Boats swinging on moorings?

[Mark J Wilme.]

Been watching this now for two hours, it's somewhat mesmerizing (or maybe I have nothing better to do).

We are on a mooring at Nantucket and there is a storm coming through tonight, so far light drizzle 20-30kn winds, forecast gusts to 40 overnight. The boat is all battened down and they use very long pendants in Nantucket so we are used to some movement like we saw the last couple of nights, but nothing like I describe below.

Next to us (or behind us) is a black mjm in the low 40ft range, what I would describe as a gentlemans downeaster style boat - stylish looking boat if you like that kind of thing. Most of the other boats are sailboats and these seem to lie to their moorings fine and behave similar to us, but not the mjm.

As I watch the mjm is far off to our port side pulling to the left of her mooring, she pauses at the far limit of her swing and then (craziest thing) the bow blows down to face us and she begins to hurtle across the water towards us, like a weight on a pendulum, seems like 10kn, probably isn't, past the apex of her swing and keep on going until she is maybe 15ft off my port stern still pointing in the same direction - not bow towards the wind or mooring, whereupon the bow slows to a stop and the stern continues to swing out, and out, and out, 180 degrees until she is literally perpendicular to me 15ft off our stern facing back the other way. She pauses there for a few seconds and then begins the whole journey in reverse.

I have watched this with coffee in hand for maybe 100-150 cycles now, always the same, never much closer then the 15ft (sometimes a little) but it seems to be very repeatable.

I have contemplated shortening our pendant but am not certain how that might make us swing differently from the other boats (except for the mjm of course), whereas now all the other boats seem to act very similarly.

What causes a boat such as this mjm to behave like this?

M
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[Rick Donovan]

On Aug 22, 2010, at 8:19 PM, Mark J Wilme. wrote:

> What causes a boat such as this mjm to behave like this?

I have witnessed this myself and it is not isolated to power boats. our 473 did this to some degree in stronger winds and we had a 38 foot Beneteau chartered years ago in the caribbean that was terrible for this when at anchor. I always called it sailing on the mooring. what I believe to happen is that the wind gets on one side of the bow and pushes it off the wind until enough pressure is applied to the anchor rode or pennant to turn the bow the opposite direction and then it repeats the process in the other direction.

although I am very far from a designer, I always suspected this phenomenon was directly related to the center of effort of the boats top side and under water surfaces. if the center of the surface area is forward of the center of effort below the water that was my best wild ass guess to explain it. on a sailboat, you at least had the option to set up a small riding sail near the stern to counter the bow falling off to leeward but the MJM you mention will not have that choice. a shorter pennant would seem to make sense to me. I am thinking that would limit the bow to its motion port or starboard and greatly improve those tendencies.

as you mentioned, altering your boats characteristics by shortening your pennant also alters its motion in relation to the other boats around you. I might also hesitate to make that change unless I was concerned about an impact after turning in for the night.

looking at the marine forecast for that area looks a bit rough for the next couple of days. stay warm and dry.

Rick Donovan
Biddeford, Maine

[mos...@aol.com]

Regarding swinging at anchor/mooring.  As Rick mentioned, we also swing significantly during heavier breezes while on anchor on our 473.  Other than a steadying sail e.g Banner, has anyone found way to mitigate the swinging?

Thanks

Mark
Alkenjibarii
B473
Edgewater, MD

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management question

[Kidd, James]

I believe that the original question was why was only ‘that particular boat’ swinging wildly…

 

Knowing Nantucket well, I’m thinking that currents that flow at up to 6kts through the Northern part of the harbor at flood and ebb could be a factor in this condition.

My boat would do some crazy things as well….

 

 

James A. Kidd

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[Denny Wertheimer]

the only true way would be to put out a stern anchor. then you have tides and current to worry about

--
Denny Wertheimer

314-330-5001

[Mark J Wilme.]

Thanks all.

It was a bumpy night but it picked up more this morning, we recorded sustained winds of 38kn at 8:30am this morning and gusts 49kn. The harbor looks more like the movie the perfect storm than the old gray lady at this point.

Interestingly when the winds picked up overnight the mjm started to pull more steadily on her mooring. I watched though (we are marina people not mooring people so this is new to us) and it's interesting how some boats ride smooth to the wind and other buck like bronco's. I am guessing its a combination of keel shape and displacement.

There is chatter on the vhf right now that a 55ft cat 'Ondine' has dragged one of the moorings and is charging its way through the mooring field, that's a shame. I don't know why a 55ft catamaran with that much windage would be on a mooring here, maybe the moorings are heavier than I thought. A few sail covers starting to come undone on unattended boats, flapping canvases, upturned dinghy's etc

Mark

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From: Denny Wertheimer <denny.we...@gmail.com>

Sender: benetea...@googlegroups.com

Date: Mon, 23 Aug 2010 08:48:22 -0400

To: <benetea...@googlegroups.com>

ReplyTo: benetea...@googlegroups.com

[Beelzebub]

A riding sail stops the problem of the boat sailing at anchor. I chose the Banner Bay riding sail, but they are relatively easy to make.

[Paul Schwab]

I used to own a MacGregor 26M.  Powerboat hull shape, light displacement and high freeboard resulted is the same behavior on a mooring that you described for the mjm.   Wild to watch in a mooring field of steady sailboats pointing into the wind.

 

I often ran a line from the mid-point on one of the mooring pennants to a mid-ship winch & shortened it just enough to so the bow was on a slight angle to the wind.  This prevented the stern from swinging free and allowing the boat to hunt back `n forth.

 

Paul

[Patrick Brehm]

You want to stop swing and have a comfortable mooring?

 

Out here in the PNW Gulf Islands calm evenings are the norm, and some popular anchorages are so packed forget about 7:1 scope, lucky to drop 3:1.  I decided to just bear with 100 feet of 3/8's chain, and in 25 feet at high tide, we're fine. But the odd blow comes thru and raises big havoc in these places, so for extra safety we drop our second anchor which has 20 feet chain, rope rode, on the V pattern. Set your main anchor, prepare your second anchor, hard left on the rudder if the port bow roller is free for your second anchor, and slowly motor, letting the existing rode "steer" the bow upwind as the boat turns to port. Stop the boat at your best guess, about even with your first anchor drop to the wind, drop the second anchor and pay out the rode so you approximate centre between the two anchors.

 

I gotta tell you, if you are going to hassle with a second anchor in the first place, do this. It is a "virtual" mooring ball. You will have the use of your windlass to weigh them both, and you will virtually eliminate swing. As long as the strong breeze is somewhat direction consistent, and strong breezes generally are, you wil be amazed how comfortable this technique is.

 

Pat Brehm

 

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From: benetea...@googlegroups.com [mailto:benetea...@googlegroups.com] On Behalf Of Denny Wertheimer
Sent: August 23, 2010 5:48 AM
To: benetea...@googlegroups.com