Mast loads for freestanding masts

This is the second time you have expressed a similar opinion. Could you please explain why an unstayed mast requires more or more complicated engineering than other types? Thank you.

 

How to use Tension Gauges - Loos & Company, Inc. - Cableware Division https://loosnaples.com/how-tos/tension-gauges/
As someone new to sailboats, I would prefer a freestanding mast after reading this. Though they could be exaggerating the importance of proper tension to sell a product...

 

Just wondering if it's a catamaran with a low chance of recovering from a rollover, would it be better to dismast before the loads increased to the level where they caused a rollover? Should the design criteria be that as the wind picks up the sequence of failures would be sheet release system / sails rip / mast breaks / rollover? What is normal design practice for catamarans? Is it different for racers who maybe like to fly a hull vs cruisers?

 

This thread seems to have mushroomed into rather more than an answer to the notion of freestanding masts in a catamaran.More or less by definition the masts will have to pass through the deck and be stepped very low down in the hull(s).Am I wrong in thinking that a basic hull skin with a deck above it may need quite considerable local bracing to withstand those loads-when they have been calculated and supplied.Would there actually be sufficient space between the mast and any bracing to allow passage through the hulls?

 

Why? It is, after all, someone who is trying to sell a product. So are many of the people who write about freestanding rigs.

All that they are saying is that a stayed rig should be "correctly" tightened to reduce loads. However, in many small one-designs the lee shroud is normally slack in a moderate to strong breeze and some of these have very simple and light rigs that very rarely break, so the fact that loose rigging can increase loads does not mean that those loads are excessive in any way.

In sailboats there are many items that have to be tightened correctly, just as in cars and bicycles. If one is going to avoid stayed rigs because they must be tightened correctly one should also avoid outboards, inboards, rudder bearings, bicycles, docks, belts, hats, and sails.

 

Some of the arguments here are a pathetic attempt at justification. You talk about outboards, belts and hats; yet totally ignore questions that have been posed multiple times.

 

Such as? Please identify them. It is pathetic that you apparently ignore the outright lies said by some advocates of free-standing rigs and identified here and elsewhere, yet choose to demand that those like me who use and respect both types must respond to each and every allegation raised by others.

It should be blatantly obvious that my point regarding outboards etc was simply that many things in our life and boats perform best with proper tensioning - and yet we don't reject other objects because of that. Why, then, reject stayed rigs because they require proper tensioning?

If you are referring to the fact that I didn't respond to one of Rob's posts it's because I don't they feel that they are balanced. For example, in his scenarios he implies that on a mono reefing requires "reaching over the lifelines to reach the end of the boom." I've never owned a boat where that was necessary so why should I respond to a question that implies a badly set up conventional rig compared to an idealised and supposedly problem-free unstayed rig?

Since I've got to finish a cup of tea I'll quickly respond to two of Rob's scenarios;

1- In thousands of miles of ocean sailing I've rarely been in this scenario so it is not of great relevance to me. However, even if the autopilot is overpowered I can just reef the jib while steering. Rob's scenario says that you luff to reef but as people who are vastly more experienced than Rob say, this is not necessary or advisable. I don't have to do it on my boat. I just reef the main on a run. No problem, no issue. Rob appears to have issues with that fact but I am NOT a liar.

Rob's scenario is that one has to pause to recover breath after rolling up a headsail and then ends up adrenalin charged, with tangled reefing lines and slipping on solar panels before falling overboard. Okay, maybe that is his experience but it's not mine; my boat has sorted out reefing lines, I don't get out of breath after reefing, there are no solar panels on deck and I've never looked like going overboard offshore.

Why should anyone be insulted for not responding to such a biased scenario? I could write a similar biased scenario of danger in almost any situation, on any boat, and it would be equally unrealistic and biased and would make any boat look bad.

One could, for example, create a scenario involving a situation where a boat must tack up a narrow coral-lined channel only one and a half times as wide as the boat is long, against a current and in heavy seas. That would not be a good scenario for a boat that requires shunting.

Anyone can construct a scenario that would make any design look bad, but that is not the way to carry out a reasoned discussion about design.


2- Most owners of stayed rigs do NOT pay to have the mast unstepped each season. Again, this is a biased scenario. I don't look up my mast anxiously every time the boat drops off a wave nor do I know anyone who does - that's just biased ********.

A significant number of yachts with unstayed rigs have lost their masts, yet I don't claim that every owner of such a rig checks it anxiously every time they drop off a wave because that is bollocks.

3- I have sailed a huge number of hours on unstayed rig of various sorts. I may well have sailed more hours with such rigs than anyone here. I like them and use them. Anyone who claims I am biased against them is a liar, pure and simple.

 

I have also cruised on boats I owned with and without stayed rigs. They both have good and bad points. It seems that some people blame their poor seamanship on the rig design.

 

This thread is not about masts made out a tree. I am familiar with junks. The comparison is between modern stayed rigs and self-standing masts.

 

Number of moving pieces is a simple measure of complexity.

This question is one for @rxcomposite, the quote won’t allow a double quoted reply. I would say, yes. The math will help with confusion if anyone does it.

I’m not attempting to overstate my credentials Ignacio, why don’t you state your own? I was only trying to help with math, but this thread went bananas.

Here is what seems to be a good thread on the subject where math is the general focus and not when to fluff the sheets!

Rotating Free Standing Mast Design https://www.boatdesign.net/threads/rotating-free-standing-mast-design.35484/

 

FG_If it is a free standing mast the cantilever load method will work as it will calculate the greatest stress at the base of the mast. It will likely be 3X overweight though as the stress tapers off towards the tip so a constant cross section mast will be an overkill as the shear and bending moment calc suggest a tapered mast.

If the calc needs to be optimized for shear, bending, and localized stress, the compound load method I posted in post #18 should be used. It calculates the stresses in two directions at a distance from the center of effort. It is easy to do in Excel. This is for unstayed mast. With this method, it is possible to size the mast for a folded and/fluted seam welded tapered stainless steel mast with local reinforcements added to the base and the pintle/boom connections.

The easiest method for sizing stayed alloy mast is to use Skene’s method. There is a lot of assumptions like the weight of the mast, weight of riggings, and Righting Moment but gets the job done.

Stayed mast requires a lot of calculations. First would be to use Euler's method and calculate the tensions on the stays. More so if you are using the NSB standard. Depending on design, the sum total of these rigging forces would exceed the displacement forces and the original stresses used in calculating hull strength needs to be recalculated. Imagine a bow with the strings drawn both ways. If it is a cat, the main cross beam has to be recalculated, the chain plate has to be evaluated for its holding capacity, and the bowsprit connection (if any) has to be evaluated. Compared to free standing mast, you need only to know the deck connection strength.

If the designer is considering using a tapered composite mast, the math catches up quickly whether it is free standing or stayed. It requires the knowledge of Classic Lamination Theory (CLT) which is matrix algebra really. The laminations will require at least 2 types of fiber (uni and biax) wound at different ply angles. Layups will be complex, diminishing in length with added reinforcement inserted in between. Because it is a segmented design, the Area Moment method is used to analyze the bending and ply schedule.

The only fabrication method that will approximate or surpass an alloy mast would be filament winding, resin infusion/vacuum assisted, or prepreg/vacuum bagged method oven cured. Clearly not for a DIY or uneconomical for a one off.

 

Which part is not true? The masts being the same weight? The forebeam, striker and chainplates being heavier than no forebeam, striker and chainplates? That these items need the hull beefed up?

The claim is not about data, it's about definition:
A rig that needs no maintenance or setting up can be neglected.
A rig that requires several wires to be correctly tensioned, with many parts which can fail cannot be neglected.

I appreciate that a lot of designers rely on copying others to get specifications, but that is not "engineering".
There is far more literature on unstayed structures than there is on stayed.
Semi experimental is like semi pregnant. No one with any experience or knowledge of boats and rigs would consider "unstayed rigs to be semi-experimental".

Which of my claims are "unsupported" and where is this "long list"?

Nirvana,
Most masts on <20 ton catamarans are designed to capsize the boat. It is too difficult to reliably calculate the loads and the materials to ensure the mast breaks first. eg safety factors, sea state, slamming, maintenance, variable payloads, etc. It's also dangerous to have a mast drop unexpectedly.

A huge safety feature of an unstayed multihull mast is that it is easy to incorporate a near fool proof mechanical anticapsize and pitchpole mechanism that will dump the sheet if the windward hull is a preset distance out of the water, regardless of point of sail or whether the sheet is cleated, on a winch or tangled around your foot.

Wet feet,
Agreed. Biplane rig space is a limitation for narrow hulls. A free standing mast will generally be further forward than a stayed one (no headsails) so the lost space in the hull will be reduced. They can also be offset. see A Dream Takes Flight | Cruising World https://www.cruisingworld.com/people/a-dream-takes-flight/ for an interesting analysis of how and why one pair of cruisers chose their rig.

rxcomposites,
Thanks for the analysis and numbers.
I've built dozens of carbon masts and provided plans to many amateur builders. Building an unstayed mast is simpler than, for example, setting up frames for a ply boat:
Mark the half round frames with a pair of compasses and cut them with a jigsaw.
Use 3 full length straight edges and a string line to set them up.
Bend fibreglass sheet or similar to make the half mould. The mould does not need to be shiny or polished, just straight.
Lay it out and infuse it (cleaner and uses less resin, but needs a couple of practice pieces) or hand laminate it and vac bag, perhaps in 2 shots if you are solo on a big mast.
Repeat.
Join the halves with 2 simple, easy to make butt straps and bulkheads at the stress points (tip, heel, deck and boom).
Glass the join, make and bond on the track, sand and paint, remembering that once it is up, no one will see the finish more than 3 metres/10' off the deck.
It's a fortnights work for someone who has the skills to have already built the rest of the boat. It will be about 70% the weight of an equivalent alloy mast (this comparison is impossible in practice due to the wall thickness taper not being possible on the alloy) and maybe 10% heavier than if it was built to America's Cup standards.
Weigh it and bench test it by supporting the ends (or the bearings) and hanging weights off it.
Using glass for the off axis, carbon for the lengthwise fibres and standard infusion epoxy, a 17.5m/58' mast for a 15m Harryproa or 6 ton 12m/40' cat materials cost is about $US3,000, consumables maybe $200 and tube weight about 120 kgs/266 lbs, which is near enough the weight of an alloy mast with 1x19 stays.

CT249,
CDog has a point. What I requested, to support what you wrote, was:
1) a step by step of how you reef solo from full sail downwind in 30 knots, ie not "I just reef the main" or, "I do it in quite a similar manner as the guys in the video, but easier", or, "my brother explains how he does it, but I won't tell you where"
2) the boats you have done this on
3) the unstayed rig boats you have "sailed a huge number of hours on"
Please add
4) details of "the significant number of yachts with unstayed rigs have lost their masts". Maybe just list the cruising boats, exclude the racing dinghies. I know of only one catamaran, the Hughes catamaran Sarabri with an aerorig, perhaps with a worn deck bearing. afaik, this is tied up in nda's so there is no information available on why it broke.

You write screeds about Laser sailing downwind on a cruising catamaran thread, but can't/won't support your unstayed mast claims. It's peculiar to claim your huge experience to justify your claims, but not commenting on specific points because you've "rarely been in this scenario so it is not of great relevance to me". We are discussing rigs, not your experience or lack of it.

My scenarios were just that. You may not identify with parts of them, but a lot of cruisers do. You could maybe have the grace to agree that reefing, maintenance and sailing generally are easier and cheaper on an unstayed rig than a stayed one.

Maybe start with what you agree and disagree with in my list in Post #87. Nit picking a couple of examples is a poor excuse for not commenting on the rest of the post, nor is it "the way to carry out a reasoned discussion about design".
Nit picking examples:
"Dropping off the back of a wave" is not the banging I refer to. I meant when the entire boat shakes hard enough to wake the off watch crew. Any owner or skipper that doesn't check the rig (and change course) after a couple of these is asking for trouble. This applies to both rig types, but there is far more to worry about on a stayed rig than an unstayed one.
There are many monohull cruisers with biminis preventing access to the boom end without being near or over the lifelines. Regardless, it will be more effort and danger than simply releasing the sheet on an unstayed rig and putting in a reef (or waiting for the squall to pass) while the boat is drifting with the main pointing into the wind.
My scenario was getting the poled out headsail on a cruising cat furled in 30 knots, solo. A fit, fearless and highly experienced seaman like you may not get tired or elated doing this, but a lot of cruisers do, which is why they sail at night under reduced canvas. Even for you, it will be more effort and adrenaline than simply releasing the sheet on an unstayed rig.
Few (probably none over 35') cruising cats drop their mast without a crane. Whether they do it annually depends on how many miles they cruise and their insurance policy. As long as it is more than once every 20 years, it will be more often than an unstayed rig.

Nobody mentioned shunting, although I am happy to discuss this if you want to.

Your scenario: "a situation where a boat must tack up a narrow coral-lined channel only one and a half times as wide as the boat is long, against a current and in heavy seas. That would not be a good scenario for a boat that requires shunting." This is absurd, but let's go with it.
I assume the channel width tapers gradually, as no one would willingly enter it otherwise. And that none of the boats have motors.
A mono without an overlapping headsail might get 2 tacks in before it tacked in less water than it drew. When the boat became vertical, it would hit the bottom, stop head to wind, the sails would flog and it would be pounded to bits in "the heavy seas".
A mono with an overlapping headsail would not have time to get it sheeted in before the other side of the channel, would not have speed to tack, would hit the bottom, stop, the sails would flog and it would be pounded to bits.
A cruising cat would try a tack, get in irons, go backwards onto the coral and destroy a rudder. It might be able to rotate and limp off downwind.
A shunting Harryproa would realise it was impossible, dump the sheet, reverse the rudders, the sterns become the bows and it sails/drifts out and waits until things improve. None of the other boats would have the space required to turn from upwind to downwind.
As I said, absurd.

 

gonzo said: ↑
All data from racing boats show that (unstayed rig catamarans are lighter than stayed rig catamarans) is not true

All of it in not true. Can you provide any data of a high performance racing catamaran that has an unstayed mast?

gonzo said: ↑
Nice claim, but there is no data supporting that one rig type is designed to be neglected while the other one isn't.

There is nothing in the Universe that needs no maintenance and can be neglected. That is sales hype and untrue.

gonzo said: ↑
Either structure has to be engineered properly, with appropriate reinforcements. In fact, literature on stayed rigs is extensive and they can be easily calculated from similar designed. Unstayed rigs are semi-experimental which requires more knowledge and time to engineer them.

Engineering is based on past experience and data. It is the foundation of the discipline
I would like to see a list of literature on unstayed rigs that is even a small percentage of that of stayed rigs. This is another unfounded claim.
Semi experimental and semi pregnant can be a long discussion on semantics that would need a new thread. As far as "no one with experience ...", that is another unfounded claim like the responses from many experienced people resonding to your thread show.

 

I'd sail on either of these boats in a heartbeat, if I could...

 

@rxcomposite has mentioned Eric Sponberg's papers for freestanding mast design and scantlings. I didn't see other links or spreadsheets for design calculations of same in this thread.

Here is a great general resource for this sort of stuff:
MIT Precision Engineering Research Group

Go here for available structural engineering spreadsheets:
FUNdaMentals Downloads - MIT MEDICAL DEVICE DESIGN https://meddevdesign.mit.edu/fundamentals-downloads/?eeFolder=Topic-8-Structures-Spreadsheets&eeListID=1&ee=1

There is bound to be one useful to any mast design problem. The wind loading model you wish to use is up to you. The cantilever beam model allows you to enter a separate linearly distributed load, point load, and an additional end moment. I'm sure you can modify if you want it to do something else.