Physics, specs, and wind...

[anotherT34C]

Physics Today has a quick, fun article about the physics of sailing;

Cookies Required

[anotherT34C]

Don't know why the link reads 'cookies required'. I think it should read 'coffee required', but in any case, the link leads to physics today online. Perfectly safe.

[GordMay]

This is athe previously referenced article on the physics of sailing in the Feb. 2008 issue of Physics Today.The Physics of Sailing:
http://
ptonline.aip.org/journals/doc/PHTOAD-ft/vol_61/iss_2/38_1.shtml

See also: The physics of sailing from other institutions:

School of Physics UNSW Sydney, Australia
The physics of sailing

Associated Students of the University of Alaska Fairbanks
The Physics of Sailing

Ca Tech
http://www.sciwrite.caltech.edu/jour...DavidArmet.pdf

[Pblais]

How do you determine how big of gear you actually need?

Consider static loads, dynamic loads, loading cycles, salt exposure, UV exposure plus types of use be it cruising or racing and it adds a lot to the mix. All vendors make different lines of products to attempt to match the major sectors and boat sizes. The last factor is also budget.

This isn't a computation you can run easily on a calculator.

Here is a good formula for measuring the area of the sail. It won't compute extra roach area but it will do any non right triangle like a jib. Where A, B, and C are the basic dimensions:

Square Root of ( .25 * ((A+B+C) * (-A+B+C) * (A-B+C) * (A+B-C)) )

Some tables will help you compute line sizes based on sail areas, but on smaller boats you just go with over sized lines since a smaller line is hard to hold.

With blocks I found Harken sizes the sheaves to match the line size so it's easy. Schaefer does not they make sheave sizes on some blocks that can't handle a large big block load but they do make multiple types of blocks with 1/2 sheaves. That means it is possible to buy a Schaefer block too light for the load. This means that with over sized lines you may not require the a block as big as a Harken block would force you into. It's about matching things to the load not so much to the size. Most products will give you a SWL (safe working load) This number is most helpful when comparing The Harken block to the Lewmar block back to the Schaefer block.

With blocks this gets even more complicated as the really large boat blocks are very expensive with all vendors. If you use the vendor guidelines at least you can get into the correct product line and then pick the block for the purpose.

You just have to watch out the price point does not distort your judgment when you really need that $80 block but the $33 block would fit the line. When you need 6 of them you can start to think otherwise. I went through almost this situation with the blocks at the base of the mast. UV had trashed 3 blocks and a face of a sheave sheared off and wedged the reefing line between the cheek and the sheave (as I tightened up the last bit on the jiffy reef point 1). They all were the same age so they all had to find a dumpster.

[GordMay]

Here’s an online sail area calculator:

Calculate Sail Area From I, J, P, and E Dimensions

[bassman1956]

I have:

Ls = SA * (WS^2) * 0.00431

Where:
Ls = sail wind load
SA = sail area, in sq ft
WS = wind speed, in knots

Granted, this is somewhat generic, without considering what-if's and corrections.
If I look a bit more, I may find the reference. But, if I remember correctly, it's the max load of the wind on the sail itself.

[Wotname]

Is Ls in pounds?

[svHyLyte]

The load is expressed in Lbs.


If you want to be really safe, take the maximum righting moment of your yacht and assume it is generated by a load applied normal to each of your sails, in turn, at the center of pressure of each. Divide the righting moment by the distance between the metacenter--the point about which the yacht rotates as she heels--and the center of pressure of each of the sails, in turn, to give a theoretical maximum potential load on each sail. Then take the moments of this load about any point on the sail to generate a point load at the clews and divide that by the cosine of the sheet angle to the surface of the sail. Multiply that load by 3 and purchase your sheets and hardware accordingly.

s/v HyLyte

[bassman1956]

Originally Posted by Wotname

Is Ls in pounds?

Sorry. Yes.