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Old 09-12-2016, 19:46   #1
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My catamaran defies the laws of physics

I have a Prout Quest 31 that often behaves in a baffling way. Has anyone had this experience:

1. At anchor, every other boat drifts back away from their anchor with the wind and tide. Mine goes inexplicably toward the anchor or drifts differently from the other boats. I'm using about 15 ft of appropriately sized chain spliced to a nylon anchor line.

2. At anchor, after pulling back and setting the anchor I later find that the bridal and anchor line are running taught under the foredeck directly under and behind the boat. Why would it take this attitude toward its anchor?

3. At a mooring ball I tried 25 of line with a bridal and wound up with the mooring ball banging into my hull. I started the motor twice to back away from the ball but the boat insists on returning to it repeatedly and stays there defying the tide and wind. I then tried using 2 - 15 ft bow lines, one to each forward cleat and for some reason the mooring ball again winds up banging into my hull...

must be a marine poltergeist!
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Old 09-12-2016, 21:28   #2
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Re: My catamaran defies the laws of physics

How long is your bridle ( I assume you do have the bridle attached to the bow of both hulls running to your anchor rode) ? If it's too long, it will be ineffective at keeping you square to the wind. you cat will then tend to "sail at anchor" and under certain conditions it will indeed work its way to windward just from the aerodynamics of the hulls.
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Old 09-12-2016, 21:38   #3
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Re: My catamaran defies the laws of physics

Even cats don't drift upwind. Either you are misjudging wind and current or your boat is defying the laws of physics.
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Old 09-12-2016, 21:39   #4
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Re: My catamaran defies the laws of physics

Maybe you discovered a perpetual motion machine.
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Old 09-12-2016, 21:57   #5
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Re: My catamaran defies the laws of physics

15 feet of chain?
I would carry at least 50 feet of chain. I would use 5/16 chain on your boat.
To prevent mooring balls banging into your hull, shorten the bridle enough to keep the ball in between your hulls.
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Old 09-12-2016, 22:04   #6
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Re: My catamaran defies the laws of physics

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Originally Posted by marlin driver View Post
Even cats don't drift upwind. Either you are misjudging wind and current or your boat is defying the laws of physics.
they do actually, proven in 50 kn winds.
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Old 09-12-2016, 22:21   #7
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Re: My catamaran defies the laws of physics

Strange thread.

Bridle too long. I normally run ~ 100-150% of beam, but some times up to 200% beam. Baloney, unless there is something very wrong, longer is better up to at least 2x. When I shorten below 150% yawing increases noticeably. though still mild.

Sail forward in 50kn? No one is going to accept that without video proof. Unless your boat is yawing just terribly (which is not what the OP is describing), the only thing that will cause this is tide. At 50 knots I'm pinned back hard.

My cat hangs back nicely, yawing no more than 10 degrees in all conditions. By last cat did the same, unless I lifted the boards.

(34' cat, all 1/4" G43 chain. I carry 100' + rope, but the rope is only needed every few years; I anchor in shallow water.)
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Old 10-12-2016, 00:39   #8
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Re: My catamaran defies the laws of physics

"Sailing up the anchor chain" is quite common with catamarans, especially cruising cats with substantial windage. So is swinging from side to side. It allows you to appreciate, more fully, the anchorage views. ☺
It can be worse in wind against tide situations.
If it annoys, try deploying a drogue from an aft cleat to hold better with the current or alternatively, shortening one arm of the anchor bridle so the boat hangs "off square".
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Old 10-12-2016, 00:58   #9
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Re: My catamaran defies the laws of physics

If you are in a wind against the tide situation, deploy a small drogue off the stern, it will help you lie to the tide.

Ann

PS. I don't think catamarans repeal the laws of physics, but I do think sometimes we fail to understand just which bit to apply.

Cheers, Mate
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Old 10-12-2016, 02:01   #10
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Re: My catamaran defies the laws of physics

My cat doesn't defy the laws of physics, but it sometimes floats over the bridle/rode and stays that way.

This is usually caused by an opposing wind/current situation, but not exclusively. When a wind from behind overcomes the opposing current, the boat moves forwards and the bridle gets swept under the trampoline. When the rode takes up the slack, the bridle resists swinging, because as soon as a swing starts, the side that is taking the load pushes it back the other way. So it stays in that orientation.

The wider the cat and the shorter the bridle the more this effect is apparent.

The solution is a longer bridle, or as Ann suggests, a drogue or something off the stern.
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Old 10-12-2016, 03:16   #11
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Re: My catamaran defies the laws of physics

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Originally Posted by arsenelupiga View Post
they do actually, proven in 50 kn winds.
had no bridle at this occasion, used middle cleat only and was bit worried. Boat sailed past mooring and then mooring pulled it back and repeat the process.

With bridle does not happen.
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Old 10-12-2016, 06:14   #12
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Re: My catamaran defies the laws of physics

Cats do strange things when wind and current are opposed. My guess would be tidal current effect.

My cat does wild things when wind and current are 180d oppossed. Ive set a stern anchor on occassion to stop her wanderings.
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Old 10-12-2016, 06:20   #13
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Re: My catamaran defies the laws of physics

In my earlier post I omitted the effect of the current on the beam.

Once in a position where the boat has ridden over the anchor, it will tend to resist swinging around to a normal orientation for two reasons.

One is the resistance as one side of the bridle takes all the load as mentioned above. The second, and perhaps stronger, is that to swing around, the boat must present its beam to the current. The wind must be strong enough to overcome this and continue to rotate the boat around against that current. If not it will settle back with nose into the current and the rode underneath and continue to "defy the law of physics".

The solution for jvolpehoo's third point about a mooring ball is to have a bridle so short that the ball is kept between the hulls and can't touch.
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Old 10-12-2016, 07:15   #14
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Re: My catamaran defies the laws of physics

As others have advised, your boat's not haunted or unique with it's erratic wandering. Palarran does the same and you need to take that into account when anchoring in crowded areas. Twice I've awoke with my boat bumping another and I'm facing the opposite way of everyone else. FWIW, both times the other boat anchored after me and way to close, but I don't care much as fenders are easy to deploy.

Many people consider the ability to anchor shallow as a huge advantage for catamarans, and I readily agree. But another is that you can anchor deep, past everyone else, with little effect to waves and swells. This is what my general choice is now in crowded harbors.
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Old 10-12-2016, 09:36   #15
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Re: My catamaran defies the laws of physics

Another option:
Is the tide actually pushing you back? Or is it pushing you forward, and the wind is just blowing the upper water layer and whatever (leaves, stick) floats on it backwards, making you think that's the actual tide?
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