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Anchoring in cyclone Pam


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I thought to share a couple of thoughts on anchoring in cyclone Pam.

We were anchored in Wairahi Bay in Great Barrier Island - a good anchorage for the conditions but subject to big gusts and wind shifts as is typical in most Barrier anchorages in strong winds. I don't have wind gear so I can't quantify the wind speed, but at its height it was not possible to see the sea surface as the air was full of spume to a height of about 2M. On the couple of occasions that I went forward to check the anti-chafe gear it was necessary to crawl, the breeze being too strong to stand up in,so I would guess the wind to be about 60 knots.

We anchored in thick mud in a high water depth of about 4.8M. The primary anchor was a 25lb Manson Supreme on 20M of 8mm chain and then warp. I would like all chain but I don't think the H28 would take kindly to the extra weight in the bows. We lay to 30M of rode. This may not seem very much but then with the shallow water and warp not providing much cantenary effect, there didn't seem much point in laying out more. Prior to the event we dropped the boom down onto the deck, took down the dodger and laid a second anchor. This may seem a bit overkill but when things get a bit dicey I really hate that feeling when you say to yourself, 'I wish I'd done so and so.....'

The second anchor was some sort of 25lb Danforth type on a similar chain/warp setup. The two rodes were at about 30degrees to each other. At the height of the breeze the boat was laying her side decks in the water and yawing through about 45 degrees in the gusts and pivoting solely about the primary anchor. The secondary anchor just dragged and to the extent that I worried that it would foul the primary one, so I let out a heap of secondary rode to take the weight off the Danforth, put my faith in the Supreme and prayed for the dawn.

According to the GPS, we didn't move at all beyond our swing arc.

So, lying to two anchors would appear to be a waste of time unless the anchors are of equal holding power. Even then, I would be concerned that if you dragged in extreme conditions, they would come together and cause a whole lot of mischief. Next time I have to anchor in a cyclone I think that two anchors on the same rode is the way to go. The Manson lends itself to having a length of chain with another anchor shackled in front of it. No doubt there is some formula to calculate the optimum distance between anchors on a single rode for a given depth - maybe someone can advise.

In closing, I confess to being very nervous during that crappy night and hope I don't see its like again.

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I'm not awfully experienced in anchoring but would have put every bit of rode out on the basis that each tiny bit of extra catenary effect and elastic effect would reduce the shock on the anchor and perhaps prevent it breaking out. I've also read about people motoring into the wind all night on the anchor in a storm, but since you didn't drag on the main anchor I guess you've learnt a lot about your system now!

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In a cyclone, where wind is going to swing full circle, that layout of anchoring is not the best choice. The two would eventually tangle. If you really wanted to use two anchors, Tandem lay is the better way to do it. Primary anchor being the first and the smaller trailing. Distance between really comes down to ease of deplyment and practicality of carrying spare chain. A rope line between works OK if one has no chain. The holding power increases dramatically with two anchors in tandem. Although I have had no way of testing how much, I have several times used this technique now and my second anchor is a very small plough, that I mainly use for stern lining. It's probably 4 times smaller than it would need to be if it were my primary. However the holding power it creates in tandem is really quite unbelievable.

       The supreme is an excellent anchor, but the genuine ploughs are good as well. However, the two work very differently and many blame the plough with not holding them, when the anchor itself may not be the problem, but the bottom type and just hopw a plough works. The Plough, as it's name suggests, worls by ploughing itself down deep, till the force against it equals the force pulling. If the bottom is softer, it can drag much further till it gets deep enough to stop moving. The supreme works more like a bulldozer, because the angle of the flukes are wider. So the result is that the two anchors would drag at different distances before equal holding power is reached.
       For every doubling of chain length, you increase the drag resistance of the chain itself, by 4 times.

More line length is not a bad thing either. In fact if the wind is so strong that the rode is tight and straight, then the more the better. The stretch of the line acts to take the shock out of the rode as the boat pulls back, just exactly the same as catenery action does. You can never have too much line or chain for that matter, when in a major blow.
       60kts? I suspect you may have been in far higher. I have stood on my deck in 72kts and could stand OK, but was pushed around. 86kts bent the blades back on the wind genny enough for the blades to hit the mounting pole and they shattered into a million pieces.

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Having been next to Crisc in Wairahi bay during cyclone Pam (luckily for us we had been offered the use of a mooring) I can confirm the force of the wind and the changing conditions as it swung from SE to SW. Like Crisc we don't have any wind gear but I would have estimated the strongest gusts to be in the region of 70 knots.There were 9 boats in the bay, the only one that dragged significantly was a large (French?) yacht that dragged maybe 100 metres, it could have become an issue once the wind went to the SW as both Crisc and I were within the arc of its swing. But by that time the wind was easing so never became a problem. Every time I went out to check on the mooring rode Crisc was out in his cockpit checking as well. In fact I think he spent most of the night out there on watch!!  It was one of the wildest nights we have spent onboard.

Had we had to anchor I would have used our Rocna (NZ made) and our kedge, a brittany anchor. (a French anchor similar to a Danforth, excellent in mud) in tandem, 

I have weathered a similar blow, also in Wairahi bay several years ago using only the Rocna and dragged maybe 25 metres. I put it down to the shallow depth, softish mud and lack of catenary in the chain.

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The most I have anchored in was measured at 108kn by a nearby French frigate. Had a Danforth in front of a Manson plough on one rode, and the storm anchor, (another plough) on another rode at 30deg.

 

Took two days to dig them out afterwards. KM tells me that the glazing I noticed on the inside of the rodes meant I was close to loosing the lot due to internal friction.

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I've personally not had any good experiences with a danforth at Barrier over 30 years or so ,and I've watched mates who 'never drag' drag out of a bay there and they had big danforths on . My theory is that the Barriers bays like Paradise or kiwiriki/ kaiarara are silted up with a fine mud and the old danforth tends to skate in it.

So back a few years I would use my own big danforth up the coast, especially when I wanted to pick in fast in a patch of sand between the kelp, and switch to the CQR for barrier. That worked fine , but now I just use a new generation anchor for everything. Rocna in my case.

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To clarify my original post..when I wrote that I lay to a rode of 30M, I refer to rode in the water. So, from water to stemhead to bollard would be about 4M. Consequently, the rope component of my rode during the cyclone was 14M, give or take. Having fairly heavy nylon spliced directly to the chain, I thought that 14M should provide sufficient elasticity. Concerning the amount or rode out, the rode itself was bar-tight during the gale, no cantenary at all. Doing the right angled triangle calculation thing, 30M of rode in 5M water gives an angle between rode and seabed of 9.6°. 40M of rode gives 7.2° and 50M rode gives 5.7°. Exceeding 50M of rode in 5M water depth gives only a marginal decrease in angle. So I concede that in lying to 30M, I was probably being a bit stingy with my warp, although there would be no benefit in exceeding 50M.

As Steve intimated, I did spend most of the night huddled in the cockpit in a state of high anxiety - the annoying thing was that my wife was quite relaxed during the whole miserable event.

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Wow, I couldn't imagine 108 knots.

I've experienced 93 (measured about 2kms away from us) on the night the Capitaine Bouganville got into strife and that is an experience that would never wish on anyone. I'm sure a little H28 named Veronica disappeared the same time also.

Unfortunately, from what I can remember, the info that we received was it could get a bit lively and we should maybe batten down a bit for the night. Obviously it got a bit worse than forecast.

We were anchored under a very high headland and the gusts coming down were frightening. Because of the headland, the wind was swirling, so we would be laying in 1 direction then get belted from another angle so hard that something was going to fail eventually. Luckily we used a mooring that was vacant, as I can't imagine any anchor setup being much use when the boat is doing laps, then charging ahead until it all takes up again from the other side. I saw the cabin side windows submerged more than once, so would have to assume the rig was getting a good wash at times.

We eventually bailed and motored out into the middle of the harbour and held station for the remainder of the night.

The anchor warp that we used to loop through the mooring and back to something more substantial than the bollard was not going to last much longer and ended up as landfill.

The memory of that has remained with me all this time as a moment that, although it was not a nice experience, it could have very easily been a whole lot worse.

It makes you appreciate how strong mother nature can be at times, and she will win if you're not careful.

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In 60kts of wind - NO waves

A H28 would see around 1100 kilos of horizontal load.

A S34 around 1500kg.

A TP52 around 2700kg

 

In 30 knots of wind

The H28 - 250kg

The S34 - 375kg

The TP52 - 700kg

 

NOTE: all of those 3 are low wooded so lowish windage. More windage means a higher load. Wave action isn't included and could mean 3-4 times those loads in the spikes, usually more like 2-3 times. Being tucked up in Bon Accord with the massive 300mm waves doesn't really count, they would add a little but bugger all.

 

Scope is not all about centenary effect. The drag thru the water of a anchor rode can be huge so that can add to shock absorption also. Just as the drag coefficient of the chain alone can also be a lot more then most realise. The biggest thing is anchors will hold the most at around 9 to 10 to 1 scope. Less than that and you are introducing a 'pulling it out of the ground' component. The deeper the water  the lower the scope can be. In shallow go for as much as you can safely get.

 

Having 'heavy' nylon in a blow isn't usually a bonus, except for the bit of extra time you'll get to see then stop any chafe. We commonly see boats with anchor rope way bigger than is optimal. The days of Go Big or go home are over and have been for a while. The gear we all use today is very very well known so we can use that to our advantage by swapping 'heavy' sizes to a smaller rope that will give more elasticity meaning better shock absorption meaning better anchor holding and better more comfortable ride for mum as we sweat it out in the cockpit all night.... which we shouldn't have to do if we have a good system which we can trust.

 

If you are set up right and have a OK technique it's only chafe you need to be worried about and these days that can be negated quite easily. Sure sh*t will still happen but if you spend your life trying to stop sh*t happening you won't get any sleep...... and the sh*t will still happen regardless. 

 

Think anchor system, not a pile of bits you cobble together to anchor with.

 

In line Tandem anchoring made super easy. Check this sexy puppy out. 3 anchors in one, very clever.

tandem-anchor-classic.jpg

 

But there are times and places where the 2 set separately is the far smarter move.

 

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