Nice to see that you are still posting
high quality, general information posts.
As you probably recall
, I am always cautious about relying on the 'numbers' when comparing boats. They really provide almost no useful information unless viewed judicuiously with regards to all of the other factors that contribute to the behaivor of a boat.
I do want to touch on a couple specific points in your posting
. In a general sense this information seems to be an older interpretation of some of these ratios than the current
thinking. I would say that much of the descriptions included would be true of boats built perhaps 20 years ago, but are less true of the better modern designs. To touch on this more specifically:
Displacement/Waterline Length - D/L :
The most recent scale on relative D/L has changed to reflect the current
trend in longer waterline lengths over older boats. This trend would make it seem that boats have gotten much lighter than they actually have. Boats of a given length on deck
weigh pretty much what they did 10- 15 years ago but their waterline lengths are much longer. When looking at more modern designs the scale looks something like:
ULDB: 90 - 110
Very Light: 110 - 140
Moderately light: 140-180
Moderately heavy 180 - 200
Heavy Over 200 or so
I also disagree with some of the statement:
"In general, a boat with a lower D/L will have better light air performance for a given sail area but it will be more sensitive to loading, likely to have a less comfortable ride in a sea and will likely need to shorten sail sooner."
This is not true when comparing the compartatively shorter waterline boats of 10-15 years ago to the newer longer waterline boats. Even though the longer waterline boats would appear to be lighter because of their lower D/L ratios, they are actually of very similar weights. That being the case they are generally more comfortable in a seaway, and can carry sail area longer than an equal LOA
, equal weight boat on a shorter waterline. Similarly, given their longer waterline lengths they can often carry more load as a percentage of their displacement than a boat than an equal LOA
, equal weight boat on a shorter waterline. In otherwords given the changes in design that have happened in the past 10-15 years you cannot simply compare L/D ratios and expect to have accurate information regarding senitivity to loading, need for sail changes, or motion comfort.
and Motion Comfort:
Both of these formulas are worse than useless. I know that I have explained this on this forum before but here it is again, both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have extremely limited utility in comparing boats that are not extremely similar.
Neither formula contains almost any of the real factors that control motion comfort or seaworthiness. Neither formula contains such factors as the vertical center of gravity or bouyancy, neither contains weight or buoyancy distribution, and neither contains any data on dampening, which really are the major factors that control motion comfort or likelihood of capsize.
I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 1000 pound weight at the top of the mast
. (Yes, I know that no one would install a 1000 lb weight at the top of the mast
.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen
formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth. That is why I see these formulas as being worse than useless.
Sail Area to Displacement:
Again how these numbers are assigned is somewhat misleading. They would be reasonably accurate for older designs which has comparatively high vertical centers of gravity and were designed to carry very large genoas.
These numbers are quite small for newer designs which tend to have comparatively low vertical centers of gravity relative to the vertical centers of buoyancy (which comes from the more frequent use of bulb keels and the shallower canoe bodies permitted by the longer waterline lengths.) The numbers are also low because better sail handling gear
and greater stability has allowed boats to stand up to a larger standing sail plan without having to shorten sail. The better modern designs are generally being designed to operate with minimally overlapping headsails which make for easier tacks, wider wind
ranges, and greater flexibility.
I would expet the assignement of these ratios on more modern designs to look more like:
Motorsailers 10 - 13 (underpowered)
Cruisers 12 - 18
Performance Cruiser-18 - 21
Racer-Cruiser 20 - 22 (high performance)
Racers above 22.
High-Performance Racers have ratios above 24
Again, this is an area where we see the 1.34 times the square root of the waterline length tossed about like it is gospel. The reality is the more moderate versions of the newer finer bow, fuller stern types have proven to be closer to 'semi-displacement' type hullforms and so routinely sail in a range of 1.5 times the square root of the waterline length.
In the most past issue of Sailing World there was an interesting couple paragraphs dealing with theoretical hull speed which touched on this very issue. I am quoting here:
“Waterline’s affect on hull speed is theoretical and not absolute. As a hull goes faster, the bow wave stretches to the point where the bow and stern wave become on wave cycle, whose wavelength is equal to the waterline length. This brings us to wave theory. “
“The speed of a wave (in knots) is equal to the square root of the wavelength (in feet) multiplied by 1.34. If your boat has a waterline length of 32 feet, the theoretical hull speed is 7.6 knots. The waterline length is thought to limit the hull speed because if the boat goes any faster the stern waves has to move further back taking the trough between it and the bow wave along with it. As the trough moves aft, it causes the stern to drop, making the boat sail uphill.”
“Except for planning designs, sailboats typically can’t generate enough power to go any faster and climb their own bow wave. But a boat with extra volume in the stern can exceed its theoretical hull speed because the extra bouyancy prevents the stern from dropping into the trough. By the same token, a fine-ended design might not achieve its theoretical hull speed if buoyancy in the stern is insufficient.” (Written by Steve Killing and Doug Hunter).
It was also thought that older designs which had longer heeled waterlines than their upright waterlines would sail at 1.34 times the square root of their heeled waterline length. That simply has not proven to be the case. While there may be some modest gains in speed when heeled, the gain is minute when compared to the speed of a boat with waterline length equal to the heeled waterline length of the boat with long overhangs.