Big Trimaran: 55, 70, or 110 kg Rocna

[pbmaise]

Looking at sizing chart:

Sizing Guide | Rocna Anchors // Rock Solid

I have a 22m trimaran with actual weight near 30 tons, gross tonnage of 70 tons, and windage from full pilot house, three hulls, and six aka.

How do you see me falling on their sizing chart?

Based on actual boat weight 55 kilo should be fine. However, GT indicates 70 kilo. Sizing up again for multihull means the 110 kilo.

To keep from dragging with my current Bruce and CQR anchors that weighs about 50 kilo I have been adding about 200 kilos of kellets.

I am hoping the new anchor will not require the kellets.

Side question, is anyone adding replaceable zincs to their anchors?

[fallingeggs]

Originally Posted by pbmaise

Looking at sizing chart:

Sizing Guide | Rocna Anchors // Rock Solid

I have a 22m trimaran with actual weight near 30 tons, gross tonnage of 70 tons, and windage from full pilot house, three hulls, and six aka.

How do you see me falling on their sizing chart?

Based on actual boat weight 55 kilo should be fine. However, GT indicates 70 kilo. Sizing up again for multihull means the 110 kilo.

To keep from dragging with my current Bruce and CQR anchors that weighs about 50 kilo I have been adding about 200 kilos of kellets.

I am hoping the new anchor will not require the kellets.

Side question, is anyone adding replaceable zincs to their anchors?

I vote you contact Rocna. You're boat clearly wasn't contemplated when they made that chart. I hear they are very responsive. At least post their response here and then let people debate whether you should upsize because Rocna is not conservative enough taste (a habit of us folks even though Rocna would probably disagree in most cases).

[pbmaise]

According to their site:

Multihulls
Our chart is intended for monohulls. In general, an anchor for a multihull should be up to 50% larger than that for a monohull of the same LOA. Commonly this means going to the next higher recommendation (one size up), depending on how close the vessel is to the upper limit of a particular size range.

.........

I decided on the 70 kg model. This follows their recommendation to go one size up and will do two things. Increase anchor weight by almost 50% and hopefully improve reset capability.


Yes I do have unique problems especially with location. For example in the Mactan channel between Cebu and Mactan Islands I couldn't safely leave the boat until I had two anchors to port and two lines to shore on starboard.

Many areas I anchor are very soft mud which clogged the claw of my Bruce. Especially Brunei.

Overall I realized:

A. Until I have any new anchoring site proven after tide and wind shifts, I will still continue to run 27/7 anchor watch.

B. Even after proving a new anchor, I still will maintain a storm watch. During a storm we run the engine to reduce anchor loading.
During a cyclone, I either will anchor to shore, or put to sea. I have done both.

C. Likely I will still add 200 kg or more in kellet weight to the chain for any long term anchoring. This is simply two 55 gallon drums filled with pebbles tied with Dyneema line.

D. The 110 kilo Rocna is too heavy for two crew to easily lift by hand. Further it would require a larger chain.

********

Note: One issue with long term anchoring I found was removal of mud from the bottom. At my current anchorage I anchored in soft mud and my kellets sank deep into the mud too. Overtime, my anchor chain kept sweeping the mud away from below my vessel. Each tidal change there was less and less mud. Eventually the chain exposed long dead corals which in turn started damaging the galvanizing. Therefore, I am having to regalvanize my chain within two years.

[pbmaise]

P.S. I have decided to modify the shank of my Rocna to accept a bolt on zinc anode. It really is a nuisance to have to find a large enough mooring while my chain and anchor are being regalvanized.

[Hydra]

If the density of stone is approximately 2.5, its apparent density in water is 1.5. Then, the apparent weight of 200kg of pebbles is only 120kg in water.

A cast iron or lead kellet would be much more effective. But IMO, all that weight would be better used in anchor and chain.

Alain

[noelex 77]

Contacting Rocna for advice would be a good idea.

It is hard to extrapolate the anchor requirements of most cruising boats with your vessel.

Kellets are a very inefficient use of weight so I agree with the aim of avoiding these.

I think one of the best rules is to use the "Largest anchor you can reasonably manage". The Kellets if attached temporally to the anchor will give you some idea of the ability of the anchor winch to cope with the extra weight. Cardboard, or plywood templates will show what will physically fit, and work with the bow roller.

Overall, despite my advice to contact Rocna I think most people place too much emphasis on the anchor manufacturers recommendations. Anchor manufacturers can only use average substrates. A fundamental rule is as you increase the size of anchor the holding becomes greater.

If you cruise over a wide area sooner or later you will end up anchoring in a poor substrate in reasonably strong wind. No manufacturers table covers this eventually. In these circumstances a bigger anchor can keep you secure, but it needs to be resonably manageable otherwise it will not be deployed when it matters.

Often it will not be obvious, beforehand, when it matters. So the largest anchor you can reasonably manage is the best option, for a long distance cruising boat.

[barnakiel]

Yes.

70 for regular work likely. 110 if you expect anchoring in a heavy blow.

This is what the table says.

b.

[CGirvan]

I've always wondered why multis should have a larger anchor than monos.
I get the increased windage but having owned both it seemed the greater displacement in gusts put a more dramatic loading on the anchor.

[Spade Anchor]

We would recommend a Spade S240 or our X240 Stainless that is on sale.

[DumnMad]

What sort of wave action do you get? Might better to look into the 70kg plus an excellent snubber/bridle arrangement instead of 110kg and so-so snubber.

[FSMike]

Originally Posted by pbmaise

Looking at sizing chart:

Sizing Guide | Rocna Anchors // Rock Solid

I have a 22m trimaran with actual weight near 30 tons, gross tonnage of 70 tons, and windage from full pilot house, three hulls, and six aka.

How do you see me falling on their sizing chart?

Based on actual boat weight 55 kilo should be fine. However, GT indicates 70 kilo. Sizing up again for multihull means the 110 kilo.
-----

You do realize that "gross tonnage" has nothing to do with the weight of the vessel? I rather doubt that Rocna's sizing guide references gross tonnage or net tonnage.

[arthur.watson.1]

Before you go out and buy a bigger Rocna, read and consider a recent post on morganscloud.com:


https://www.morganscloud.com/2016/08...hts-on-vulcan/

[Olddave]

That's a serious tub. It sounds like you don't need to get to hung up on how much weight you add to the bow, although that is usually a big concern for multis. I would go for the biggest anchor manageable dependant on how the bow is set up to carry the anchor.
It's a bit hard to comment without knowing how much chain you carry. If you are using a lot of nylon then that is another reason to up the weight of the anchor. I presume you run a bridle out to the amas/floats to reduce shearing.
Lots of variables here, a photo of the vessel or design info would help us to help you with ideas. I am guessing that you just want to kick a few ideas around. Anyone in charge of a serious vessel like that probably knows what they want anyway.
Dave

[knockabout]

30 kg x2'' x1mtr 4'' galv plate 3mtrs from the anchor strong swivel on the top end of plate called link weights /ad 2-3 or track frame off d6 caterpillar bull dozer

[Gaia]

No one is suggesting Aluminum. Even Spade and they make nice Al anchors. Great holding power relative to weight so equal weight anchors should have higher holding. Might that not be a solution when reaching maximum handling weight?

Jim