[Cruisingdad]

Here is a new Anchoring THought/Discussion. It comes in part from Don Casey's Idea of using a Snatch block to keep your rhode from fouling your prop/keel/rudder when you do a Bahamamian moor.

My thought was a little different (Though, using his method). Use the Snatch Block and let her rest just up off the ocean floor. Connect one side to the chain and RUN YOUR LINE RHODE through the block. THis would be done AFTER you have set both anchors at the appropriate length. Illustration is such:

Also, here is a close-up:

Now, keep the chain slack. Make sure the tension stays on the rope line. As the wind blows the boat back, the rope line will pull its anchor toward the snatch block. Notice the heigth at which it pulls (lowers your scope and center of effort). Also, the chain anchor will be pulled, like the anchor line, at the snatch block.

Illustration:

Haven't you effectively managed to split the pulling force between the two anchors, yet still have a bahamian moor for when the tide/wind shifts?

Thoughts? Could this be used in a hurricane situation?

Again, the basic idea (at least as I understand it) is Don Casey's. I do not claim credit for it. I am simply modifying it to split the force on the anchors.

- CD

 

[erps]

I've done something like that up here in the PNW in benign conditions. I also had quite a mess after the boat had been turning circles a couple of days.

 

[sailingdog]

Good luck hauling those two anchors aboard, after the two rodes have twisted together.

 

[Cruisingdad]

I have already commented on the same thing. I agree, that is a problem. However, that will be an issue with ANY Bahamian moor. For those of you that have not had to deal with strong, daily tidal changes and strong storms, it can be quite different and interesting, to say the least. However, there are some places where you almost have to do a Bahamian moor - whether because of swing or strong tidal changes. As D Casey said, you can generally use the tender to turn you around. THis system also has the advantage that it will keep your lines from getting wrapped up in your prop/rudder/keel (which we had happen a couple of months ago and is a real PITA).

Any other thoughts? Would it work?

Brian

 

[lbdavis]

This would likely complicate matters further, but:

Could you attach a line to the snatch block for retrieval/weighing?

 

[Cruisingdad]

This would likely complicate matters further, but:

Could you attach a line to the snatch block for retrieval/weighing?

Easily, but I do not think it would ne necessary. It is already fixed to the chain.

- CD

 

[JimHawkins]

CD,
The forces in your picture are not accurate. With the boat pulling toward the left, and assuming the boat is tied to the yellow rode, I think the force on the chain is 2x the load, not 1/2 load. If you are tied to the chain, the force on the chain is 1x load, but unless you are going to retie your rodes every time the tide shifts, when it goes the other way you will be back to 2x load on the yellow. When the wind/tide shifts 90 degrees the forces are going to be even higher. I think it depends on the angle between the two anchors. The greater the angle, up to 180 degrees, the higher the loads. I can probably work out a formula for this, but I'm sure others here can tell us more accurately and faster than I can. The problem really comes down to the snatch block failing. The forces on it will be greater than normal anchoring forces.

 

[Cruisingdad]

CD,
The forces in your picture are not accurate. With the boat pulling toward the left, and assuming the boat is tied to the yellow rode, I think the force on the chain is 2x the load, not 1/2 load. If you are tied to the chain, the force on the chain is 1x load, but unless you are going to retie your rodes every time the tide shifts, when it goes the other way you will be back to 2x load on the yellow. When the wind/tide shifts 90 degrees the forces are going to be even higher. I think it depends on the angle between the two anchors. The greater the angle, up to 180 degrees, the higher the loads. I can probably work out a formula for this, but I'm sure others here can tell us more accurately and faster than I can. The problem really comes down to the snatch block failing. The forces on it will be greater than normal anchoring forces.

You may be right. It would take someone wit ha Physics background to work it out.

The Rope (yellow) rode goes from anchor, around block, to boat. It is the main holding line. THe chain is attached to the block also. However, it does not slide through the block. It is "permanently" fixed.

Also, since the block is close to the ocean floor, the pull on both of them should be relatively horizontal. Not sure the pic does it justice.

Thoughts? WOuld that be 2x on the chain still, or 1/2 on both?

- CD

 

[TomRay]

I have already commented on the same thing. I agree, that is a problem. However, that will be an issue with ANY Bahamian moor. ...As D Casey said, you can generally use the tender to turn you around. THis system also has the advantage that it will keep your lines from getting wrapped up in your prop/rudder/keel (which we had happen a couple of months ago and is a real PITA).

Any other thoughts? Would it work?

Brian

I have never tried this method, but would be worried about having a twisted mess down on the bottom. Although twisting is an issue with Bahamian mooring, at least the twists are usually found right at the pulpit, not down on the bottom. That also means that there is usually a slack anchor line, also accesible from above the surface. If you have a pair of tight anchor lines leading to a twisted mess on the bottom, how do you undo that without getting wet? Or, how do you undo that in a swift current? With a knife, I suspect.

Call me old fashioned, but I think lots of chain on both rodes and/or a kellet are better ways to keep the slack rodes clear of the boat, and I'd prefer to retrieve an anchor with the dinghy and then untwist the two rodes by doing laps around one than swim down and deal with a tangled mess under water, probably in a swift current.

 

[scottyt]

i do it the lazy way, throw an anchor of the back, motor forward and set it. then i run up front throw and anchor tied off at like 9 to one scope, run back pull boat backward by hand till front line is tight then use motor to set it. i then adjust the 2 rodes to where i want em, and cleat off. i have been doing this all winter with danforth anchors, i have not dragged including a bunch of 30 knot days with 50 knot gusts. my boat stayed exaclty where i left it when i could not get to it for over a month when i hurt my shoulder.

why worry about twisting lines in the wind when the boat does not rotate

the only ***** is getting them both up, let out line on the back, pull boat forward, cleat line, motor forward to break it free. then pull backward let the wind spin me and motor to break rear anchor loose.

 

[TomRay]

why worry about twisting lines in the wind when the boat does not rotate

The point of a Bahamian moor is to allow the boat to rotate to face the wind or the current, whichever is stronger. With 4 tide changes per day and passing thunderstorms that might send a gust from any direction, the boat will end up rotating all the way around eventually. If you stay someplace a few days, it's not uncommon to have a twist or three.

 

[Craig Smith]

JimHawkins is correct; your forces are all wrong. You cannot decrease the horizontal force applied to the anchor out front, and tension applied with a second opposing anchor can only increase that force.

Moreover, I don't see from your diagram that this tension from the second anchor will lower the vertical lifting force on the main anchor. It will remain the same, with only greater horizontal force (hence the lower angle deceiving you). Conversely the vertical force on the boat will be greater (sum of that of the two rodes).

The numbers range from "no difference" (with no tension applied to the second anchor) to a doubling of horizontal force on the main anchor, assuming the secondary anchor can sustain the same (with rope and chain tensions equalized).

~

The only way to relieve the force on the primary anchor is to use additional anchors (assisting in the same direction). The only way to ensure equilibrium between such multiple anchors is to daisy-chain them in a tandem rig, whereby the force on the primary anchor is relieved by the opposing force of the attached tandem out front. This is speaking in theoretical terms, and may or may not be necessary.

 

[SVAuspicious]

It would take someone wit ha Physics background to work it out.

Have you ever jumped a halyard? The physics are the same. Your diagram "jumps" the connection between the anchors so that the force pulling the anchors together is a significant multiple of the force you see at the boat.

Not so good.

Simple is better. One anchor whenever you possibly can, a classic Bahamian moor when you have to.

 

[AdamLein]

Alternatively, you can treat the system as a block-and-tackle.

If the current is setting towards the chained anchor, so that the chain is slack and the rope is taut, your snatch block can ride around on the rope as much as it likes and it's as though the chain isn't there, except insofar as the chain is acting like a kellet on the rope.

If the current is setting towards the roped anchor, there are two subcases:

(a) The rope is slack. Similar to above, except without the kellet feature.

(b) The rope is taut. This requires that the chain, from snatch block to chained anchor, is also taut. This is identical to a two-to-one purchase: if you imagine your boat is on the hauling part and the roped anchor is on the fixed part, then the snatch block is the moving block which has two parts coming into or out of it.

So the practical use I could see for this is, if you wanted to really make sure your (chained) anchor will hold against forces that are twice as strong as what you would normally apply when setting, you could rig it up as you've displayed and set your anchor with a two-to-one purchase.

 

[Cruisingdad]

It was at least worth a discussion.

I aagree with Simple... but there are simply some places (as noted) that you cannot get by with a single hook. Still, good discussion and thoughts.

Brian