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Go Back   SailNet Community > General Interest Forums > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 03-08-2005
e31 e31 is offline
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rode rider idea

I know that reducing the angle of an anchor helps increase it holding power, but in anchorages where the scope needs to be short, this is difficult to do.

A friend told me he use to hang the mushroom anchor from his dingy on the shackle between his chain and nylon rode, this would help increase the holding power without having to buy another gadget.

Does anyone else do this?
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Old 03-08-2005
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windship has a little shameless behaviour in the past
rode rider idea

e31,
What kind of scope are we talkig here? 3:1-4:1?
Dennis
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Old 03-08-2005
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rode rider idea

In some places doing this is called using an angel. It can be a good idea if you don''t have room in the anchorage to let out as much scope as you''d like. Usually you lower it down the rode with a long pennant so you can keep it from fouling the anchor itself. They can be a hassle if the pennant wraps around the rode too much.
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Old 03-08-2005
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rode rider idea

e31,

This weight is commonly referred to as a "kettle" or a "sentinel," for the obvious metaphorical reasons.

The weight certainly can keep the pull on the shackle of your anchor more parallel with the seabed, and therefore reduce the need for scope.

Connecting the sentinel where the chain shackles to the rode is one way to go. Those using all-rope rodes achieve the same effect by snapping the sentinel around the anchor rode and running it halfway down on its own retrieval line.

Note: This is a dynamic set-up: wind & current against chain and the weight and the placement of the sentinel. When the wind and/or current increases, the force on the anchor rode increases, and the sentinel may lift off the bottom (if it is not already suspended off the bottom). The stronger the force, the more defeated the sentinel becomes and the "straighter" the entire anchor rode becomes. Swinging radius should always be calculated as if the sentinel did not exist.

I''m sure others will be eager to add to my crude remarks. And I''m sure a little poking around the archived articles (behind "features" tab @ top of page) will yield some at-length discussion on the topic.
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Old 03-08-2005
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rode rider idea

E31, this is IME quite a common technique and especially so with those boats that, to avoid bow heavy loading conditions, don''t carry a full chain rode on their #1 anchor. So...to depress the catenary and make the pull more horizontal to the seabed, one lowers the sentinel down the rode using a secondary line. This works in medium strength winds and becomes all but ineffective in higher winds, as the weight of the sentinel must be such that it''s easily handled by the crew (30#? 20#?) and that''s not enough to counter the windage of the boat.

In the Pacific with its deep anchorages, often the reverse logic is used and a buoy is attached to the rode. In light winds, little pull is felt by the anchor but, as the wind builds and the boat starts to set back on its rode, it must first submerge the buoy. Larger buoys provide more resistence, yet are no more difficult to carry, inflate and attach; it''s a clever alternative.

You can make your own sentinel easily by filling a 4" ID PVC pipe with dense, heavy material (e.g. old wheel weights from the local tire store) with one end cap drilled to accommodate a loop of line in the end.

Jack
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Old 03-09-2005
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rode rider idea

Thanks all for the input...I did not realize this is a common practice. The scope I am looking at could be as little as 3or4:1 as the tide is close to 8'' and the little harbors can get quite packed with boats.

This though arose from an expirience last summer in which 6 out of 8 boats dragged in an blow one night which went more northerly than we expected.
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Old 03-09-2005
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rode rider idea

e31,
With a scope of three to one and shallow water, you could use a Bruce anchor and chain.
I never rely on a sentinal and the shorter the scope, the less the effect. With a scope of three to one, you''d have to have alot of weight on that sentinal for it to have any effect.
But again, if you really like this spot and want to go there and anchor safely, you might just as well make-up a rig you use just for that place(or any skinny water). Go with the Bruce and chain and watch everybody else drag.

Dennis

Dennis
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Old 03-10-2005
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rode rider idea

A sentinal, of a given weight, has more effect, the further it is from the boat.
Force = mass x distance.
The sentinal force does not depend on the total rode length (scope).
This force is not neutralized once the rode is straightened, and the sentinal elevated.
A sentinalís downward force, tends to keep the rode parallel to the bottom (improving the anchorís holding power), whereas the bouy tends to lift the rode, increasing the anchorís tendancy to plow. Both (sentinal &buoy) provide elasticity to the rode.
FWIW,
Gord
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rode rider idea

Gord,
A sentinal does depend on the lenth of the road. The longer the rode, the better the effect. What would the effect of a sentinal on a 1:1 rode be? It would be zero in any weather. Now, how about a 10:1 rode?
Once the rode is streightened and the catenary is non-existant, the effect of the sentinal is eliminated. The same goes for chain or rope.
I don''t understand why sailors would rely on a sentinal instead of having the proper ground tackle.
Sentinal''s were big in the time of wooden anchor''s and long scope''s. With the proper, modern ground tackle, sentinals are a thing of the past.

Dennis
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Old 03-10-2005
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rode rider idea

In a crowed anchorage a sentinal can mean the differance of safety and a wild night on anchor watch all night. dropping the angle of attack of the chain to the anchor can make a 5:1 hold like a 7:1. It is a good idea also to ask what scope you anchorage neihbors are using as well. If everyone is 5:1 a sentinal is a godsend.
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