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perchance · 25-12-2008, 17:49

What is the best type of anchor for use in rocky bottoms?

AnchorageGuy · 25-12-2008, 18:24

A Fisherman.

S/V Illusion · 25-12-2008, 18:34

make sure he's properly weighted down as some of them have been known to float and swim a while

AnchorageGuy · 25-12-2008, 18:47

we can only hope we would get yet another anchor thread.

**noelex 77** · 26-12-2008, 03:09

If forced to anchor in rock the best option is a grapnel type. The multiple flukes stand the best chance of grabbing something. Make sure the flukes are strong enough some are designed so that the flukes will straighten out when the anchor is caught.
As the design is not very critical and the chance of loosing the anchor is high a cheap locally made design is often used. Talk to the fishermen.
Of the commercial designs a fisherman or variations on a fisherman are the best and also a useful option in thick weed. Of the general purpose anchors possibly the Bruce with its 3 flukes will be best.
If you can dive on the anchor wedging any anchor (or even the chain) by hand into a rock crevice is often the best way. The anchor does not have to be large, but take into account that rocks weigh less underwater and a "substantial" rock may move more easily than you imagine.
If possible always choose another anchorage as anchoring on a rock bottom is a lottery.

**Pblais** · 26-12-2008, 03:37

Quote:

but take into account that rocks weigh less underwater and a "substantial" rock may move more easily than you imagine.

I suppose if you were on a different planet they might. While a rock falling into a pool may travel slower through water than air it still has the same mass. Weight is commonly the measure of acceleration due to gravity. It's nominally the same everyplace on the planet. The fact that the acceleration falling through water is less than air does not change the mass of the rock. A crane under water could not lift a rock it could not lift on land nor could it pull one out of the water. Same is true for anchors. Chain is also the same strength in or out of the water.

We agree anchoring in rocks is a lottery. I go around the point and anchor in the sand or mud. Never select a poor anchorage when you don't have to. Navigation errors tend to create such circustances.

**GordMay** · 26-12-2008, 04:38

Objects DO weigh less under water than they do out of water.

Don’t conflate mass with weight. Mass is a measurement of the amount of matter something contains, while Weight is the measurement of the pull of gravity on an object.

Archimedes’ Principle states that: “the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object.”

Hence, the weight (holding power) of a submerged rock (or concrete mooring block) is less than that same rock above water, whilst it’s mass remains constant.

exranger · 26-12-2008, 06:14

So I need a mass of anchors? lol.

I know there are a lot of anchors threads, read most myself, I think. But how about this: Maybe a thread where we list the 4-5 most common bottoms, and one or two GOOD anchors (not the "best") for each? When I first started, i would drop a plow onto a rocky bottom, or a danforth on some bottom it won't hold, etc. Maybe we can come up with some general guidelines on some that work on mud, some on sand, some on rock, and conversely, some advice on what we generally agree won't work on these??

**Pblais** · 26-12-2008, 07:34

Quote:

Hence, the weight (holding power) of a submerged rock (or concrete mooring block) is less than that same rock above water, whilst it’s mass remains constant.

Not really. The mass always is the same so the force to move it is the same. F= mA (force = mass times acceleration). Buoyancy is not an issue here as rock is denser than water and has zero buoyancy. The density of water causes the rock to sink slower in water because it is a denser material than air. The fact that air and water are different does not change the rock. You can not lift anything under water that you could not lift on land unless it is unaided by buoyancy. What it's weight is means nothing. A submerged rock or anchor has no buoyancy what so ever. There is no other force pulling the rock to the surface to counteract gravity. It does not float on land for the same reason it does not float in water.

The concept of displacement is only the difference between the density of air and water. An object floats on water because it's net density is greater than air (or it would float above the water) and less than water (or it would sink to the lowest level). Gravity is the constant in all cases.

The problem is that weight is measured in pounds and force is measured in Dynes or Newtons in the English system. Since we are air breathing mammals that live on land, pounds will work for an English weight system that is too often used to speak about force. Weight has nothing at all to do with force. Anchors have everything to do with force.

An anchor holds by more than it's mass and those that don't usually prove to be the worst anchors. None of them float.

AnchorageGuy · 26-12-2008, 07:39

Quote:

Originally Posted by exranger

So I need a mass of anchors? lol.

I know there are a lot of anchors threads, read most myself, I think. But how about this: Maybe a thread where we list the 4-5 most common bottoms, and one or two GOOD anchors (not the "best") for each? When I first started, i would drop a plow onto a rocky bottom, or a danforth on some bottom it won't hold, etc. Maybe we can come up with some general guidelines on some that work on mud, some on sand, some on rock, and conversely, some advice on what we generally agree won't work on these??

Because you will get 2,000 posts on which ones are "good" and then the anchor vendors will jump in and there goes the bandwidth for the entire site.

CaptHead · 26-12-2008, 09:13

I use dead fish to weigh down my fisherman anchor. That way I might catch a big one when I weigh anchor. You have to knock them out first. That is the trick.

**noelex 77** · 26-12-2008, 10:32

Quote:

Originally Posted by Pblais

Not really. The mass always is the same so the force to move it is the same. F= mA (force = mass times acceleration). Buoyancy is not an issue here as rock is denser than water and has zero buoyancy. The density of water causes the rock to sink slower in water because it is a denser material than air. The fact that air and water are different does not change the rock. You can not lift anything under water that you could not lift on land unless it is unaided by buoyancy. What it's weight is means nothing. A submerged rock or anchor has no buoyancy what so ever. There is no other force pulling the rock to the surface to counteract gravity. It does not float on land for the same reason it does not float in water.

The concept of displacement is only the difference between the density of air and water. An object floats on water because it's net density is greater than air (or it would float above the water) and less than water (or it would sink to the lowest level). Gravity is the constant in all cases.

Paul I am afraid Gord is correct.
Mass is constant. An object has the same mass everywhere (even on another planet), but its weight will vary.
Think of a spring scale. This measures weight. A rock will register less weight on the moon than the earth. It will also register less weight underwater than on land. This is because all objects displace some water and so have some buoyancy, even if they sink. Note the mass is unchanged in all cases.

So a person can lift an object that is submerged of greater mass than they could if it was out of water. The difference is small for a dense object like a lead brick or anchor, but significant even for objects like rocks. If an object has a density only slightly greater than water a small child might lift a mass of many 100’s of kilograms because its weight underwater will be much less.

saumur · 26-12-2008, 11:00

I was taught to use a fisherman in rocky areas as they are cheap. If you have to dump it when it gets stuck then it is not such a nightmare as ditching your spanking new Rocna.