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-   -   How to calculate scope (http://www.trawlerforum.com/forums/s42/how-calculate-scope-47127.html)

Woodland Hills 10-17-2019 09:03 AM

How to calculate scope
 
When calculating anchor scope do you add in the distance from the anchor roller to the water? My roller is 9 feet above the water so I have been including it in my calculations. Last night we anchored in 11 feet plus 9 feet up to the roller for a total of 20 feet times three equals 60 feet of chain for a three to one scope. Is this correct? Or do I actually have over 5 to one out?

rslifkin 10-17-2019 09:05 AM

Yes, you include height straight up from water to roller in the calculation. So your case, scope would be be based on 20 feet, not the 11 feet of water depth.

djmarchand 10-17-2019 09:26 AM

As noted above, scope includes the height of the bow roller above the water. In your case it is 9' above the water which either significantly reduces available scope in crowded anchorages or requires you to put out double or more rode to achieve a reasonable scope.


BTW, I consider 5:1 to be the minimum scope I would routinely use. The anchor tests done here on TF and others indicate significantly reduced holding at 3:1.


So consider adding a fitting near the water line on your bow to decrease the height above water line to a foot or so. This should be well reinforced with proper backing inside the fiberglass. It takes a bit of rigging to make it work. You can use a short painter that feeds through the bow fitting with a block on it for the main rode so that you can raise or lower the attachment point to raise your anchor. A bit of a PITA for sure, but I would really consider it if I had a bow 9' above the water.


David

O C Diver 10-17-2019 09:53 AM

It depends.
I start by using water depth to calculate scope. Then the next issue is your snubber and where it attaches, and if it's a bridle. When I used a bridle, it was fed through 2 hawsuer pipes probably 8' back from the bow roller. So if the chain straightened out in a blow with 7:1 scope, the point where the chain entered the water was maybe 10' ahead of the bow roller. The other option if using a snubber is too spool excess chain out, maybe until it almost touches the bottom. This keeps the snubber hook pulled down and reduces the amount of scope required on normal days.

Ted

jimdavi 10-17-2019 10:26 AM

Don’t forget tide depth change as well.

DDW 10-17-2019 10:43 AM

Yes add the roller height. 5:1 is reasonable in 9 feet of water, not so reasonable in 100 ft. There is a good argument that (at least with a chain rode) the deeper the water, the less scope is needed.

tiltrider1 10-17-2019 10:52 AM

Is your depth gage reading from the bottom of the boat or is it corrected for actual depth? I can see 11’ +3’ + 9’ = 23’ On a 3:1. That would be 69 feet vs 33 feet if you only took your depth reading.

jclays 10-17-2019 11:10 AM

Quote:

Originally Posted by Woodland Hills (Post 812076)
When calculating anchor scope do you add in the distance from the anchor roller to the water? My roller is 9 feet above the water so I have been including it in my calculations. Last night we anchored in 11 feet plus 9 feet up to the roller for a total of 20 feet times three equals 60 feet of chain for a three to one scope. Is this correct? Or do I actually have over 5 to one out?

Yes your calculations are correct. Three to one scope is good if you are just hanging out having dinner or watching a concert.
Five to one is more appropriate if you are spending the night and sleeping.

caltexflanc 10-17-2019 11:18 AM

Quote:

Originally Posted by O C Diver (Post 812093)
It depends.
I start by using water depth to calculate scope. Then the next issue is your snubber and where it attaches, and if it's a bridle. When I used a bridle, it was fed through 2 hawsuer pipes probably 8' back from the bow roller. So if the chain straightened out in a blow with 7:1 scope, the point where the chain entered the water was maybe 10' ahead of the bow roller. The other option if using a snubber is too spool excess chain out, maybe until it almost touches the bottom. This keeps the snubber hook pulled down and reduces the amount of scope required on normal days.

Ted

The snubber is just part of the rode. So I just let out the appropriate amount of chain as if there was no snubber, and just count the length of the snubber as bonus rode. Another 10 feet never hurt any one. But if you need to be pedantic, and want a very precise amount of rode out, then let out the chain in an maonunt less than the length of the snubber. The only kind of snubber that affects rode calculation are those that attach to an eye at the water line.

I too use your method of letting out a big lazy loop of shain behind the chain hook/plate.

Woodland Hills 10-17-2019 05:32 PM

I am using a 55kg Rocna Vulcan with 250’ of chain. Last night I had 75’ out in 35mph gusts with no movement. If I used 5 to 1 it would be a problem in tight or crowded anchorages given the difference in rode lengths. We were the only boat in the anchorage so I could have used any scope I wanted, but nearly 4 to 1 worked just fine. I only use my bridle if there is going to be a big blow as the boat sails around less if I have a single attach point. Of course I lock the chain and relieve tension on the windlass.

I like your idea of loading up the bridle with a giant loop of chain to keep it down and will be trying that next time I use it.

dhays 10-17-2019 07:44 PM

Quote:

Originally Posted by tiltrider1 (Post 812116)
Is your depth gage reading from the bottom of the boat or is it corrected for actual depth? I can see 11’ +3’ + 9’ = 23’ On a 3:1. That would be 69 feet vs 33 feet if you only took your depth reading.


Good point. I have my depth sounder set to give me water under the keel. From my anchor roller to the bottom of the keel is conveniently ~10'. So I just add 10 feet to whatever the depth sounder tells me and use that to calculate scope. Figuring exactly where I'm at in the tidal cycle is more of a trick in some places.

Spinner 10-18-2019 01:53 AM

I have 300 feet of all- chain rode. This last summer, I anchored twice in 100 feet of water....put out most of the rode and slept like a baby! Also note that my Vesper AIS gives me an anchor alarm on its cell phone app. It’s not feasible or needed to carry 700 feet of chain!

Bigsfish 10-18-2019 08:09 AM

Ted and others

I also use a lazy loop on my chain when using a snubber. But your remark that that loop reduces the amount of scope needed on normal days, doesn’t that loop become straight in a blow so there is no advantage? At least that’s the way my feeble brain works.

caltexflanc 10-19-2019 09:19 PM

Quote:

Originally Posted by Bigsfish (Post 812390)
Ted and others

I also use a lazy loop on my chain when using a snubber. But your remark that that loop reduces the amount of scope needed on normal days, doesn’t that loop become straight in a blow so there is no advantage? At least that’s the way my feeble brain works.

You are right, the loop is not part of scope. What it does is add extra weight to the system for the boat to move against, and, in most cases, aid the security of the hook to the chain. I suppose you could argue that it reduces the need for additional scope, but I have always seen it as icing on the cake, so to speak.

koliver 10-19-2019 11:05 PM

Quote:

Originally Posted by Woodland Hills (Post 812076)
When calculating anchor scope do you add in the distance from the anchor roller to the water? My roller is 9 feet above the water so I have been including it in my calculations. Last night we anchored in 11 feet plus 9 feet up to the roller for a total of 20 feet times three equals 60 feet of chain for a three to one scope. Is this correct? Or do I actually have over 5 to one out?

Here is a good example of the east v west difference.
Here, we would never dream of anchoring in 11 ft of water. From the look of your boat, that would leave only 5' or perhaps a lot less, of water beneath your keel.
Scope calculations should take conditions into account. In shallow water, any wind waves will cause proportionately more movement than in deep water, so more scope should be required than in deeper water.

Here in SW BC, average anchoring is in 50' of water, and few use more than 3 to one.

FF 10-20-2019 06:34 AM

There should be at least 2 choices of snubber diameter , perhaps 3/8 for fine days and 1/2 for modest wave action.

It takes about 15% of the lines break force to start to get some stretch , to smooth the ride , so thin line is required on nice days.

The stretchy snubber also helps the anchor stay set on nervous boats that dodge from side to side.

psneeld 10-20-2019 08:04 AM

And when I anchor in the Indian River in Florida often there is less than 2 feet under my keel.

Different situations, different boats and different captains.... and no particular method is right or wrong.

Codger2 10-20-2019 10:38 AM

Quote:

Originally Posted by djmarchand (Post 812088)
BTW, I consider 5:1 to be the minimum scope I would routinely use. The anchor tests done here on TF and others indicate significantly reduced holding at 3:1.

I completely agree! Too much info is complicating a relatively simple task. :hide:

Maerin 10-20-2019 10:39 AM

Quote:

Originally Posted by koliver (Post 812920)
Here is a good example of the east v west difference.
Here, we would never dream of anchoring in 11 ft of water. From the look of your boat, that would leave only 5' or perhaps a lot less, of water beneath your keel.


You'd be apoplectic in the Bahamas, where we routinely anchor with 2ft under the keel, travel across the bank with 1-1/2 ft under the keel. We had west coast owners join the group at our 2011 Selene rendezvous, they were freaking out at the water depths. :lol: It's all in what you're accustomed to!

rslifkin 10-20-2019 10:49 AM

Quote:

Originally Posted by Maerin (Post 812986)
You'd be apoplectic in the Bahamas, where we routinely anchor with 2ft under the keel, travel across the bank with 1-1/2 ft under the keel. We had west coast owners join the group at our 2011 Selene rendezvous, they were freaking out at the water depths. :lol: It's all in what you're accustomed to!

Yeah, I would not be happy with that! I get nervous with 3 feet under the props and by 2 feet I'm at idle and sweating.

As far as scope goes, I generally treat 5:1 as default unless conditions require something different. In good weather, deep-ish water and a tight space, I'm ok with a bit less. In questionable weather, I'll tend towards 7:1 if at all possible.


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