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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > The physics of a mooring ball...
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Thread: The physics of a mooring ball... Reply to Thread
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Topic Review (Newest First)
04-01-2013 11:59 AM
chris_gee
Re: The physics of a mooring ball...

A 2' diameter buoy could if submerged displace 200 lb of water roughly.
The simple way of thinking of it so far as I see it is that in strong winds with lengthy scope of say 10;1 for simplicity the wind force is predominant and is trying to make the warp a horizontal line. It can't in effect because the buoyancy holds one end up.
Simplifying it with a long warp and say depth of 20' approximately 10% of the force is downward.
With a wind force of say 2000 lb that is the equivalent of 200 lb plus the weight of the chain say another 200 lb which is fair force downwards. Without the buoy you would get greater plunging down more or less equivalent to someone standing on the bow. But equally the bow would rise less so it would be wetter.
I don't think it would make much difference to the caternary as one end of the chain is supported by either the buoy or the bow and the weight of the chain remains the same.
When the wind is strong enough the chain is taut i.e at a fixed angle.
As it lessens in lulls with any stretch from the warp but not chain would relax giving some forward motion due entirely to the horizontal component. This would not be influenced by the buoy.
So on this sleepless night I don't see much change in the to and fro but in the bow being less inclined to dig in but more inclined to lift ie less wet. Of course if the waves are of reasonable size compared to the depth the rise means the boat moves forward and back with the fall so that motion would be slightly more but not that much if the waves are much less than the depth.
04-01-2013 02:26 AM
capta
Re: The physics of a mooring ball...

Not exactly your question, but possibly of interest?
Cruising off the grid (uncharted atoll interiors) we found we could not use traditional anchoring techniques when anchored in sand with large coral heads rising to near the surface (10 feet down in 30 to 50 feet of water) around the sandy spot. As the tide changed we would wrap the chain around one or more heads in a day and end up with zero scope.
I switched to a short chain of 30 feet and line from there. After setting the anchor I would use a large fender to float the chain as high as possible and place smaller floats on the line at intervals of about 10 feet.
It worked like a charm, never dragging and no more wrapping around the coral heads.
04-01-2013 01:13 AM
One
Re: The physics of a mooring ball...

MedSailor, I found this on another forum, which might interest you (it did me):

Quote:
Although the hardcover Chapman Piloting occupies a place of prominence in my home sailing library, the smaller paperback (expendable?) reference book I keep onboard is Tom Bottomly’s Boatman’s Handbook 1985 edition (now available as The Boater’s Handbook to be more PC, $13.97 at Amazon.com). In the chapter on Seamanship there is a section called “Basic Anchoring Techniques” which lists 3 ways to prevent anchor dragging. The first suggestion is to try more scope or a bigger anchor. The second is to use a sentinel weight (kellet) but there are warnings that wave resonance may cause rode erosion. The third alternative is to use a buoy (anti-kellet?). That paragraph is confusing so I’ll quote: “3. An alternative to the sentinel is a buoy on the anchor line. This carries the vertical hold in the anchoring system and limits the basic load on the boat to that required to hold the boat in position. The buoy permits the boat’s bow to rise up easily over wave crests, rather than being pulled down into them, increasing the loads on both anchor and rode.” What I get from this is: 1) with the bow up, the boat will be dryer; 2) By reducing the shock load cause by wave action, the anchor won’t be jerked loose.
Or here from the horses mouth himself:

Boater's Handbook - Elbert S. Maloney - Google Bøger.

I think he has a good point when it comes to the weight of the chain coupled with the angle pulling the stem down into the water.
04-01-2013 12:15 AM
MedSailor
Re: The physics of a mooring ball...

My anchor bridle is between 15 and 20 ft (haven't measured). I was thinking (as Outbound says) that for it all go to bar taught, the buoy would be submerged. Also, my bridle attachment point is AT the water line, so it definately would either be submerged (possibility for adding snub-factor) or, it wouldn't be submerged, and my boat would be snatched up horizontally 20 ft behind the ball (possibility for trading pitch for surge).

I guess I should have clarified in my first post that my bridle setup wouldn't be the same as a typical mooring ball.

Good point about it banging against the hull. I did think of that, and was also thinking "I'd only employ this method if I thought it would be a windy anchorage with big waves".... Then again, if I knew that were the case I wouldn't anchor there would I??

Still, the physics intrigues me. And there are some harbors that will be rolly that I will want to anchor in.

MedSailor
04-01-2013 12:02 AM
outbound
Re: The physics of a mooring ball...

Maine - as usual yu're right ( grin). Sure need a hell of a lot of chain. I just carry 200'. But if placed at scope you desire on the chain and enough lt out afterward might work. Maybe Medsailorwill let us know. I don't think I would place my boat at risk. If I did this wuld still rig conventional snubbers to protect the windlass etc. Ifyou had to move in a hurry would just be another thing to mess with.
03-31-2013 11:17 PM
Maine Sail
Re: The physics of a mooring ball...

Quote:
Originally Posted by outbound View Post
Wonder if med sailor's idea has merit. If the ball was far enough away fom the boat when the penant was "bar tight" the float would be brought under water and quite probably serve the function of a snubber to sone extent if ball had significant displacement when submerged. Issue would be to have sufficient distance from boat but not too close to anchor as to effect it's function. Lets say with 7 to 1 have the float in the 5 to 1 or 6 to 1 positon. In a mooring field much shorter scopes are usually employed so practicality of his idea would be limited unless two floats employed ( and held together by spring loaded line when not under load) and there was sufficent area to swing.
But if you want 7:1 scope then placing it at 5:1 would only serve to increase the scope angle.... I think in order for that to work you'd need to set the ball at 8:1 then you at 9:1 or 10:1... You'd need a lot of swing room. I was picturing it used at a typical pendant or "snubber" length based on the statement "at the end of my anchor bridle".......
03-31-2013 11:10 PM
outbound
Re: The physics of a mooring ball...

Wonder if med sailor's idea has merit. If the ball was far enough away fom the boat when the penant was "bar tight" the float would be brought under water and quite probably serve the function of a snubber to sone extent if ball had significant displacement when submerged. Issue would be to have sufficient distance from boat but not too close to anchor as to effect it's function. Lets say with 7 to 1 have the float in the 5 to 1 or 6 to 1 positon. In a mooring field much shorter scopes are usually employed so practicality of his idea would be limited unless two floats employed ( and held together by spring loaded line when not under load) and there was sufficent area to swing.
03-31-2013 10:06 PM
RobGallagher
Re: The physics of a mooring ball...

I can't imagine a mooring ball offering any sort of help as ground tackle.

It's job is to allow you to find the darn thing when you get back. It might double as an aid to keeping the rope bits out of the mud or a place to put your identification.

You don't really "need" a big buoy unless someone with authority tells you that you have to.

I've used both a big round floaty thing and a lobster pot type float and found the only difference is that the big round floaty thing will bang against your hull when it's dead calm and keep you up all night.
03-31-2013 09:34 PM
Maine Sail
Re: The physics of a mooring ball...

Quote:
Originally Posted by DonScribner View Post
MS,

So . . . what's your point?!!!??! Where is that anyway? PH?

Don
My point is the photo shows that a typically rigged mooring ball will not provide any shock absorbing in a good blow.. That is Falmouth at 30-35 knots of so...
03-31-2013 09:01 PM
DonScribner
Re: The physics of a mooring ball...

MS,

So . . . what's your point?!!!??! Where is that anyway? PH?

Don
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