Scantlings IRL

[TheOffice]

Avram,

We dragged a couple times this summer.

Luckily, we set an anchor alarm on my phone and no damage was done to property. Only egos were harmed.
We made several rookie mistakes. Chain was not marked, so I was estimating scope based on angle of the chain. When calculating scope, I did not add the distance from the bow roller to the water, and finally, we did not back down hard enough. We were generally anchoring in10-12 feet of water, so the scope calculation was way off.

We have now traded in our 100 foot chain for a 250 foot piece and marked it with orange paint every 50 feet. That, with 150 feet of 8 plait rode should hold us if we don't repeat our mistakes.

Joel

[avramd]

Yikes, you have me spooked now... Tell me if I'm being naive here please:

I simply eyeball a 15º angle where the rode penetrates the surface; I double-check this by visually comparing the base & height of the surface triangle that I can see. If the distance from the stem to where the rode penetrates the surface is >= 4x the height of the chock above the surface, then my scope is a little more 4:1. Of course I do this with some load on the anchor, so it if there is no breeze or current, then I add light reverse throttle.

At the beginning, I did measure rode out on my deck, but then it occurred to me that if I did it by angle, I would never need to measure. 4:1 scope corresponds to about a 15º angle at the anchor. The right triangle formed by the rode and the surface of the water is proportional to the one formed by the rode and the "seabed", so if the one I can see has 4:1 scope, then so does the one that matters.

This all hinges on the idea that the surface of the water is perpendicular to gravity, so if it's really choppy, this wouldn't work very well. But I also wouldn't anchor anywhere that was that choppy if I had any choice.

Other caveats:

• My boat only weighs 2500 lbs
• It's a trimaran, so I can go out near the float bow and asses the surface triangle with effectively no parallax
• I do hard-reverse and watch my water-speed to verify that it's zero (haven't had to anchor in significant current yet)
• I'm definitely a princess in terms of how much protection I required to spend the night
• I usually anchor in 8'-10' of water
• I use a Spade anchor

If I had to do it in rough conditions, I would watch it for a little while, and look for the maximum height surface triangle over the course of a minute or two, and use that for my assessment.

I also basically stopped using anchor watch electronics. My calculus goes like this: "If I don't trust my set enough to go without an alarm, then I don't need an alarm because I'm not going to sleep anyway." I haven't found an alarm yet that doesn't yield false alerts. One alert whether it's false or true means I won't sleep the rest of the night, so anchor-watch means no sleep, and no sleep means anchor somewhere else, or just accept that I'm the anchor-watch. I will sometimes set a pin on my iphone, and occaisionally check that I haven't strayed far from it.

[avramd]

Joel - and Steve for that matter,

This came back to haunt me twice this year. As many here may know, I brought Empathy to the FL Keys last winter. I had a major dragging incident in eel grass there.

Then again this summer, all three of the Newport trimarans that went to the Vineyard for the Black Dog Dash dragged in the basin at Cuttyhunk on the way there. Eel grass again. I was the first to drag, before the wind had clocked, so for me I was actually somewhat protected. It was coming from the west, over the hill. Here's my drag path:

Cuttyhunk-anchor-drag.png

The other two didn't drag until the wind was out of the northeast, coming right over that isthmus.

I anchored in roughly 6' of water at about 7pm. At around 4am I noticed the our swing started to feel irregular. We were in the marsh, half a boat length or less from the east isthmus.

I'm in a whole different class from big boats & even big multihulls. My boat is 23' long and only weighs 2000 lbs. I race and only do coastal cruising, so I'm not going to carry 100 lbs of chain. I carry probably 20' of fairly small chain maybe 20 lbs, and the rest is rode. I don't calculate my scope, I just eyeball it - my chock is about 2' above the waterline. I admit my rode is not marked, I just imagine that I can tell what 5x the vertical looks like. When I'm backing and she stops, I figure the rode is a straight line, so I don't need to know what's true under the water, it's the same triangle that I'm looking at above the water. Is this naive?

The problem I find with eel grass is that the anchor holds just fine (I have a Spade A60), but it rips up a huge clump of seabed, with a root system that doesn't shed. Then it's not a spade any more, it's just a ball, and a fairly light one. Would more scope really change this?

In looking at Spade's sizing chart now, it does seem that I undersized my anchor slightly. They have a note there that I missed when I bought it, "It is recommended to move up to the next size anchor for Catamarans." The A60 is rated for 24' & 4800 lbs. While I'm <1/2 that weight, I have three hulls tugging, and probably 3x the surface area right at the waterline. So that spells A80.

[TheOffice]

I'll defer to Steve, but I'm not sure a bigger anchor would have helped in eel grass. Its basically a sandy bottom with growth. You probably don't have enough horsepower to back down hard, so getting the anchor to set is always going to be a challenge. Make sure you use an anchor alarm. Maybe anchor in deeper water with less grass. Holding ill be better and you won't chew up the bottom.
Maybe a Danforth anchor would work better? Steve?
Joel