Dismasting of CS 20 Allecat....

[Rob Blackburn]

http://homepage.mac.com/blackburns/Expeditions/PhotoAlbum74.html

The pictures tell the story but essentially it came down to this.

A gusty westerly blowing we decided to trailer the boat up the river and sail down to Ballina and go back for the trailer. We figured it would be good running and reaching. We had a fairly uneventful trip until a few bigger gusts started reaching us.

We were on a broad reach and had 2 x 85kg on the gunwhale and saw the gust coming. Without sheeting off we put the other 2 up on the weather rail and as the gust had lifted us I steered up a bit to keep the pressure on. The thing with westerlies is they are gusty and the gusts come from different angles.

I looked to the wake and said .."now that is planing" only to hear a funny sound as the mizzen went over the side.

We then sailed downwind to the lee of a nearby island to sort ourselves out.

I have long marvelled at the way the mizzen stays upright in a breeze when you look at the relative lever of the thing... not a lot of mast in the boat and lot of sail up in the wind.

Our consensus is to have a bit of wood lathed up and put it inside the new lower section up to above the start of the sail to spread the load and reduce the arm.

Think I will do the same in the main as well.

The other thing that became evident was we really should have had a hacksaw on board in case we could not have got the mast out of the boat as it had buckled quite a bit.

In summary I reckon we overloaded it by.. 1. big gust with change of direction, 2. did not sheet out ... 3. drove harder into the gust rather than back off...4. lot of weight on the rail putting more stress on the whole thing...

But this is the place to find it.. in the river, not 50 miles from town at Fraser Island..

Rob

[Designer]

Rob,

I'm sorry to hear about your mishap. So that we can learn from your dismasting can you give us the specs on the mast, diameter, wall thickness and the grade of alloy. While the mizzen has more leverage than the main, it has less sail area and works out about the same load.

It is normal to calculate the load on the mast when hard on the wind at maximum stability. With the CS 20 I used 3 people on the rail in 20 mph wind and gave it a safety factor of 3. I can see that you probably went way beyond that and that I need to re-think the way that I calculate free standing masts for hard driving skippers.

[Tom Lathrop]

Rob,, I detect a bit of understatement in your story. With somewhere in the neighborhood of 700lbs on the rail and a gusty blow, the mast was apparently loaded to a fair thee well. Oh well, at least now you know the limits of that particular set up and that the boat is strong.

Hope that the repair works out well and you get back out soon.

How fast do you suppose you were running?

I remember the same thing hapening to me in a Thistle with five on the rail in a thunderstorm. Of course that is a stayed mast with double diamonds. Expensive thing too.

[Guest]

Sounds like to me you did a great job on building your hull. You can make it without a mast, but..................................

Alls well, that ends with the blue sky overhead. 8)

[brian watkins]

Rob, I'm very glad to see that you're safe and no one was hurt, especially since you sail with kids so often! Thank you for sharing the information on your dismasting. This is time I've heard of this type of mast breaking, has anyone else done it?

[mustangermatt]

Sorry to hear about your trouble.

I think the wood insert will go a long way to avoid another incident like this. Looking at the photos it appears that the compression side of the tube failed, not the tension side. By putting a "core" in there it will force the tension side to let go at the same time or before the compression side.

[Guest]

Here is a for what its worth department. I am not sure if anyone uses this method. Reviewing your shots of removal of the broken mast, this may come in handy for all of you that uses a mast in a wooden seat thwart. We drill a larger than mast size and use small hardwood square wedges, to take up the slack in the hole. Some days we need two, three or four of them, depending on the changes in the holes with the weather. We also place a small line through them that allows for us to removed them quickly, just in case we need the mast down in a hurry. It also will make your wife happy, if she is the one helping you in your rigging department.

I have two sets, with them all linked together, and hang them on the cleats for the sheets, when not in use.

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[Rob Blackburn]

Hello again,

I will get the data on the alloy from my mate the "welder" who got it for me today if possible. At the time I remembered a discussion on the group about various grades and I think the one wanted was "T5?" or something but we could not get it here. Somebody posted the fact that alloy tempered itself over time and got harder I think. Either way we broke it but will get all that today in town.

I am also going into the sailing club to look at my Laser as I suspect it's lower mast section has a liner in it too.

I have been thinking about it a bit and am wondering if an aluminium liner might not be lighter. If you can't get one to fit neatly inside you could lathe up from hardwood some "donuts" to slide over the ends to make it a closer fit.

I also wonder if this would not be a result of fatigue over time but then it is not that old but then boats the world over drop masts so it is not too significant I don't reckon.

At the end of the day we were having so much fun and the boat was going so well that we will probably do it again so shall have to be prepared.

Tom asked how fast were we going.. well I dont' know but the thing was seriously on the plane so you would have to suspect better than 12 knots.. that is a guess. I took my gps but the batteries were flat typically. Yep it moves along.

See you all later.

R

[Rob Blackburn]

Well the bottom sections are 80mm x2mm T5 6060 extrusion.

The new section is the same but 3mm and are going to put a sleeve in it to about 1.8m....

Also going to do the same for the main mast I reckon.

I guess we run with this as it is all we can readily get and if we break it again shall look at it then..

Rob

[Garry]

Increasing the section thickness from 2 mm to 3 mm should make a tremendous difference in resistance to buckling even without the addition of the wooden reinforcement. I would think that, over time, the wood could shrink a little therefore adding marginal strength. I would think the increased thickness would add plenty of strength.

[Rob Blackburn]

Not going to use the wood plug but some 65mm 2mm aluminium tube with some collars turned up and slipped over and glued to give it the right diameter. Thinking of 4 collars over 1.8m or so?

Rob

[Bruce Leonard]

Sorry to hear of your mast problem, Robb. Last week I had a problem with my recenly launched CS20 #51. I was singlehanding in Narragansett Bay., Rhode Island, in 10- 15 knots breeze out of the West. Here, West winds coming over land are unpredictable and gusty. I was just chasing other boats reaching (on a plane) when I could . I headed up to tack when a strong gust hit me. If I had been paying closer attention I would have seen the gust on the water. It laid me way over on my side/knocked me down. I tried to bear off, but should have realized that the rudder was already out of the water. As I was about to go for an unscheduled swim, I finally followed Graham's advice and released the main sheet. The boat popped back up and off I went. I was pleased with the way the boat behaved in this situation. As the boat laid over on its side, the mast bent and spilled wind.

In the future, when overpowered, I will release that mainsheet.

I like my CS 20. The boat goes!

[Joe Nelson CS#35]

This is an amazing testimate to the boat...gust that would break the mizzen rather than tip over the boat. Maybe this should be considered by those who pose questions as to the stability of the boat.

If the 2 masts have equal leverage, are we attributing the mizzen failure to the fact that the mizzen support is lower than the main? Also consider that the main is in a tube. I wonder if the mizzen was in a tube if it would help to spread out the load more?

[PJPiercey]

From the information I can find, T6 6061 has almost twice the tensile and compression strength of T5 6060. It may be the alloy is the problem. T6 6061 is referred to as "structural" and T5 6060 as "window frame material".

With all of the boatbuilding in Australia I would think there must be T6 6061 available somewhere.

Paul

[Rob Blackburn]

Well, I remember at the time that I rang 4 different suppliers, one in Brisbane and none could help... So if it is here it must be well hidden.

I have the new 80mm x 3mm wall here with the 63mm for a sleeve. So I guess I will know soon enough.

How much stronger will the 3mm be compared to the 2mm one? Can you work that out?

Rob

[Frank Hagan]

Shine, a contributor here in our Main Forum, has a company called BoatBuilderCentral.com that sells a carbon fiber mast kit. There's a thread with a link to it in the Classifieds section at http://www.messing-about.com/forum/viewtopic.php?t=2574. The webpage listed in the bottom of the prior message is actually done with one of the kits from BoatBuilderCentral.com.

[Aaron_G]

How much stronger will the 3mm be compared to the 2mm one? Can you work that out?

Rob

Based on the ratios of the moments of inertia for the 2mm and 3mm tubes: the 3 mm walled tube will have about 2/3 of the stress due to bending than the 2 mm tube (so it will handle 1.5 times the bending moment). This is a rough estimate since the mast will likely fail due to local buckling and not yeilding due to bending stress.

This is all based on the simple tube w/out your 63 mm insert.

[Designer]

Robb, I calculate that your new mast section will be about 46% stronger than the previous one. The new section works out to be about the same strength as the plan specs.

I have bought plenty of 6061-T6 in Aus. but it often takes a lot of digging to find who has a particular section in stock. I would think that your original section would be perfect for spinnaker poles, so a sparmaker would be a good starting point.

Someone asked how many Core Sounders have been dismasted? As far as I know three aluminum masts have been broken. Yours is the only one broken in the water, one drove under a tree with the mast up and a Travel-lift drove a over parked boat and forgot that the mast was up.

[Rob Blackburn]

I have to be really honest in that we broke the mast. It did not break itself. That might sound funny but 4 big blokes, a big black gust on the water and no letting off on anything is what did it. I think the other factor was that as we drove up into the gust we would have increased the apparent wind further as we had been on a broad reach.

Do you think the lesser temper alloy will eventually break further up.? ie should I replace the whole bloody thing? or will the thicker bottom and sleeving do?

R.

[Rob Blackburn]

Turns out that we have bent the main mast as well. It did not come down but has a bulge on the leeward side and has a 5-10deg kink. So off to replace that tomorrow.

Pity I did not start out with the you beaut tempered alloy but then how dull would it all be if things worked perfectly.

Spent the weekend helping Col over the road convert his sloop rig dory into a junk rig dory.

Should be interesting...

Rob

ps. we are wondering if the web is populated by dreamers who are seriously outside of all the boxes you read about in the paper. What box are we in?