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Go Back   SailNet Community > Skills and Seamanship > 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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  #21  
Old 05-01-2002
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maxcontax,
Good morning.
I think you made the right choice, especially with the Bruce.
I do alot of anchoring and the Bruce has never even come close to failing. Yes, it does reset itself because it tends to ''roll'' and dig in with the side flukes to remain set in wind shifts and tidal shifts. And it does work quite well in sand and mud also. By the way... I have just installed a windless set-up an a twenty-seven foot power boat. I set him up with a seven point five KG. Bruce so you should be good in winds well in exess of forty-two Kts. depending on the bottom.
Watch out with the danforth. If it drags it will trap weeds and or rocks in it''s flukes and then your all done. It won''t reset itself. If it breaks out it might not reset itself.
Also, the Bruce can be used with much less scope if need be, and be sure to use about one boat lenth of five sixteenths chain for your boat.

Dennis
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  #22  
Old 05-01-2002
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Dennis,
Your remarks appreciated. The anchor matter is quite important to cruisers, esp. in a little boat where I can anchor in thinner water than you: my choice of oversized anchors is in part due to my preference to anchor closer to shores. It is nice to spend time at anchor, away from the cost and traffic at the dock. My charterboat experience is like yours: the Bruce''s never failed, the two times I dragged anchor were with Fluke anchor designs in weedy/rocky bottoms, and the most dramatic anchor drag was watching the "Spirit of Chemainus" (100 ft heritage vessel) drag her plow anchor across the clay bottom of Silva Bay (Gulf Islands, Canada) in 35 knot winds, in the night, unattended, rescued by the rest of us in our tupperware boats. I had a Bruce down with all chain rode @4:1, so wasn''t in the game. I still think the anchor needs to match the bottom to be effective, hence my two different anchors.

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Old 05-17-2002
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Thanks for all the astute comments and experience..I learned a lot. I myself dragged anchor in Key West on my 34'' O''Day sloop
setting a Danforth in the sandy bottom. I woke up at 2 am aground on a sandbar about 4 miles from where I had anchored. The boat dragged through a small flotilla of anchored boats and by the grace of God missed them all until coming to rest. Boat US Tow boat rescued me and got me back to a better anchorage. My problem was obviously not enough rode, a high tide, and strong current and winds. From then on I let out a lot my anchor line til the scope was at leat 6 or 7 no matter where I was in the Keys. No more dragging anchor I am pleased to report. The Danforth is great in the Chesapeake so far--with sufficient SCOPE.
Jerry
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Old 05-17-2002
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WOW....4 miles?!.....This is a great reason to set an anchor alarm on a handheld GPS that you keep next to your bunk if you are ever concerned that will happen again.

These handheld GPS units are getting cheaper and cheaper BoatUS sells the Garmin eTrex for just $119. Much cheaper than a tow or any other damage you might have incurred.

The anchor alarm can be programmed to trigger on whatever range you set. They are so accurate these days (with the WAAS enabled ones) that if you set it to low it will trigger when the boat just swings on the anchor (So you then need to back off on the sensitivity)
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Old 05-17-2002
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Four miles is a WOW!

If you log your time/distance as skipper, you now have "miles under power", "miles under sail", and "miles while at anchor." Could make for very intersting conversation.

Glad it turned out as well as it did, Jerry.

Duane
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Old 05-19-2002
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I like that very much...sounds like a Robert Frost poem. And yes, I am glad it turned out well also. Did I say that Key West was a great place to visit? We gave up anchoring and took a slip at a local marina...expensive but worth some peace of mind as the winds can blow hard and steady down there..Now, want to hear about trying to catch the Gulf Stream back to Miami?
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Old 05-22-2002
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It seems that cruisers often go to short scope in popular moorages to limit swing--largely due to crowded conditions. I suspect this is one of the causes of dragging anchor. What about the use of kellets when using short scope? any experiences or techniques to share on this aspect of anchoring?
Size of kellet, use of a leader along the anchor rode, distance from anchor, style of anchor? Any critiques on whether it is worth the fuss?
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Old 05-22-2002
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I think a kellet (or sentinel) can really help. Especially when using a short scope on a mostly nylon rode .

If I am forced to shorten my scope (re: limited swing in crowded anchorage). Then I always think its worth it and it has worked for me and made me sleep better at night. I think almost anything could be used as a kellet and you needn''t pay alot of money for a fancy one. You could just get a small mushroom anchor and a carinber Hook to attach to your rode (and a retreival line).

I like to get the kellet about half way down the rode, which also helps dampen the boat from "surging" due to swells if you get them. Further down might help more with regards to keeping the anchor shaft horizontal.

Take a look here for some technical information about the catenary effects and keeping the anchor in.

http://www.johnsboatstuff.com/Articles/anchor.htm

And here is a manufacturer of Kellets (Sentinals) with some info. Personally I would just get a 20lb mushroom anchor if needed.

http://www.anchorbuddy.co.nz/technic1.html
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Old 05-22-2002
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It seems to me that there is starting to be to much emphisis on using a sentinal.If you choose to anchor in an area that is crowded, mabey to crowded,remember that a sentinel is suppose to work because of gravity pulling it downward. This effect is most prominent on a rode that is perfectly horizontal. No anchor rode is perfectly horizontal right? Because the anchor is on the bottom, and the boat is on the surface, the rode is diagonal and as the rode becomes more vertical , as with short scope, the effect of a sentinal lessens. A sentinal works because it changes the angle of the rode at the anchor, like chain does...thats why we HAVE chain down there. Picture your boat anchored on three to one scope, the wind at fifteen Kts. Your rode would be streight and tought from the bow to the anchor chain. Do you think a twenty Lb. sentinal is going to make a difference? It won''t. If you anchor in these conditions,use heavier chain. Or all chain. Or mabey avoid these places. It sucks, but I avoid them because even though I know I won''t drag, I know other people do. They do because they aren''t anchored properly. I also think anchoring isn''t a time to cut corners.

Dennis
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Old 05-23-2002
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Dennis,

Not trying to put too much emphasis on the Sentinal. But it can help and that is a fact that has been used by seaman for centuries. But nothing can fully replace adding more scope or having an all-chain rode.

But there are a few things you said that are not correct.

"Picture your boat anchored on three to one scope, the wind at fifteen Kts. Your rode would be streight and tought
from the bow to the anchor chain. Do you think a twenty Lb. sentinal is going to make a difference? It won''t".

YES it will help, the question is- Is it enough for the conditions and there are too many variable (anchor type, weight, bottom etc). to determine that. But there is most definitely an added benefit. Let me go into some detail with real calculations.

I am not sure you saw or read the website I listed before, but it has actual calculations and an excell spreadsheet that you can download and then input the variables yourself (Depth of water, rode length, chain length, sentinal positioning, sentinal weight).

It is here for anyone who wants to use it http://www1.iwvisp.com/download/pub/spreadsheet/anchor.xls it takes a little knowledge of using Excell to use it though.

(Having a lot of sag in the rode reduces shock loads and helps keep the anchor dug in by reducing the angle between the rode and anchor. The
best you can do is an angle of "zero", which exists when the anchor rode is flat on the bottom, and all the pulling forces are horizontal. As tension increases, it reaches a point where the weight of the rode is overcome, and the angle becomes positive. Positive angles make the anchor work harder at keeping set, since the rode is now pulling up on the anchor. If the tension continues to increase, the catenary reduces to a straight line, and eventually the anchor pulls out.)



Lets assume you are in 15 feet of water and you have 15 feet of 5/16 chain and 30 feet of 5/8 3-strand rode (3 to 1 scope) (no sentinel)

It would take only 33.816 lbs of force at the boat to *begin* to lift the anchor rode off the sea floor at the point of attachment to the anchor shaft (not necesarrily where the anchor pulls out. But this will be our reference point)

Now lets add a 20lb Sentinel 30 feet down the rode.
It would take It would take 66.657 lbs of force at the boat to *begin* to lift the anchor rode off the sea floor. That is 2 times force needed than before. A 100 % increase. It WILL make some difference

Now lets assume you changed to all chain with the same parameters (3-1 scope in 15 feet of water no Sentinal).
Now it would take It would take 54.27 lbs of force at the boat to *begin* to lift the anchor rode off the sea floor. The reason this is not as high as before with the sentinel is because the heavy weight of the Sentinel in relation to the short amount of added chain (30 feet) was located further down the rode (30 feet)

Lets assume you are in 15 feet of water and you have 15 feet of 5/16 chain and 75 feet of 5/8 3-strand rode (6 to 1 scope) (no sentinel)

Now It would take only 95.87 lbs of force at the boat to *begin* to lift the anchor rode off the sea floor. Almost a 3 times increase (even though only doubling the rode) .

Now lets add a 20lb Sentinel 30 feet down the rode.
It would take It would take 131.38 lbs of force at the boat to *begin* to lift the anchor rode off the sea floor. That is only a 0.5 times (50%) increase in force needed to *begin* to lift the anchor rode off the sea floor.

Now lets assume you changed to all chain with the same parameters (6-1 scope in 15 feet of water no Sentinal).
Now it would take It would take a whopping 248.34 lbs of force at the boat to *begin* to lift the anchor rode off the sea floor. The reason this is so high is that now the added weight of the chain overtakes the benefits of the 20lb sentinel .


So what does this all mean? (plagiarized from the aforementioned website)


Select the size of your anchor based on boat length and weight.
Chose an anchor type suited for the kind of bottom you expect to encounter.
Use as much scope as practical.
Add 20-30 feet of chain to an all nylon rode.
Consider placing a Kellet about half way down the rode.
For the ultimate in holding power, switch to all chain rode.

Using these techniques to increase rode sag and improve anchor performance is only one factor in keeping your boat anchored. Adding a sentinel or some chain
wonít compensate for a fouled anchor, inadequate scope, failing to set the anchor properly, or using the wrong size or type of anchor. Also adding a kellet has bigger influences on a shorter scope. So understand the importance of rode sag and apply it to your anchor system, it will improve the holding power of whatever system you chose, and maybe even let you sleep a little easier when anchored in your favorite cove.

Tom


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