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Old 07-13-2015, 07:40 PM   #21
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When you think about the loads you can generate

Hanging off a single anchor seems simple enough, When you very from the norm it appears things can ramp up very quickly. Say having a stern line to shore in a tidal bore with a bow anchor into the wind but tidal flow from the beam. I have a friend who in the early 60's father wanted to take his family down the Sacramento river from Red Bluff to the bay. He was a San Diego fireman who wanted run most of the major rivers in the US. He ended up running most including the great loop with a 16' outboard. This trip didn't end up as planned. The Coast Guard recommended rafting as the safest way to do the trip. My friend father decided to build a large raft of telephone poles banded together Huckfin style with a large turning oar at the stern. Fitted out with a redwood picnic table, green umbrella tent, coleman coolers, white gas lanterns and stove off they went down the river. The trip went well for several hours with the exception of the steering oar which just spun the raft in circles. About 1 pm the raft approached a large snag in the river. No problem the raft will just glance of the snag. This was not to be, the raft hit the snag head on, the stern of the log raft slumped down in the current and that raft flipped end for end like a paddle wheel all 60' or 70' All was lost including my friends transistor radio. Fortunately no one was hurt. That has always been a lesson to me about the force of water.
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Old 07-13-2015, 07:43 PM   #22
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Smart Man

Quote:
Originally Posted by markpierce View Post
I have no desire to anchor fore-and-aft, cross-current to face 20-knot winds, let alone accompanying several hundred other boats attending a Fourth of July fireworks. That's why you wouldn't see us anywhere near there until the Fifth.

Single-anchored in the Delta on the Fifth:





Regardless, how did it all work out for you? Anchor held? What weight, what type?
plow, came with the boat, no recommendations from me on type. 3/8 all chain rode helps big time.
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Old 07-13-2015, 07:47 PM   #23
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plow, came with the boat, no recommendations from me on type. 3/8 all chain rode helps big time.
I'm also a 3/8 kind of guy.
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Old 07-13-2015, 07:51 PM   #24
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I'm also a 3/8 kind of guy.

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Old 07-13-2015, 08:13 PM   #25
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Can't answer this on a phone at an airport bar...

...but I actually did try to calculate this once. If anyone is familiar with the "Powell River Hulks", I re-moored them ~13 years ago.

I was relatively close. I hired a Coastal Engineer with a Masters in Engineering Physics model and do the real math.

IIRC, the most significant load wasn't current or wind, but wind generated waves. Modeled several different significant wave heights.

I'll dig out the file when I get home.
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Old 07-13-2015, 09:13 PM   #26
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A chain rigged tight across 2 posts (anchors) with 1000lbs (boat) pulling from the center exerts 2000lbs pull on each post not 500lbs as most would assume.
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Old 07-13-2015, 10:44 PM   #27
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If I wanted to know the force exerted on my rode, I'd connect a digital hanging scale to the rode and record the force. Any calculated values would be hypothetical and suspect.

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Old 07-13-2015, 10:49 PM   #28
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I would agree with 1000# on each 'post'.
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Old 07-13-2015, 11:05 PM   #29
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I would agree with 1000# on each 'post'.
I think you will find 2000# is the correct number. Actually, if a chain could be placed straight between 2 posts, the force would be much much higher than 2000#, 2000# is the figure for 15 degree sag or 165 degree included angle. That is why anchoring in that manner is doomed to failure when current or wind picks up.
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Old 07-13-2015, 11:10 PM   #30
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Dang! I had a 50/50 chance....or did I get that wrong also?
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Old 07-13-2015, 11:15 PM   #31
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Easy. Answer is 7. I used the same formula king authur used to answer " what is the air speed of an African swallow laden with a coconut?"
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Old 07-13-2015, 11:17 PM   #32
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On my first ship we were anchored in a 4 point moor with 10" Sampson braid rope. We snapped two of the during a heavy storm that blew in on the beam. 4 20k anchors with 12" chain on 6 ton buoys that never drug. They weren't Rockna's.
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Old 07-13-2015, 11:17 PM   #33
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I too have all chain and am little help with this.....but can see there are some knowledgable folks on this board! Great info.
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Old 07-13-2015, 11:24 PM   #34
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No worry. There's 11 and a half months left to do the calculations when they will be needed.

The simple answer is: "Don't anchor where you will have to short scope (like at Mandeville) under extenuating circumstances." More is better. Heavier chain is better. Using the engine to ease the load until the anchor sets and the boat heads into the current/wind is key also.
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Old 07-13-2015, 11:38 PM   #35
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That's got me wondering what size my chain is. I'll have to go check.

Richard
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Old 07-13-2015, 11:42 PM   #36
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That's got me wondering what size my chain is. I'll have to go check.

Richard
That reminds me of a joke
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Old 07-14-2015, 06:13 AM   #37
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The hassle is it would take lots of tending to STAY at a 90deg angle to the current.

Any side loading causes the boat to move (think sailboat) which will cause one set of anchor lines to have a higher load.

Even a SQ barge will surge one way or the other.
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Old 07-14-2015, 07:34 AM   #38
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Ok, did I miss something here? Is the OP using two anchors "bow and stern" ?
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Old 07-14-2015, 11:29 AM   #39
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I was trying to discuss how rapidly we can increase anchor load,

It's all exploring how quickly anchor loads can ramp up with current, wind and way you set up your anchors. As soon as you get away from just dropping a single anchor how adding a second anchor can actually add to anchor loads. Something I hadn't really thought about. Another point I wanted to make is how rapidly current and wind velocity increase anchor loads.
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Old 07-14-2015, 12:03 PM   #40
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Quote:
Originally Posted by Brooksie View Post
A chain rigged tight across 2 posts (anchors) with 1000lbs (boat) pulling from the center exerts 2000lbs pull on each post not 500lbs as most would assume.

Chicken butt.
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