Fw: [Rhodes22-list] Anchor Chain Question

[Roger Pihlaja]

----- Original Message ----- 
From: Roger Pihlaja 
To: sanderico at earthlink.net 
Sent: Tuesday, March 15, 2005 6:13 PM
Subject: Re: [Rhodes22-list] Anchor Chain Question


Rik,

The author has done a force balance wherein he has compared the aerodynamic wind loading on the boat's hull and rigging vs. the force required to pull the anchor up out of the bottom.  If the wind loading force is less than the force required to pull the anchor out of the bottom; then, the anchor is successful.  This is a valid engineering approach.  I only had good enough data sets to check the author's calculations for the mud bottom and hard sand bottom cases.  The numbers for wind loading force and anchor holding force check reasonably well for those two cases.  So, despite the weird combination of engineering units, apparently the author did his units conversions correctly.  I still can't get used to force being expressed in kg?  Oh well, maybe that's how the Aussies do things.

Here in the Great Lakes, we commonly get sand, mud, rock, and weedy bottom types in our anchorages.  The author's nomograph & theory apply reasonably well to homogeneous bottom types like mud & sand.  But on a rock bottom, my experience has been that the anchor's holding power is essentially zero until the anchor happens to get caught on or under a rock ledge.  Then, the holding power can be as high as the yield strength of the steel in the anchor's flukes!  That's why anchor types that have a lot of "hooks", like the grapnel, are preferred on a rock bottom, because they are much more likely to get hooked up than other types.  The author's theory and nomograph don't present a realistic picture of anchoring in this situation at all.  The reality of anchoring in this situation is more like a digital "go" or "no-go" type of answer vs. the continuum implied by the author's theory.   

Similarly, on a weedy bottom, a standard Danforth type anchor will simply "skip" along the weed bed, shearing off weeds, & collecting them in its flukes until it looks like a ball of vegetation.  The Danforth anchor is very unlikely to be able to penetrate the weed bed to dig into the bottom.  Whereas, plow type anchors, like the Bruce & CQR, will tend to penetrate the weed bed & get into the bottom.  But, even a plow type anchor may not penetrate the weed bed every time and several attempts may be required to properly set the anchor.  The anchor's holding power is near zero if the anchor doesn't penetrate the bottom and I'll accept the author's predicted results for holding power if the anchor does penetrate the bottom.  Knowing your anchor is plenty big enough to keep you in place would be small consolation if you couldn't get it to penetrate the weed bed & every time you pulled it up it looked like a giant ball of tangled weeds.  Again, the author's theory and nomograph just don't present a realistic picture of actual anchoring on a weedy bottom.
  
A few years ago, Practical Sailor magazine did an anchor comparison test which included the Flook Flying Anchor.  As I recall the test, the Flook anchor didn't perform as advertised.  It didn't "fly" thru the water like it was supposed to, didn't set especially well compared to Danforth & CQR types, and the holding power was no better than those types.  Practical Sailor magazine concluded the Flook anchor was snake oil.

Bottom line, the theory and nomograph are useful for basically sizing the anchor for your boat.  But, you still need to carry sufficient different anchor types to handle the various bottom types in the area where you sail & you need to know when to use what type of anchor.  The theory presented is really only valid for homogeneous types of bottoms like sand or mud.  The more nonhomogeneous the bottom type, the less well the theory works.

Roger Pihlaja
S/V Dynamic Equilibrium   

      
----- Original Message ----- 
From: "Rik Sandberg" <sanderico at earthlink.net>
To: "Roger Pihlaja" <cen09402 at centurytel.net>
Sent: Tuesday, March 15, 2005 9:23 AM
Subject: Re: [Rhodes22-list] Anchor Chain Question


> Roger,
> 
> I'm afraid, doing the math is a bit over my head :-) But, after having 
> read many anchoring articles from various people, I find the logic (in 
> terms of improving the rode caternary, weight placement along the rode 
> and shock absorbtion) in this one quite good when viewed from my usual 
> perspective ..... just plain old horse sense.
> 
> I hope you will be able to find the time to check this guys math and let 
> those of us who are numerically challeged know if it all still makes 
> sense when you're done.
> 
> I will look forward to seeing your results.  :-)
> 
> Rik
> 
> Roger Pihlaja wrote:
> 
> >Rik,
> >
> >I downloaded the article you referenced, very interesting!.  The author's
> >use of the theory is sound.  However, for some reason, the author uses an
> >odd combination of engineering units: knots, pounds mass but not pounds
> >force, meters, feet, both mass and force in kg?  I was expecting to see
> >force in newtons when the rest of the calculation was done in the MKS metric
> >system.  I haven't had a chance to check his math.  But, it's been my
> >experience that calculations made with inconsistent engineering units tend
> >to have mistakes in them.  Normally, what's done, is to do a calculation all
> >the way thru to the end in one system of engineering units.  If the results
> >are to be presented in more than one system of units, the conversions are
> >made at the end on the final results.  So, be warned; even though he's got
> >the theory correct, his numerical results may be wrong because of this units
> >conversion issue.
> >
> >Roger Pihlaja
> >S/V Dynamic Equilibrium
> >
> >----- Original Message -----
> >From: "Rik Sandberg" <sanderico at earthlink.net>
> >To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
> >Sent: Monday, March 14, 2005 4:02 PM
> >Subject: Re: [Rhodes22-list] Anchor Chain Question
> >
> >
> >  
> >
> >>William,
> >>
> >>Here's an excellent paper on the physics of anchoring. Might be more
> >>than you really wanted to know, but I'd say this guy knows his stuff.
> >>Sounds like the best combination is a half, give or take, chain, half
> >>nylon anchor rode. here's the link.
> >>
> >>http://www.dulhunty.com/dmp1.htm
> >>
> >>Hope this helps.
> >>Rik
> >>__________________________________________________
> >>Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> >>
> >>
> >>    
> >>
> >
> >
> >
> >  
> >
>