Anchor Rope Length

[White Knight]

OK, I have a 17 foot Trophy CC and I take it out to the Nothern Channel Islands to SCUBA dive and spear fish. As I read the instructions for my anchor it quotes the Coast Guard in saying that I need to let out six to ten times the depth in anchor line? As a rule, I usually let out four times. So.....my question is this, "Letting out that much line is problematic for many 'tight' dive spots. What are your feelings about this?" I have a very heavy chain that I hook to my anchor and I am somewhat nervous about my boat drifting a way while I am under. Again, what is the prevailing wisdom? Thanks, Bruce

 

[dpbishop]

The reason for "scope" ( length of anchor line out / depth) is to get the pull on the anchor as close to horizontal as you can. Vertical effort pulls anchors up, horizontal effort sets the anchor further in the ground (if it's a soft bottom). Anchor chain, if you have enough of it, will lie on the bottom and keep the vertical pull caused by the boats motion from acting on the anchor. One rule of thumb for boats under 30 ft. is the rode should be 4 times the depth of the anchorage and the chain should be twice the height of the expected seas. Anchorages at the channel islands tend to be pretty deep, and the swells can get pretty big. When I dive the Southern Channel Islands I have 100' of chain plus 100' of line for a 35' boat. I always start the dive by visually checking the anchor.

 

[theITdiver]

I just finished my ohio boaters license and in the classes it recommends to let out 7-10 times the water depth (plus the chain on the anchor) as well. Main reasons I can see is for tidal changes and wave action to keep from sinking your boat when the tide or a large wave comes in. i am FAR from an expert on the subject, but I imagine in an area where this length would either put your vessel in harm of impacting a nearby structure or obsruction the passage of other vessels, shorter rope length would be ok. As long as you are SURE you have enough length to allow proper movement wiht waves and tidal changes.

 

[Charlie99]

Shorter scope and lighter anchors (such as a "lunch hook") are fine if the boat is attended.

The scope of 7-10, a good length of chain, and an oversized anchor are what you should have if the boat is unattended. You can lessen the hassle of hauling in a heavy anchor and chain by attaching an appropriately sized lift bag which you inflate as you start your final ascent. It should be small enough that it can't float the anchor and chain, but big enough that it counterbalances most of the weight.

 

[OBXDIVEGUY]

4 to one worked great for us, until any waves or boat wake hit us and then the anchor immediately started slipping. That was with a typical dansforth anchor. We now have a large grapnel hook anchor, and that thing holds like crazy! Especially if we're on a wreck and I physically tie it to the wreck. When we use that anchor, we can shorten up the scope quite a bit.

 

[Empty V]

As far as rope to chain ratios go we go 3:1 on our 47'. We usually let out about 6x the depth but we usually don't anchor in over 100' of water. If you fear the anchor unhooking and the boat drifting have someone on the boat. Personally I don't dive off any boat without someone acting as surface support.

Billy

 

[ScubaBob92651]

Personally I don't dive off any boat without someone acting as surface support.

Billy

Amen Brother. For boats big enough for two buddy teams, it amazes me why so many people think they need to be in the water at the same time. If you are diving off a P.B. for the day and each person is going to do two dives, assuming you are actually planning your dives and as such, are including an adequate S.I. - the most you are going to gain by having everyone in the water at the same time is approx 1 hr. Not nearly enough savings for your potential loss, including your boat and the safety of people aboard your vessel for the day.

 

[JMdiver]

Amen Brother. For boats big enough for two buddy teams, it amazes me why so many people think they need to be in the water at the same time. If you are diving off a P.B. for the day and each person is going to do two dives, assuming you are actually planning your dives and as such, are including an adequate S.I. - the most you are going to gain by having everyone in the water at the same time is approx 1 hr. Not nearly enough savings for your potential loss, including your boat and the safety of people aboard your vessel for the day.

That's good advice.

John

 

[White Knight]

Thanks to all who replied to my anchor question. It would appear that most of you comply with the standard anchor rules. I might mention, however, that my boat is only 17ft long and only two people with SCUBA gear fit in it, so leaving an extra person on board isn't an option. All that said, I get it: Make sure you have enough chain and rope. But I am curious, if you were to dive Painted Cave on Santa Cruz Island, and assuming you would anchor just outside the opening to the cave (that is 70 - 80 feet of water last I was there) in the cove how could anyone possible let out the recommended amount of anchore line and chain and not slam up against the rocks? Hate to beat this thing to death, I am simply hoping for a more feasible option. Thanks.....and again, a huge thank you for your advise so far

 

[Empty V]

Thanks to all who replied to my anchor question. It would appear that most of you comply with the standard anchor rules. I might mention, however, that my boat is only 17ft long and only two people with SCUBA gear fit in it, so leaving an extra person on board isn't an option. All that said, I get it: Make sure you have enough chain and rope. But I am curious, if you were to dive Painted Cave on Santa Cruz Island, and assuming you would anchor just outside the opening to the cave (that is 70 - 80 feet of water last I was there) in the cove how could anyone possible let out the recommended amount of anchore line and chain and not slam up against the rocks? Hate to beat this thing to death, I am simply hoping for a more feasible option. Thanks.....and again, a huge thank you for your advise so far

How about a thinner rope to save space? Some spots might not be suitable for a boat that small too.

Can't you bring someone on board who is marine savvy that isn't a diver to have as surface support? Sorry if this is harping too much but there's gotta be a safer way to do things. I also might be overly cautious.

Best of luck and wish I had a better answer for you.

Billy