Re: What size nylon rode?
Posted by: Bil
Date: February 6, 2019 07:29AM
Category: Anchors, rodes and ground tackle
Excellent questions, Jonathan, about the contradictions between safe anchoring and stowage on a BCC.
To answer you directly: Zygote has two hybrid rodes, one for the small bower (aka secondary bow anchor or kedge) and one for the stern anchor. Both of those hybrid rodes are made of 5/8" (0.625 inch) or 16 mm 3-strand nylon plus a chain pendant of about 26 ft (8 m). The bitter end of the nylon is made with a bowline to the port bitt (down below); the working end terminates in an eyesplice with a thimble, and a bow shackle to the chain pendant.
So not quite equivalent to your setup.
Z's small bower is a Danforth 20H (20 pounds or 9 kg). Her stern anchor is a Danforth 12H (12 pounds or 5.5 kg).
New England Ropes reckoned that its 5/8" 3-strand nylon when new, dry, and clean had a tensile strength around 11,650 lbs (5.28 tonnes). Of course being wet, being stained with rust, being riddled with salt crystals and sand grains reduces that. NER quoted the lineal mass of its 5/8" rope as 9.6 lbs/100'. For the small bower, Z carries 600 foot (184 m) of that rope, with the idea that carrying 57.6 pounds (26 kg) of nylon and an 8 metre pendant of chain with a mass around 14 kg allows us to anchor in about 20 metres depth at a 7:1 scope.
The more significant mass is in the best bower (aka primary bow anchor), a 15 kg Rocna, and its all chain rode (a bit less than 300 feet/92 m of 5/16" 8 mm Grade 30 BBB that has a mass or about 164 kg or 330 lbs.
Several years back, when I was writing Zygote's Stability Information Book and Stowage Book, I did some simplified calculations of the inertial moment generated from the ground tackle.
Having changed PCs a few time between then and now, I don't have all the calculations. But I found a crumpled envelope after I read your e-mail and did a little estimating on its back.
We stow Zygote in a couple of modes. Main one is voyaging. But we also have a mode for sheltered waters. In Sheltered Waters mode, we'll often hook the Rocna around the bobstay. But in voyaging mode we carry the Rocna on deck, in line with the scuttle hatch. In all modes, the Danforth 20H sits on its side on deck with most of its chain pendant ranged in a box that pins both of them inboard of the port bulwark, with the centre of gravity about level with the forward end of the deckhouse.
That of course means Z does not have a self-launching anchor. To anchor, the foredeck crew has to unlash whichever anchor is to be used and lower it over the side. And the reverse when weighing.
The disadvantages of not having a self-launching anchor are obvious. The benefits are in minimising the contribution to inertia. If I talk about approximate centres of gravity (and stick to metric measures for conciseness), the 9 kg Small Bower ( SB ) is stowed 0.7 m forward of the notional CG of Zygote. The 15 kg Best Bower ( BB ) is stowed 1.6 m forward of the notional CG of Z.
The 26 kg of hawser (H) is in the most forward compartment (the Hawser Bin, forward of the No. 1 bulkhead), with its CG 3.83 m fwd of CG of Z.
The chain rode for the best bower is stowed in two lumps. One lump, the Anchor Rode Fwd (AF, is under the Sail Bin (the compartment between Bulkhead 1 and BH2, just aft of Z's sewage holding tank) about (and I probably overestimated this) 2.92 m fwd of CG of Z. The second lump, Anchor Rode Aft (AA), spends its time in the most forward part of the bilge, sitting under the sole directly under the scuttle hatch (that fwd bilge is also a shower bilge, although to date no one has used the forepeak shower). That puts it 0.7 m fwd of the CG of Z.
To calculate the pitching moment of inertia, the easy formula is mass x (distance from the CG of Z)^2
where ^2 means to raise to the power or 2.
I won't bore you with the resultant calculations (because what is relevant to you is your own calculations). I'll just close by attaching a diagram with the locations of H, SB, BB, AA, and AF (plus G, the centre of Gravity of Zygote; M the metacentre; and B, the centre of Buoyancy) marked.
Edited to stop parenthetical BB turning into an emoticon and correct typos! But I put the marker for AA in the AF location and vice versa! Sorry.
Edited 4 times. Last edit at 02/06/19 07:40AM by Bil.
ZSIB - anchors rodes.jpg (74kB)