Boat numbers and performance - SailNet Community
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Boat numbers and performance

Hello,

I am fairly new to sailing and I''m trying to learn more about boat designs as it pertains to performance. I have been researching lots of different racer / cruisers in the 34-36'' size.

What is the relationship between things like LOA, LWL, Displacement, Sa-D, and hull speed relate to performance (as measured by PHRF ratings).

For example, according to Sail Calculator Pro (http://www.image-ination.com/sailcalc.html), a catalina 36 has these numbers:
LOA - 36.3, LWL - 30.25, displacement - 13500, sail area - 555, SA-D - 15.66, hull speed - 7.37, and PHRF rating of 138.

A C+C 36 (late 70''s to early 80''s) has these numbers:
LOA - 35.7, LWL - 27.7, displacement - 12000, sail area - 600, SA-D - 18.32, hull speed - 7.05, and a PHRF of 129.

Why is the C+C rating faster? Is it because the SA-D is higher?

Comparing a catalina 34 and C+C 34 (again, early 80s) is similar. The Catalina has a greater LWL, greater hull speed, 2000 lb greater displacement, a little more sail area (530 vx 517), a slightly worse SA-D ratio (17.3 vs 17.7) and a slower PHRF rating (150 vs 144).

So is the SA-D ratio the most important thing to measure performance of a boat?

If you were looking for a cruising boat, but valued performance, would you draw a line at SA-D or something like that?

Thanks,

Barry
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Old 04-23-2005
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Boat numbers and performance

SA/Disp. ratios are a little like horsepower ratings for a car. They relate how much power (sail area) is available to move the boat (displacement). If you compare cars, you can find a big one with 300 hp and a small one with 300 hp. The smaller one will tend to be "sportier". Boats are harder to tell apart than cars, as far as displacement goes. Two boats could be tthe same size, but have very different displacements. This makes the SA/Disp. ratio very useful in doing what you''re doing, comparing boats to one another.

There are other criteria that can also affect performance, even if two boats have similar size, displacement, and sail areas. Hull type and shape can make a big difference -think about full-keel, fin keel, and centerboard hulls. Because of this, if you want to measure performance, it makes sense to use a tool that tends to take all these factors into account. PHRF (or also IRC) does this. On a triangular course, the Catalina 36 would be expected to sail 9 seconds per mile slower than the C&C 36. You may not sail triangular courses. On one point of sail or another, or in certain conditions, the Catalina may actually be faster than the C&C. (In light air with a bit of chop or powerboat slop, the Catalina''s heavier displacement may tend to keep them going, while the C&C gets stopped more easily by the waves because of its lighter displacement.) Overall, however, the rating will give better insight into the performance of a boat than any single ratio could, because that is what it is designed to do.
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Old 04-24-2005
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Boat numbers and performance

The bait is out: will JeffH be able to resist? :P
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Old 04-24-2005
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Boat numbers and performance

phrf numbers are a good "simple " way to compare boat performance.... however it shouldnt be the only decision maker.

Phrf numbers are also in direct relation to the people who race that particular boat. Plus they vary from region to region. In other words, if a lot of rockstars are reacing that boat the phrf numbers will get"faster" ( lower) and if only bad sailors ( lets pretend for the discussion sake, that there is something like that ) the numbers get slower ( higher )

are those boats raced double handed halfways close to their numbers or only with 6 people full crew ??? Remember also that the phrf numbers are based on a well prepared race boat....
Phrf numbers might just become totally obsolete if you replace the 4 titanium sporks with a complete set of 6 persons eating utensils ...
( race weight versus light cruising weight )

Thor

its a cool easy start to use phrf numbers though...
thorJ30 is offline

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