Mast loads for freestanding masts

Just think about this selected quote for a moment.Knockdowns can and do happen,using a breakable mast as a kind of mechanical fuse would not be a good move.
I have wondered about the general direction of this thread,but not any more,and I think I will avoid it in future.

 

Apparently you weren't paying attention in the first place. It's a catamaran, a knockdown would not be recoverable. In either scenario how is it acting as a "mechanical fuse"? If you don't have anything constructive to add then why comment? If you think what I'm saying is so wrong say what it is and tell me why so I can learn something. I really am here to learn but I'm not going to just accept everyone's statements without reasoning and information I can use to make my own deductions.

Perhaps you are right, this thread is going nowhere.

I should not have made such a strong statement about stayed masts, I apologize for offending anyone, it wasn't a personal attack on you or your boats.

Thank you to those of you who genuinely tried to help me.

If someone wants to tell me if the proposed masts will work or not and why I'd appreciate it.

 

Can you tell us the steps you take, please? Assuming the worst case scenario, which is full main, solo at night in a squall which makes rounding up your catamaran dangerous.

 

I don't understand why you couldn't haul in the mainsheet. I have never had any problem. In fact, that is routinely done for a controlled jibe.

 

I said nothing of what you call standard rigs. I have taken down sails on self standing rigs by sheeting the sail in too. It would be really dumb to take down a sail and let it fall in the water.

 

I never had a problem before either, but, 4-1 purchase on the mainsheet and 45+knot wind didnt jive.

 

There is always the compromise between the effort you need to input and the speed. Too much reduction and it takes forever.

 

Getting knocked down with any kind of spinnaker is very common. Some people consider getting knocked down in the 'wrong' direction on a J24 something of a rite of passage. The logic about the highest righting moment being a good rather straightforward place to calculate maximum load on the mast does seem sound. I will say however that your rig SHOULD be able to tolerate the entire load. I have been been dismasted on a knockdown followed by a round up and kite flog before. You can also quite easily have the kite balloon in the water and cause further issues.

Additionally, I think I read somewhere in this thread, an argument made against mast taper, citing weight gains. In my experience the bigger advantage of a tapered mast is its ability to bend more at the top than in the middle.
On a standard fractional rig arguably the biggest advantage is the fact that the top fraction of the rig is freestanding, tapered and bends in the puffs and gusts thereby spilling air and auto-depowering. It adds to the comfort and overall sailability of the boat, otherwise your trimmers would be working much harder.

I think it might be prudent on some level to think of traditional stays and shrouds as a way to "control" mast bend and sail shape. Controlling your mast shape is an essential part of sailing well. It is quite common for racers to tune rigs to the estimated wind speed and common for any sailor to use backstays to bend the mast back to flatten the mainsail. The mast SHOULD be bendable to a certain extent. How much is usually controlled by the inters and lowers aka the checkstays.

Additionally to further add some value to this conversation I think there was an interesting interview with Juan K about the Club Swan 36 with its radical-ish design, he does mention a few things about righting moment and the design of the keel.



I should also probably mention that I am definitely more a sailor than any kind of designer and that I don't sail catamarans.

 

Cats if designed properly will have about 20 degree of heel. The sail area (and the resulting force in the wind speed it was designed for) will cause it to heel to maximum (the maximum righting moment, point of no return). For a high performance sailing cat it is designed so that it will heel a little less than 20 degree with one hull out of the water.

Sails are designed so that the maximum force it will experience will not cause the boat to heel more the maximum (or the maximum righting moment), otherwise it will capsize.

Your beam is 22'. 1/2 beam is 11'. Your VCG at 2' seems a little bit high. I would assume it is right below the crossbeam or at deck level. If the length of the sail is 14 feet, 1/3 of that is 4.7' (the centroid of area). By geometry, your moment arm is about 12'. Close enough to 1/2 beam. Your force/sail is 200 lbs thus 200 x 12 = 2,392. Multiply by 2 as you have another sail at opposite end. Thus there is total 4,875 lbs acting on the end of the arm.

Where did you get the 6,000 lbs righting moment?

 

From everything I've read the typical cruising catamaran will heel around 5 degrees under normal operation, and not too much more before a hull lifts out of the water. I imagine this depends on the waterplane area of the hull, weight, and beam. It seems most cats would have one hull well out of the water at 20 degrees.

I am using the formula: weight x center to center beam (divided by) sail area x the height of center of effort, take the square root of this and multiply by 14 and that gives you your static stability in knots wind speed, then multiply 0.6 and that gives you your dynamic stability, or the maximum wind speed you can keep full sail to account for wind gusts and other factors. For my boat as currently designed this means I have a static stability of 43 mph and dynamic stability of 26 mph, meaning I should reef before wind speed reaches 26 mph.

I'm not sure you read my prior post properly. My centerline to centerline beam is 11', not 22 feet. The VCG is an approximation based on how I envision the weight distribution. There will be a bridge deck cabin, and no accommodation or storage in the hulls, most of the storage will be at deck level, hence the higher VCG.

The height of the *rectangular* wing sail is 12', the bottom of the sail will be approx 3' off the waterline, hence the center of pressure will be 9' off the waterline, a moment arm of 9'. The length of the mast from its deck support to the top of the sail will be just under 14'. The center of pressure will be about 8' from the deck support, so a moment arm on the mast itself of 8'. These are all approximations. Each sail will be just shy of 50 square feet.

The righting moment comes from taking 1/2 my centerline to centerline beam and multiplying by the weight of the boat, which yields a result of 6600 lbs. The 6000 lbs figure was conservative.

The overall design has changed significantly since some of my earlier threads as I have narrowed down my requirements for the intended purpose of this boat.

 

I'm not on a catamaran and the OP and the link didn't mention cats and I don't know why rounding up in a squall is dangerous on a good cruising cat. However, on cats we have done just the same as on monos - pull the mainsheet in a bit, take down the leach and luff reefs gradually. No problem.

Yes, the mainsail lies against the stays. So what? That's what they do anyway.

These guys are doing it (although I have a different setup so I don't need to get so high to use the luff reef, thankfully)



John Kretschmer says that after 161 offshore training passages aboard Quetzal and more than 150,000 miles, he ALWAYS reefs when heading downwind.
Reefing Off the Wind | Cruising World https://www.cruisingworld.com/how-to/reefing-off-the-wind/

 

I'm still having major trouble trying to work out how a boat of that size will move properly in anything under very heavy conditions. Even a boat as small and slow as a West Wight Potter 19 has over 50% more sail than that.

Of coure evidence is needed - to make claims with zero evidence in such an area is utterly illogical, particularly when many many of the claims made by proponents of unstayed masts are simply wrong and therefore unreliable.

If the design, construction and maintenance is perfect than no rig will fail, stayed or unstayed. Comparing an ideal unstayed mast to a poorly designed or maintained stayed mast is simply not logical. Stayed masts have more parts but that can lead to redundancy (they don't always collapse when a stay or spreader breaks, for example) and it is also in many ways a very elegant solution that allows other benefits.

Tradition is a factor, but so is the tendency to think "most people are sheeple but I am smarter than they are" and "ooh, people like Eric Sponberg abuse others and therefore he must be right even though I haven't actually investigated his claims". Around here we see a lot of people who fall for the trap of unquestionably believing those who write stories about how dumb everyone else is.

What is depressing is that any story that implies that most people make the wrong choice and are therefore inferior beings seems to be swallowed completely by people who don't do the simplest checks - for example you seem to still be happy to claim that people normally choose stayed masts for fairly poor reasons but as you say yourself, you haven't given any proof of that at all.

And there is no real evidence that tradition in sailing favours stayed masts. The most popular small-boat classes (Laser, Optimist, Sunfish, Snark) are unstayed so vast numbers of sailors have far more experience in actually using unstayed rigs than many of their exponents. The longest-lived of Olympic classes (the Finn) is unstayed. Many sailors grow up spending years moving through more than one class with an unstayed rig - any idea that they will suddenly become scared of unstayed rigs and unaware of their strengths is just weird and illogical. Why would guys like Ainslie and Slingsby become "traditionalists" and gone from unstayed rigs to stayed rigs at the same time that they moved away from tradition and into foiling? Why would someone like Elvstrom have gone from unstayed Finns and then become a "traditionalist" stayed-rig man at the same time that he did such un-traditional things as inventing the Trapez and becoming a cat sailor?

Unstayed masts were the norm in mid-size boats in traditionalist parts of the world like the north-eastern USA, where catboats were extremely popular - why on earth would people who often had their roots in unstayed-rig boats suddenly find a traditional preference for stayed rigs? The very first international small-boat racing class had an unstayed rig - why would people suddenly acquire a "traditional" preference for stays?

If tradition was such a strong cause in preference for stayed rigs then why did so many of the greatest designers and designers of the most successful unstayed rigs, like Bruce Kirby (Laser) and Clark Mills (Optimist) and Nat Herreshoff all move back and forth from stayed to unstayed rigs? It is weird to imply that Kirby, for example, was ruled by tradition when he created his Int 14s with stayed rigs, then free of tradition when he made the unstayed-rig Laser, then ruled by tradition with his SJ24 and other designers, then suddenly free of tradition when he made the Norwalk Island Sharpie unstayed cat ketches, then victim to tradition in his later designs.

The undeniable truth is that hundreds of thousands of sailors have chosen unstayed rigs in situations where they work best, and chosen stayed rigs in situations where they work best. Making sneering comments about them is just a way to ignore technical realities.

I have seen the video but I don't know to what you are referring not see any reason to accept whatever the commentator says as true. Some stayed rigs have troubles but so do some unstayed rigs.

 

Reducing the rm estimate (max, for tansl) is the opposite of conservative.

Being conservative would move the needle the other way.

 

Conservative in terms of how much sail area I can carry in a given wind speed, is what I meant.

 

That is a very pudgy little boat. My catamaran will be MUCH more easily driven.

I have analyzed my speed requirements for cruising the Bahamas and arrived at the conclusion that 100ft2 will be enough. The wing sail should give me better performance on certain points of sail over the conventional rig. When the wind is really light I can motor or just wait for proper conditions. With the exception of a few, all my passages will be under 50 miles, I should be able to complete most of these in under 10 hours in daylight. I will have a L/D ratio of 50 and SA/Disp ratio of 14.2. Wave making drag will be practically nil. I will have 10KW of battery storage and 600 watts solar(possibly more if I can find a way to fit it), this should allow me to motor for 15 hours at 6mph(90 miles) in a go in calm conditions, if necessary. I have enough RM to carry much more sail, but I don't see the point. I may rig up some simple way of putting up a modest spinnaker for downwind. I am trying to keep everything as light and simple and cost effective as possible.

I'll respond to your other points later.