Anchoring mathematics for very small boats

[Greenheart]

I always enjoy thinking about anchor design, cable catenary, scope, etc...

...so a web page which I found, looked like interesting reading...mainly, it aims to focus on dinghy-anchoring, and explains (in the author's view) why light unballasted craft require as much chain/anchor-weight/rode elasticity as much bigger displacement craft.

I found this page after midnight, and I'm very sleepy, so possibly I'm mistaken...

...but do the quoted mathematics strike anyone else, as a tad improbable? Here's an example, lightly tweaked by me:

"The forces on a ten-foot dinghy are roughly equivalent to those (on) a twenty-five foot displacement boat. That will (require) 50′ of 5/16″ chain, or 29′ of 3/8″ chain. Add to that 300′ of 1/2″ three-strand anchor line and a 20lb standard anchor and you’ve got an adequate anchoring package for up to 45′ depth."

An Avon Redcrest, carrying as much chain and line as the Folkboat she belongs to?

Granted, the author endeavours to pre-empt doubt such as mine; and doubtless his approach is on the cautious side, which must be wise amongst divers who anchor little boats in deep water. It still sounds excessive to me, though.

Here's the website address: http://www.my-boat-works.com/blog/?p=597

 

[Seajet]

Dan,

whatever that author is smoking, I'd stay to windward if I were you !

I note along with full ground tackle dinghy tenders are supposed to have splash / flood resistant electrical systems, well that's me rowing to and from the mooring knackered then.

 

[Rossynant]

whatever that author is smoking, I'd stay to windward if I were you !

"I simply love kelp" could be a clue...

 

[Conachair]

This is the best one I've come across..

http://alain.fraysse.free.fr/sail/rode/static/static.htm

 

[vyv_cox]

I have communicated several times with Professor John Knox over anchoring matters. He has derived expressions for various anchor and chain loadings. Amongst those is his finding that to use the length of a boat, as opposed to its cross-sectional area in wind loading calculations, is surprisingly accurate. Hence his expression: Force (in kg force) = (1/500) x (LOA in metres squared) x (wind speed in knots squared), backed up by measurements using his Anchorwatch load cell.

 

[Lakesailor]

I always enjoy thinking about anchor design, cable catenary, scope, etc...

Whatever you feel happy with.......

In respect to the question posed. I can't imagine how he could justify that. A Mirror dinghy exerts the same force on the anchor line as a Westerly Centaur-sized boat?

All sounds a bit TB to me.

 

[Greenheart]

A Mirror dinghy exerts the same force on the anchor line as a Westerly Centaur-sized boat?

I believe the chap was referring to inflatable dinghies...I got the feeling he regarded the lightness and instability of a flat-bottomed inflatable (which refuses to dig into a wave's surface) as causing greater tendency to 'snub' a small anchor out of the ground...

...his conclusion seemed far-fetched even as I was nodding off, but I wondered what you gents thought.

I suppose however mad his heavyweight set-up might seem, it's less foolish than the contemptible undersized hooks and mostly-rope cable, on a short scope, used by some people to secure much bigger boats.

 

[Barnacle Bill]

Thing is, anchoring mathematics is quite complex, and not really suited to real decisions on the day.

The guy who wrote the original blog is clearly a diver, and one of the problems with 'do-it-yourself' diving is that good dive sites make bad anchorages, and vice versa. (Try anchoring at the Eddystone for example.) In my view the best strategy for diving is to leave one person in the dinghy and not to anchor: and yes, that includes if there are only two of you, because an experienced diver is probably safer diving solo and knowing there is a reliable boat on the surface. Plus you'll get a much better dive if you aren't constrained to coming back to the same place at the end (you can, for example, drift-dive with the tide which is huge fun.)

If you are going to 'anchor' for diving, the first task when you reach the bottom is to attach the rope chain anchor or grapnel securely to rock or wreckage, and the final task before ascent is to undo it so it can be hoisted. This is entirely unlike anchoring a larger boat.

It's extremely instructive, if you are a diver, to pop down and have a look at how the anchor and chain of a big boat behave when the sea/wind is getting up a bit. It is a very dynamic situation. You can have loads of chain out, but still there will be occasions when the 'random' motion of the boat on the waves lifts the chain off the ground almost up to the anchor itself. You are highly likely to go back on board and let more chain out ...

 

[Greenheart]

It's extremely instructive, if you are a diver, to pop down and have a look at how the anchor and chain of a big boat behave when the sea/wind is getting up a bit. It is a very dynamic situation. You can have loads of chain out, but still there will be occasions when the 'random' motion of the boat on the waves lifts the chain off the ground almost up to the anchor itself. You are highly likely to go back on board and let more chain out ...

Great reply, thanks Bill! Non-divers certainly put blind faith in cable weight, length and anchor-manufacturer's claims, when dropping the hook...

...how long till we get an 'anchor-cam', pointing down the shank to show those on deck how tenuous their grip is, on the seabed?