Joined: June 2005
||Posted: Jan. 24 2008,14:01
Since our last posts on the subject of the potential speed difference between the Marlin and the Dorado, Jimbo and I have bounced several e-mails back and forth. When I read his response to my post, I was, and still am, concerned that the use of the FOUNDRY computer program in the way it was used was inappropriate. The Marlin, indeed, may be faster and have less drag resistance than the Dorado, but it’s not the conclusion that concerned me, it’s that the results of a misused program were offered as proof.
Specifically my concerns include:
1. The FOUNDRY program calculates drag resistance from hull design. Jimbo used the generic hull design, changing it only to conform with the LOA and beam of the two hulls. First, its unlikely that any specially designed yak conforms to a ‘generic’ yak. Specifically, the generic hull is not as full at the ends as the Kaskazi’s and the wetted surface area, which is critical in calculating the frictional resistance, was more than one square foot more in the program than provided by Kaskazi previously. Thus, the program’s estimate for the wetted surface of the Dorado becomes questionable, as well.
2. According to Kaskazi, the maximum load of the Dorado is 88 pounds greater than that of the Marlin. I failed to appreciate that in my post, and that may in fact suggest that the hull designs for the Dorado and Marlin are different, because a 2-inch difference in LOA and a 1/2- inch difference in the beam is unlikely to increase the maximum load by 88 pounds. My guess that the ends of the Dorado are fuller and/or the hull is deeper. But, in any case, utilizing the same hull design by changing only the LOA and the beam, as was done, is subject to question.
3. Jimbo’s post shows that the performance of the Dorado is 93% of that of the Marlin both at the maximum load of the Marlin and the maximum load of the Dorado. When overloading a yak the performance will deteriorate, but a simplified program such as FOUNDRY or KAPER does not generally calculate the effect of overloading. I question whether the performance of the Marlin will remain 108% that of the Dorado when over-loaded, and if it does not, then the predictions of the program are in error.
4. Unfortunately, the outputs of computer programs are too often assumed to accurate to the Nth degree, rather than reporting a margin error, which is inherent in all computer outputs. Outputs are subject to accuracy, and the output can be no more accurate than the inputs and the simplifying assumptions of the program. In addition, just because a program gives, in this case, drag resistance or speed to two decimal places does not mean there is not substantial error that should be reported. Too often we measure with an axe, record with a crayon, and report with a sharp pencil. In my opinion, it would not be unreasonable to assume an error of plus or minus 5% in this case, because the inputs to the program are gross assumptions for all of the necessary input parameters. If this is the case, then an 8% difference is with the margin of error and there may be no difference in performance.
Again, I don’t argue with conclusion that the Marlin is faster than the Dorado ---- only that the results of the FOUNDRY program do not substantiate the supposition.