So, we've had great success anchoring in general. However in the recent gusty winds (25knts +) and the bigger tidal range of the Whitsundays, we've slipped a couple of times.

What would you do to improve our setup?

Our primary anchor is a 8kg Lewmar high hold delta. We have two plough back up anchors.

Primary rode is 27m 6mm chain plus 13m 12mm rope.

The obvious options are :

More anchor chain
An anchor kellet to improve the caternary
A different anchor
or don't worry about it and accept the odd reset.

What do you think would be your first improvements?

Oh, and the boat is a little compass northerner 28.

Cheers

Bristle

8kg. Too light. Got a 16kg off sir g for our tophat. Hasnt budged in 30+ with just 9m chain

Well, after replacing the 27lb cqr - which is keeping SectorSteve and KiaKaha happy at the moment somewhere around Mollolabah - with a Rocna 10, my gear is:
35lb Manson cqr,
25lb Rocna,
17lb Fort sand anchor,
33 meters of close link 3/8 chain,
12 meters of 3/8 chain, ordinary,
1.5 meter of 25 mm very heavy chain,
330ft of 14mm nylon rode,
100ft of 14mm nylon rode,
2 lengths of 8mm nylon ropes for trip lines 20m each,
a 10' foam float,
a big orange inflatable float,
an 'easy lift' anchor lift device.

If that's not enough, l deserve to be blown away.
The boat is an 8.7 meter Adams and it is 3.8 tons dry.
I keep the long chain at home less l am going to Qld. with coral and rocks.

Okay....so the answer is never as simple as a bigger anchor, and is usually cable rather than anchor. If you are dragging, veer more cable - always. If you don't have more cable to veer - you don't have enough cable to anchor in that depth of water.

So having said that I owe at least some sort of an explanation.

In the commercial maritime world the formula used to calculate how many shackles (1 shackle = 27.5m) to veer when anchoring is given as:

Shackles = 1.5 x SQRT(depth of water in meters)

This is based on an all chain anchor, so to anchor in 5 MOW (meters of water) I would need 3.3 shackles (about 92m of chain).

Okay, so 1; obviously not possible on a small yacht and 2; probably overkill for a small yacht - even though our cable is smaller.

We also have a compromise that the major ships don't have - we can use rope as part of our anchoring system. This rope doesn't add to the weight of the anchor system and help with holding but it does absorb the shocks (from gusts etc) that a large cable scope is required for in large ships. Okay so positives and minuses.

Right so what do we do about it in a small yacht. From here I have to give a good part of the credit to Dave1258 on yachting and boating world forums - he has written a very comprehensive post of the maths behind anchors. (It can be found here: www.ybw.com/forums/showthread.php?33805-Anchor-Chain-length). I'm not going to repeat his work here but I'm going to use some of his formulas.

I'm going to ignore the rope you have for the moment for most anchoring calculations are done assuming an all chain catinery.

Using Dave1258's formulas, to determine how much chain you need in a given wind speed based on your yacht:

Chain required: 1/16 * yacht length in m * wind speed * SQRT(depth/specific chain weight)

Yacht length = 8.53
You have given us wind speed of 25+ let's call it 30 for safety.
Chain weight = 0.8kg

Therefore for a 5 MOW anchorage you would need:
Chain = 15.9375 * SQRT(6.25)
Chain = 39.84m

This is for a constant 30 kts and doesn't include any gusts.

Working back the other way, using the amount of cable you already have the wind limit for you would be about 20kts in 5MOW.
Now your rope give you a little bit of flexibility, but more in wind speed gusts rather than anything else.

So in answer to your original question, I would add an additional 13m of chain, or change to a heavier chain.

Hope this helps.

PhoenixStar said..

Bristolfashion said..
So, we've had great success anchoring in general. However in the recent gusty winds (25knts +) and the bigger tidal range of the Whitsundays, we've slipped a couple of times.

What would you do to improve our setup?

Our primary anchor is a 8kg Lewmar high hold delta. We have two plough back up anchors.

Primary rode is 27m 6mm chain plus 13m 12mm rope.

The obvious options are :

More anchor chain
An anchor kellet to improve the caternary
A different anchor
or don't worry about it and accept the odd reset.

What do you think would be your first improvements?

Oh, and the boat is a little compass northerner 28.

Cheers

Bristle

8 kg is a little light on for a compass 28, I would give some thought to a Vulcan 12 or Rocna 10.

cmpgroup.net/

Catenary doesn't really matter, when you need it cos it's blowing hard the rode is bar tight and there is no catenary. Manson supreme, Vulcan and Rocna set every time without needing a reset. My Vulcan 15 held the Star (10 m, 5 tons) in 55 knots for 3 days without budging an inch. I would suggest that 40 m of rode is a bit optimistic, that's only 10 m of depth including tide and distance from the water to the stem head, so not much really.
Avoid Sarca like the plague. So sue me.

I agree. Start with a good anchor. Sell all the plow type anchors for scrap.

Bristolfashion said..
So, we've had great success anchoring in general. However in the recent gusty winds (25knts +) and the bigger tidal range of the Whitsundays, we've slipped a couple of times.

What would you do to improve our setup?

Our primary anchor is a 8kg Lewmar high hold delta. We have two plough back up anchors.

Primary rode is 27m 6mm chain plus 13m 12mm rope.

The obvious options are :

More anchor chain
An anchor kellet to improve the caternary
A different anchor
or don't worry about it and accept the odd reset.

What do you think would be your first improvements?

Oh, and the boat is a little compass northerner 28.

Cheers

Bristle

Had a Delta, dragged a few times (once is too many). Brought a Rocna (about the same weight) and put it on the same chain/rode. Never dragged again.

And I have always put out plenty, unto 6 or 7 times the depth and all chain. Even this didn't get the Delta to work where as the Rocna has been used in tidal situations where the reversal of the current requires the Rocna to reset itself twice a day. And it did it by itself without a hitch. Slept like a baby.

I have a 15 pound Manson Supreme on my 2000kg Spacesailor 24 and have about 35 feet of chain, then warp. I've never dragged once. It's set and forget.

AshleyM said..
I have a 15 pound Manson Supreme on my 2000kg Spacesailor 24 and have about 35 feet of chain, then warp. I've never dragged once. It's set and forget.

Anchored once with a 35 lb Manson Supreme on a Bristol 40. Tossed the anchor and then backed off to straighten the chain and dig the anchor in. Nearly got whiplash injury when it dug and straightened the chain.

Value for money I think the Manson Supreme is the best of the modern anchors.

Get a Rocna, none of them are as good!

Bushdog said..
While it may be sour grapes, it's worth reading this if you have a few minutes to spare...
www.petersmith.net.nz/boat-anchors/manson-supreme-anchor.php

An interesting read, however in my experience, having replaced an unreliable delta anchor with the recommended size Rocna, whilst a vast improvement it proved not to be all that Peter Smith claimed. The anchor did drag in soft mud on a number of occasions in different locations with at least 5 to 1 rode, nearly always came up with a ball of mud attached and has since been replaced with with a Sarca Excell, an anchor that so far has not failed me.
For those considering an upgrade I can only suggest that the Sarca product be considered, I found it to be superior to Rocna.
Cheers Bob.

Whilst awaiting new anchor and rode, I'm carrying out a little experiment.

Tonight I have 27m chain and 13m rope out as my rode. I've shackled my 2nd rode to the point where the splice meets the chain. The second rode has 12m chain and then rope. The idea is that it firstly adds weight and improves the caternary and secondly means that I've effectively got chain all the way in case of stray coral.

What do you reckon? I've never heard of it as a technique.

If you see me drifting past in the night, it hasn't worked!

Cheers

Bristol.

Little bit of a tangle when I pulled it up, but seemed to work well.

Bristle