SailNet Community

SailNet Community (http://www.sailnet.com/forums/)
-   Gear & Maintenance (http://www.sailnet.com/forums/gear-maintenance/)
-   -   Anchor Rode Spectra? (http://www.sailnet.com/forums/gear-maintenance/103658-anchor-rode-spectra.html)

Anchormanagement 09-13-2013 11:50 AM

Anchor Rode Spectra?
 
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:eek:) 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?

overbored 09-13-2013 01:25 PM

Re: Anchor Rode Spectra?
 
If I were you, I would contact a manufacture of the type of line for their recommendation. there are many types of line made using the new UHMWPE materials known as Sprectra/Dyneema. it is braided into line that has no stretch to lines used for moorings with stretch. Spectra is a trade name for rope made by Novabraid. just saying I am using spectra rope does not meam anything to me as the specs vary widely depending on the specific line that you are using. the material is being use in commercial fishing and towing with great success but it needs to be the right line for the job to work well.

zz4gta 09-13-2013 03:46 PM

Re: Anchor Rode Spectra?
 
You don't need to worry about it breaking. It'll tear hardware out of the boat before the line snaps if you sized it right. I would also talk to a manufacturer about it. Sounds like the system is overbuilt which would further reduce any shock absorbtion.

Anchor rodes should stretch so it doesn't snatch the anchor off the bottom.

SlowButSteady 09-13-2013 03:48 PM

Re: Anchor Rode Spectra?
 
Your friend who warned you not to use Spectra is right on the money. Nylon is normally used in large part because of its ability to absorb shock loads. Such loads can easily multiply the forces to which your ground tackle and boat fittings will be subjected in choppy seas. Lots of chain and an anchor kellet will also help to reduce shock loads, to a point. But a 100+ feet of nylon is hard to beat as a shock absorber.

svHyLyte 09-13-2013 04:21 PM

Re: Anchor Rode Spectra?
 
Quote:

Originally Posted by Anchormanagement (Post 1088211)
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:eek:) 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.

FWIW...

Anchormanagement 09-13-2013 06:18 PM

Re: Anchor Rode Spectra?
 
You know, every boat I ever fished on over 50 feet uses chain and cable. I don't think that cable stretches. I did call Novabraid before I posted. The guy there seemed to think that because the working load limit of the rope was so much greater than that of the chain that it should be fine as anchor line. He did not try to talk me into going back to nylon. He said I was correct in assuming no shock mitigation but that the line would not break. I'm thinking that you guys are correct. I don't want to stress out the entire system. If I go with 5/8 double braid my chain will likely end up being reduced to around 50' and my rode to 150'. That's half the available scope that I have now. I'll look into down sizing the chain to 3/8 grade 43 to regain some scope. As far as plait goes... I just now looked that up. Working load limit for 5/8 is only 932 lbs. http://www.unirope.com/fiberropes/fr...lonbrait.shtml That's much to light.

manatee 09-13-2013 09:31 PM

Re: Anchor Rode Spectra?
 
Could you make a nylon 'shock absorber', suitably sized, so that a bight of Spectra hangs loose and the nylon stretches to absorb shock gently, instead of the sharp snub from Spectra?

Something like this:

Anchor Shock - Heavy Shock

SlowButSteady 09-13-2013 10:11 PM

Re: Anchor Rode Spectra?
 
Quote:

Originally Posted by manatee (Post 1088419)
Could you make a nylon 'shock absorber', suitably sized, so that a bight of Spectra hangs loose and the nylon stretches to absorb shock gently, instead of the sharp snub from Spectra?

Something like this:

Anchor Shock - Heavy Shock

That might work, to a very limited extent. Nylon, or any material, absorbs energy over its entire length as it stretches. Use five feet instead of 100 feet and you'll basically have one-twentieth the shock absorbing ability.

svHyLyte 09-14-2013 10:30 AM

Re: Anchor Rode Spectra?
 
Quote:

Originally Posted by Anchormanagement (Post 1088349)
You know, every boat I ever fished on over 50 feet uses chain and cable. I don't think that cable stretches. I did call Novabraid before I posted. The guy there seemed to think that because the working load limit of the rope was so much greater than that of the chain that it should be fine as anchor line. He did not try to talk me into going back to nylon. He said I was correct in assuming no shock mitigation but that the line would not break. I'm thinking that you guys are correct. I don't want to stress out the entire system. If I go with 5/8 double braid my chain will likely end up being reduced to around 50' and my rode to 150'. That's half the available scope that I have now. I'll look into down sizing the chain to 3/8 grade 43 to regain some scope. As far as plait goes... I just now looked that up. Working load limit for 5/8 is only 932 lbs. : : : UNIROPE LTD. : : Fiber Ropes : : : That's much to light.

I believe you may be mistaken as to the strength of plait. For example, NE Ropes 5/8" mega braid mooring cable is rated at 10,300#. Take a look at New England Ropes - Product Details
I believe that the SailNet Store, appearing to the left of this frame, may also offer mooring line with similar capabilities.

cupper3 09-14-2013 10:52 AM

Re: Anchor Rode Spectra?
 
Neither chain or cable stretch. The difference is one can use the catenary as a shock absorber with chain, which effect is minimized with rope.

Would using snubbers help?


All times are GMT -4. The time now is 08:49 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012