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Physics of beachcat capsize recovery


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When righting a beach cat, you can set up bungee rope systems that are really quick to use from the corner of the crossbeam right _inside_ the hulls (aka Hawaiian system). Or you can swing a rope _over_ the hull, this takes some fiddling about, and being quick is ofter a big win (as the tip of your mast sinks slightly). 

Is there significant mechanical / leverage advantage to slinging the rope _over_ the hull? In particular with the hardest bit which is to get the tip of the mast out of the water. 

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1 hour ago, martin 'hoff said:

When righting a beach cat, you can set up bungee rope systems that are really quick to use from the corner of the crossbeam right _inside_ the hulls (aka Hawaiian system). Or you can swing a rope _over_ the hull, this takes some fiddling about, and being quick is ofter a big win (as the tip of your mast sinks slightly). 

Is there significant mechanical / leverage advantage to slinging the rope _over_ the hull? In particular with the hardest bit which is to get the tip of the mast out of the water. 

Yes, quite significant on the water if getting the mast up is on the critical edge. Leverage is everything even if its just 200mm more of height for the single hander.

Basic routine of swimming the boat ( or just standing well forward on the hull ) so that the mast is perpindicular and facing the wind, not forgetting to release your downhaul, releasing the travellor half out and using your trapeze hook to hook onto the uphaul rope, are all the things you need to do to get that sticky flat top sail tip out the water. Even with my 90kgs of fat arse in below 5 -6 knots my F16 was a real pain. As soon as the wind got up and you got the mast perpindicular I could just about pull it up by hand.  If your mast is filling up at all with water, even a few drops, then you need to seal it ( fill it with water and see where it come out ).

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There is no difference between having the rope come from the keel or the beam root, to the righting moment you can apply.  The righting moment comes from the height and weight of the sailor, and how low and far they can project (similar to trapping low with shoulders back), it makes no difference where the 'trap' line is attached, it could even come from the dolphin striker without affecting righting moment.

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2 hours ago, maxstaylock said:

There is no difference between having the rope come from the keel or the beam root, to the righting moment you can apply.  The righting moment comes from the height and weight of the sailor, and how low and far they can project (similar to trapping low with shoulders back), it makes no difference where the 'trap' line is attached, it could even come from the dolphin striker without affecting righting moment.

Correct with regards to the ease with which the boat will come up there is no difference.  There is however a massive difference in the tension in the righting rope.  And therefor the load on your arms/legs etc. If you could get the rope vertical then the tension would be equal to your weight.  As the angle increases (attachment point getting lower) the tension goes up.  The result is with a lower attachment it is physically harder to right the boat even though your body weight is applying exactly the same righting moment.  For most boats though it’s nothing a correctly placed loop in the righting rope for your trap harness can’t fix.

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48 minutes ago, SCARECROW said:

Correct with regards to the ease with which the boat will come up there is no difference.  There is however a massive difference in the tension in the righting rope.  And therefor the load on your arms/legs etc. If you could get the rope vertical then the tension would be equal to your weight.  As the angle increases (attachment point getting lower) the tension goes up.  The result is with a lower attachment it is physically harder to right the boat even though your body weight is applying exactly the same righting moment.  For most boats though it’s nothing a correctly placed loop in the righting rope for your trap harness can’t fix.

Now we're talking. So the righting power is the same, but because we're pulling a rope, it's harder on the hands/arms. Unless you have a harness hooked to it.

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One difference that can be important is time.  I was flipped backwards sailing out through surf, had a righting line rigged up that I could have just grabbed from under the hull, but was stuck on first flipping it over the hull.  I neglected that the water was only about three meters deep, and the mast was stuck on the bottom.  Between the extra load from me leaning on the beam so I could get the line over and the extra time of waves bouncing the mast off the bottom, it broke.

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For some reason it is easier to get your weight further out with the rope over the top and that’s where the easier righting comes in, the system set up under the boat is certainly quicker if it rights quickly but I used to be leaning back straining for a long time, so I changed to over the top but it depends on your boat as well. Long masts and narrow beams are easy to right, whereas short masts (relatively)  have the boat further tipped over 

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On 12/22/2020 at 8:21 PM, SCARECROW said:

As the angle increases (attachment point getting lower) the tension goes up.

But if your body weight is in the correct position with yourself hooked onto the trapeze, will only exert say 25kgs of pull at the inside of the hull and the sail won't release from the water, what would you do to increase the force applied to the mast end ?

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Before I got fat (well I was always fat but adult sized fat) I used to carry a plastic bag tucked into my life jacket that I could put into the water behind me and pull on.  The advantage of having a righting pole or going over the hull is your righting rope isn’t pulling you into the hull as much so you can actively jump away from the hull to get some kinetic help.  Even if you can’t get you feet of the hull because of the effect of the righting rope bending and straightening legs while throwing arms back can make a big difference.  Momentum is a wonderful thing.

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22 hours ago, Waynemarlow said:

But if your body weight is in the correct position with yourself hooked onto the trapeze, will only exert say 25kgs of pull at the inside of the hull and the sail won't release from the water, what would you do to increase the force applied to the mast end ?

If there is enough wind to capsize a cat, there is enough wind to right it?  Some talk of using righting poles, or ballast, I think both would add risk and complexity.  I would say any well designed modern cat with a modern light rig,  should be no problem to right, using nothing more than righting line, wind, and good technique.  If the question is, how can a singlehanded sailor right a 2 man cat, or a small sailor right a big cat, I say, just don't, it is not only the righting problem that will make the boat a miserable and limited ownership proposition, get the boat that suits your size.

I did once capsize in a rain squall, then lacked wind to right it, I just waited half an hour for the breeze to re-establish, then got away.  Mast leaks are bad.  Modern carbon masts make righting way easier.  Add a solid tramp, and the boat almost wants to right itself. 

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Quietly noted above but absolutely essential to release any sail controls, mainsheet not cleated, traveller off. Downhaul and vang off. The boat won't come up otherwise.

A guy with a paper tiger had a neat little pocket in the front of his tramp that held a small rope ladder for climbing back on (he had a heart condition).

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  • 2 weeks later...

If you are doing the Wild thing in light air there may not be enough wind to get the cat upright again. Seen that done with a Tornado and lightweight crew had no chance until the wind filled in ..  Never fill a mast with water.. it can stay in there forever because of the foam manybuilders install to compartmentalise mast internals, especially carbon ones.. There was, from memory, a US righting plank you attached to the front beam and loosened and swung out to give huge leverage as a righting tool for even the lightest crewe.

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