Ground Tackle & Anchor System Sizing TABLES & Swivels

[Ron Hill]

Guys : A shackle will deform first before it actually breaks.  I'm not too sure exactly what they use for their strength numbers in a shackle ?! 
As Mainsail said "your weakest link is the swivel"  That is a true statement !! 

[waterdog]

Steve, you have a very important piece of missing information:  What wind strength did you size your system for? 

Stu, when designing for wind speed, what assumptions are made about sea state?    If I'm in an anchorage that is protected from waves, 30kts of wind just loads up the gear, and it's almost static.   It's the unexpected 2-3 foot slop that stresses the hell out of the gear.   It's the damn waves that want to rip off the bow roller, chew through the rode, and snap the cheap Chinese cast shackle.   They're as much influenced by fetch and beach as wind. 

Does the "wind strength" design approach assume flat seas?  Throw in a margin for waves?   40 kts in Smuggler's Cove is a quarter of the problem of 40 kts at Turtle Bay.   

[Stu Jackson]

Steve,

Assumptions on sea state are included in the table above, which BTW is ABYC.  Calder's text, which is too long for my fingers to reproduce/retype here, notes that the table includes wind strength, sheering, current, sea state and shock loads.  He notes it is a conservative table "...if this table is used to size ground tackle, it will provide a significant margin for dealing with dynamic (surge) loads and other complicating factors."  

He also notes that dynamic loads can sometimes be absorbed by scope, but indicates that it is a very important issue, as your Smuggler's Cove vs. Turtle Bay comparison points out.  Having read the text, it's included in the table.  

Also, if there is any concern about this, do the calcs going one step higher on the first table and see the results.  If you go up one boat size, not wind strength 'cuz wind strength would make too much difference,you'll find that the determining factor may well be the available component strength.  This is true in most real life engineering comparisons: do all the calcs you want, but know that the conclusions for selection will be based on the strength of the shackles, rodes and chains that are out there on the market and on your chandlerey shelves.

That approach could be a better "engineering variables" analysis than it would be to simply go up one size in your anchor (only) selection, which could well be unnecessary, overkill, hard on your back, etc.  Which is what I did, I used the 40 foot boat at 42 knots, which is partway between 42 and 60 knots for the 35 foot boat.  Who says engineering is "specific?"  

I see so many boats with 3/8 inch chain with small anchors or, for that matter, with anchors the size our boats would normally use but with well over sized chain.  Unless specifically selected as you did Steve, over sizing chain or any other component is a waste of hard earned $.  Over sizing and mismatching system components and equipment, now that this information is available, is simply wasteful.  Even just a small bunch of years ago, it was not so easy to find this kind of summary approach.

[Hawk]

Last year I purchased a ss swivel that toggles up and down as well. But I then read the packaging and observed that it was made in China. Since then I haven't had the guts to replace the existing shackle that has worked just fine. I have read that chinese made swivels could be suspect.
On reading the posts below I am even less inclined to use it....wonder if I can get my money back!

Hawk

[pbyrne]

I realize this is an old thread, however Manuts has tables that seem to be very similar to the ABYC quoted by Stu.  Might be a little bit easier to sort through the various options for chain/shackle/anchor etc.

https://www.mantusmarine.com/mantus-anchor-knowledgebase/anchor-sizing/