Anchoring off... working out distances.

[Nostrodamus]

Is there a technique you use for working out the nearest place you can anchor off a cliff or beach or anything?

Do most people just guess?

Sometimes you like to anchor in as close as possible when tides or winds change and swing you round you don't want to end up hitting something or going aground.

 

[AMCD300]

I suppose it depends upon whether you run a shore line as this overcomes swinging radius concerns.

I had to run really close to the beach in Meganisi recently as the drop-off rate is so high and you need to get close to get the anchor shallow enough. I made sure, however, that I was being blown away from the shore (what wind there was) otherwise it may have been a little more risky.

As for distance, that would depend upon forecast winds and my ability to react if it all went 'Pete Tong'........

My main concern, assuming I had the swing distance, would be the anchor dragging and, with the anchor upwind, me dragged toward a lee shore.

I saw a number of yachts anchored far too close to danger (in my opinion) so as you suggest everyone must have their own method of assessing risk.

Andy

 

[Seajet]

Is there a technique you use for working out the nearest place you can anchor off a cliff or beach or anything?

Do most people just guess?

Sometimes you like to anchor in as close as possible when tides or winds change and swing you round you don't want to end up hitting something or going aground.

Nostradamus,

well if the place has a charted height like a lighthouse and you know that the height of tide is you could use a sextant, the tables for distance off are in the almanac ( probably just the big expensive version now, I haven't done it for years ! ).

More likey I'd use a hand bearing compass for a fix if there are charted marks available, combined with the depthsounder and seabed contours on the chart.

Or just GPS...

 

[vyv_cox]

Drop your anchor in the position you estimate it will finish up, backing towards any obstacle until you have laid out sufficient chain for the depth of water. I try to think in boat lengths as a rough scale. If you run aground on the obstacle your anchor was too close!

Also, use your kedge to ensure you don't swing where you don't want to go.

 

[sarabande]

cheapy plastic sextant for horizontical angles ? Or make up a device from ply with a moveable wooden arm (similar to the Dambusters bombaimer 'sight'). The it is to hand to check if you are still in the same place.

http://en.wikipedia.org/wiki/The_Dam_Busters_(film)

About halfway down on the right.

 

[prv]

Nostradamus,

well if the place has a charted height like a lighthouse and you know that the height of tide is you could use a sextant, the tables for distance off are in the almanac ( probably just the big expensive version now, I haven't done it for years ! ).

More likey I'd use a hand bearing compass for a fix if there are charted marks available, combined with the depthsounder and seabed contours on the chart.

Or just GPS...

Mm - I read the question as more "deciding where to anchor" rather than "establishing where you are".

Not really much to say beyond keeping an eye on the depth. Obviously in an onshore wind (assuming you're still comfortable anchoring here) you'll want to run in to your minimum depth, then head back out again to lay the anchor, then fall back to where you were.

Sometimes if necessary I will tootle about doing what amounts to a mini survey of the surrounding depths (in my head, though, I've never yet tried to make a little chart) before picking my spot to drop. Feels a bit like a dog turning round and round before flopping down to sleep

Pete

 

[macd]

+1 with prv. Tootle in towards where you expect to be closest to shore, hazards, whatever and see what the sounder says. If suspect ground, have a spotter on the bow. I wouldn't bother, but there's nothing to stop you setting a waypoint at to use as horizontal datum for anchor position.
To Seaject: Nostradamus didn't say as much, but I believe he's in the Med so tidal issues are of little concern.

 

[Seajet]

One thing worth bearing in mind is the ' night breeze ', the opposite to the shore breeze; as we all know in daytime the wind flows onshore to replace the hot air rising from the land, in a convection current.

At night if there's no other significant wind this is reversed and the air goes out to the ameliorating sea; more of a consideration in rivers with banks to both sides though than beaches, apart from ensuring there's enough scope out to avoid being blown out to sea.

 

[Nostrodamus]

Drop your anchor in the position you estimate it will finish up, backing towards any obstacle until you have laid out sufficient chain for the depth of water. I try to think in boat lengths as a rough scale. If you run aground on the obstacle your anchor was too close!

Also, use your kedge to ensure you don't swing where you don't want to go.

This is what I have been doing.
Tonight I am anchored between two massive rocks and a beach. There is no wind to speak of and not a lot forecast tonight.
What wind there was has me facing away from the beach and cliffs.
Good news is I can see the anchor well dug in although with the little amount of wind there is I suspect i may not move more than a few meters.
I did back up so I know if I swing at all I will be clear of everything and will have enough water.

Just out of interest how hard does it have to blow to stop the chain keeping you where you are so you are relying on the anchor. I know it depends on the length and weight of chain out, windage etc but I was wondering.. when does the anchor take over from the chain?

 

[prv]

One thing worth bearing in mind is the ' night breeze ', the opposite to the shore breeze; as we all know in daytime the wind flows onshore to replace the hot air rising from the land, in a convection current.

At night if there's no other significant wind this is reversed and the air goes out to the ameliorating sea; more of a consideration in rivers with banks to both sides though than beaches, apart from ensuring there's enough scope out to avoid being blown out to sea.

Good point, especially as (in this country at least) we especially tend to anchor close in to beaches in hot, settled weather when there's little other wind to disturb the land/sea breezes.

Pete

 

[Nostrodamus]

One thing worth bearing in mind is the ' night breeze ', the opposite to the shore breeze; as we all know in daytime the wind flows onshore to replace the hot air rising from the land, in a convection current.

At night if there's no other significant wind this is reversed and the air goes out to the ameliorating sea; more of a consideration in rivers with banks to both sides though than beaches, apart from ensuring there's enough scope out to avoid being blown out to sea.

Had that last night. Pretty amazing to feel huge gusts from the cliff. Even whistled through the rigging but the sea was almost flat.

 

[vyv_cox]

Just out of interest how hard does it have to blow to stop the chain keeping you where you are so you are relying on the anchor. I know it depends on the length and weight of chain out, windage etc but I was wondering.. when does the anchor take over from the chain?

Surprisingly little wind will drag the chain across a sandy bottom. It takes time but you can hear the chain dragging, if you don't have a snubber to damp the sound, in not much more than a force 1. On a weedy bottom it can blow old boots but the chain run can still be curved after days.

I am anchored in a narrow and quite shallow bay at present. The forecast is north westerly wind all night, which is fine, but it has gone quite still. Just in case we have a reversal of direction, which could put me on the beach, I have put the kedge out astern immediately before replying to this post.

 

[Blueboatman]

Congratulations Nostro, soon anchoring will I hope just seem perfectly natural and marinas bloody noisy dirty and expensive unless carp weather ..
Try setting a depth alarm and or GPS position alarm. I should, but don't!
The mini survey advice is spot on, as is noting the bearing for clearing out if it comes to blow onshore in the night ( just plod out a couple of miles and heave to, another thing that will seem perfectly natural to delay an arrival until daybreak etc.
Isn't the water so much cleaner at anchor eh? Early morning swims natch'

 

[Seajet]

Nostrodamas,

may I explain about depth sounders and anchor watch alarms.

Most modern sounders have 2 alarms, shallow and deep; you set the shallow one at a depth before you go aground giving time to make your escape, the deep alarm for in case the tide has come in to lift the anchor warp / chain unreasonably so risking dragging.

Some GPS units have an ' out of position alarm ' too .

Anchoring is simple and very pleasant when you get used to it.

As for depth alarms I remember seeing a tip in PBO to place a frying pan on the cockpit coaming with a line attached to the handle with a weight dangling overboard; if the boat became near going aground the weight would hit the seabed and stop pulling on the pan, the clatter would hopefully wake the skipper !

 

[prv]

As for depth alarms I remember seeing a tip in PBO to place a frying pan on the cockpit coaming with a line attached to the handle with a weight dangling overboard; if the boat became near going aground the weight would hit the seabed and stop pulling on the pan, the clatter would hopefully wake the skipper !

It's a rare balancing act that would work like that and yet not go off when the wash from a distant ferry sets the boat rolling

Pete

 

[Seajet]

It's a rare balancing act that would work like that and yet not go off when the wash from a distant ferry sets the boat rolling

Pete

I wasn't saying I'd rely on it, I just love the idea !

Maybe one for when the electrics fail.

 

[Nostrodamus]

All good ideas and thank you but it was not the original question.
The orginal question was on trying to work out how close you can anchor to an object ie rock or beach without going aground or hitting it if the boat swung.
Anchor drag is something else.
I wondered if anyone did something different apart from setting the anchor and reversing back to the obstical in question to make sure they wouldn't reach.

Vyv... thank you for the reply.. I just thought weed was a pain but it can obviously help in a blow

 

[noelex]

I think a laser rangefinder might be a good tool.

I usually snorkel and check the anchor and if there is a hazard if swim towards it from the anchor at a steady pace you can measure distance reasonably well.

A simple way is to time your swim (say by counting one cat and dog) from the back of your boat to the anchor (providing the chain is reasonably stretched out) and then swim the same time towards the hazard (rock or shallows) fom the anchor. This will be where your stern will finish when you swing that way.

Adding a bit for error is a good idea, but done with care it is reasonably accurate if there is no current. It is a useful means of gaining reassurance in those cases that look marginal.

 

[Twister_Ken]

If you know (bearings or transits) where you dropped the hook, and where the worrying obstacle is, and you know your scope, then a circle, radius the scope, drawn around the hook will show whether your swinging circle might put you in danger.

As others have pointed out you can restrict your swinging circle with a second anchor, lines to the shore or other immovable objects.

 

[NornaBiron]

We have just purchased a laser range finder and use this to check our distance from various points as we anchor. It doesn't replace all the usual techniques but is an added extra which we have found gives peace of mind. Distances are difficult to judge when there are few reference points.