# Righting moment to aluminum tubing size?

Discussion in 'Multihulls' started by emf, Jul 21, 2006.

1. Joined: Oct 2005
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### emfJunior Member

I've learnt the hard way that lots of force is exerted on outrigger connections. So i'll try to calculate the forces exerted and work out what sort of connection is needed.

Umm, basically the design calls for maybe 200 kg of weight 2 m from the middle. So that would work out to 2000 Nm? Roughly what sort of connection would I need and what size of aluminium tubing would I need to support these loads? I was thinking of using epoxy and fibreglass to join the aluminium tubing together and to the boat. Would that work? Or is 2000 Nm too large a force and require welding a proper support frame. Thanks in advance!

2. Joined: Dec 2004
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### cleblancJunior Member

200kg at 2 meters gives the following :

200kg * 9.81 m/s2 * 2.0 m = 3942 NM

There are alot of pressure from the side stays and it also has to be calculated.

An estimate should be fairly easy if you know the weight, centre of gravity and the beam, of the trimaran. Draw the trimaran with an heeling angle just sufficient to fly two hulls and do a force diagram. This should allow you to have a rough estimate of the forces.

You can do a few simplification like the sidestays and mast are rigid.

Once you have the value of the forces, you can choose the section based on the moment of inertia and modulus of your crossbeam section and the maximum deflexion that you want or the maximum stress that you want.

As far as the connections, many sport catamaran gave a fiberglass connection from the crossbeam to the hull and they work well. The fiberglass connection is ok as long as it is engineered with the proper dimension to take the load.

Charles Leblanc