Catamaran Structural Resistance to Materials Fatigue

[Factor]

Originally Posted by neilpride

Well in the bar we say Fountaine Crackjot , nahh just kiding...

Which bar would that be

[barnakiel]

Originally Posted by Dragon Lady

Stopping flexing can often increase fatgue failure not reduce it.
Compare a fishing rod to a broom handle.

Yes it can. When we stiffen part of the structure while another part remains free to flex. A hard point is created and that's where the thing fails.

Making the structure stiffer as a whole (no hard points) limits flex and all other factors equal reduces cycle amplitudes and extends life.

In a post-built aspect, often the only way to go would be by adding support and making the unsupported panels smaller, e.g. adding stringers in the hull, spreaders in the mast, etc..

At build phase the choice is often made based on how the thing is to be used. A boat built for extended offshore use will be built stiffer, stronger and may end up heavier unless lighter materials/built techniques are employed.

b.

[Eleven]

Don't worry. Do a check from a dink when you are next in a comfy place just to be sure, to be sure.This furniture in a bendy plastic shell has long been a problem. Does the designer use the furniture to stiffen the hull (Old Prouts do this with Ply Bulkheads at the front and back of the cabin, with some cracking seen at the bottom of the door openings in the rear bulkhead) or try to isolate it by either not connecting it at all (an inch gap to the roof) or connecting it soildly in one plan only (i.e. the back, leaving the top and bottom free to move).
The boat will flex, it's how this is handled by the designer, and your's has gone for tidy joints to the hull which don't transmit any great forces or load the GRP skin unevenly. There are more expensive ways of doing it, but a retro fix might be difficult.

[Dragon Lady]

Originally Posted by barnakiel

Yes it can. When we stiffen part of the structure while another part remains free to flex. A hard point is created and that's where the thing fails.

Making the structure stiffer as a whole (no hard points) limits flex and all other factors equal reduces cycle amplitudes and extends life.

In a post-built aspect, often the only way to go would be by adding support and making the unsupported panels smaller, e.g. adding stringers in the hull, spreaders in the mast, etc..

At build phase the choice is often made based on how the thing is to be used. A boat built for extended offshore use will be built stiffer, stronger and may end up heavier unless lighter materials/built techniques are employed.

b.

Yes you are right, but is extra weight or expense a desirable outcome?
Especially in a cat?