Spectra vs. Samson double braid for anchor rode.

[Anchormanagement]

Hi Stu and other community members.
I have been perusing the anchoring, anchor sizing, and anchor tackle threads. I am looking for practical and theoretical evaluations comparing the working load limits of double braid nylon vs. spectra. I own a 38 foot combination fishing boat in southeast Alaska. In the winter months this boat serves as a harvest dive platform. We spend 70% of our time anchored up. Expected conditions during the working day while anchored with divers and hoses (both diver airlines and 2" water lines) extended out to 350 feet can be extreme. It is not uncommon to be working in 40 -50 kt winds and an 8 foot sea with attendant 2-3 foot chop.  In between openings or waiting out a blow it is not uncommon to be anchored up inside a protected bay and have winds blowing a steady 70 or 80 kts with higher gusts. In these conditions you are always expecting to drag. My old anchoring system was a 60 pound bruce style anchor, 60 feet of 5/8 chain, 125 feet of 5/8 double braid. Many times I've seen that 5/8 double braid stretched down to 3/8 of an inch. This system was limited by the size of my anchor winch. I had a bigger winch built for the upcoming winter harvest season. (its still not big enough but I cant get anything bigger w/o compromising the floatation of the bow)  My new anchor is a 60 pound Manson Supreme. Chain is 100' of ½ " galvanized. 300 feet of 3/8 spectra. Normally I size my tackle by looking at the different parts and say to myself "holy cats, that's some huge chain that ought to work." I chose the 3/8 spectra by actually comparing its strength to the old 5/8 double braid. They are very close. The spectra has a breaking strength of 14k lbs. and the Samson braid is 15k lbs. I have some concerns beyond the 'jeez that stuff looks small' factor. One of my fellow harvest boat operators told me that spectra is terrible for anchor rode because it wont stretch and this will cause it to snap due to shock in heavy weather. "Its good everywhere on the boat but don't put it on the anchor" he said. I was considering going up to 7/16 diameter spectra to give myself some peace of mind. Then I found the sizing and working load spread sheet on these forums. Now I'm very concerned. There is no rope size on the chart that meets design standards when u plug in a 40 foot boat in a 60 knot blow. According to the spread sheet, and the posted tables from Calder's, double braid has a breaking strength of 6,375 lbs. and a working load limit of 1,275 lbs. Is it possible that those figures are kilograms? Samson's tables gives their 5/8 double braid anchor line a breaking strength of 15,200 lbs.   Despite this I am still going to use a rode above the chain to get some scope. I would like to use spectra because its strength to size ratio allows me to get more of it on the winch. Its working load limit is 5% its breaking strength, or 13,490lbs.  Rocna's anchor sizing chart suggests a 3/8 inch chain for the 55 and 73 pound anchor. It should follow that the rode strength be very close to the chain strength. The strength of 3/8 chain according to the spread sheet is 16,200 lbs. This is for grade 43 chain. ( I wish I knew that before I bought.) To further muddy these murky waters I pulled up a chain strength and grade table. My ½" chain is grade 30 galvanized. The table says its breaking strength is 18,000 pounds and a WLL 4500 lbs.   If I go up to 7/16 diameter spectra the breaking strength of the rode and the chain are close and the WWL are very disparate. If I keep the rode I have (3/8 spectra) the working load limit is double that of my ½" chain and is not then the weak link. Here are some pictures of my old anchoring system and the new one.https://www.facebook.com/MansonAnchors . The boat is the Ocean Point.  I am not really worried too much about dragging anchor any more. The Manson Supreme seems to have solved that problem. I am worried about the rode snapping. I keep the old ground tackle in the lazz. Should I be very concerned using spectra as opposed to Samson double braided nylon.

[Stu Jackson]

am,  I haven't looked into the charts and strengths, but IIRC the photos I posted from Calder's book had both pounds and kilos.

You wrote:

One of my fellow harvest boat operators told me that spectra is terrible for anchor rode because it wont stretch and this will cause it to snap due to shock in heavy weather. "Its good everywhere on the boat but don't put it on the anchor" he said.

He's right.  All anchoring texts I've ever read say that anchor rode NEEDS to stretch.

[Ron Hill]

Anchorman : I've used Sampson braid on braid anchor line for over 20 years. 
I've been in a few blows even rafted with another C34 on my Sampson double braid with out a problem.

An anchor rode need to stretch!  My thought

[Fred Koehlmann]

Three strand twisted nylon rode has served myself, my father and my father in-law well, for over forty-five (45) years. No broken rodes, no lost anchors.

Nylon naturally has a stretch to it, which is why it is deadly to use as a tow line (the recoil when snapping could kill you if you were in its path). This stretch helps deaden the jerks and tugs the line gets when you bob in the waves.

Plenty of people use braided line for rode, since it will fall more nicely and be kinder on your hands. However twisted line also has better stretch than braided, so historically it has been the preferred rode.

As they say on this forum, your boat, your choice. Cheers.

[DarthOccam]

AM,

You are asking good questions, and as you are finding out, there are no one size fits all answers.  One reason is that you are trying to satisfy conflicting goals and constraints.  The perfect rode has a huge WWL, absorbs shocks with reasonable damping, is impervious to abrasion and corrosion, simple to use, easy to store, compatible with your windlass, light weight, and cheap.  (If you find something that has all of these, let us know  .)  Did I mention cheap?

A chain/nylon combination rode is a good choice for many as they optimize for these goals/constraints for their situation and expected conditions.  As the desire for a higher WWL becomes more important, as in the use case you describe above, some people transition to an all-chain rode, with snubbers or other approaches to adding shock absorption.  This is a trade off against some of the other goals, as it can add complexity of use (or maybe not, depending on your windlass and bow roller setup), weight, and cost.

The question of 3 strand nylon vs. double braid is another example of optimization.  3 strand tends to be less expensive and has more stretch.  Double braid can be easier to handle and store.  What's more important to you?

Spectra isn't used in anchoring because, as other have pointed out, it doesn't absorb shock as it is designed not to stretch.   You either need to find a way to compensate for this or find a material that is a better fit for your goals and constraints.

Regards,

Michael

[mainesail]

Spectra can be a good choice to get over the bow chocks and minimize chafe but you'll still want 5/8" or larger as it is not 100% immune from chafe. I would personally end the use of Spectra after getting past the chocks.. You will want some elasticity in that rode but breaking strength should not be the real concern shock loading should be the real concern.

You might check out Nova Gold by Nova Braid it has excellent strength and stretch characteristics and is stronger than comparable three strand.

We don't know how big your boat is but I punched up numbers for a 38' foot sailboat. This would be much worse than a power boat...

38' Long
3.5' Freeboard
3.5' Draft
22' of water
100' 1/2" chain
100' rode
70 knots of wind

In an exposed anchorage the theoretical worst case load on this boat, with these criteria, is about 5000 pounds. This takes surge and wave action into account.

In a protected harbor the theoretical worst case load on this boat with these criteria is about 1840 pounds. This takes minimal surge and wave action into account and only counts wind.




http://www.novabraid.com/rope/novagold.html

[Anchormanagement]

I have read all of your replies and and considered them carefully. I think the key thing here...the crux of the problem is this: how much shock can a given material withstand before it will snap. Common ground tackle for commercial vessels in the Pacific NW has always been chain followed by cable. In order to mitigate shock the placement of a buoy along the rode can be deployed. Is it possible to determine how much shock a particular rode is likely to experience in the most horrendous conditions? 3/8 spectra has a working load limit of  around 13000 lbs which is 3x that of 5/8 Novabraid. The only reason i want this to work is because I can fit 300' on my winch. I want that scope. Price is not a factor here. One thing is certain. If I do decide that spectra is the best fit for my particular operation I will certainly be deploying a buoy on the rode. 

[Ken Juul]

Thats a new one, AM, can you explain "deploying a bouy on the rode"  Never heard of that one.

[mainesail]

Keep in mind that a buoy on the rode cab shorten your effective scope angle in lighter winds. This would mean figuring the scope you want from the buoy rather than the bow of boat.. In stiff winds the rode will become near straight anyway...

[lazybone]

This is a strange thread.
Anchormanagement man seems hell bent finding justification for using spectra for the sole reason that it allows for more linear foot storage on his windless. 
Maybe the windless is being used wrong or is of the wrong type?