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MedSailor 03-31-2013 06:40 PM

The physics of a mooring ball...
 
I've been floating an idea in my head for a while (pun intended) ;). The idea in question, is using a fairly large ball fender at the end of my anchor bridle to act as a portable mooring ball. Why you ask? 2 possible reasons.

1: I recall reading, though can't find it now, that a mooring ball itself helps add shock absorption and ultimately holding power to an anchor when a boat is in big waves and wind. Is this true, and if so, through what mechanism? Sinking of the ball acting as shock absorption much like the weight/caternery of chain being lifted, or is there more to it than that?

2: The motion might be better in an anchorage with big waves. Again, is this true? What I imagine is that instead of my anchor chain trying to violently pull my bow down, only to have it rise up again, I might be surging back and forth towards and away-from the ball instead of just pitching at the bow.

Anyone know if either of the above is true and if so would adding a big round fender 15ft forward of the boat at the end of my stretchy bridle be a useful thing to do?

MedSailor

Maine Sail 03-31-2013 06:53 PM

Re: The physics of a mooring ball...
 
Quote:

Originally Posted by MedSailor (Post 1010477)
I've been floating an idea in my head for a while (pun intended) ;). The idea in question, is using a fairly large ball fender at the end of my anchor bridle to act as a portable mooring ball. Why you ask? 2 possible reasons.

1: I recall reading, though can't find it now, that a mooring ball itself helps add shock absorption and ultimately holding power to an anchor when a boat is in big waves and wind. Is this true, and if so, through what mechanism?


A picture is worth a thousand words...
http://www.pbase.com/mainecruising/image/129493299.jpg





Quote:

Originally Posted by MedSailor (Post 1010477)
Anyone know if either of the above is true and if so would adding a big round fender 15ft forward of the boat at the end of my stretchy bridle be a useful thing to do?

MedSailor

The above photo was with very heavy mooring chain, not dinky little anchor chain. Still lifted the whole thing near bar taught.;)

DonScribner 03-31-2013 08:01 PM

Re: The physics of a mooring ball...
 
MS,

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

Don

Maine Sail 03-31-2013 08:34 PM

Re: The physics of a mooring ball...
 
Quote:

Originally Posted by DonScribner (Post 1010496)
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...

RobGallagher 03-31-2013 09:06 PM

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.

outbound 03-31-2013 10:10 PM

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.

Maine Sail 03-31-2013 10:17 PM

Re: The physics of a mooring ball...
 
Quote:

Originally Posted by outbound (Post 1010539)
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".......

outbound 03-31-2013 11:02 PM

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.

MedSailor 03-31-2013 11:15 PM

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?? :rolleyes:

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

MedSailor

One 04-01-2013 12:13 AM

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.


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