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Old 24-02-2019, 07:47   #1
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Mooring ball buoyancy?

Hello all, I am currently piecing together the parts to place a mooring for my 51 Endeavour. I will be purchasing the SK8500 skrew anchor from spadeusa. Attachment to the anchor is 1/2Ē ACCO galvanized mooring chain and 5/8 shackles. I will be placing this in approximately 18 feet of water. The chain and shackles weigh approximately 50lbs at that length. My question is, how much buoyancy do I need in the buoy? Standard 18Ē balls from west marine say 105lbs buoyancy. Is this enough or do I need the next size up ball?

I have purchased 30 feet of chain and thought about purchasing a smaller float to add that will help support the weight. This would be affixed at around the 12 foot mark so would always be submerged.

Iíve never built a mooring so any input would be appreciated.
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Old 24-02-2019, 12:55   #2
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Re: Mooring ball buoyancy?

Is that 50lb for 18ft of chain?

You need more than 18ft of chain for "approximately 18ft of water".

What about tides?
You also don't want the mooring chain to be vertical.

If the chain weighs 50lb in the air, it will weigh about 43lb in water. 105lb of buoyancy will mean that about 40% of the buoy will be submerged. Sounds OK for that amount of chain.



If you are planning on using the 30ft to allow for tides etc, the chain would then be 83lb in air and 72lb in water, in which case the buoy would me mostly submerged. I'd go for the next size up.
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Old 24-02-2019, 13:15   #3
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Re: Mooring ball buoyancy?

I think he's counting only the chain that the bouy has to support, which is the 18' plus a bit (under normal circumstances). When the wind pipes up and there is more chain off the bottom, then there will also be a strong upward pull from the boat. With the boat gone, there shouldn't be much catenary.


Kalimnios, there's a good description of what you need for a mooring on the West Marine site: https://www.westmarine.com/WestAdvis...manent-Mooring


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Old 24-02-2019, 14:01   #4
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Re: Mooring ball buoyancy?

Yes I have purchased 30 feet of chain but the average depth is approximately 18 feet, 20 feet at high tide. So I calculated 20 feet of chain at 2.53lb/foot = 50.6lbs.

I don’t mind if the buoy goes under while the boat is moored to it. As long as it will have enough buoyancy while just supporting the max of 20feet of chain.

I realize the easy answer is to just buy the next size up buoy , however it is a substantial enough price jump that it makes me wonder if I really need it.
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Old 24-02-2019, 14:34   #5
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Re: Mooring ball buoyancy?

Kalimnosjohn, you are going to want about 1.5 X the water depth of heavy chain - most folks buy some (BIG) cheep Chinese chain (like 1" for your application) with the water depth of lighter (and better grade) chain shackled to it. (so, 20' of your ACCO 1/2"). This will give you enough scope that you shouldn't put undue stress on your anchor under storm conditions.


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Old 24-02-2019, 14:53   #6
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Re: Mooring ball buoyancy?

If your mooring area has significant current you need more buoyancy to keep the ball visible at high current.
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Old 24-02-2019, 16:23   #7
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Re: Mooring ball buoyancy?

Quote:
Originally Posted by Hartleyg View Post
Kalimnosjohn, you are going to want about 1.5 X the water depth of heavy chain - most folks buy some (BIG) cheep Chinese chain (like 1" for your application) with the water depth of lighter (and better grade) chain shackled to it. (so, 20' of your ACCO 1/2"). This will give you enough scope that you shouldn't put undue stress on your anchor under storm conditions.


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Hmm, I was thinking since the chain I have purchased is 1.5 times the max water depth that would be sufficient for length. Especially since I would obviously be using a pennant to connect to it and would be able to lengthen it accordingly in storm conditions. As to strength I recently went to the west marine mooring page linked earlier and it says that a 50 foot sailboat in 100kt winds would impart a load of 16,000lbs. My 1/2Ē chain has a rated breaking load of 27,600lbs. If I am way off base here feel to correct me. Like I said this is my first mooring construction. I am definitely on a budget but donít want to do anything wrong on something as important as a storm rode. And when we are finally cruising ready (hopefully next year) we will be bringing this setup with us specifically for storms.

Quote:
Originally Posted by pjShap View Post
If your mooring area has significant current you need more buoyancy to keep the ball visible at high current.
After a bunch more searching I decided to use a polyform a-4 series ball for my main buoy. Itís displacement is 22.5 gallons which if I did my math correctly should equate to almost 192.5 lbs of buoyancy. Hopefully that will be enough. I decided to go this route because itís able to be deflated when we finally set off cruising.
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Old 24-02-2019, 17:11   #8
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Re: Mooring ball buoyancy?

so how are you going to be sure to 75ft motor yacht doesn't use you mooring ball in a storm? forget legal here..just reality..
it could happen..
it happened it a sail group that I'm part of here in NC.. we no longer have
a mooring in Taylors creek. someone with a too big a boat moored on it in a storm.. dragged it to place not known..


otherwise..sounds good. a 2nd ball on the pendant..good..


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Old 24-02-2019, 17:35   #9
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Re: Mooring ball buoyancy?

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Originally Posted by dkenny64 View Post
so how are you going to be sure to 75ft motor yacht doesn't use you mooring ball in a storm? forget legal here..just reality..
it could happen..
it happened it a sail group that I'm part of here in NC.. we no longer have
a mooring in Taylors creek. someone with a too big a boat moored on it in a storm.. dragged it to place not known..


otherwise..sounds good. a 2nd ball on the pendant..good..


-dkenny64
Itís a screw in type mooring that is removable by simply unscrewing it. Iíll be taking the whole setup with me cruising and in the event of a storm my gf and I put on the scuba gear and place the mooring wherever we are. Iím not sure how any vessel but mine would have the opportunity to get to it before me. I guess the Main problem Iíll actually face is weather or not the bottom composition allows it to be fully screwed in or not.

I am worried about other boats taking the mooring while Iím still living in Florida though because we will be doing 1-2 week Bahamas trips. I guess Iíll just take the buoy with and let the chain sink to the bottom until I return.
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Old 24-02-2019, 17:56   #10
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Re: Mooring ball buoyancy?

The reason for the 1.5 plus 1 times the water depth is for scope of the chain on the anchor under stress. Under normal conditions, the heavy chain won't even get moved much - but when a storm occurs, you don't want the boat pulling the anchor around
At any rate, that's the combination I've seen in several "build a mooring" articles.



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Old 25-02-2019, 02:50   #11
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Re: Mooring ball buoyancy?

Quote:
Originally Posted by Kalimniosjohn View Post
... As to strength I recently went to the west marine mooring page linked earlier and it says that a 50 foot sailboat in 100kt winds would impart a load of 16,000lbs. My 1/2Ē chain has a rated breaking load of 27,600lbs...
Your chain, rated 27,600# B.S., only has a Safe Working Load of about 5,500 - 9,000 (at most) pounds.
Every piece of chain, rope, and end hardware has its own number associated with both its Working Load Limit (Safe Working Load) and Breaking (Tensile) Strength.

Break strength refers to the point at which any new/undamaged section of a given piece of rigging WILL FAIL.

Working Load Limit is always less than the breaking strength. The manufacturer's recommended working load is determined by taking the tensile strength and dividing it by a factor (often 3 to 5) that more accurately reflects the maximum load that should be applied to a given rig to assure a comfortable safety margin and longevity of the rig.
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