Originally Posted by estarzinger
All the curves I have seen show a quite linear function up to about 10-15 degrees (for cats, and to 20 degrees for tris). I attached this curve several posts back. I even put a big red x on it where I would set the fuse. There is very clearly a lot of room to set a fuse to not break under 'normal' cruising (say up to 5 degrees of hull lift = 8ton-ft for the below curve) but still to easily break at hull lift (=25ton-ft for the same curve).
In your example if the hull is lifting to 30 degrees on a cruising cat, I at least, would want the fuse to have already blown (near the top of the curve, whatever angle that is).
I get that people don't want extra complexity and that's an argument I understand. I just happen to honestly not consider a spectra fuse to be 'complex'. We all have fuses in our electrical system
to prevent fires. This 'load fuse' seems equally sensible (for a multi). But I am not selling fuses today
, so I will leave it there.
As you said, we use fuses in our electrical
system, an impulsive load in our electrical
system can cause damage, just like on the French cat an instantaneous force could cause component failure. These two scenarios are analogous. This approach makes a lot of sense.
This logic does not work in terms of capsizing. A large multi cannot be capsized 'instantaneously' by forces that wouldn't rip the mast
clean off. Go back to your curve. The sum of the area under the curve represents the energy it takes to actually tilt the boat to that position. Your spectra fuse would blow at less than 1/5th the energy it would take to capsize
To put this in perspective imagine a 10ft. (about 3m) long beam of wood laying on flat ground. You are standing in a hole under one end of the beam so that it is over your head
but close enough you can reach up to lift it comfortably. You lift the beam up just of the ground. Feel the weight of it, that is the max righting moment. You decide to be a gust of wind and punch the board so you'd feel the same pressure you felt on your hand when you lifted it up ever so slightly. You punch the board, it only tilts up a few inches, if that, before it clatters back down. Now imagine how hard you'd have to strike the board to make it go vertical and topple over. The difference is power is pretty large if you are even strong enough to do it (not to mention probably hurt your hand so don't seriously try this...without a glove or something). The point is, you can hit the board quite a bit harder than the max righting moment without ever being in remote danger
of capsizing it.
So what does this mean for a fuse?
Well it means that if you have the fuse fail every time it felt the max righting moment, it will fail constantly when you are in no danger
of capsizing. That would make it impractical at best.
How then to design a fuse that will work as intended (snap when you really are in danger of capsizing)?
You gamble. You capture a LOT of weather
data and data on wind induced multihull
capsizes. From this you figure out how often gusts occur that may cause 'false positive' reactions form your fuse, and how strong they are. You look at sustained conditions that are capable of capsizing the boat with and without gusts. There will probably be a large overlap between the force of sustained weather
conditions with little variation in gusts and sudden violent gusts able to knock your boat over, however, that is your general range. You might then want to compare this range to catamaran
accidents to get an idea of the difference between 'safe forces exceeding the max righting moment' and 'dangerous forces exceeding the max righting moment'. From there its a judgement call and you will still probably have the fuse break often when you're in no danger. Or even worse, potentially not prevent a capsize.
Long story short: A spectra fuse that actually prevents capsizing but doesn't fail often for no reason is extremely complex and tricky to make.