mast downward loading how to calculate.

[waynemarlow]

Can anyone recommend a simple free programme to calculate the downward mast loading at the base of the mast or better still can anyone do the following calculation ( I cannot get my head around this one, it should be easy ).

Mainsheet load is max 450 kilos @ 2.06m metres from mast, front stay is 1.4 from base of mast.

Now what happens to the loading at the mast if I shorten the forestay to 1.2 and ideally 1.0 metre.

The reason I ask is the class of boat I have runs without a jib and the further I get the stay back the lighter I can build the hulls in front of the beam.

Thanks.

[Petros]

from a simple ratio stand point, it would be something like 1377 kilos with the fore stay at 1 m, and 1223 kilos at 1.2 meters.

The problem is there are other loads on the rig than just the main sheet tension.

The sail drives the boat forward, loading the back stay, or the shrouds if you have no back stay. And the lateral sail loads (that heels the boat) can also put tension on both the shroud and fore stay, deeding on the geometry of the rig and sail, that will put downward force on the mast. There will also be tension contributed to the fore stay from the shrouds. And then there are the dynamic forces from the wt. of the rigging when the hull is pitching, back and forth, among others.

Usually the design loads are based on rules that have evolved over many centuries and follow simple ratios developed by Lloyds or other rating organizations. These are based on many years of experience rather than calculations, and use tables and oversimplified formulas with very large safety factors built into the design formula. This results in very heavy (and durable) designs, and because of that I suspect these formulas are not used for racing designs.

A look at the text book by Larsson and Eliasson will give you the standard rigging design process for pleasure yachts.

[Richard Woods]

you could try beamboy ver2.2 It's free, it calculates beam sizes so maybe will do what you want

Richard Woods of Woods Designs

www.sailingcatamarans.com