Increasing a rode's caternary

[DanW9494]

A 7:1 scope on a set anchor usually means a good night's sleep. The purpose of a 7:1 scope is to give adequate caternary of the rode to improve the angle near the bottom. This "angle of attach" is improved by using more chain or a kellet weight, as long as the wind is moderate.

Another way to increase the caternary would be to add a modification to a stern anchor. Use the stern anchor to pull the bow anchor rode into a greater caternary. You need two rode shackles, and old heavy duty block, and a length of nylon line smaller in diameter than your bow rode. Shackle the nylon line to the bow anchor chain rode at least the water depth or near the end of the chain, whichever is less. Set the bow anchor as usual, playing out the nylon line. Place the bitter end of the nylon line through an old heavy duty block connected to a rode shackle. Attach that shackle to the stern anchor chain rode at about the same distance from the anchor as done for the bow anchor. Set the stern anchor as usual, and equalize the scopes. Then pull up the nylon line to increase the caternaries on both rodes and cleat it off. You might let out the bow and stern rodes a little to be able to tighten up the nylon line more if desired.

If your anchors are set, this should keep them set, as well as snubbing the rodes as well.

This worked well for me last summer several times, but the winds never really tested it well.

I guess this is a modification of Bahamian mooring. It wouldn't be typical, but could be used when you expect increasing winds, you don't have room for 7:1 scope, or when other boats are stern anchored and you can't get 7:1 scope. How could this be improved?

 

[tsmwebb]

"improved bahamian moor"

http://www.sailnet.com/forums/miscellaneous/25495-better-bahamian-moor.html uses a similar method. It should reduce your swinging circle but you still need adequate scope. If you set it someplace with a strong reversing tide expect to spend some time untwisting things when it's time to recover the gear... Since it will make you swing differently than boats hanging on single anchors it is a good idea and polite not to use in close proximity to conventionally anchored boats.

--Tom.

 

[Ross]

If you anchor in ten fet of water and have four feet of freeboard at the bow you can simply anchor to a ten to one scope and add only forty-two feet to your swing. If that gets in somebody's way they are too close. I have never had anyone drop the hook within shouting distance of us.

 

[DanW9494]

This is Don Casey's idea!!!!

http://www.sailnet.com/forums/miscellaneous/25495-better-bahamian-moor.html uses a similar method.

--Tom.

Wow! Thanks Tom. Well at least now we know it works in the big leagues. And using snatch blocks is certainly faster and more convienent to use than rode shackles. My version only has the advantage of being cheaper.

Has anyone used it or wondered if they should?

 

[Franklin]

I always thought that you can get away with much less scope with the improved bahamian mooring. The tighter you pull the aft rode, the lower the block goes and so the less angle of pull you have on the two anchors.

I have a bullwagga that will reset itself every time so I don't ever worry about flipping it, but I do know that I need two anchors to do the bahamian mooring if everybody else in the anchorage is doing it too. Otherwise, I'll reverse into them when the current changes (if it's strong enough...manytimes the current just changes the direction the boat is pointing but the wind still pushes it).

 

[Paul F]

Anchor Kellet

Could an anchor kellet do? I use a home made anchor kellet from 40 pounds of weights and plastic hose covered chain attached with a quick link. Using this kellet attached around both fore and aft anchor rode might also accomplish the goal of snubbing and setting the two anchors.

 

[tsmwebb]

parallel evolution

It's encouraging to see folks coming independently to the same solution to a problem. In a bit of propinquity I'd been thinking about the improved Bahamian moor because we're in La Paz and the anchorage outside of the town has reversing currents to 5 or 6 knots and when the wind blows it tends to be parallel to the stream. That's why I had the link handy. This would seem to be the ideal place for the IBM but nobody does it despite the fact that during the "La Paz waltz" (change of tide) boats bump into one another when the wind pipes up... Strange. I've anchored in a few other places iaround the Pacific where an IBM would seem like a good idea (eg. the river in Opua, NZ and Brisbane Austrailia's river and broadwater) but never seen anyone use the system. Maybe it's because most folks in the tropical Pacific anchor on all chain or maybe it's just that most SP anchorages don't have reversing currents and so we're just not in the habit. It's not like we don't all carry multiple anchors and even occasionally set stern anchors or take lines ashore and even set two anchors off the bow. We just don't seem to do the Bahamian moor... Any ideas why? Anyway, if I can find a spot where I will not be in anybody's way I'll give it a try and see if I like it.

At to kellets, yes, they are great and much simpler to set and recover than extra anchors. I occasionally use one that I make up of spare dive weights and have even hung a spare anchor as a kellet, but not that often... I found a kellet absoultely required to prevent keel/rudder/sail drive hang-ups when I experimented with a floating rode in Brisbane.

--Tom.

 

[RichH]

There is a 'problem' with the 'improved' Bahamian Moor technique when anchored.

This implies a 180 degree angle for the second anchor .... Then if nature AND trigonometry come into effect as the NEW wind comes at **right angles** instead on that 180 ... the forces generated into the two 'tight' anchors goes towards infinity and you now have Two anchors breaking loose!!!!! Its OK when one of the previously set anchors is being 'worked' but when the forces of the wind/current are acting on TWO anchors (with a 'tightly set combined rode', and the boat 'in between' .... expect BOTH anchors to break out. For this situation when the boat is pulling at 90 degrees at BOTH anchors .... you must remove that 'snatch-block'/rolling hitch, etc. pronto and allow a great deal of slack between those two anchors.

 

[tsmwebb]

This implies a 180 degree angle for the second anchor .... Then if nature AND trigonometry come into effect as the NEW wind comes at **right angles** instead on that 180 ... the forces generated into the two 'tight' anchors goes towards infinity...

It's a good point. As I understand the set-up once you're in place and the anchors are set you drop enough rode to make sure the connection between the anchors is below the keel and then make both rodes fast. If you only want to halve the longer axis of the singing oval you will only double the stress on the rode at 90 degrees. In an emergency you should be able to slack off the rode that runs through the block and ride to the other anchor. In practice would this be worse than having a cross wind when you have a line ashore or a stern anchor? This is a common enough occurance and it doesn't typically break gear or pull anchors. Still, one of the reasons that 99% of the time we ride to one anchor is that you've got more options and less of a mess when things go wrong...

--Tom.

 

[Franklin]

ummmm.......NO!

you don't want to provide slack. You want the rodes as tight as possible. The tighter they are, the lower the block and the lower the rodes.

Also, the pull on the anchors doesn't increase, it decreases!. The pull never changes as the boat will always have the bow towards the wind or current and the pull will be the same regardless if it's 90 degress or 0 degrees or 180 degrees. The only difference is, when it's 90 degrees, the force is divided between two anchors instead of one. The direction of the pull is also mostly the same as long as you have the two rodes tight. The more slack you have in the rodes, the more of sideways pull you have.

 

[RichH]

No Franklin

Applied stress that is PERPENDICULAR to the axis of a rope, cable, etc. etc. produces towards an INFINITE reaction load in that 90 degree member and there is little deflection of the rope because of the initial stress ('pulling the 2 anchors 'tight' towards one another). Simple trigonometry - as this a special case where you are approaching dividing by the trigonometric SIN of 90 degrees, which result is ZERO and anything divided by ZERO is infinity.

 

[Dave Groshong]

We offer the Rode Rider here in the chandlery:

http://shop.sailboatowners.com/detail.htm?fno=400&group=1049

 

[Franklin]

No Franklin

Applied stress that is PERPENDICULAR to the axis of a rope, cable, etc. etc. produces towards an INFINITE reaction load in that 90 degree member and there is little deflection of the rope because of the initial stress ('pulling the 2 anchors 'tight' towards one another). Simple trigonometry - as this a special case where you are approaching dividing by the trigonometric SIN of 90 degrees, which result is ZERO and anything divided by ZERO is infinity.

infinity....that's funny. So a 2 knot breeze blows from the side and two storm anchor can't hold. BS. If your math was correct, this method would never work and so it would never make print. If the force was "infinity" then all it would take is a mere 2 seconds as the wind shifted from bow to stern and the anchors would be out.

 

[Franklin]

To prove my point....do this. Get 4 milk jugs and fill with water. Tie one to each end and run each over something so they are balanced pull on each other. Now put two jugs on the line in the middle. You will see that the two jugs on the side will rise up to the level of the two in the middle of the line. Now friction is part of this equation but only a little and it can be reduced by using a little oil or using blocks, but the point is, if you have 100 lbs of pressure on the boat via wind and current, then you have 50 lbs on each rode if you are pulling sideways.

 

[tsmwebb]

Interesting. It seems to me that one could control the depth of the bock by varying the tension of the rode to which the block is lashed relative to the rode that runs through the block. Perhaps there is an optimal tension. It seems to me that the trade off is between strain on the connection between the anchors and the amount that the anchors have to veer assuming that the swinging "circle" is acceptable.

Also, the pull on the anchors doesn't increase, it decreases!. The pull never changes as the boat will always have the bow towards the wind or current and the pull will be the same regardless if it's 90 degress or 0 degrees or 180 degrees.

No, if the pull on the boat stays the same the pull on the anchors varies. The aspect of the boat will also vary. Broadside to the wind or tide is not uncommon in a wind on tide situation.

Rich is correct that the boat pulling at 90 degrees to the anchors initially has virtually infinite mechanical advantage. This is exactly the same trick that people use when they jump halyards. Of course, just as with jumping halyards, the advantage will quickly dissipate as the angles get larger. My initial take is that this is probably not a major problem in reasonably protected anchorages but I do think it is worth some more serious thought.

--Tom.

 

[Franklin]

There is no mechanical advantage. You can only get mechanical advantage with a rope by using blocks....aka...pull one foot on this line will result in 1 inch pull on the end point. That is impossible with just one block as the IMB calls for.

With both rodes going off the bow (not the original post but the IBM), then as the wind shifts, so does the boat. Now I'm not including swing on anchor because you have that anyway unless anchored from the stern.

Don't believe me, do the test I posted. I just did a few mintues ago.

 

[Franklin]

The pulley system on the left in the attachment shows how to get mechanical advantage from a rope. The pulley system on the right shows no advantage at all, just added resistance. Now I'm not going to do the math to come up with exact numbers, but if you pull on the bottom rope of the left one a foot, the top end will move only about 4 inches but will with much more force then you pulled with. If you pull one foot of line on the right one, the end point will move one foot. No mechanical advantage!

 

[Franklin]

Basic rule of mechanical advantage

The basic rule is this, if the end point moves less then the start point, then you have positive mechanical advantage. If the end point moves more then the start point, then you have negative mechanical advantage. If both points move the same distance, then you have zero mechanical advantage.

It is very hard to do a 3D drawing for me using Paint so I kept it simple, but the point is clear. If the boat is pushed back one foot, both anchors will move one foot towards each other as both rodes are going through only one block and attached to the bow. That means zero mechanical advantage.

I hope the picture helps clear it up.

 

[Franklin]

My mistake

You can have mechanical advantage with one pulley if it's like the picture which of course IBM is not.

If one end of the blue line is tied to something and you pull on the other end of the blue line a foot, then the red line will move 6 inches with twice the power that you pulled on the blue line with.

 

[tsmwebb]

is there an engineer in the house?

Franklin,

Thanks for all of your work. Nice stuff.

Perhaps I was speaking too loosely. I'm sorry for any confusion that I caused. The question we have at hand is one of statics and is typically set-up as a free body diagram. A quick google search has come up mostly empty. I'm not totally happy with this link but it's illustrative of the issue: http://www.mytreelessons.com/Pages/Rope Angle Leverage Calculator.htm.

--Tom.