Another anchor

[noelex]

It is worth looking at the above photos of the Rocna and Mantus “M1” and noting the similarities. The contention that they adopt a very different setting angle underwater is entirely wrong.

I have owned and extensively used both models.

 

[CAPTAIN FANTASTIC]

I reed all anchor posts with great interest, so, carry on posting anything to do with anchor design. Also, don't forget to look at the underestimated Kobra 2.

 

[zoidberg]

A friend with something of a 'thing' about anchors sent me this pic of a recent acquisition, alongside an Fortress Fx-16.

I have refrained from comment!

 

[Kukri]

I can’t help thinking that Professor Taylor and Simpson Lawrence were “onto something” with the drop forged shank of the much derided CQR. Many new generation anchors seem to rely on simple flame cut plate shanks and the others have complex built up section shanks which are bound to corrode from the inside out over time.

The ‘I’ section drop forging gives the best combination of strength, light weight and durability. I don’t think there has been a single case of a CQR ( not a CQR “B” or a clone) failing at the shank.

So why has no new generation anchor got one?

 

[Neeves]

If you take a drawing or side view picture of a Mantus and orientate the image such that the long of the shank is in a horizontal position then the fluke is at an angle of 16 degrees to the long of the shank and the long of the shank (in Noelex picture) is parallel to the seabed. Look at Noelex picture above - the shank is horizontal - the fluke is therefore at 16 degrees.

You can of course argue black is white - but its easier to take a picture, a ruler and protractor and measure the angle yourself.

So Noelex - who says his anchor is the same as Vyv's Rocna.

Make the drawings - show me, and other members, I am wrong.

It is slightly more difficult to analyse other anchors - you need to look at underwater pictures carefully to define 'horizontal' and even more difficult for Spade. If you make a similar interpretation of a Rocna, Spade, Excel etc you will find the fluke is at 30 degrees. If you do the same for the M2 - you will find a fluke/seabed angle of 23 degrees and the video clearly illustrates what happens - it drags and drags and drags. The M2 has a fluke almost identical to the original Mantus - I would expect the 2 flukes to act similarly. Note the only, or most of, the pictures of a Mantus are of a huge anchor, originally 50kg, and most Rocna, Spade etc that you encounter are 15/25 kg. In contrast Rocna, our Excel and Spade simply lock up in about a shank length (which would be more than a metre - were we to have the need for a monster anchor).

The NCEL, the US Naval Civil Engineering Laboratory (that's the research centre for the US Navy) have a simple analysis of fluke angle, hold is directly proportionate to the Sine of the fluke seabed angle. Sine 30 being 2 times that of sine 16 - has twice the hold. Its not quite that simple - if the seabed is a bit hard and you were to extrapolate from 16 to 30 to 45 - then a fluke designed for 45 degrees might not engage - at all - on a hard substrate. As long as we work in sand - and common seabeds - 30 degrees is the optimum.

This is what Charles Reid said of his anchor choice in a post here a couple of years ago:

Another Anchor thread :O but its simple (hopefully)
I've used a CQR cast copy, that came with the boat, for 9 years, until the shank fractured during a blow in a Spanish port.
This was a blessing, as the replacement 25lb CQR was a far superior replacement, though how well you were anchored was very dependent on technique, requiring about one boat length to set.
That anchor finally wore out (the pin joint enlarges and setting becomes dodgy). I finally replaced the anchor with a 25kg Mantus (I suspect the others such as the Spade, Rocna and Ultra are indistinguishable in performance if not in price). My kedges are a Delta, SWMF Danforth and a Fortress.
The Mantus sets in about 3m reverse, digs into any soft substrate and resets easily. Its good for 42 knots constant, on a 5:1 scope. In soft substrates it only just beats the Danforth, but resets better. The Fortress is as good as the Danforth, much lighter. The Delta is as good as the CQR was, but easier to stow.
Choice of anchor has to be down to the composition of the sea-floor, (round Bardsey a grapnel or fisherman has to be favourite, in Alvor the largest fluke area anchor is key).

end quote.

Anchors when they set may start with a fluke seabed angle of 30 degrees, or 16 degrees. As they dive (bury) they need to drag the shank and chain with them. In the Mantus case because the shank disappears last then when the shank starts bury - with some chain there is suddenly a rapid increase in the upward tension on the rode and that 16 degrees very rapidly reduces - and will quickly reach the point where the anchor will drag freely though the substrate (just look at that video of the M2). Anchors that start with an angle of 30 degrees have a long way to go before they even reach 16 degrees.

This underlines why anchors have used thin flame cut shanks for the last few decades - its to cut down and reduce the upward pressure on the fluke and allow it to continue to dive with as high a seabed/fluke angle as possible. Its also another reason behind using smaller, rather than larger, chain, not using a swivel (better a Boomerang) etc - anything that has area or volume will reduce the performance of the fluke. Note its the fluke that causes the anchor to bury, the shank, chain, roll bar, shackle, swivel (and Boomerang) all are a hinderance to burying. The whole raison d'être of an anchor is - to bury.

Historically the way to impart strength was to use that 'I' beam shank - we now have steels that allow us to dispense with those beefy shanks and allow the anchor to bury more deeply. Peter Bruce knew all about this - his shanks were chamfered at the leading edge to allow the shank an ability to make an easier penetration. Spade, Ultra and Peter Smith have the same ideas, a shank narrow at its cutting edge.

It merits note that as chain buries it develops a reverse catenary and the angle of the rode has little influence on the increased shackle angle (that rotates upward as the chain is buried). The rode may have a low angle at the seabed - say 15 degrees under tension - but your shackle - which determines the angle at which that tension is transferred to the anchor may be at 45 degrees. It also merits note there is, 'inexplicably' (its very complex) no relationship between your scope (or rode angle) above the seabed and your shackle or tension angle. The tension angle is dictated by the depth your anchor has 'dived' to and the shear strength of the seabed + other variables. There are formula to calculate shackle (or tension) angle - but you need to know shear strength, development of shear strength with depth (normally shear strength increases with square of depth - which is why you want to bury deeply) - and there are too many unknowns for 'us'.

But the deeper your anchor, the more chain you bury and the less impact vessel movement will have on the anchor.

So Noelex - anyone with the time can work out your fluke/seabed angle (16 degrees). The photograph is there - a child could make the analysis of the seabed angle. Please explain why you have never mentioned this fundamental difference and why this is better than Rocna, Ultra, Excel or Spade at 30 degrees. Tradewind provided the picture of the huge Mantus, above, - I believe yours is a 60kg model now - on a yacht where Rocna would recommend a 33kg model. Your 60kg Mantus has the hold roughly of the 33kg Rocna - excuse me if I find this 'slightly' unusual - if not contradictory - as you don't emphasise the need to double anchor size (with commensurate need for a bigger wallet).

You are more than happy to damn a Boomerang that you have never ever seen, never ever seen one in action. You are happy to damn a Kobra, or other modern convex anchor (some of which you have never ever seen in action) yet you studiously ignore a fundamental characteristic of you own anchor. Excuse me if I am incredulous. I am sure there is a term, slips my mind, for someone who denigrates a product, sight unseen and ignores unusual characteristics of a product they champion.

If anyone reads your 'picture' thread (the link is in GHA's post above) they will find hundreds of pictures of your anchor - self same oreintation - long of the fluke parallel with the seabed, they will find critical comment by you of other anchors, they will not find one word on fluke angles. They will find people praising the thread and confessing that as a result of the thread - they too have bought a Mantus (that has not one of iota of published holding capacity data (a similar omission of the M2).

Who needs a salesman if they have your level of marketing - intentional or otherwise.

But now we have the images - a simple series of explanations would explain how you can damn a Boomerang and a Convex anchor - neither of which you have seen and omit that key characteristic of your own anchor.

And of Kobra - an excellent produce, good fluke seabed angle. Sets quickly and reliably. I did not like the folding mechanism and on ours had the shank welded up. I don't like the shank - its a bit wimpy. But its cheap - possibly the cheapest, by far, high performance anchor available and as long as you are cognisant the shank is a bit wimpy - an excellent product. It proves and underlines the actual cost per kg of a decent anchor - compare the Kobra to the cost of a Rocna, Mantus, M2, all of which have similar manufacturing costs (all made in China) - and you will soon see how much goes to marketing and profit. I have no issue with the high prices - if that is what the market will pay (especially if you have someone doing the marketing for you) - go for it!

Finally - if that is not enough - We have a commercial mooring near us composed of 3 Danforth, or danforth copies, of 1.5t each, set at 120 degrees to each other. They were originally installed to secure Murdochs 3 masted super yacht (I think they might have sold it since - but the mooring is still there for superaychts that visit Xmas/New Year). The mooring is professionally designed and approved by one of the Classification Societies, Lloyds etc for insurance purposes. The anchors are set using a large mooring barge - but to allow the mooring to be serviced the flukes are set to allow a 22 degree seabed fluke angle to allow 'easier' removal.

Possibly you can explain Noelex the background to your recommendation that a large anchor allows security at short scope - when the CS seem to say the exact same opposite.

If this thread were about baking bread or the type of antifouling - I would have no emotive input - this is about safety and safety equipment. People take notice, as evidenced in Noelex thread, people need to know exactly how that safety equipment performs - its not a game. Why people buy anchors without any hold data beggars belief - not everything people say on the internet has any basis in fact (and if I get something wrong - let me know). If we make claims - people might believe them, we should be able to defend our claims without question. To me an undefended claim is marketing fluff (and potentially dangerous).

Jonathan

 

[Neeves]

For anyone who wants to read a bit more about anchor angles I can offer this link (which I have posted previously)

https://www.practical-sailor.com/issues/37_74/features/An-Inquiry-into-Anchor-Angles_12153-1.html

There is much more 'out there' on the internet (as GHA states) but you need to be pretty fanatical to search and read. There is a lot of work conducted as part of PhD studies in many of the Universities, Perth (Australia), Houston and Southhampton being just 3. Fluke/seabed angle has been covered in enormous detail and is known internationally - simply think of the Fortress variable shank angle that was introduced in the late 80's. The river barges on the Mekong appear to have decided on 40 degrees as the best angle. For very hard substrates the US Navy recommendation is reduce the angle, from 30 degrees, though they don't mention by how much, and sharpen the toe. I think there may be a bit in the Vryhoff manual on angles. its not rocket science.

Some of the work has focussed on shank design, length, thickness of plate etc and some related work has been done on the influence of the thickness (resistance to burial) of the rode

https://www.practical-sailor.com/issues/37_38/features/Anchor-Rode-Report_11321-1.html

If you look at Vyv's Rocna and Noelex' Mantus one has buried chain one not - any guesses which might be better if the wind (and thus yacht) are veering. Most 2nd generation anchors, Kobra, Supreme, Ultra, Excel, Rocna, Spade bury chain and toe simultaneously and when the anchor has almost disappeared then 2-3 metres of chain is also buried. Mantus and Fortress perform differently - because the shank tends to be horizontal chain does not bury until the whole anchor had been buried. We have found burying the correct sized Fortress needs much more tension that we can develop - and have downsized one of our Fortress, from FX23 to FX16 - for just this reason (keeping a big, FX37 for squishy mud).

One reason we like buried chain - I've mentioned this before (as has another member) - touch an anchor when the chain is off the seabed and the anchor twitches (which will result in the shear strength of the seabed immediately adjacent to the anchor being reduced. This effectively means the hold is reduced). Have the anchor more deeply set with more buried chain and the twitching is reduced. Its subjective but something that anyone who cares to dive on their anchor can detect. If the anchor is so deep you cannot actually touch it - a long metal rod (screw driver will do, I have some galvanised thin, 5mm, reinforcing rod) prodded into the sand - find the anchor - you can still feel it. We mark our chain, 1m marks for 5m, so that we know exactly how much chain is buried

Jonathan

edit,

I had a few moment so I thought it would help if a provided a link for any seriously interested in anchor design (again it is a link I have posted before). The work was conducted by a Korean research student. Kim. The research, from memory is focussed on fluke anchors, and at our scale like Bruce, Bugel, Fortress and the original Mantus. it does not encompass, though you can extrapolate, to ballasted anchors like Rocna, Spade, M2 and Excel.

Kim has a series of sections in his report where he looks at the crown location, shank thickness, length etc. If you check the sections you don't need to read the whole thing and can simply look at the section that appeals to you at the time. Some of it is heavy going - other parts - dead simple.

In view of this being a Mantus thread and Mantus is based in Houston - the work was conducted at the Texas A and M University. There is a useful reference list at the end of the report.

it is a bit dated, 2005 - but it makes a good start

Kim; Upper Bound Analysis for Drag Anchors in Soft Clay

https://core.ac.uk/download/pdf/4271814.pdf

end edit

 

[Bouba]

I have a Rocna, which I’m very happy with, but I’ve never managed to bury the whole anchor and I’ve certainly never buried the chain.
Is this a problem of the sea floor (sand not deep enough) or haven’t I subjected the anchor to enough force (I’m a fair weather motorboater)?

 

[vyv_cox]

I have a Rocna, which I’m very happy with, but I’ve never managed to bury the whole anchor and I’ve certainly never buried the chain.
Is this a problem of the sea floor (sand not deep enough) or haven’t I subjected the anchor to enough force (I’m a fair weather motorboater)?

It's an interesting question. I have never fully buried my Rocna despite having anchored in sandy bays like the one shown above and being subjected to gale force winds for days on end. Yet I know of other Aegean sailors, also using a Rocna but bigger than mine to reflect the fact that their boats are bigger, who feel that there is something wrong if they can even see the top of the hoop above the seabed. So far as burying the chain is concerned, the photo above is fairly typical of winds of force 4-5 and less but above that the whole chain will be out of the seabed at the end of each yaw and drop down in a shallow loop between.

This photo shows a big Manson Supreme on a very windy evening, around force 7. I think it is a 33 kg anchor with 12 mm chain, attached to a 55 ft boat. Anchor certainly not buried, nor the chain


And here is my Rocna 15 kg with 8 mm chain taken at the same time.


The pattern in the sand in the Manson shot shows how well it accommodated wind direction change. Initially the wind was about F4 from the SE, veering to NW about F7. The Manson (and the Rocna although this shot does not show it) rotated and reset virtually without dragging any distance.

 

[RichardS]

I have a Rocna, which I’m very happy with, but I’ve never managed to bury the whole anchor and I’ve certainly never buried the chain.
Is this a problem of the sea floor (sand not deep enough) or haven’t I subjected the anchor to enough force (I’m a fair weather motorboater)?

I can tell when my Rocna is totally buried because the chain is vertical but when the windlass is blipped to bring it up all that happens is the bows drop down towards the sea, although it only happens after a couple of days at anchor in a blow.

Richard

 

[Neeves]

Anchor burial is contingent on the tension applied to the chain, engine or wind and the size of the anchor.

If you have grossly oversized your anchor you are less likely to bury it.

Without an idea of how you set your anchor, engine size, size of vessel, length (and 'type') and size, weight of anchor it would be impossible to comment.

We are a 38' x 7t catamaran with the windage of a 45'yacht. We have 2 x 20hp engines and 3 bladed folding props. We use 15kg anchors (and 6mm chain). Our anchors are all aluminium and weight 8kg - but the 15kg size./weight is that of the steel equivalents. We can bury our anchors and 5m of chain after a bit of a blow. Our anchors would be considered small by some, maybe many - we don't regret the choice.

We used to carry a FX23 anchor but when we set it under power the stock always stood proud and there was fear that if we used it and there was a wind shift, or the tide changed, we might lift the anchor (or the chain might lift the anchor) as the chain swept under the exposed stock. This fault, or comment, of a Fortress is well aired we have simply minimised the risk. For this very reason we have moved to the FX16 - a size smaller than Fortress might recommend (and have no regrets). We do have a FX37 for thin mud.

I am most envious Vyv of the clarity of water in your images - it allows very good photography.

Jonathan

 

[Bouba]

Gentlemen, thank you for your thoughtful answers:encouragement:
Jonathan has said something that might ring true, my Rocna is big! On a Beneteau Swift Trawler 34 motorboat with 425 hp single engine, total weight I’d guess to be 9 or 10 tons, but very high compared to length and lots of windage up front. We have eight millimeters chain and a twenty kilo steel Rocna, no swivel or banana. When setting I try and do as it says on the instructions and just lay the chain on top of the anchor and then let the wind do the work. However in the summer there isn’t room for a lot of scope so I reverse (very slowly with minimal power) to set it. I always look for a sandy patch.

 

[noelex]

When setting I try and do as it says on the instructions and just lay the chain on top of the anchor and then let the wind do the work. .

Rather than trying to lay the chain on top of the anchor, a better idea is to lay the chain out in a nice straight line.

Drop the anchor and as the wind takes the boat backwards lay out the chain at the same speed. If the wind is very light you might need an occasional nudge in reverse. Try not to put any force on the anchor until you have laid out enough chain for reasonable scope (say 3:1). On the other hand, don’t let the chain just pile up on seabed. It is a bit of balancing act.

Once you have laid out the full scope, unless the wind is strong gradually add some reverse engine force to dig the anchor in.

Perhaps I am misunderstanding your description, but it sound like you may be achieving something like seen in the attached photo. Most of the time the wind pressure will sort everything out, but it is not ideal:

 

[Kukri]

Sounds like the standard method of laying a CQR.

 

[Bouba]

Rather than trying to lay the chain on top of the anchor, a better idea is to lay the chain out in a nice straight line.

Drop the anchor and as the wind takes the boat backwards lay out the chain at the same speed. If the wind is very light you might need an occasional nudge in reverse. Try not to put any force on the anchor until you have laid out enough chain for reasonable scope (say 3:1). On the other hand, don’t let the chain just pile up on seabed. It is a bit of balancing act.

Once you have laid out the full scope, unless the wind is strong gradually add some reverse engine force to dig the anchor in.

Perhaps I am misunderstanding your description, but it sound like you may be achieving something like seen in the attached photo. Most of the time the wind pressure will sort everything out, but it is not ideal:

Yes, that’s how I do in summer when the anchorage is rammed packed but the Rocna instructions definitely (yes, I just checked) say just pile out the chain and let nature take its course!

 

[sailaboutvic]

Gentlemen, thank you for your thoughtful answers:encouragement:
Jonathan has said something that might ring true, my Rocna is big! On a Beneteau Swift Trawler 34 motorboat with 425 hp single engine, total weight I’d guess to be 9 or 10 tons, but very high compared to length and lots of windage up front. We have eight millimeters chain and a twenty kilo steel Rocna, no swivel or banana. When setting I try and do as it says on the instructions and just lay the chain on top of the anchor and then let the wind do the work. However in the summer there isn’t room for a lot of scope so I reverse (very slowly with minimal power) to set it. I always look for a sandy patch.

I don't think your 20 kg anchor is over sized for your boat , we had 20 kg Rocna on our old 385 9 ton and never had a problem .
I don't under stand this , for as long as I can remember we been told one size bigger is best , now we being told a smaller anchor is best,
Well I personally will stick to the old school of thoughts and I very happy with my 25kg Rocna . On my 13 mts boat all of 13 tons ,
I think you need to look at the way your setting your Anchor , As Noelex have said .
Good luck

 

[Bouba]

I don't think your 20 kg anchor is over sized for your boat , we had 20 kg Rocna on our old 385 9 ton and never had a problem .
I don't under stand this , for as long as I can remember we been told one size bigger is best , now we being told a smaller anchor is best,
Well I personally will stick to the old school of thoughts and I very happy with my 25kg Rocna . On my 13 mts boat all of 13 tons ,
I think you need to look at the way your setting your Anchor , As Noelex have said .
Good luck

Yes, I agree, I’ve always believed in the biggest anchor that will fit!
Can I just reiterate that I’m very happy with my Rocna, it’s never slipped. It’s just on these anchor threads, the talk is of burying the anchor and chain (just like smaller is better!)

 

[vyv_cox]

When I bought my Rocna, a long time ago when they were made in NZ and there were no UK importers, I was told by Craig (remember him?) that the sizing figures on their website took all conceivable conditions into account. It is never necessary to go up a size. Since then they have modified their advice a little taking displacement of the boat into account but otherwise I believe they still make the same claim.

 

[sailaboutvic]

Bouga boat comes in at 10.5 mt considering it's a motor boat with more windage then a sailing boat of the same size
We call call that 11 mts , plus his weight,
A 20 kg anchor is about the right size .
I wouldn't want to go down a size with that type of size and make boat .
When I referred to the anchor size being bigger , I wasn't talking about Rocna , more that over the years books , magazine articles where alway insisting one size bigger anchor would be better , of cause we didn't have the NG of anchor we have now , but even so I wouldn't want to go smaller unless I was a weekend sail with a marina where I could run too when thing get really bad .
I hold my hands up and say I couldn't say if my anchor chain has ever been buried , because I never go out of my way dive on my anchor .
So I can't answer your question .

 

[Neeves]

Classification Societies have designated CQR, Bruce and Delta as High Holding Power anchors. Rocna, Excel and Excel have been designated Super High Holding Power anchors. The difference between HHP and SHHP is a factor of 2 - the designation is independently verified. The 2nd generation anchors have twice the holding power of the 1st generation.

If you take the average of spread sheets defining anchor size vs vessel size for 1st Generation anchors, CQR, Bruce and Delta and do the same for 2nd generation anchors, Supreme, Rocna and Excel and then plot the 1st gen v 2nd Gen - you find the two plots are basically - identical. Ultra, Fortress and Spade are also rated SHHP.

Despite claims of superiority of the 2nd gen anchors over the 1st gen anchors the weight of an anchor for a 40" yacht is the same for a 1st gen anchor as a 2nd gen anchor - even though the 2nd Gen anchor has twice the hold.

Go figure.

Until the introduction of 2nd Generation anchors, say 2006 - many heroic voyages, say in the second half of the 20th Century,, including anchoring, were made relying on CQR then Bruce and finally Delta. The owners returned to tell the tales - with little reference to ground tackle (presumably it did what was expected and was - unremarkable).

Classification Socities allow a 30% reduction in size, weight, for a 2nd gen anchor replacing a 1st gen anchor.

The 2nd gen anchor makers recommend the same size of their, better, anchors over the 1st gen anchors.

Go figure

The internet is full of people who are self made anchor experts - Peter Smith a boat builder and the designer of Rocna has possibly spent more time anchoring in southern high latitudes than some gurus have spent on their yachts. Not everything you read on the internet has much basis. But who needs salesmen when you have self appointed anchor experts and gurus

Anyone buying a 2nd Generation anchor chooses their anchor based on the excellence and performance of the design - and commonly buys one size bigger, sometimes 2 sizes bigger and some almost twice the size - ignoring the recommendation of the anchor maker (see post 43 above). so - they buy because they think the design is good - and then ignore the recommendation of size. some of these people then go on to praise their own choice - omitting to mention the anchor they praise is bigger than the CQR that is recommended for their yacht.

Go figure

A thread was raised on this forum asking for incidences of dragging of 2nd generation anchors - specifically anchors bought of the recommended size. Basically the conclusion is - 2nd generation anchors are reliable, even those that are of the recommended size.

Now ask the gurus to justify their statements and you will variations on

"its obvious'

'It stands to reason'

'its heavier it must be better'

'Dashew says so'

'An anchor i not big enough until people laugh at the size of your anchor - see Tradewind's picture, above, and that single word.....

probe a bit deeper of their claims - and you will not get an answer at all.

Go figure.


But there again maybe people have lots of money and it really does not matter.

1st Generation anchors require skill and patience to set, 2nd generation anchor tend to be - with no reference to anyone - idiot proof (which to me is a good thing (because I obviously do not know what I am talking about). But this has nothing to do with size.

Interestingly dragging of anchors and vessel loss has not figured with insurance companies, lightening strikes create bigger attention than losses from anchor failure.

But the internet gurus - without exception -'Buy a bigger anchor', look at me 'my anchor is twice the size'

I sometimes think anchor makers only need rely on the fear factor - who needs marketing gurus.

Jonathan

 

[thinwater]

What do you consider the "design wind load" of your anchor system? I often hear people describe windy as 30 knots which really, is only 1/4 the load imposed by a very strong squall (60 knots). I have not seen that many times, but I have anchored through a few in the 50-70 knot range, as well as the TS outskirts of one cat 1 huricane. Certainly anyone cruising cyclone areas is at risk of this strength, even if they are only on the fringe of a storm.

What do you consider your design wind speed? I offer that 60 knots is a reasonable basis for US east coast sailors. What about other places?

And then what safety factor do we apply? Chain and fittings are reasonably straightforward (ABYC or other rating group tables are conservative and well proven). Anchor holding capacity is quite variable and ratio of holding capacity to MWL is variable. First, the WLL is based on the load at which the anchor doe not creep, which can range from 25-70% of the maximum load carried (that is what the literature says and that is what I have observed--Knox also studded this). Then a factor is applied beyond that, typically about 35-50%. But we're not looking for the long-life that construction guys are looking for, so the creep factor is typically only 55-80% and the total safety factor need only be about 2:1, since we aren't worried about years-long problems. But what do we really know about the soil?

Just food for thought.