[Rhodes22-list] What's in a Number?

[Mary Lou Troy]

Bob,
I've played with similar calculations in the past and don't remember much about my conclusions but you should note that the waterline beam for a Rhodes 22 is very different than the trailering beam of 8'. I think think Stan suggested 6' as a reasonable number. I'll see if I can find it later. Even then equation doesn't account for a flared hull at all. 

There is an online version of these calculators at 
http://www.image-ination.com/sailcalc.html
Ignore the error messages, the results seem to come up anyway. The online calculator gives a very different number than you got for sail area to displacement even using less than 300. Is there an error in your calculation someplace?

Good luck,
Mary Lou


> 
> From: "Bob Keller" <r22yankeeclipper at hotmail.com>
> Date: 2004/03/25 Thu AM 11:33:51 EST
> To: rhodes22-list at rhodes22.org,  rhodes-list at sailnet.net
> Subject: [Rhodes22-list] What's in a Number?
> 
> Hello all,
> The current (April) edition of Sail magazine has an interesting article 
> titled "Crunching the Numbers."  It explains the commonly used numeric 
> calculations to define and compare different boats and designs. The article 
> discusses some of the shortcomings of these values, but also tells how to 
> calculate them.
> 
> So, I ran the numbers for the Rhodes 22, and the conclusion is that the R-22 
> is a moderate displacement, tender boat with a low-powered sail plan that 
> does not point to windward well at all.
> 
> Here's how the calculations were done:
> 
> D/L (displacement/length ratio) = (displacement/2,240)/(0.01 x LWL)3 (third 
> power) = (3,200/2,240)/(0.01 x 20)3 = 1.43/0.008 = 184 - this puts it in the 
> "moderate" category.
> I used 3,200 displacement as a guesstimate for my 1995 boat with motor, 
> three batteries, marine head, water tank, etc, etc.
> 
> SA/D (sail area/displacement ratio) = sail area sq ft/displacement cu ft 
> (displacement lb/64) = 300/50 =6 this is considered low, with 15 being the 
> the upper end of the low range!
> I used 300 sq feet considering IMF main and 175 genoa.
> 
> BR (ballast ratio) = ballast weight/displacement = 700/3200 = 0.22 this 
> falls into the tender category with anything below 0.3.  This means 
> (according to the article) that the boat is tender with limited upwind 
> capability.
> 
> L/B (length to beam ratio) = 22/8 = 2.75 the article states that "a wide 
> beam increases stiffness, which in turn reduces the ballast required, 
> enabling lighter construction to be used all around."  Also, "lightweight, 
> beamy boats often perform poorly to windward in choppy conditions because 
> they get thrown all over the place."  The author looks for a L/B of 3.0 or 
> higher.
> 
> Some of this seems to be off the mark, but I can identify with some of it, 
> as well.  In particular the last comment about going to windward in choppy 
> seas.
> 
> Comments anyone?
> Bob K
> Yankee Clipper
> 
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