Conference unifix::sailing

See Note 373. The load on an anchor rode is a function of the wind speed
and the area of the boat (topsides and rigging) perpendicular to the
wind (assuming no waves). The load in mild conditions is rather small,
actually, so that a very small diameter line (say 1/4 inch) would be
sufficiently strong. 

There are considerations other than strength, however. The rode should
be elastic to reduce shock loads. Shock loads can be very high, and thus
a larger diameter rode (say 1/2 inch diameter) is needed for safety in
rigorous conditions. Too large a rode (say 3/4 inch) may have
insufficient elasticity. Chain rodes may need nylon line shock 
absorbers. 

A smaller diameter rode chafes through much more quickly than a larger 
diameter rode. 

A larger diameter rode takes a larger volume to stow, is heavier, and 
more expensive. 

Theory aside, we have four 1/2 inch diameter nylon rodes and one 5/8 
inch diameter nylon rode for our 32 foot, 12000 lb cutter.

Alan

    My understanding is that the key metrics are the surface area presented
    to the wind and the speed of the wind. I don't think displacement
    figures into it much at all. The use of LOA is a way of (gu)estimating
    surface area.
    
    Ed 

    This note was .1  I removed and edited it due to some typo errors
    I missed.
    
    As with the choice of anchor, the size anchor line has more to do
    with the load placed on it than anything.  I think most charts try
    to generalize displacement and windage into LOA. They probably do
    not error too much doing this, and they usually add some disclaimer
    about going one size larger than indicated for severe conditions.
    
    I would say that for a single all purpose rode, a line of nylon twist
    should be about 5/8". This has a typical a tensile strength of 10,000 
    pounds and a working strength rating of 2,000 pounds which is probably 
    more than your boat can generate short of full hurricane conditions.
    
    I use this size line on our 10,000+lb. 33'. It was recommended by the
    manufacturer and provides a very comfortable ride (the component
    of stretch that reduces shock loads).
    
    Walt

re .4: 

According to the formula in Note 373, it will take a 117 mph wind to 
generate a 2000 lb load on a 33 foot boat. It will take a 108 mph wind 
to generate a 1700 lb load (20% of the breaking strength of 1/2 inch 
diameter nylon braided line). Quite a huuricane!

The problem is, I would think, really with shock loading due to wave
action. It is shock loads in severe conditions that break rodes, pull
windlasses off decks, and do other interesting damage. 

Consideration should be given to the strength of the anchoring SYSTEM, 
not just a single component (ie, a rode). Steel thimbles should be used 
in eye splices, not plastic, splices should be served (a good activity 
for a foggy day), shackles should be saftey wired, large backing plates 
must be used under cleats, etc. Above all, use large anchors and large 
scope. 

    RE: .5    "SYSTEM".  Amen.

    
    RE: .4     
    
    I used displacement in my first paragraph in addition to windage
    because of the secondary effects of wind (waves, sailing on the
    anchor, etc.) which add to the stresses caused directly by wind
    and windage.  This isnt particularly scientific on my part. Rather
    it is a compilation of various "recommended" charts (like the 
    West Marine Advisor and the Danforth holding power), plus my
    boat makers recommendation (Tartan tends to be slightly conservative
    - IE my "33" is 33'8" - 2" shy of the Sabre "34") and my own experience
    with the 5/8" line on the hook and as dock line (my boat held the
    remants of "C" dock and several boats in place at Masthead Marina
    during Gloria - but thats a different story). 

    Walt