# converting moments of inertia into metric

Discussion in 'Multihulls' started by Corley, Dec 18, 2014.

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### Corleyepoxy coated

I'm looking for a rotating mast section for my F40 project and have been given a moment of inertia value for the two axis in imperial units just wondering if anyone can give me the heads up on how to convert to a metric measure? These are the values I've been given least axis 27 in^4 and other 63 in^4.

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### NavalSArtichokeSenior Member

Well, which ones? m^4, cm^4, or mm^4 have all been used at one time or another.

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### TANSLSenior Member

1 inch = 2.54 cm
so
(1 inch)^4 = (2.54 cm)^4 = (2.54)^4 cm^4 = 41.62314256 cm^4
can that help?

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### rcnesnegSenior Member

I would try Unit Analysis, a cool trick where you convert from one unit to another.

Ie: 1 inch (2.54 cm/1 inch) = 2.54 cm. The inches cancel out, you multiply by everything on top, and divide by everything on the bottom.

Another example:

52 mph = 52 miles/hr (5280 ft/1 mile)(1 meter/3.28084 ft)(1 hr/3600 s) = 23.2460792561 meters per second.

If you have squared units, or to the fourth, or whatever, just use that conversion.

Ie: (1 inch)^4/(2.54 cm)^4 or (1/41.6231), etc.

The easy way: Use this thing: http://www.unitconversion.org/unit_converter/moment-of-inertia.html

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### Corleyepoxy coated

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I'm curious. Do you regularly quote values down to 9 decimal places in plate thickness for example, or the scantling of a transverse frame when designing a boat?

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### gonzoSenior Member

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### TANSLSenior Member

No, it would be nonsense, I only use 8 decimal places. See my post.

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### gonzoSenior Member

Garbage in- garbage out. Pay attention to the sig figures.

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