Un-stayed mast flexibility

[Eric Sponberg]

Originally Posted by Inquisitor

Eric,

I have many simplifying assumptions... off axis plies are not accounted for in the EI (conservative). The stress concentrations (nice crack tip initiators) where the carbon tows end along the mast are ignored (non-conservative). So, I only consider the spread sheet as a far-field (away from the actual boat joint). Its good to know that a S.F. of 3 can handle those. As far as the joint, I'll do an FEA model of the region and use the far-field loads.

One run of the spreadsheet, I got the Carbon fiber portion down to 42 lbs for a 45' mast, with 85K ft-lb moment at the base. That was using Rob's 7% tip deflection criteria. I'll also incorporate you're information. I'm concerned about that designs validity... with the wall thicknesses and local buckling. I use to have some closed form equations for orthrotropic shell buckling prediction, but seem to have lost that box from the old days.

Inquisitor: That seems awfully light for that much load, and I wonder what wall thickness you do have. I have found over the years and with LOTS of testing that for composite tubes that are primarily UDR, regardless of material, that minimum a wall thickness of 3% of the diameter is necessary to prevent local column buckling. If your wall thickness is thinner than that, then your mast is going to buckle before the stress reaches maximum.

From Cavalier mk2:
Aren't commercial s.f./FoS 5? Would you need the higher amount if the boat was to be used for charter?
Junktolee, it is true that modern cloths such as Dacron, Mylar etc...are of a fairly rigid shape and won't conform to deflection. I suppose the old cotton sails stretching with the spars would luff less. Of course we can say the sail was designed to spill the wind in such a manner, if it was designed to set well with deflection it would look pretty awful when the spar was straight! Maybe there is a market for sail cloth that mimics the natural fiber stretch, though that might be hard on the manufacturers after all these years!

Cavalier mk2: One has to be careful with Factors of Safety--they can be too high. I have always felt that if you need a factor of safety greater than 5.0, then you probably don't know what you are doing, engineering wise, and you are wasting material and money.

Also, although current Dacron sail cloth is better than it was 20-30 years ago, it will still stretch over time. Mylar sails will eventually delaminate, and with delamination they will stretch. Cuben fiber sails, although expensive, are practically like flexible metal--that is, they are foldable, but they look and act like thin metal sheets when deployed. My design Saint Barbara has cuben fiber sails and the owner is getting remarkable results for performance on his free-standing wingmast sloop rig. Any sail will have to be cut to an average bend because the wind and the mast bend always changes. It is possible to find that "sweet spot" for the proper cut for the vast majority of wind conditions. That is where the sailmaker's talent really pays off.

Junk2Lee: Masts with appropriate stiffness (not too much, not too little, but just right--we call this the Three Bears approach) will not flog as the boat rolls, at least not really any more than a conventional sailboat. If the masts do seem to whip as the boat rolls, then the masts are too flexible.

Another word for those reading. Fiberglass-only masts can be made strong enough but they are way too flexible--think fishing pole for the kind of bend that you would see. They would also be way too heavy--you need a lot of fiberglass to get the best strength and stiffness, and this is why carbon fiber is, so far, the best material for free-standing masts.

Eric

[cavalier mk2]

Thank you for the information Eric, I'm interested to learn about Cuben fiber sails, I'll do some research. The commercial guys aren't happy about the 5 safety factor either, I was checking to see if any regulations had been strapped onto commercial freestanding mast applications.

[Inquisitor]

I know the price is dependent on cut and all... but do you happen to know the ball park price of a Cuben sail for something with a 50' mast? Are we talking $1K, $5K, $10K, $100K?

What little I've looked up says its for "well healed" teams only. If it falls into the category... "If you have to ask, you can't afford."

[Eric Sponberg]

Originally Posted by cavalier mk2

Thank you for the information Eric, I'm interested to learn about Cuben fiber sails, I'll do some research. The commercial guys aren't happy about the 5 safety factor either, I was checking to see if any regulations had been strapped onto commercial freestanding mast applications.

Cavalier, you will find that there are few to no regulations concerning sailboat rigs in general. The US Coast Guard does not review or certify commercial sailboat rigs, nor does the American Bureau of Shipping classification society. The American Boat and Yacht Council (ABYC), which establishes voluntary standards for boatbuilders and equipment suppliers, has no standards on sailboat rigs of any type. Everyone, all designers and builders, are on their own. I am unsure about ISO standards in Europe and Det Norske Veritas, who may have standards or regulations, and perhaps some of our European correspondents in this forum can enlighten us about that.

From Inquisitor: I know the price is dependent on cut and all... but do you happen to know the ball park price of a Cuben sail for something with a 50' mast? Are we talking $1K, $5K, $10K, $100K?

The sails for Saint Barbara came from Elliott/Pattison sailmakers in Newport Beach, CA. (Website: http://www.epsails.com/). I don't know what the owner paid for them, but all you have to do is call them up to ask. The owner is extremely pleased with the cut and fit of these sails. The same is true with any sailmaker, and I am sure a number of them make cuben fiber sails. Call and ask! It does not cost you anything.

Eric

[magwas]

Germanisher Lloyd have this on rigging:
Part 4 * Rigging Technology
Chapter 1 - Tall Ship Rigs I-4-1 1997
Chapter 2 - Guidelines for Design and Construction of Large Modern Yacht Rigs I-4-2 2009
Chapter 3 - Guidelines for the Type Approval of Carbon Strand and PBO Cable Rigging for Sailing Yachts I-4-3 2008

And I remember reading in Principles of Yacht Design that the rig sizing rules are coming from the than yet to issued ISO standard.

I nearly forgot to tell how glad I am that there is at least one NA here who knows that sharing useful information is a good thing for everyone involved. Thank you Eric!

[cavalier mk2]

Thank you again Eric and magwas, the sea sorts a boat out faster than any regulations, now if only all the committees understood....

[rob denney]

Safety factors are better named fudge factors. They are there to take out all the possible load cases not in the calculations. These include materials not to spec, lousy workmanship and user abuse.

Materials properties are usually pretty reliable, and if the laminate is consolidated under pressure and elementary qc used (accurate resin mixing, (keep a sample of each mix), correct temperature, humidity, etc) it is difficult to mess it up. Misalignment of plies is very easy to do, and must be watched and checked, ideally by a second party. However, if the mast weighs what it should, and flexes as it should (very easy to check with an unstayed mast), then you can be pretty confident the build and materials are correct.

User abuse is a different ball game! On a stayed mono rig, flapping sails, broaches, uncontrolled gybes, poor set up, mistreatment when the mast is off the boat each winter and rig imbalance are all factors which can break a mast. A fully battened mainsail on an unstayed rig on a multi sees none of these loads so the fudge factor can be much lower.

rob

[Eric Sponberg]

Rob, I know another cause of mast failure, and I cannot believe how often I see this: Getting run over by a fork lift (mistreatment of the mast off the boat, in your categories). It seems people leave masts lying on the ground, and fork lift drivers don't watch where they are going. Eventually, the two have an unfortunate, and catastrophic, meeting.

Eric