Calculator for Boat Speed, Performance, Stability, etc

ash · Post by **ash** » Tue Jun 13, 2006 11:50 am

Reading through some of the threads on the PBO/YM forum, I came across a link to a webpage which provides a calculator of the various ratios which are of interest on a sailboat.

Sail Area would normally be calculated using 100% foretriangle, but using actual foresail area probably gives a better comparison between boats of different eras. Using an estimate of actual cruising displacement rather than design weight will also be more realistic.

Give it a go - we could have a mine's bigger than yours discussion - though I suspect that we'll only beat Superstrath on AVS !

Try HERE for the link.

Ash

cpedw · Post by **cpedw** » Wed Jun 14, 2006 9:55 am

When we were looking for a new boat, I made up a spreadsheet with several of these ratios on it for a number of boats. It now contains information on about 180 types in the 32 to 36 foot range. We specified a permitted range of values for these parameters based on what we had read in magazines. While it was a useful filter of boats we'd never heard of, I have just checked and the boat we finally chose failed the test on 2 of the 5 parameters. We are very happy with the boat so it doesn't do to take these things too seriously.

But if I am taking them seriously, I would use the 100% triangle area. It's fairly fixed for a given boat - the size of the mast, whereas sail area is a user-adjustable parameter. Another consideration in choosing a boat might be how adjustabel the real sail area is.

I was about to quote the formulas I used but I see the link you gave describes them all in detail.

Derek

Post by **Silkie** » Thu Jun 15, 2006 8:43 am

There's a bunch of stats already worked out on the Alberg 22 site here. These are mostly for American designs but there are some old favourites too. The Hurley entry includes the 2300lb ballast figure which is commonly quoted but is wrong IMO as well as an overall length of 23.08' which is also very strange!

ash · Post by **ash** » Thu Jun 15, 2006 1:34 pm

cpedw wrote:I would use the 100% triangle area

I hear what you are saying, and if you were comparing similar boats it would be OK.

If you were comparing a boat from the 70's which was designed to be driven by an overlapping head sail and a relatively smaller main with a very recent design which had a self tacking jib and relatively larger main, then using 100% foresail would not be realistic.

SA/D would only be a rough indicator in any case, there would be other effects on performance - wetted area, aspect ratio of sails for instance.

The designer, Per Brohall, of my boat, an Albin Vega has provided advice on suitable sail combinations for sailing to windward in various wind strengths.

Using the design weight (5070 lbs) gives the following SA/D

- 8 knts....................... 21.79
8 - 14 knts.................. 20.11
14 - 20 knts.................18.35 *
20 - 28 knts.................13.28 *
28 - 34 knts.................10.63 *
34 - 40 knts....................8.13 *
40 - 45 knts....................6.07 *
45 - knts.........................3.14
100% foretriangle..........16.1
IOR Measured Sail Area..18.32

* I have estimated the area of reefed mainsail

This for a static LWL of 23' which gives a theoretical max displacement speed of 6.43 knts. The LWL will increase slightly when heeled. In practise, it is fairly easy to achieve 6 knts boat speed through the water.

If I had similar information on another design, then I could make a fair comparison on likely performance.

Ash