Windlass Problem Using A Length Of Chain Between Anchor and Rope

[KiwiKen]

"The CQR is all chain and the Danforth has 150' of rope and 25' of chain at the end. The problem is that the rope drum on the windlass won't wind the chain."

"You could consider doubling the length of chain so your anchor is pretty well always going to be on the bottom still, while you are swapping the rode from the drum to the gypsy, to make that job easier. Theoretical solution only - never tried that."

A rule of thumb is that the chain length should be one boat length, the chain serves two vital purposes.
It ensures that the pull on the anchor is horizontal and thus unlikely to break it out,
and secondly, the chain acts as a shock absorber, when a load comes on the anchor due to wave action etc, then the chain lifts absorbing the energy of the additional load on the rode, thus ensuring that there is no shock load going on the anchor that could break it out.
It is essential that the length of chain is sufficient to ensure that the pull on the anchor is still horizontal ie increasing the length of the rode in bad weather.

[double u]

Originally Posted by Jim Cate

FWIW: If you take two turns of the chain around the drum and tail firmly, it will likely work just fine. It is likely to scar the drum, but it will haul in the chain. Oil rig workers use this technique to screw together and unscrew pipe sections.

Jim

had me standing to attention in a nanosecond in the bunk when a kiwi next to us did that in teh wee hours of the morning

[Just Another Sa]

Originally Posted by KiwiKen

A rule of thumb is that the chain length should be one boat length, the chain serves two vital purposes.
It ensures that the pull on the anchor is horizontal and thus unlikely to break it out,
and secondly, the chain acts as a shock absorber, when a load comes on the anchor due to wave action etc, then the chain lifts absorbing the energy of the additional load on the rode, thus ensuring that there is no shock load going on the anchor that could break it out.
It is essential that the length of chain is sufficient to ensure that the pull on the anchor is still horizontal ie increasing the length of the rode in bad weather.

Unfortunately that old theory was and still is totally wrong.
One meter of 10mm chain has mass of approximately 2.3kg, and it's effective weight under water is 20 Newtons, due to it's buoyancy.
If the boat has an all chain rode with length of 100.045 meters between connection points at the anchor and boat, and said connections are 12.565 m vertically separated, and the horizontal load on the chain is 8000 N, the connection points of the chain will be 100.000 meters horizontally apart. The rode will be oriented horizontally from the anchor, but the chain will not touch the seabed from anywhere else except the anchor connection. It will curve upwards immediately after that point. If the chain would have infinite strength and stiffness, and you would pull it perfectly straight, while maintaining vertical distance of connections the same, the horizontal distance would only increase by 0.260 m. In the real world the boat will move far less than that, which means the chain will not absorb any significant amount of energy due to catenary effect! Only dragging anchor will allow for any significant movement of the boat!
If the chain is only as long as the boat, the effect will be much less than that, and totally irrelevant magnitude to be worth even considering.
The only time chain will act as shock absorber due to catenary effect is then the load is really small, and no shock absorbing is therefore even required.
When it is most needed, the chain will allow more movement while going slack during lulls, resulting higher boat speeds, with associated momentum and kinetic energy until the horizontal load on the chain finally matches windage and the boat begins decelarating. Unfortunately there is practically no catenary effect anymore available to absorb that energy, as the chain is so close to being straight at the time the boat begins decelerating. The result in the real world, supported by actual engineering calcs, is that the catenary effect increases substantially the shockloads on the boat and anchor, instead of reducing them! A snubber is the way to go for all chain rodes, or alternatively rope rode is needed. Both can handle shock absorbing fine, even with the chain doing all it can to make it more difficult for the snubber to accomplish that!

The short length of chain has practically no effect on the angle of the pull on anchor either, another bogus claim with nothing to support it. A long and heavy chain can and will change the angle, but then the chain must really be long, and almost the same effect can be achieved with 50% of the rode closer to the boat being rope, and only the other part closer to anchor chain.
The 100 m chain in the example is the minimum length to direct pull on the anchor horizontal in that water depth. In shallower water somewhat shorter chain will do the trick, but the catenary effect is even smaller, and that 0,26 is already far too little compared to what is required for shock absorbing. A shallower water with same length (100 m) chain will reduce the catenary effect.

The vital function of the short chain with mostly rope rode is chafe protection, not either of the ones you claimed.

[double u]

"... will not absorb any significant amount of energy due to catenary effect..."
&
"When it is most needed, the chain will allow more movement while going slack during lulls, resulting higher boat speeds, with associated momentum and kinetic energy until the horizontal load on the chain finally matches windage and the boat begins decelarating"
- observed many times while snorkeling/anchorchecking
very well put JAS!

[double u]

my thinking goes towards lighter chains with higher load capacities to reduce this "sailing-@-anchor" with the associated "fetching-up" of the boat after drifting back in the gusts which puts heavy lods on the anchor (this was very pronounced when using 10mm all-chain rode on our 37' boat)