a way that no rule can adjust for.
There are abbreviated forms of these VPP based formulas, (example Americap). They greatly simplify the measurement and scoring process but at the price of accuracy and fairness. For example, they eliminate the multiple ratings for different wind speeds. As a result, the boat that is optimized for light air will be hard to beat on a light air day, and the heavy air boat will be hard to beat on a windy day.
Then there are simple measurement rules. These use a greatly reduced number of measurements to try to predict the relative performance of a boat. (Example: CCA, IOR, and MORC) The problem with these measurement rules is that the measurements are taken at very specific locations and in very specific manners so the rigs, and hulls are distorted to try to beat the rule. These strangely distorted vessels are referred to as ''rule beaters'' and they are shaped by a edict of man rather than the common sense of the sea.
When you get past all of that you have PHRF (Performance Handicap Rating Formula). PHRF is not a measurement rule in any shape or form. PHRF looks at past performance over a period of time and from that past performance a single rating is assigned. PHRF ratings will vary with region since they are supposed to be adjusted to the particular boat''s performance in the prevailing conditions. They vary with politics of the regional PHRF board. And the Vary with what I will call ''typicals''. By this I mean that some boats are clearly designed for speed and so typically attract experienced sailors who further optimize the boats for speed by having a well-maintained hardware, a smooth bottom and good sails. The past performance reflects this fact. Then there are boats that are not really intended to be race boats. The typical person who would buy one of these boats really does not care about speed and so typically these boats have not been prepped or optimized. Just like the high performance boat their typical past performance reflects this fact. So if you buy a high performance boat and don''t optimize it, it is hard to beat your rating or if you buy a slower boat and really optimize it you can really beat the rating pretty easily. That''s what I mean by beating the ''Typcials''.
The PHRF number that you see reflects the relative time expressed in seconds that any boat is supposed to be faster or slower than a 12 Meter Class Yacht. (The original PHRF of zero was assigned to an 12 meter class America''s Cup Boat.) a boats with a PHRF rating of 198 owes a boat with a rating of 174, 24 seconds a mile which, in the world of cruising or racing, translates as actually a whole lot of time as things turn out.
A boat that is anything longer than about 23 feet with a rating of 198 is really pretty slow.
You also asked about L/D (Length to displacement ratio). The L/D ratio is a way of equating the relative weight of a boat to its waterline length. The formula is a bit more complicated than simply dividing the length by the weight. This is done so that a fair comparison can be made of boats of differing lengths. While the definitions vary from source to source, in a general sense, an L/D under 100 or so is called a ULDB (Ultra Light Displacement Boat), 100 to 130 you some times see as a VLDB (Very Light) or LDB (simply Light Displ). 130 to 160 is generally referred to as moderately light and 160 to 200 is seen as moderately heavy. Over 200 is generally seen as heavy with anything over about 250 is seen as very heavy. (As I said before there is no universal agreement but these ranges should give you are relative sense of this works.)
So a D/L of 245 suggests a very heavy boat. This combined with SA/Disp (sail area to displacement ratio) are good predictors of a boat''s general performance. A boat with a high D/L would tend to be slow in light air and in heavy air, with its best performance somewhere in between. and would tend to roll a bit more than a lighter boat which would tend to have a quicker motion. Weight in and of itself does nothing good for a boat. It does not make a boat stronger, or more stabile, or more comfortable. It is simply weight! Weight means higher stresses and loads on the various parts of a boat. It means having to carry more sail area to achieve equal performance. Which brings us to SA/Disp. To have good performance in light to moderate conditions a boat needs somewhere around and SA/D over 20. On a heavy boat that is a lot of sail area.