I have always set my depth offset so the fathometer reads the actual water depth. It minimizes the chance for me making careless mistakes in arithmetic because I spend a LOT more time comparing my fathometer reading to the chart than anything else.
Other people do other things, of course, but I did make the (possibly rash?) assumption that when I talk about "water depth" to sailors they understand how to convert their own fathometer reading to a water depth. If anybody is using the depth under their keel to calculate anchor scope, they have probably already lost their boat.
I guess I was not clear in making my main point. I did not mean to imply that I could anchor with ONLY 60 meters of chain in 10m of water. I said I could anchor in nearly 10m of depth (@ a scope of 7) without the splice touching bottom. That was apparently unclear, so here is what I meant...
If I am anchoring in 10m of water, and adding 2m for the height of the bow roller (mine is actually 1.6m, but let's round up just for fun!): I need 7*12=84m of scope. if I let out all my chain that's a scope of 5:1, and if I let out an additional 12 meters of line, I am at 6:1 and my splice is JUST touching the bottom in dead flat calm conditions. Of course if I have let out 12 meters of line, there is no need to add extra for a snubber.
I hope you'll grant me that I wasn't too far off when I wrote that I can go to NEARLY 10m of depth without the splice touching bottom.
Of course if ALL you have is 60m of chain, then 10 meters of water depth is out of range in anything except reliably mild conditions.
Great Guana Cay, Abacos, Bahamas
---In amelyachtowners@..., <yahoogroups@...> wrote :
I am writing this to clarify something in your recent post about anchor scope. I am sure you know this, but there wasn't enough detail in your post that newbies will certainly need.
10 meters indicated on the depth instrument is actually 10 meters of water, only if you have the depth Datum set to water depth rather than actual transducer depth, or keel depth. If you have the Datum set to keel depth (as many do), you need to add this to the scope computation. Unless the Datum is set to indicate actual water depth, you are probably closer to 12 meters. You can find information on the Datum offset in your depth instrument manual.
FYI, I was recently in an Amel that someone had attempted to set the Datum to indicate the depth of the keel. That person then relied on his depth instrument in shallow water, but because he set the Datum wrong, when his keel touched bottom, his instruments indicated 3 meters. When this boat was hauled out, there were more than 10 large chips on the cast iron ballast and a large chips in the rudder.
Additionally, your bow roller is about 2 meters above the water, so when computing scope you'll need to add this to the computation. The easiest way is to add it to the indicated depth.
Also, many of us like to have a lazy loop of chain between the snubber connection to the chain and the bow roller.
Chances are that with 10 meters depth indicated on your depth sounder, you need to use somewhere between 12 and 14 as depth in the scope computation. I doubt that you can get 7:1 in 10 meters of water with 60 meters of chain...I believe that the most that you could have is between 4 and 5:1.
CW Bill Rouse
Admiral, Texas Navy
Amel School www.amelschool.com
720 Winnie St
Galveston Island, TX 77550