Wednesday, November 14, 2018

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.

Monday, January 26, 2015

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.

Thursday, July 3, 2014

Weather to go? and the herd instinct...

Pondering the weather?
I think we have all been there, in that anchorage or port - pouring over forecasts , grib files and weather maps, trying to decide whether to go or not. Somehow the perfect weather window always seems to have just gone or to be another week away. Our options seem to contain many uncertainties, despite (or maybe because) of the much more accurate long range forecasts available these days. The decision to go is often complicated by other boats all thinking about heading of as well.

The Herd Instinct

It always slightly amuses me how much sheep thinking is coded into our DNA's (or is it human thinking by the sheep?) and I realise I'm not immune to this either. You get 2 or three boats in an anchorage waiting to depart, and everybody starts in subtle ways to find out what the other boats are thinking, and when they are going, there is much at stake in these discussions, ego, fear and respect... This doesn't only apply to other vessels, a similar dynamic can happen between individual crew on a boat.

Ego, comes into it because people don't like to be thought of as wimps, so (typically) the blokes like to make light of the forecast... 25-35 knots, no problems, been in much worse than that many times, we'll be right.. Another factor with the whole ego thing is that it can trap you, if you have stated loudly that you plan to go tomorrow- to other people who thought the following day might be better, it can be hard to then change your mind to agree with them when they are right...

Fear, it’s funny how travelling in groups somehow makes us feel less fearful, watch a group of penguins about to jump into water that may contain a hidden leopard seal to see the same type of reactions... Be careful that you are not leaving against your better judgement because you are afraid of being left behind... Or not leaving on a good forecast because no one else is...

Respect, saying we are going to do something different from others is like saying you think they are wrong... once we are locked into group discussions this becomes a powerful factor, it can be hard to say no.

The factors to consider between different boats
Factors to be brutally honest about are our boat, our crew and our rush factor.
Our boat- Is it bigger, smaller, faster, slower, well found or dodgy
Our crew - it is the hardest one to be objective about; we all like others to think we have heaps of experience, it's that ego thing. And they may be playing the same game, the blind leading the blind…

The rush factor - How much of a rush are we in, have we got pressing deadlines, have we been waiting for weeks already. How much do we motor vs. slop around in light air? Are we ready to go yet or do we actually want to spent a few more days in this place....
Ultimately the decision to leave should be an individual one, and one that you are comfortable with. Don't let that Gung-Ho delivery crew make you feel like you are cowards. Or that boat that has been waiting for 4 weeks for a perfect patch of weather make you feel worried about the conditions as long as you are confident the weather is OK for the boat and for you. Also try not to put pressure on other boats to follow your lead; they need to make up their own mind...

In saying all this there can often be advantages to discussing things with others, such as

  •          local knowledge
  •          A bit of a reality check
  •          other boats may have better or different weather information
  •          It is social and you get to know the crew from other boats.
You may even end up sailing at the same time and this can be fun (or a nuisance as they call you up every half hour to give you a position report and then expect you to slow down and wait!) If you are lucky you can get some good photos of each boat sailing…

Getting it wrong

I stuffed it up badly once (actually more than once but this one sticks most in my mind). I knew I was making a mistake at the time (Stupid hey...) I had a deadline, impatient crew and a bad forecast. Another boat was going, they reckoned it would be fine, and they had no engine - my ego didn't like that... My crew were starting to think I was a wimp and were really keen to go. I knew I could handle the conditions ok and I had faith in the boat, but I really thought the next day was a much better idea... It looked very unpleasant out there and the clouds were racing overhead. With a sense of foreboding we sailed. The crew got greener and greener as we left the lee of the land but their insistence on sailing meant they couldn't really bottle out, I should have taken the initiative and pulled the pin - it’s the skipper’s job, but I didn't want to be the one to bottle first. 

To windward the sky got darker and then without warning (actually there was good warning, I just ignored it) a solid squall came through flattening the boat. The seas rapidly built, steep and nasty, one throwing us well over and making me think the windows should have popped out. It didn't ease up any, so after a good struggle I clawed down the spitfire jib (we already had the third reef in) and we slogged our way back to the anchorage, motorsailing, arriving very wet and tired at midnight with our tails between our legs. The other boat ended up being towed in...
The next day the wind had eased somewhat and it looked much healthier but the crew were now very wary, they took some convincing before we headed out… We ended up having a great crossing, why I had headed out the day before with that forecast I don't know (actually I do, It was my ego, and I should stop listening to it.)  Now I must say that despite this incident they were a fantastic crew, and I enjoyed sailing with them. It was my error and ego that led to a miserable night.

Doesn't look so bad? Don't take the red arrows lightly...

Getting it right

I remember sitting at the Melchior Islands in Antarctica. We had a shocking system coming though the Drake Passage; the Grib files had it intensifying rapidly into 50 knots or so of southerly. It is about a 3-4 day passage and the trick is to try to time it to avoid a nasty westerly at Cape Horn, it's not normally too hard but in this case the timing would have to be perfect. Too early and the southerly could be dangerous, too late and the next westerly could make it difficult to lay the shelter under the lee of the horn... To make matters more complex five crew had flights from Ushuaia in the next week - And the new crew were to join shortly after...
I was pretty happy with when I had to leave, but I wanted to talk to Eric from Vaihere, he was heading off soon and had much more experience in the area than me. Vaihere is a bigger and faster boat than us and he was planning to leave 6 hours after me. 

It was good to talk to him, because it showed me that he didn't much like the situation either (very bad...) so there was some justification for my nerves... But we were both pretty confident that we could time it right for our respective vessels, and cope with the situation if it didn't pan out how we thought. He waited and I left. He arrived at the horn 2 hours after me, and I was probably an hour or so later than I should have been (because it was starting to get very nasty); it blew like stink that night. Vaihere kept going and looked magnificent sailing past in 50 or so knots of westerly. But I was glad we were tucked up in a safe anchorage.

What worked well about talking to Eric is that we had both dropped our ego's, he admitted that it looked nasty, and he was worried about it, as was I, we weren't trying to impress each other. We both had a pressing need to get away, but we were prepared to wait if needed. And we were aware of the different speeds and capabilities of our boats. My plan was to head out, and assess the situation in 12-24 hours, if I thought I couldn't reach the horn before the westerly’s started to crank up I would head back to Antarctica and cancel the flights... I had not used Eric to prop up my decision, but I wanted the benefit of his local knowledge of the Gribs and their relationship to the local weather systems near the horn.

Tuesday, March 5, 2013

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



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

Tuesday, November 27, 2012

A new boat!

Like a hermit crab for a while now I have been feeling like I have outgrown my shell, she feels too small and not exciting enough... Well I have just bought a new boat, or at least I will have in a few days when the bank processes my money transfer!

In my last post I had a list of a bunch of desired features of a new boat, most of which did not make for a cheap boat. Looking back over the list I think I have ticked most of the boxes, or at least the boat has the potential to tick them one day.

Thinking some more about it I guess one of the main goals is to be able to lift my sailing average on coastal passages from about 50-60% up to 70-80% so good light airs and windward performance are important. Also more space for a fully separate toilet and shower and at least 2 separate cabins and room for a kayak and a decent hard dinghy on deck was needed.

Those fellow boat nerds out there will immediately recognize the dreaded lines of an old IOR Two Tonner. yep.. Hrmm OK so they roll like pigs down wind don't they? err yep.. and.. they Broach at the blink of a hat right? (slightly red faced).. Yes so I am told.

Well that's the negative, the positives are the fantastic heavy air windward performance,  the huge amount of room, the strength of all alloy construction and good light air performance. Did I mention that I could also afford her, a big factor for someone with a simple lifestyle and aversion to debt like me, and I can afford to do what is needed to make her a simple fast cruising boat.

She wants a proper interior with insulation (one day), new rigging (urgent), a bigger engine (eventually), a redesign of the cockpit area plus dodger, and the sailing gear sorted for shorthanded work.

Strong 6,5 and 4mm Aluminium, exceptionally
well built by Noel Wilde in Melbourne.
A basic but functional interior. Those chainplate stays look annoying and she will drip condensation everywhere in tassie... 

One day maybe a lift keel could be retrofitted, so I will build the interior to suit a centerboard case in the distant future.

The bad downwind habits may be able to be improved by dropping the spinnaker before the wind gets over 15 knots and reverting to my slow trundling downwind with a poled out headsail. A wind-vane and auxiliary rudder will also help. A friend of mine has a boat with a couple of dagger-boards down aft for running. Apparently they also help considerably...

Anyway no rush, I will sail her for a few years before I do anything to drastic, She is quite serviceable as she is, It will take much time and money but I think she could be a great fast and fun cruiser to sail offshore and inshore.

Oh, by the way I now have an exceptionally tough, go anywhere red yacht for sale at a giveaway price to the right adventurous soul...

Edit, Snow Petrel has just sold, the new owner is Dean, he will cruise around Hobart way, learning the boat before heading off further afield. Sad in some ways, but it opens up a new door, and now I can afford to do some of the improvements to Sunburst.



Monday, September 24, 2012

That dreaded POX

I think I have contracted that dreaded scourge of boat owners. I have all the symptoms, excessive time on the computer, eagerly awaiting emails and attachments. Carefully scrutinizing picture after picture of boats, a dreamy look on my face, with screwed up sketches of possible modifications strewn around.

Poor Karen has to put up with me constantly asking if she likes this or that boat... I almost wish she was suffering from the same illness.  For a while I thought I was alone with this syndrome, but I heard some friends are also suffering the same malady, it must be going around. They have even found a name for the sickness, the dreaded 6 foot-itis

Of course Mike and Larissa deserved to get it, spending years living on a 34 footer with two kids is bound to bring on such an nasty affliction, but me.. What did I do to deserve it?

Anyway look on the bright side, it could be worse, it could be 10 foot-itis or even 20 foot-itis.

So yes it is official, I am looking at other boats, nothing to serious yet, it's a big leap. But I always saw Snow Petrel as more of an interim boat, at some point I need to get serious about things and make the plunge. Boats being so cheap at the moment is certainly a factor. I just have to make sure 6 foot-itis doesn't develop (as it so often can) into that most fatal of diseases, the dreaded Dreamboat-itis that will leave me crippled, my back broken forever under a mountain of debt.

I have looked at catamarans, old racing boats, even older rusty cruising boats. My wish list is as follows :
  1. Cheap
  2. Strong
  3. Fast and easy to handle
  4. Shoal draft
  5. Beautiful
  6. Low maintenance 
  7. Good light airs performance 
  8. Roomy and comfortable at sea and in port
I am guessing I will be looking for a while, but if you hear of such a boat please let me know right away.