Anchors for Catamarans

[Mr B]

Hi Noelex 77,
According to that chart, My anchor has a holding power of 2052 Lbs,
How do you calculate the 2052 into wind strength and boat size resistance,

Or how much square footage of the boat above the water line can be converted to to the 2052 Ibs of wind resistance,

Bit vague, Not sure how to describe it,

[noelex 77]

Originally Posted by Mr B

Hi Noelex 77,
According to that chart, My anchor has a holding power of 2052 Lbs,
How do you calculate the 2052 into wind strength and boat size resistance,

Or how much square footage of the boat above the water line can be converted to to the 2052 Ibs of wind resistance,

Bit vague, Not sure how to describe it,

Hi Mr B

The holding power is only for one particular design of anchor in one particular substrate. It shows the relationship between holding power and anchor size that Rocna expects for their anchor (This was data from the original Rocna design not the Vulcan).

Your anchor design could be significantly better, or worse. There is no way to tell without some testing.

However, the relationship between anchor size and holding power seems to roughly hold irrespective of the design, scope or substrate. Thus we can conclude if you built an otherwise identical identical anchor, appropriately scaled, that was half the weight it would have roughly half the ultimate holding capacity and if you built an an otherwise identical identical anchor that was double the weight it would have roughly double the ultimate holding capacity. These are rough rules of thumb rather than absolute mathematical constants, but they are useful principles nonetheless.

There are formula that predict the force a boat at anchor might experience at various wind-strengths. Unfortunately there is wide variation in the numbers. The popular ABYC table is widely thought to be pessimistic, or the worst case, but the data is easy to find.

This link is very good:
Forces

BTW I have anchored many times in the area where your boat is based. The substrate is nice and soft. Providing the anchor has enough size it will hold wel, but it is not a good testing ground for the ability of an anchor to penetrate difficult substrates such as very hard sand or weed. Be careful when you venture into the South Pacific and further afield, the substrates can be very different.

Anyway, congratulations on designing and making your own anchor, that is more than I have ever done.

[CaptRehab]

With an anchor that would not hold the boat in the mud in Woburn Bay and limited choices available we opted for a 25kG Rocna Vulcan. Once installed we then spend 3 months locked down on a mooring. Well Maybe 2021 will be better. I will report back with my thoughts when we get the chance to move about and anchor once again.

[fxykty]

Originally Posted by CaptRehab

With an anchor that would not hold the boat in the mud in Woburn Bay and limited choices available we opted for a 25kG Rocna Vulcan. Once installed we then spend 3 months locked down on a mooring. Well Maybe 2021 will be better. I will report back with my thoughts when we get the chance to move about and anchor once again.

Is there any way to move the roller assembly away from the hull and more to the centre of the beam? I’m sure we’re not the only ones to see our anchor chain loaded and off to one side or the other when dropping and particularly when retrieving the anchor. I would allow for up to 45* in any direction from the roller.

When hoisting and the anchor shank comes up over the first roller there’s probably a swing as the anchor rights itself and gets pulled in and seated. And imagine hoisting in a bit of a sea with the boat rocking back and forth and the swinging anchor as it comes up out of the water.

At very least if you keep the roller where it is then install a big SS plate to the protect the side, front, and underside of the hull.

Also, the anchor seems to very low, such that it will catch waves in any sort of seas. Is there any way to rotate the roller assembly so the anchor is stored more horizontally?

Enjoy local cruising again once you’re out of boating lockdown. Any progress on that?

[Manateeman]

This is not a comment on anchors or chain.
Anchoring gear is a system, each component is important. The three most often overlooked are: final attachment point...snubber...anchor roller.
I was asked to fabricate a better roller system for your model catamaran.
I passed because just too much engineering was required.
Thank goodness several previous posters were generously in providing photos and solutions as the issue is very serious. The main cross beam is set aft of the bows and will have to take all the stress if the snubbers/ bridals fail and the roller system was attached aft of the bow only one one side. I could see why the owners wanted something different and why others have changed their boats.
Catamarans behave very differently than monohulls when anchored in a storm.
It’s just a fact. Keeping the anchoring system weight low is only one issue.
Finding attachment points...plural...is another. You must also have an anchor roller system which is strong and minimizes point loading and chafe.
I suggest you consult a marine architect and bring as much information as possible both on your vessel and photos of how others have designed their rollers. Once things get serious, you will not be physically able to crawl out on the bow and do much about it. It’s a system, not one component.
Happy trails to you :thumb
Captain Mark and his “we’re not rowdy orcas” pizza & beer manatees

[CarlF]

The Leopard cats have the windlass and roller in a fairly shallow spot with a hatch. There's not a lot of room for a tall shank and still close the hatch.

To my eye, the Excel shank seems to sit lower than the Spade shank and might fit better.

Does anyone know?

BTW - the picture isn't my boat - just grabbed it off the internet

[chrisr]

we have an excel and dimensions are given here on their website

SARCA Excel No 5 - Anchor Right Australia

(this is the size we have on our 12m / 7.5mt cat)

i don't know about the spade but suspect they will give the info on their website likewise

cheers,

[Morph]

Originally Posted by Factor

SARCA - EXCEL. Job done

Yep--SARCA saved my boat in 60 knots of wind swinging at 170 degrees...I would not cruise on a boat without one!



and just to provide some sort of third party verification--look at SV Panope's youtube site. He has done an amazing job!

[Boatguy30]

Originally Posted by CaptRehab

With an anchor that would not hold the boat in the mud in Woburn Bay and limited choices available we opted for a 25kG Rocna Vulcan. Once installed we then spend 3 months locked down on a mooring. Well Maybe 2021 will be better. I will report back with my thoughts when we get the chance to move about and anchor once again.

I Found the Vulcan to perform pretty poorly in soft mud. Also drug in a squall in Abacos when wind shifted 180 and was able to get clogged with weedy silt.

I’ve gone back to a slightly oversized 44 delta which over the last 12 months has done well. Notably boats with oversized Rocnas drug in nearby spots in different bottom types while we have held.

Of course, could easily drag tomorrow!

[Mr B]

My Girl suggested that she cant pull pull up either of my anchors by hand, If I get incapacitated in any way,
I cant do what you can do,
So she bought me an anchor windlass,
Its now fitted on the bow,
It will make life easier for me,
Instead of getting just a good spot to anchor,
I can now pull it up and change spots repeatedly,
It has a 220 Kg lift and a 630 Kg pull,

Cheers, Brian,

[At Peace]

Originally Posted by Rimica

Just for the record. I have a Spade on a 51 feet catamaran. It held no problem for 4 days in 50 knots winds in Greece. I was in 5 meters of water. I put 50 meters of chain and the bottom was mud.


I now have trust in my anchor. Nothing like experience with a bit of kit to judge if its a good piece or not.

Rimica, if I may ask, which SPADE anchor do you have and what is your boat weight?

Thank you!

[Rimica]

I have an Outremer 51. Probably 13 metric tons fully loaded. I have a 25 kilos spade.

[fxykty]

Originally Posted by Rimica

I have an Outremer 51. Probably 13 metric tons fully loaded. I have a 25 kilos spade.

We had a Spade S140 - I believe that is 30 kg - for our 12 metric tons Outremer 55L (2003 - not one of the new O55s). If you have the S120 on your boat that really seems a bit small, or rather just enough. We sometimes felt our S140 was a bit small.

It worked just fine and we only had one dragging incident in the 4 years till now, but now that we’re living aboard full time we’ve upgraded to a Excel 9 50kg anchor to help ensure the boat doesn’t move when we don’t want her to.

[Alistair242]

We have a Sarca Excel #6 on the bow and a fortress fx23 as a kedge. The former is oversized for the boat on 80m of chain so we sleep well. The Fortress is very quick and easy to deploy on the boat or from the dinghy. Some may think it is undersized but it is perfect for what we want it for.
The alloy Sarca Excel looks the goods at half the weight but is a bit pricey for us.

[Rimica]

Our anchor has served us well in a variety of bottoms, wind speed and change in directions. I am comfortable with it. If I did something it would be to add more chain.