Performance ratio

[kenJ]

I was re reading an article I printed off the web several years ago. It concerns the various numbers and ratios that can be used to predict a boats performance, stability, etc.

The un named author uses what he calls the "overall performance ratio" to help summarize his findings. It is a combination of SA/Disp, Ballast/Disp and Disp/Lwl, but he did not publish the formula. He says normal values of this ratio are in the 1-2.5 range for most sailboats.

The website no longer exists and I have googled in vain. Anybody ever heard of this?

[Eric Sponberg]

Go to the thread on Center of Flotation, which you can find here:

center of flotation calculation and implications?

I don't think I am the original poster that you had in mind, but the above thread covers similar ground. In that thread I discuss various design ratios related to boat design and performance, including SA/D and D/L ratios. At post #198, I discuss a concept called the "S Number" (S#) which is a way to rate sailboat performance on a scale from 1 to 10 using SA/D and D/L ratios. The equation for S# is given, and I include a spreadsheet of a database of boats where I calculate S# and chart it in charts. You may download the spreadsheet for your own use. Follow the discussion afterwards, too, for additional insights. In the next week's post, I also talk about Motion Comfort Ratio, and give its equation. In the last post this past week, I talk about Bruce Number for multihulls, which figures into the range you mention, 1 to 2.5 or so.

I have two more topics to discuss on that thread, which will be the displacement speed formula next week, and finally Crouch's planing speed formula the week after that. When the series is over, I will collect all twelve topics in a single pdf document and post it there for free download.

I hope that helps.

Eric

[u4ea32]

Eric, sounds like a great document!! Thanks in advance!