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How do I calculate righting moment?

2020 July 1

Dear Boat Doctor,

I recently replaced the original standing rigging on my 1968 Islander 34. I wanted to make sure everything was correct, so I confirmed the wire sizes with my own righting moment calculations. Could you please share your methods for performing
these calculations.

Don Stead

Homer, Alaska

Dear Don,

Let’s start with the basics: your boat wants to remain upright, the wind wants to blow it over, and your standing rigging transfers those forces between the hull and the mast. The standing rig and mast must be strong enough to transfer those forces without failing.

Righting moment is the leverage your hull exerts to stay upright, a force acting on a lever, the force is the buoyancy of the hull, the lever is the half beam of the hull. The more your boat is heeled by the wind the more it wants to stand back up. The righting moment at 30 degrees of heel is the standard calculation point. Most of your sailing occurs within 30 degrees and for most hulls the force to go from zero to 30 degrees is linear. You can determine the righting moment by doing an incline test, heeling your boat over with weight on deck, or you can estimate with tables developed over many years.

I like to use the righting moment table from “Skene’s Elements of Yacht Design.” On that table, a boat with a 25-foot waterline would have a righting moment at 30 degrees (RM30) of 20,000 foot-pounds. Do be aware this entire exercise is an estimation, but perhaps a useful one.

We can then take that righting moment and translate it into an overall shroud load. First, we multiply the load by 1.5 to approximate the rest of the righting moment beyond 30 degrees, and divide by half of your 10-foot beam. This yields a shroud load of 6,000 pounds.

The overall shroud load is divided up between your shrouds.  On a boat like yours with single uppers and double lowers, the uppers see about 45% of the load and lowers about 33% each.  We also need to use in a safety factor of 2.5 to allow for unexpected loads and degradation over time. This yields an actual wire strength of 6,750 pounds tensile for the uppers and 4,950 for the lowers. Your headstay is likely the size of the uppers and the backstay will typically be a size smaller because it sees less load. This would translate to ¼-inch wire for the uppers and headstay and 7/32-inch for the lowers and backstay.

I am hoping this lines up with your calculations and kudos for digging deep into your re-rig.