Breaking Strain

[naggi]

My boat is 26' and has a displacment of 2.2 ton,
the ground tackle I use is 60m of 10mm x 8 strand nylon braid rode (braking strain of 2.4) and 4m of 8mm chain using various anchors. All the gear is in good order.
My question is,
Do you think that this setup is ok, or am I traveling to light?
cheers Tim

[GordMay]

An approximate rule of thumb suggests that Rope should be roughly double the diameter of the Chain to which it's matched.

A typical 26' boat might use a rode rated from 500 up to 1000 Lbs (2.22 - 4.44 kN), requiring 7/16" - 5/8" (say 1/2") 3-strand nylon (don't have specs on 8-plait handy), coupled to 1/4" (6.35 mm) PC/BBB “G30" Chain.
I'd recommend a longer chain; at least 10m (& much longer if anchoring in a lot of rock or coral).

Another Rule of Thumb:
Working Load Limit (WLL) of Chain in kiloGrams = Diameter x Diameter (in mm) x grade ( L = 30) x 0.3
8 x 8 x 30 x 0.3 = 576 Kg
576 x 2.2 = 1267 Lbs

[klem]

Another way to look at it is what the expected load is. If you are only going to be anchoring for lunch on calm days, the load will be so small that just about anything would work. However, if you plan to anchor in storms, you will need very strong gear. Depending on where you are, if you have severe thunderstorms or squalls or something like that, I feel that the gear should be capable of handling these as a minimum. For reference, the ABYC gives a suggested mooring load of 1500 lbs (3/4 MT) for your boat which should cover most sudden severe weather events but not things like hurricanes or typhoons.

With chain, you simply need the safe working load to be below the expected load, they have included a safety factor for you. If you have old chain that is rusty or worn, remember, it will no longer be as strong.

Nylon line is rated in terms of breaking strength and it is wise to use a safety factor of approximately 5 on this. There have been many documented cases of nylon lines breaking at much lower loads than the rated amount due to cyclic loading. Also, it will weaken a lot with age and use. I would recommend a line with a breaking strength of 4 MT.

[David M]

Safe Working Load is what you want to rate things for. The SWL for different materials varies with the material and the application.

[Bash]

I second GordMay's advice. You should have at least 8 meters of chain on that boat. Consider that an absolute minimum. If it were me, cruising, I wouldn't be comfortable with anything less than 15 meters.

[GordMay]

Originally Posted by klem

... For reference, the ABYC gives a suggested mooring load of 1500 lbs (3/4 MT) for your boat which should cover most sudden severe weather events but not things like hurricanes or typhoons ...

NACM Chain Specifications
➥ NACM Chain Spec's (pg.11) G3, G4, & G7 - National Association of Chain Manufactuirers (NACM) "Welded Steel Chain Specifications" (13 pages total) Goto: http://www.nacm.info/Downloads/NACM_Welded.pdf Cruisers & Sailing Photo Gallery

ABYC Recommended Working Load Limit for Anchor Rodes (Rope & Chain)
➥ WLL for Anchor Rodes - ABYC Section H-40, Table 2 Cruisers & Sailing Photo Gallery

ABYC Design Loads
➥ Design Loads for Deck Hardware - ABYC Section H-40, table 1 Cruisers & Sailing Photo Gallery

[naggi]

Thanks for the links and comments, much appreciated,
they confirm that my rig is on the light side and should be upscaled to suit our boating enviroment down here.
The last night we spent aboard, it got fairly blowy (20-25kn), the anchor had set well on a mud bottom, but my confidence in the anchor line was mininal and made for an uneasy night.

[MasterMariner]

Get yourself some spectra rope as strong as cable for equal size and weighs nothing. It's not cheap but will last if you take care of it.

[Adelie]

If you interpolate the ABYC anchor load table for your boat length you can expect anchor loads of 540 and 1080lb in 30 & 42kt of wind speed respectively. I don't know the beam of you boat which might give a different answer. There are number of alternative analytic methods, such as outlined in the Heinz book on anchoring, but that is more work than I care to put into this and frankly I don't have enough details of your boat to even begin. Also ABYC table is pretty conservative so any answer coming out of it will probably suffice.

Since you are on the cruisers forum I will assume that you intend some long distance sailing, coastal or offshore, meaning that you probably should use the higher value of 1080lb.

1/4" (6mm) Proofcoil/BBB/G30 chain should have a Safe Working Load (SWL) of 1300lb assuming SWL is 25% of failure load. It seems to me your chain size is just fine.

Fiber line is a little trickier. In the US the Cordage Institute recommends a 10% SWL for nylon rode. In his Cruising Handbook Nigel Calder suggests 25% is more realistic. At 10% you would need 16mm, at 25% you would need 11mm. According to John Vigor's "The Sailor's Assistant", for a boat your length he suggests 6mm chain and 14mm nylon. My feeling here is that if you are staying in relatively protected waters what you have is fine size-wise. If you start venturing further afield you might want to beef up your nylon line.

I concur with GordMay that 4m of chain is a bit short. The rule of thumb I have heard is that the chain length should be at least as long as the boat. My personal preference is for 35-50m of chain plus 100-200m of nylon for the main anchor. For the secondary and backup anchors I would use 10m or so of chain.

If you use a swivel in your anchoring system, do not connect the swivel directly to the anchor even if it seems most convenient, there needs to be a shackle between the anchor and swivel or the prying action of the swivel on the anchor shank will break one or the other when a very heavy wind veers.

[Adelie]

Originally Posted by MasterMariner

Get yourself some spectra rope as strong as cable for equal size and weighs nothing. It's not cheap but will last if you take care of it.

The problem with spectra is the almost non-existent stretch, without significant stretch, shock loads in a bumpy anchorage may break the anchor loose, break shackles, pull out deck hardware.

[MasterMariner]

Originally Posted by Adelie

The problem with spectra is the almost non-existent stretch, without significant stretch, shock loads in a bumpy anchorage may break the anchor loose, break shackles, pull out deck hardware.

What do you think the chain is for?

[Adelie]

Originally Posted by MasterMariner

What do you think the chain is for?

Chafe resistance on coral.
To prevent the boat from sailing back and forth at anchor.

The chain will provide some shock resistance up to 20-25kt. By the time you get to 40kt there is essentially no give left. For a detailed explanation of this check this post I did: http://www.cruisersforum.com/forums/...tml#post737667.

If you use an even lighter anchor rode, what give there is will disappear even earlier.

[MasterMariner]

Originally Posted by Adelie

Chafe resistance on coral.
To prevent the boat from sailing back and forth at anchor.

The chain will provide some shock resistance up to 20-25kt. By the time you get to 40kt there is essentially no give left. For a detailed explanation of this check this post I did: http://www.cruisersforum.com/forums/...tml#post737667.

If you use an even lighter anchor rode, what give there is will disappear even earlier.

Ah maybe with enough chain as you should have if you are a prudent mariner the rope does not matter. Do ships use rope or large yachts? No. The chain acts as a shock absorber and a weight to hold the anchor/boat hence why you put out a scope of line proper to depth of water. For a small boat 60' or less you would be just fine using spectra with chain. It does not absorb water so it's a nice light alternative to a heavy rope and it's small and storable but with supreme strength. You're boat will be breaking before you drag anchor if used with the proper tackle.

[Adelie]

Originally Posted by MasterMariner

Ah maybe with enough chain as you should have if you are a prudent mariner the rope does not matter. Do ships use rope or large yachts? No. The chain acts as a shock absorber and a weight to hold the anchor/boat hence why you put out a scope of line proper to depth of water. For a small boat 60' or less you would be just fine using spectra with chain. It does not absorb water so it's a nice light alternative to a heavy rope and it's small and storable but with supreme strength. You're boat will be breaking before you drag anchor if used with the proper tackle.

No ships and very large yachts do not use nylon. I notice that they don't use spectra either. As vessels get very big scaling effects make nylon unsuitable, the nylon line would be so big as to be impractical. Instead very large vessels use other techniques such as running the engines in forward thru a storm in order to take some of the load off the rode. Multiple anchors is also a normal occurrence on very large vessels when heavy weather approaches. A large crew makes much of this possible.

Have you used spectra as anchor line on your boat? If so how did it work? What is the heaviest winds and waves you have anchored in?

If not, do you know anyone or know of anyone that is using spectra for anchor line?

[MasterMariner]

I have indeed on 18' runabout worked great. Also on a 40' custom steel yacht worked great as well. The reason ships don't use it is as stated it is expensive/ too light for large applications but a small boat no problem. Been at anchor in 3' seas no different then nylon as long as the proper scope with chain is used. I'm simply offering a opinion I think it is great stuff for small boats. If you are at anchor in a small boat in over 5' waves with high winds you prob. should be heading for the harbor anyways.