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Join Date: Nov 2008
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Re: Anchor Rode Spectra?
Originally Posted by Anchormanagement
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. It’s around 14 net tons. 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. I replace this every other year whether it needs it or not. This system was limited by the size of my anchor winch. I had a bigger winch built and installed 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. (it sure looks small
) 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. Samson’s tables gives their 5/8 double braid anchor line a breaking strength of 15,200 lbs. 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 on the winch (3/8 spectra) the working load limit is double that of my ½” chain and is not then the weak link. Is that right? Here are some pictures of my old 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. I guess the crux of the problem is this: Is there a way to determine the peak shock load a rode can withstand before it parts? Is the peak load on the rode different than the peak load on the chain? I think the spectra is stronger than the chain. Keep in mind that spectra is new technology and very, very strong. I really do not want to go back to filling up the winch with nylon double braid because it takes up so much space. I can easily deploy a buoy on my rode using a longline snap. Will this work as a shock absorber and do I need it?
The problem you may have is that although Spectra is remarkably strong for its size/diameter, it has poor energy absorbing capability. In essence, its lack of elasticity works against it in extremis. While a buoy might provide an adequate damping effect to counter this disadvantage/weakness with long enough rode and deep enough water, absent doing rather extensive computations, that would be a difficult judgment and even with would be rather heavily biased by one's assumptions. Given your gear, I would think that 8 or 12 plait nylon brait would be/would have been preferable to Spectra.
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