Tang Bolt and mast strength

Jean-Luc Van Den Heede's recent problem with his lower tang pulling down through the mast after a severe knockdown during the GGR got me thinking about the loads the lower tangs see. I've heard of example's of the tang bolt or tang breaking but this failure mode is less common and it acts as a useful data point for the loads in a rollover, Since the rest of the rig survived we can narrow down the loads and failure points.

This is a rough drawing of the setup he had, plenty of unknowns but I have made some educated guesses

8mm rigging wire
16mm tang bolt
4.8mm Monel rivets

8mm wire fails at around 4.6 tonnes . Since there are two shrouds the maximum load would be unlikely to have exceeded 9.2 tonnes and was probably a lot less than this. But the wire yields and would begin to show permanent elongation at approximately half the breaking strain, or in this case a total load of 4.6 tonnes, though it is unlikely the load is evenly shared between both shrouds.

The failure mode was the tang bolt pulling down through the mast 50mm on one side, presumably the rivets must have sheared off and all the load went onto the aluminium wall.

Here is the results from a quick spreadsheet I knocked up.

So if we assume a 16mm tang bolt we see that its Shear strength is about 5.7 tonnes. But since it is in Double shear due to the external strap it should be able to take about twice that load, more than enough to cope with the maximum shroud load to failure.

Look now at the bearing strength of a 4mm wall section mast, It yields at 2.5 tonnes and fails at 3.9 tonnes. The other side attached by 7 rivets fails at between 2.3 and 3.8 tonnes depending on the size of the rivet. (rivet breaking loads from Stanley). There is also likely to be some clamping force helping to secure the plate to the mast, but this is very hard to calculate.

My guess is that the mast wall yielded slightly, putting more of the load on the rivets. this may have happened earlier, as the mast yields at a reasonably low load. When she got knocked down the Pop rivets sheared placing the full load onto the mast wall which tore down, exceeding its bearing stress. the load may also have permanently stretched the wire, and in his latest phone call, he mentions the D2 (Diagonal 2's, discontinuous intermediate shrouds) needed tightening indicating they also may have yielded slightly. I am guessing they are 7mm wire so 3.5 tonne break, 1.8 tonne yield.

We can also see the required wall thickness to support the full load of the bolt alone, in the case of a 16mm bolt we would need a 9.1mm thick bearing surface, but if it's in single shear you would need to upsize to a 19mm tang bolt and make the wall 11mm thick to match the strength of the shrouds.

This mast was absolutely bulletproof for all normal sailing loads, but the incredible shock loads placed on the mast when it hit the water found the weakest parts.

Of course, the failure of the mast wall may have relived the loads enough to prevent the shrouds from failing. Perhaps if the tang had been stronger the whole rig might have been lost?

I am not an engineer, I do have some basic tech school training in these types of calculations (1.5 years at Polytechnic studying a diploma in mechanical engineering and drafting) but not enough to be happy if you use these figures to design anything without first having them checked by a Qualified and competent engineer. Also working with figures past yield or maximum design stress is not good practice. To really nut out what happened the mast needs to be analysed and the calculations made with much more advanced non-linear techniques like FEA.

I'd be interested to hear any real engineers comments and observations.

A dinghy to row. Galifreya

Most dinghies today aren't designed to row, Even ones that you might think are. I was lucky to grow up with two nice rowing dinghies and no outboard motors.

Part of the decision to go for a bigger boat was to be able to get a bigger dinghy on deck that will row much better than the 8 footer on Snow petrel. I can fit a 12 footer onto Snow Petrel II.

Anyway, A good friend was going to build an Auk for his beautiful atkins cutter. I couldn't help myself designing a 8.5x4 foot rowing dinghy that was more suited to him.

Here's the result. I called it Galifreya to go with his yacht Galifrey.

 It had to look good, especially upside down on his deck, And fit under the boom.

The dinghy also had to row exceedingly well, He has a bolger light dory and that was the standard that it was being judged against. But more stability was needed, and better towing. He wanted high freeboard to get a comfortable rowing position,

I chose to pull the quarters up to reduce wetter surface and transom drag when heavily loaded. The waterline beam is also pretty narrow, but wide enough to carry the weight and give about twice the initial stability of the bolger light dory. Most small rowing dinghies have broader low sterns to carry more weight and give more stability. Good for outboards and sailing but very very bad for rowing, especially when loaded.

The bow is fine underwater but rapidly flares to a fuller bow to lift it over any waves and stop bow steering when towing. Coupled with the high freeboard the dinghy is very dry, The sheer is pretty flat to give the comfortable rowing position he wanted, And to give more capacity for deep loading.

 The keel is long and straight with a big skeg. It makes it track very well. possibly two well for my liking, I would probably take 25mm of the skeg and 10 mm of the keel to make it more nimble. But it is a joy to row, You don't need to focus on keeping it going straight. Even when gliding or with a beam wind. It surfs well down the smallest waves. And is stable enough to stand up in.

 Happy! A smooth wake and nice curl to the bow. The dinghy rowed better than any other I have tried in this size range. Next we need to do some speed trials against other dinghies and row it in all sort of conditions but our first trails were better than I could have hoped. Just needs some paint and a few finishing touches and she's done.

The lines plan, the waterline shown is roughly with two light to medium weight people onboard and no gear. It was not a quick and easy dinghy to build, but it is within the capabilities of most people. I generated offsets for the hull planks which where mostly pretty close, except at the bottom of the bow, where they were about 10mm off. It is 6mm ply over bulkheads and temporary stations.

Anchor Ratios for the bored

Warning, This post contains maths, and other boring concepts, it some of the information has been superseded by better formulas and more information. another post on this will soon follow.

I have been playing around with Excel, you can do interesting things on it if you are bored enough (I was on a plane, planes are boring). Anyway I had been pondering a thread on Cruisers Forum regarding is a big anchor better than a smaller one?

I was keen to see how the law of Mechanical Similitude  applies to anchors and boat sizes.

I found it very interesting, much more interesting than the plane, although Karen, sitting next to me did not share my excitement...

Anyway, for me the discovery's were interesting, if you take the view that holding power is proportional to the anchors blade area (edit: I have since found that it needs to include depth of set as well, so it is normally proportional to the weight, or area times depth of set) then the benefits of going larger are not as great as you might at first think. In my case going from a 20 kg anchor to a 30 kg anchor then the weight increases by 1.5 but the anchors area only increases by 1.3. Useful, but not as big a benefit as at first you might think.

Now to get a bit controversial and way out of my depth, but thinking about what actually holds anchors into the seabed in some bottoms it occurred to me that the shear strength of the seabed might well play a part. and if that's the case then the circumference of the anchor would play a part in it's holding by defining the shearing boundary. Now the little bit I read on soil dynamics on Wikipedia confused the crap out of me so don't go thinking I am any sort of expert...  But the bad news for big anchors is that up-sizing the weight to 1.5 only gives you 1.09 more shear strength. Interesting, not sure how relevant it is, in soft mud area is critical, in clay or firm mud maybe shear strength comes into it? Any soil technicians out there? (edit:still havent found out anything about this, but probably not a big factor)

Thinking more about it the shear strength would also include the thickness of the substrate it needs to pull through, so it may increase as an area rather than a length, the same as the surface area of the anchor...(edit: now I am getting closer... see the next anchor post, comming soon)

I was also curious to see how the colossal anchors carried by some bigger cruising boats like Morgans Cloud and the Dashews stack up against the boat weights and windage. Now boats don't scale properly, no smart designer is going to directly scale up directly. What normally happens is boats get relatively narrower, lower and lighter as they get bigger, so my scaling is rather crude, but it is interesting to see what happens if I scale Sunburst up to 60 foot (18m) by a factor of 1.50. the displacement goes up to a whooping 33 tonnes, 3.37 times more to about 34 tonnes. The windage only goes up by a factor of 2.25. This would require a 70kg anchor to keep the ratios about right. (edit: I think I have this sorted out now, anchors scale in proportion to their weight. so not quite such a big anchor would be needed)

The biggest winner with a big anchor is the tip loadings which scale with the surface area, double the weight gives you 1.59 times the tip load, provided your dip is as sharp as the smaller anchor. This will help your setting in some bottom types since the extra weight of the anchor helps drive this sharp tip into seabed.

Now it also occurred to me in a light bulb moment during a restless nights sleep pondering exciting things like anchors that we compare yachts ratios with numbers such as the Sail Area/displacement ratio, and Displacement/Length ratio. So why not compare anchors to boat size the same way, this would stop half the internet arguments about if your anchor is to big or to small, we could exchange our numbers and we would know exactly where we stand on the continuum of permanent mooring to toy anchor.

So here are a few ratios to play with, they might make sense number wise or just be weird, maths isn't my strong point, I would be interested to hear your Ratios.(edit: I now have a better ratio Length squared/Anchor Wt, should be about 35 or less, 7 or less for metric)

Displacement to Anchor weight ratio = Disp (kg)/Anchor weight (kg)

Displacement to Anchor Area = Displacement (tonnes)/(Anchor fluke area (cm^2)/1000)^(3/2)

These are the easy ones, windage is another critical factor, probably much more important than vessel displacement, depending on the situation. I got a crude idea of my windage in m^2, took side windage and head on windage and averaged them to reflect the normal anchor ripping out angle of 45 degrees. made a bunch of crude assumptions about rigging drag and then even I got bored... So I have a crude guess really. keen for any input on better ways. I need to refer to a few yacht design books to see if I am ballpark but hey a guess done with maths is much better than a guess done by.. well just guessing, isn't it?

So the ratios for windage are

Windage to anchor area ratio = windage (m^2) / (Anchor fluke area (cm^2)/1000)

Windage to anchor circumference = windage (m^2)/(circumference (m))^2

Anyway, even I am getting bored now, My guess is the first few people with a real maths and engineering background will die laughing and probably make spot a heap of mistakes in my logic or my math, but if by some chance the laughter isn't fatal, please make comment so it can be improved or deleted.

Here are my results after some tweaking of the original formula to get the brackets in the right place.. DUH

Cheers

Ben

PS can't work out how to add the spreadsheet to this blog. Any ideas?

Snow Petrel... The boring details

A few keen observant and possibly quite bored people have noticed the extra page added to the blog.. well done! It's next to the videos page on the top of my latest rambling blog if you aren't one of those observant ones..  I kind of slipped it up there to see what would happen. I wrote this a few years ago to go in the back of a book my old man was writing, Its incomplete, caustic and a few things have changed since then, but if you have nothing more important to do, and are curious it is up there... I plan to slowly add stuff to it, but may well not get around to doing so, maybe this blog is my attempt to add to it... Any comments on the page can be posted here.

Cheers

Ben

Snow petrel battery power

Two solar panels getting iced up.. Not much charge going in..

Yesterday at lunch break my fellow builders and I discussed batteries and solar panels and the like. We talked about using wind, solar and the other option of trailing a water generator. At this point I should come clean and reveal that my fellow builders are all sailors as well. My dad (head honcho) with NZ maid, two of my brothers with assorted boats like the Proa (see the videos page.. Top left), And another friend who is rebuilding and living aboard a 34 footer.

Living aboard a boat for any length of time (away from the marina) quickly shows that power in a battery is your lifeblood. And there are plenty of vampire like appliances to suck this lifeblood away, If you are silly enough let them get anywhere near your batteries.

I used to manage quite well with one 40 watt solar panel, It kept the basic lights running and the radio playing, or occasional bit of watching a small LCD TV. But things could get a bit desperate on cloudy weeks, and I reckon the 50% discharges haven't done my batteries any good.

I had a wind generator (an Air Marine) on the last boat, but found that athough it worked well in (Windy) Wellington and did OK at sea, It was not as good as the solar panels at running a base load, I either had no power or far to much, In general the average good anchorage was too gusty to get any useful power. The 40watt solar panel was a huge improvement, trickling in a good reliable charge most days. With my small two battery system I cannot store the huge amount of power that the wind generator puts out in any decent wind, So after a couple of hours of charge the wind generator was switched off for peace and quiet.

So if I spend the money on another wind generator it will probably be something like the Rutland 504 which puts out a pitiful amount of power but is small, quiet, safe and works well in strong winds. Most of the bigger ones have to be shut down for safety in gale force conditions, right when the solar panels are often under cloud, and when you don't need any power issues. Saying that, in some places like the trade winds or with a good reliable sea breeze wind generators may make more sense?

My father uses a tow generator at sea and loves it. It powers the autopilot and the small fridge, and works well whenever the boat is moving. This was our backup for the trip to Antarctica, but we didn't need to use it because the solar panels provided plenty of power. Obviously the tow generator only works at sea, and not at anchor.

The system that works for me now is two 40 watt panels on tilting mounts and with a top quality regulator/battery monitor and two combined starting/house 70 amp hour batteries. The batteries are standard wet cell lead acid, They are not sealed, so I can  top up the fluid and check them with a hydrometer every so often (actually not often enough). They are cheap and seem to last well even given considerable abuse (I nearly boiled them dry last year), They are stuffed now, after 6 years, and are only on life support due to the solar panels. This is a good innings (I think three years is about average) and I feel I have got my moneys worth out of them. At this stage the jury is still out on the sealed AGM and Gel cells. I think I will just use cheap standard batteries again, the modern charge regulators pulse charge them, equalize them occasionally and generally keep the batteries much happier than in the past, extending their life considerably.

If you look at this link It will give you some idea of just how hard it is to run a large complex boat electrically. I avoid alot of these problems by being a power control freak, and by having minimal power draw, no fridge/freezer, no wanky wind and boat speed instruments (can draw more than you think), no big chartplotter, and a windvane rather than autopilot, It Is a good feeling to know I can cut my needs down to just one small light and a book.

I have a third 40 watt panel to install, and I will probably put in a standalone engine start battery, With this in place and new house batteries I should have plenty of power for my computer.. With Modern LED lighting I may even have some power to spare..

Sextants and GPS Take II

After all The Iphone stupidity I now have my Big 800w inverter dodgily clipped onto the batteries . Its happily running my computer, But like a vampire it is slurping the life blood from my old pair of geriatric batteries (I can tell it's enjoying the feast because the fan is buzzing away happily). The voltage is rapidly dropping from 12.2v down toward the 12.0v mark... But if I type real fast I may just get this post finished before the interior lights begin to fade.  The joys of being on a mooring.

Now back to the Sextant issue, I now finally have a photo to add.

see I told you it was a Beauty, A Freiburger, never been used. It's a very close relative to the Russian SNO-T sextant (The T stands for tropical apparently). I know this because My friend also lent me a very interesting book called "The Nautical Sextant" that gives in-depth details of the inner workings of every sextant since Sir Isaac Newton first thought of the concept. It will keep me awake again tonight if the batteries hold out long enough...

We discussed my sextant today at work over lunch (I worked as a builders labourer today) and we compared it to a GPS, Most of my fellow builders reckoned they wouldn't cross an ocean without a sextant and almanac, but aside from the rather remote possibility that all the satellites might fall out of the sky.. Or a lightning bolt might fry all 4 GPS units we would have replacing the sextant. No one could come up with a realistic scenario that didn't involve some sort of doomsday apocalyptic situation requiring the sextant.

That being said, I wouldn't cross a reasonable chunk of water without a sextant, maybe for superstitious reasons as much as anything else, I mean it is really tempting mr fate isn't it. There is also more to the sextant than just a backup, somehow carrying one makes you feel more independent, you are not relying on high tech satellites to get you from A to B. You could do it on your own if you wanted to. The satellites become just a convenience, not a necessity. For me at least that is one of the things I like about being at sea, a feeling of self reliance.

The other thing about carrying a sextant is that it links you to the seaman past, to Cook, to Columbus, and even in some ways to the amazing Polynesian navigators, they all had to navigate using the stars, sun and moon. You are a part of a tradition, and it feels right.

The last 1000 NM of the trip from Tasmania to Chile was done using Celestial Navigation only (except Dave who had the odd secret GPS peak for safety). The landfall was the most exciting ever, knowing we had found our position and navigated by the sun and stars. This is something every sailor should feel at least once.

Anyway the vampire has sucked the life blood down to 11.8 volts so if I don't stop soon I will kill the batteries for good. Since you've missed a few photo's I will give you two today, I have finally found a use for Snotty the sextant, It makes a great mouse table, and every thing on a boat needs two uses ...

Cheers

Ben

Ps If you can think of a plausible reason for 4 waterproof, isolated GPS units (including one in a Faraday cage) to simultaneously die, for more than a couple of days, I would very impressed. (note this is a hypothetical situation)

Edit 15/2/11 After discussion with Chris at Brilliant star see here for more details and also the comments on this post. It seems we have our answer, there is a remote possibility a large enough solar flare could compromise the GPS, Galileo, glonass and irridium satellites and radio for long enough to make carrying a sextant across an ocean a good idea?

From the geology.com website they suggested that normal flares would only reduce the accuracy by 50 meters or so, not a big deal if you are navigating defensively. And they mentioned an 11 year solar flare peak 2013…

However the chances of an large enough solar event to wipeout these systems for any significant period seems remote, And it would also cause chaos ashore, so it falls into my doomsday apocalyptic situation. By coincidence today has the largest event this solar cycle according to (see I am turning into a journalist..) these guys

BUT I will still carry a sextant... And I will also keep an eye on the space weather and also be aware that a strong aurora and poor radio propagation may also indicate reductions in gps accuracy.

A Big thanks to Chris onboard Brilliant Star for the Info

for further info see

The 500 year flare.. NOAA
NOAA flare scales

Iphones and Idiots

Had a premature posting problem, so now you know I have an Iphone, and I am also not very good at driving it.. So yes I come clean, I am one of those Latte drinking (actually a flat white) Iphone types.
So today due to being on the boat with a flat computer battery I thought I would try to email in a post about my lovely new sextant. But while halfway through composing my masterpiece of modern literature, my far to fat forefinger acidentally tapped the far to small screen in the spot called send.. so off went my half finished post into the interweb with that stupid iphone signature that I nomally delete..
Anyway the thing doesn't support flash or something so It won't let me go back in and edit the post on my site, hence this longwinded explanation, but anyway you can still answer my question in the last post, what to you think, are sextants like tits on a bull? (ie useless)
While on the subject of phones, for us power starved cruisers this little phone is a marvel, internet at your fingertips, draws next to nothing powerwise. Stores songs and brilliant sailing podcasts from Furledsails.com (you can look them up) And it also stores charts, for $15 (Navionics) I have the whole of Australia stored on It. (it even updates them for me!) Its now a pocketsized chartplotter (It has a built in GPS). Absolutley amazing.... now if only those screens were bigger.

Cheers from Snow Petrel

PS I'm sure the other brand smartphones are just as good, and infact the 3G part of this phone has died so its reception is pretty bad out of town. And in my defense mine is a crappy old 8 meg 3G not the way cooler 3gs or iphone 4, but if anyone from apple is reading this I do accept sponsership...

Sextant and Tits on Bulls

I just bought a Sextant today, I had absolutly no intention of getting one, It just sort of happened.

I brushed the cobwebs off the hatch of Snowpetrel and shook the cockroaches out of the sails, then sailed across to Bruny Island for BBQ. The next day I popped in to see a mate who showed me a sextant he had aquired in a garage sale. I checked it and adjusted out the slight errors, and just held it, unlike my old plastic davis mk 15 this had a "Real" sextant feel, so much like the sextants I learnt to use on the ships. Anyway I bought it.. 500 bucks for a virtually new Freiberger with a lovely wooden case. Why are they so expensive asked karen? 500 bucks I could have bought about 4 GPS units, which would do the same job quicker, more accurately...etc.

So are sextants about as much use as tits on a bull? should they be resigned to antique stores with the walker log?
Sent from my iPhone

Cyclone Yasi.. and old car tires

Katabatics Starting to ease off. Photo Matt Tucker

My thoughts go out to all those in the path of cyclone Yasi.

It's got me thinking about what I would do if Snow Petrel was up there, Having seen the strength of the Katabatics which I estimated gave us 80 or so knots for 3 or four days really makes me aware of what you guys must be going through. Fortunately we had no storm surge to deal with and lots of solid boulders to tie our 5 shorelines into.

My only slightly original thought (actually think I have seen it in a book somewhere.. like most of my `original` thoughts). I'd head to the nearest garage and get all the old tires possible, to use as shock absorbers on all lines, and fenders around the boat.

The humble tire makes a great if somewhat dirty and heavy fender (actually the weight is good, light fenders blow out of position). Tied into the line they absorb shock loads better than any snubber, and if the worst were to happen they make a good pad on the bilge if you end up on a beach. Tires are used by yachts during panama canal transits see here for some good photos.

Yes they can even be useful at sea, making a great emergency drogue, even better tie a couple into the line to make a crude series drogue.This has been called "de-spare tire..."  GROAN..

By the way I have never actually used tires as a drogue, it has been done before by others. I have only used conventional drogues and warps. Maybe I need to test the theory.... 

I am sure I don't need to remind you to make sure the rope is well protected from chafe, ie with chain or plastic tube covering, and if you plan to keep them choose small mini tires, paint them a nice topside matching color and cut some drainage holes in them with a hole saw. My parents Gaff ketch is painted black, just as well because we couldn't afford proper fenders so tires were used instead, and the black didn't mark the hull.

All the best

Ben