Friday, February 24, 2017

Portsmith Number for the Omega 14 with my comments.

The Omega 14 Portsmith Number

I thought I might write about the Omega 14 from a standpoint of the Portsmith Number and compare it with some similar size boats.  The Portsmith number is used sometimes to handicap racing sailboats of various designs. The results of races are reported to an international organization and new numbers come out from time to time.

When you look at the numbers it seems like a really good idea and a well thought-out system.  But when I consider I have beat several other class boats with lower numbers (Puffer, Catalina 14.2, Finn, and Lido 14 for example) I realize that the number may not be as important as the skill of the crew, the age of the sails, the cleanliness of the bottom, and the weight of the crew. One class of boat may sail better in light air, another in stronger winds.  Ad infinitum.   Put another way: I am certain boats that have had a higher number (supposedly slower)  have beat me.    Getting a good start and reaching the favored side of the course first puts all but the slowest boat in good stead.

I tried to look up the Portsmith Number for the Omega 14.   However the most recent lists did not show the Omega.    I did find a 2008 publication that did.
This particular Portsmith Number is called the D-PN (Dixie Portsmith Number).    The numbers differ greatly from the system that seems to show up more recently, the RYA Portsmouth Yardstick (PN).
In both cases lower numbers indicate faster boats.
The PN system numbers seem to be about 11 to 13 times the  D-PN numbers.

Class          PN              D-PN Ratio
Optimist   1665           123.6 13.47087
Finn      1042 90.1          11.56493
Comet    200 91.7       13.08615
505        912 79.8          11.42857  (Sorry, the table didn't come over well from excel)

So if I want to compare the Omega 14 with other boats I can do it in the older D-PN system or I can attempt to find a comparable boat in the PN system and use that as a guide to compare boats.  
For example in the D-PN system we have the Omega 14 with the number 110.6 and the Alpha with the number 110.4.  The Alpha is slightly faster.

The alpha is rated 1182 in the PN system so perhaps the Omega number might be at about 1184 in the PN system. (I did the math and that is how it came out.   However, as far as this article is concerned I don’t see much point in using the PN system.  That assumes both systems are fair.

So next I'll list some one-design boats in the same size with their respective weights, sail areas and D-PN numbers and see if there are any unexpected results.  Perhaps it will provoke a discussion.  I see on a blog or two that some skippers think their boats should have higher numbers ("I can't possibly win a race with such a low number") while others think they should have lower numbers (why my boat is much faster than that, why does it have higher number than the other class?).  These things are usually easily explained by waterline length, sail area and weight.   But I did find at least two real anomalies when I looked at nine boats similar to the Omega.

                           Feet                   Pounds     SA         D-PN
Widgeon                12.33          318           90          121.6
Puffer                     12.50          190           90          116.1
Spindrift                 13.33          205           100        112.8
Blue Jay                 13.50           275           90          110.7
Omega 14              13.75           295          110.61    110.6
Rhodes Bantam     14.00           325          123.75     98.3 ?
Javelin                   14.00           524         125          111.8
Lido                       14.00           310         111         100.9 ?
Albacore                15.00           240         125           92.3

Why is the Rhodes Bantam number less than the Omega?
  That would make the Bantam about 10% faster than the Omega.  Is that possible?   The Bamtam is roughly 2% longer than the Omega, but that only helps at hull speeds and at that is only proportional to the square root of the Water Line Length (WWL).  That seems insignificant.   The Bantam is 26 pounds HEAVIER so that would make it slower, not faster, if anything.   The sail area (SA) of the Bantam is about 11.9% higher than the Omega.  That may explain the difference.  The beam of the Bantam is about 2% less (1.54 inches) than the Omega.  However I doubt that the two boats differ that much, if at all, at the waterline.  The Bantam does have a plumb (vertical) bow and that would give it additional waterline length, but again, that is not a factor below hull speed, and not proportionally advantageous after that.  Conclusion, the SA is the only significant difference that would make the Bantam faster than the Omega.

Now look at the Lido 14. The D-PN suggests it is nearly 10% faster than the Omega.    Compared to the Omega it has nearly identical sail area.  Certainly insignificantly different if the SA numbers are correct. The Lido weighs slightly more (5% more) which would suggest it were slower, but I doubt 15 lbs. Makes a significant difference. The Lido is slightly longer, but as above, that shouldn't be significant in the majority of cases.  The stats say the Lido is 4% wider at the beam, so that if anything should make it slower, not faster. So the only advantage for the Lido seems to be it's slight length advantage which is probably made up for by it's wider beam, and more weight.  I have sailed along side a Lido in several races and it (they) could not keep up with the Omega (me) on any point of sailing. Small sample size, I know.   Conclusion: I cannot see how the nearly 10% D-PN number advantage of the Lido is justified.  Lido racers should complain. My guess from this, and experience, is that the two boats could (should) race head to head. The Lido should not have the disadvantage of a lower number.
Wouldn't it be interesting if there were a handicap rating system that took all the similar length boats with similar sail areas and gave them all the same rating so they could race head to head.   Maybe they could even allow those who consistently lost in local races to start a minute early, or some other advantage.  
By the way.   I measured my Omega main and Jib and got 40.3 SF for the jib and 68.25 for the main.  The main is listed as 70 SF somewhere and I would accept that.  The jib is listed as 40.61 somewhere and I would accept that.
That totals 110.61.   However the site says he total sail area is 108 SF.

No comments:

Post a Comment