anchors
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 allchain 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 3strand 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 (31 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 3strand 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 (61 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 2030 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
won’t 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
