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Go Back   SailNet Community > General Interest Forums > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 04-14-2010
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Why must you let out boom quickly after jibing?

Perhaps I'm a little dense, and I'll admit that I haven't jibed in 30kts. It seems to me that if you can bring the boom into the centerline with mainsheet before the jib and have full control of the boat, you can then ease the wheel or tiller just a little to get wind on the opposite site of mainsail. The sail goes over just a little and the situation is exactly the same as it was just before you jibed. Then ease the boom out and get the jib sheeted in on the new course. Or don't ease the boom out for a while...you'll still be under control, no great stress is applied to the rig. It's the same as it was just before you jibed, except the wind is on the opposite side of the sail. What am I missing? If you let the boom out quickly (meaning fast) do you impose lots of force on it when you bring that fast movement of the boom to a stop?

Last edited by NCC320; 04-14-2010 at 04:59 PM.
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  #22  
Old 04-14-2010
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NCC -

Those of us who sail in the San Francisco Bay often see 20-30kts every summer day. It's been my experience that if you don't ease the main immediately after jibing, the boat will round-up (broach). If one thinks about the boom as a giant lever arm connected to the boat's z-axis (parallel to the mast) and the wind is the force at the center of effort (the sail), then keeping the main in enables the strong wind to spin the boat quickly.

By the time the boom is nearing the shrouds, the pressure on the sail and the turning moment are reduced significantly so that the load on the main sheet and the boom are much less than if it were left sheeted in.

Forgive this explanation from an electrical engineer.. I'm sure someone more mechanical can explain it better.
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  #23  
Old 04-14-2010
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I'd point out that if you have an autopilot, you can leave the helm, since most autopilots will auto-tack if you enter the right command. This means you can deal with the mainsheet and jib without worrying about the helm. The helm will "tack" the boat 100˚ or so... so if you're sailing with the apparent wind at 130˚, it will bring you to where the wind is 130˚*on the opposite tack.

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Originally Posted by bacampbe View Post
Right--that's why I was talking about single or short handled gybes. That is, what's the best procedure when you can only do one thing at a time. So, doing all that single handed (and ignoring the jib for a moment), we're talking about bearing off, sheeting in the main, gybing, then running up and easing the main, running back to the wheel to arrest the turn, etc.

Chicken gybes are sounding better and better, at least when there's too much wind to just throw the boom over by hand.

I do have an autopilot, but I don't think I can trust it to steer to a new course and hold it under gybing conditions. (It does a pretty good job of autotacking, but it seems to get confused by the boat response during a gybe)
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  #24  
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Quote:
Originally Posted by erikdj View Post
NCC -

Those of us who sail in the San Francisco Bay often see 20-30kts every summer day. It's been my experience that if you don't ease the main immediately after jibing, the boat will round-up (broach). If one thinks about the boom as a giant lever arm connected to the boat's z-axis (parallel to the mast) and the wind is the force at the center of effort (the sail), then keeping the main in enables the strong wind to spin the boat quickly.
I think the question that both NCC and I had was why the moment arm is any different right _before_ the gybe, when the boom is still on the old side. You've got the exact same forces trying to make you round up in the _other_ direction, right?

From the responses so far, I gather the difference is that right after the gybe, you've got some instability from shock loading on the boom and rudder. I suspect it may also be because you were already steering into the gybe, which works against a pre-gybe round-up, but in favor of a post-gybe round-up.
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  #25  
Old 04-14-2010
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Right it's a combination of the transient shock load combined with heeling which lifts the rudder up and changes its angle of attack that all combine to start the broach. The rudder becomes less effective as the boat heels, etc..

Sorry I missed that part of his question.
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  #26  
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Quote:
Originally Posted by sailingdog View Post
I'd point out that if you have an autopilot, you can leave the helm, since most autopilots will auto-tack if you enter the right command. This means you can deal with the mainsheet and jib without worrying about the helm. The helm will "tack" the boat 100˚ or so... so if you're sailing with the apparent wind at 130˚, it will bring you to where the wind is 130˚*on the opposite tack.
That works for 130˚ to 130˚. It doesn't work well for gybes between deeper reaches or runs. (Also, my autopilot has a gybe inhibit feature when autotacking. I can turn it off, but I'm not sure I want to keep it turned off.)

I've tried putting it in windvane mode, and try to go from around 160P to 160S and vice versa. But the weirdness in apparent wind you get in that range seems to confuse it. It might work better if I switched it to true wind mode. Even when steering to the compass, the autopilot also will not steer hard enough to overcome the jib if I let it back until the main gybes. That might be something I can fix by adjusting the response.
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Old 04-14-2010
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I agree with SD's earlier comment on controlling the main as you turn through the wind. I was taught, and still practice it, to turn through the wind and pull the main to center just as the jib collapses. Waiting for that moment you can easily control the main by hand and be ready to let it out quickly on the opposite tack. I release the jib as I go through and have it somewhat pre set for the opposite tack. When I have the main under control I trim the jib and then the main. Never gybed above 30 knots, but never had a problem up to that wind speed. I have a wheel and auto pilot...and SD's post about auto pilots brings me to my question to add to this post.

My auto pilot will auto tack, but not auto gybe. I have tried to find a way to override the brain, but unlike my own, which I over ride to my detriment often, I can't seem to find a way.

I have a Raymarine ST4000 Autopilot on a Catalina 34 Tall Rig with Raymarine ST60 instruments all talking to each other. The autopilot recognizes the wind speed and direction and will not let you tack if it reads a gybe. My question: Has anyone found a way around this issue? I can't call it a problem as I understand the reason, but I would like to be able to over ride it as I often sail alone and auto is your friend...especially above 20 kts of wind speed.
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  #28  
Old 04-14-2010
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Auto pilot? I've yet to sail with one. I would imagine it making it easier. Easier to screw up too....
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  #29  
Old 04-14-2010
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Quote:
Originally Posted by CharlieCobra View Post
Auto pilot? I've yet to sail with one. I would imagine it making it easier. Easier to screw up too....
Until I can give it verbal commands to correct problems like I can with my crew, I'm not sure I'd want it jibing my boat
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Old 04-15-2010
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Quote:
Originally Posted by erikdj View Post
Until I can give it verbal commands to correct problems like I can with my crew, I'm not sure I'd want it jibing my boat
As I've mentioned, mine has so far not been reliable for gybing. But on the other hand, when single-handing, the wheel is not so great at taking verbal commands either :-).
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