Anchor weight

[purplerobbie]

I have a 34ft 11.5 tonne boat that has a 15kg bruce on it which i think might be a bit small??
Is the weight of the boat more important than the size or is it the other way round?
More boat in the water to drag about?
I would of thought i would be looking at at least a 25kg?
Any thoughts?

Rob

 

[Talbot]

weight is less important than size. However for steel anchors, weight is normally the indicator of size! for example a 10 kg fortress would be more than enough - but it is made of aluminium!

As far as your boat is concerned, I would have a larger anchor. You dont say what size chain you are using, and I reckon this has a bearing on how big the anchor has to be.

The other consideration is how do you weigh anchor, if you use a windlass, going larger is less of a concern than if you are doing it by hand.

 

[SkipperMarv]

look HERE

 

[GMac]

I'm in with Talbot.
A 15 is OK but one bigger defineatly wouldn't hurt. Unless you have a nice big pile of chain then the 15 should be sweet.

Talbot is right about the Fortress but should have added 'It will be physically VERY big' /forums/images/graemlins/smile.gif

The Jimmy Green chart looks to have been taken from manufacturers specification 98% of which are well known to be absolute minimums, at best. Don't use if planning on boating in heavy conditions. I'm in the game and no way would I recommend anchors of that smaller sizing unless you only day sail in nice weather.

The 'general rule' is 1lb per foot when talking plows, Bruces and most older style anchors. The Delta is a plow but just has a fixed shank. Some of the newer designs can be a bit lighter. From there you tweak a bit for the individual boat and users desires.

 

[Salty John]

You are OK with the 15kg but you might have better peace of mind in heavier weather with one size up.
http://www.saltyjohn.co.uk/resources/Anchor%20system%20selection.pdf

 

[rogerthebodger]

On your chart why is there such a big junp in size between 14 m to 15 m for the Delta and Danforth anchors but not CQR and Bruce.

 

[Salty John]

Because there is a Delta 10kg and a Delta 16kg but nothing in between. I think the same situation goes for the Danforth.

 

[TigaWave]

MCA advice on anchors and cables for coded boats
annex 5.
http://www.bluemoment.com/codeofpractice/annex5.html

 

[rogerthebodger]

So the Delta goes from 20 Kg to 40 Kg and the Danforth goes from 19Kg to 32 Kg then

 

[Salty John]

Danforth goes 19, 32, 45 (metricated equiv.) and Delta goes 10, 16, 20, 25, 40. There are many Danforth copies of different weights and Delta may have added intermediate weights since I did the chart.

 

[PCUK]

There is still a lot of doubt about modern lightweight anchors. Anchor weight is still the key to safe anchoring.
A 25Kg would be right with minimum 8mm chain.

 

[craigsmith]

He's talking about the jump from the 20 to the 40, which isn't sensible - the Delta 25Kg should go in there somewhere.

I also note there is no Rocna size in the 15m column, according to your other choices probably the 33 should fit there.

And hey, what if I have a 17m boat? /forums/images/graemlins/tongue.gif

On the side, I believe you should probably make it more clear that these are your sizing recommendations, not endorsed by the manufacturers, as it isn't obvious, and most of the time these tables (e.g. in product catalogs) are just copied from the companies' recommendations (so some will assume the same here).

 

[craigsmith]

[ QUOTE ]
I have a 34ft 11.5 tonne boat that has a 15kg bruce on it which i think might be a bit small??
Is the weight of the boat more important than the size or is it the other way round?
More boat in the water to drag about?

[/ QUOTE ]Size determines windage but weight determines momentum and so force on the rode when the boat's moving about.

Our own sizing chart takes into account both factors (for monohulls).

We'd recommend a 20Kg anchor but that doesn't apply to a Bruce!

That's conservative and you could get away with 15Kg for most anchoring, but again only if it's something better than said claw.

A 15Kg Bruce is too small no matter how you look at it.

 

[ditchcrawler]

Just out of interest I sold my spare 10kg Delta through the R2R website & when I posted it(unwrapped) it weighed 11.3 Kg.The actual weight of anchors may be nominal,at least for Deltas.

 

[TigaWave]

Having experienced anchoring in pretty testing conditions in a wide range of boat/hull types I have a theory which tends to go against the often quoted wisdom that a heavier boat needs heavier anchor.

Lets compare
1. a light weight boat, very little displacement huge buoyancy, probably high windage, tall mast, high topsides.

2.heavy boat low freeboard, short mast but huge heavy keel and large under water profile.

In the usual extreme anchorage situation, you'll be experiencing high winds and usually no current but increasing swell and thus snatch on the anchoring system. Of course you are trying to limit this snatch to make life more comfortable and help the anchor hold.

Wind and no waves.
Light boat has more windage and less mass so will react to gusts by surging back and also skating to each side. Snatch load will be high as will sheering.

Heavy boat will resist the relatively short gusts and will be more likely to sit still, Less snatch and sheer forces.


Wind and waves.

A light boat, as a wave passes under the boat, the huge bouyancy and lack of mass allow the boat to be lifted quickly and snatch at the anchor, sometimes violently.

A heavy boat by virtue of its inertia will tend to allow the wave to pass over the boat as it won't react quickly enough to the passing wave = less snatch? The weight of the boat resists movement by wave action.

This indicates that for improved holding a better anchor is required for a light boat than a heavy boat, of roughly equal size?

Discuss? this should be interesting....goes for lunch.... I'll log on again before I go home and see what you all think!

 

[tobble]

I would tend to agree with your argument, but you haven't mentioned the effect of the anchor warp. A nice heavy chain will sink and bow more and therefore absorb lots of a shock load in its inertia more than a lighter chain or rope. the boat will have to move more before the slack is taken up and the full force of the windage is loaded onto the anchor, so it is lees likely to come out.

Having said all that, you (I) still wouldn't use a 5 kg anchor to hold a 50 ft boat in any conditions...

 

[TigaWave]

I'm thinking same size boat same ground tackle and the light boat could produce greater snatch loads on the anchor.

Thinking about on the limit situations when the snatch load has lifted and straightened all the chain. I think the light boat will do this first and hence either needs heavier ground tackle or larger anchor than the heavy boat to hold in the same conditions.

But this goes against the graph from some manufacturers, but they don't give their reasons?

Time to go home now so I'll follow up on Monday...bye

 

[purplerobbie]

[ QUOTE ]
As far as your boat is concerned, I would have a larger anchor. You dont say what size chain you are using, and I reckon this has a bearing on how big the anchor has to be.

The other consideration is how do you weigh anchor, if you use a windlass, going larger is less of a concern than if you are doing it by hand.

[/ QUOTE ]
I have 46m of 10mm chain and 50m of warp and a lofrans electric windlass (if i can get it working)
Rob

 

[Talbot]

[ QUOTE ]
a lofrans electric windlass (if i can get it working)
Rob

[/ QUOTE ]

That is the crux of the matter, cause if you dont, and want to anchor in more than 10m, the weight of 10m of 10mm chain, and the anchor are going to be a significant element on whether you can actually lift it off the bottom!

 

[GMac]

I'd agree with the above in average to strong weather. Once it really starts to pipe up it swings the other way.

Assuming the 2 boats are on 14mm rope and 10mm chain rodes and the same. Both boats are say 35ft.

When it's really going off the lighter boat will put a smaller maximum load than the heavier boat. The chain weight and drag effect will have a lot more baring on how she hangs and moves. She will pull up faster as the chain kicks in hence a higher shock resistant set-up. The heavier boat will take time to get going but will soon be banging a lot harder on the rode. Inertia. The longer or bigger the chain the bigger the dampening effect, to a point. To big and it goes against so many things.

BUT as most lightweight 35fters usually run 8mm chain, not 10, the chain effects would be reduced a bit. On the heavier 35's the warps are more 16mm usually, even seen some 20mm.

All along the rope has been stretching back and forth. On the heavier boat the rope works better than on the light boat. Higher Inertia uses more of the stretch in the rope so gets a softer ride from that. The light boat usually doesn't have the power to get in to the real good stretch zone of the rope. Bigger and less stretchy the rope the less the effect, again to a point. Go too small and the rope will cook and go bang, less chafe resistance as well.

As I always say, having a good anchor is fine but you still need to tune your rode.

I have a 32ft light weight 1 class design boat. A couple of months back i was anchor next to another. I have 15mts of 7mm chain to 12mm polyester 8 braid (Octiplait to you northern types). The other had 9mts of 10mm and 14mm laid nylon. These boats would rate in the all time top 3 for sailing at anchor. My ride was noticeably smoother. The longer chain even though smaller and smaller stretcher rope just was having a ball, loving every minute of it. He just was too oversized.