# Mast Compression Post Question

Discussion in 'Sailboats' started by Jetboy, Apr 23, 2015.

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Ok, so are you referring the the magnitude of the loads to be applied to a known mast/rig or once the loads have been obtained how to design a mast/rig to account for said loads?

If you're referring to the later, then a simplified explanation is shown here:

Well, that's encouraging

Without a load, it is pure guesswork! Ergo the loads must be estimated/calculated. Perhaps the people you have discussed this with do not know how to calculate/estimate the loads and just guess??

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### SukiSoloSenior Member

Not that uncommon, GP14s' use around 400lb as an example and they are old school.... Most common deformation is the shrouds pulling the sides of the boat together, just wind up an old Lark.... The Int 14s' use even more tension something like 800lb shroud tension and they have had some problems with leaking hulls, but not mast compression post failure. More like triangulating the forces involved slightly poorly, for the very high loads involved travelling at speed in the sea. Worst load condition for a rig is mostly probably, not quite pitchpoling a dinghy/sailboat with water in the boat and all over the deck, just before they 'pop' onto the plane.

The compression post for all in classes that use them ie deck stepped rigs such as say Merlin Rockets, Enterprises etc still don't need to be particularly big. Again it's down to compression of material (properties) and beam bending/buckling. Not forgetting that the 1 square inch of Sitka mentioned before, can take 3,600 lb of compression before failure (in column and assuming no bending).

So we still don't have a shroud tension parameter from Jetboy? or a species and compression shear value of the existing compression post?

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### latestarterSenior Member

The link below you might find helpful (or send you to sleep)

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### DCockeySenior Member

gggGuest and SukiSolo, are the various tension numbers you give the "pre-tension" set by tightening turnbuckles, etc without the sails set, or the total tension in the stays while sailing?

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### gggGuest...

That's base static tension. Add the dynamic loads as per your diagram, the effects of the weight of the crew on trapeze wires, transient loads bouncing of waves, and frankly I get to numbers that make me think I was operating with a rather smaller safety margin than might be considered optimal.

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### SukiSoloSenior Member

David, the tension is usually measured static with sails up. Most use the Loos tension gauge, even little Cadets' use them to set the jib luff to whatever the sailmaker recommends. So it is shoreline measurement, you could consider it a 'static' condition. Obviously as ggg comments the actual dynamic loads are a bit different, although a fair bit of the time in light and moderate wind speeds, they will be quite close. Once you start bouncing off waves, or on a trapeze (see the 470s' pumping on the wire) loads can get a bit higher.... Despite all the tension, it is common to have a moderately slack leeward shroud in a breeze upwind, though it still can have significant tension. More a function of mast bend in both planes.

On a positive note, it is rare for modern dinghies to have structural failures. However trying to get an old boat to take modern tensions is not a good recipe. It is also possible to have too much tension, and the boat will sail slower as a result, it seems that there is an optimum elasticity window for each class. This has certainly been the case for some 12 and 14' boats I am familiar with. It also pays to tune the sails a little to find this optimum where luff tension etc can be controlled. I'm still very impressed by the reliability of the Mach 2 Int Moths considering the capsizing etc a lot of good thinking in the construction of boats at the cutting edge, IMHO.

Note also that classes like the RS100 only has stays to take the extra load of the asymmetric kite, doubtless the extra 10.2sq m does almost double the forward (at an angle, I am aware) load. Upwind no forestay is required.

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### JetboySenior Member

Thanks. Kinda odd wording - probably not English first language author. I'm a physicist by education. The point of the article is very similar to the one i was making. We use rough approximations because we simply lack the necessary data or modeling ability to provide actual mast loading in a dynamic environment. Every mm of the mast has different stresses. Most of our knowledge base and design guides are based on empirical evidence of what fails and what doesn't.

It's probably plausible with unlimited computing power, but I don't think anyone has actually done such a model.

Fortunately the mast compression post appears to require a much smaller section than does the mast. And I have a bunch of different sticks of aluminum tube in my scrap pile that are more than adequate for this purpose.

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### JetboySenior Member

I don't think we can accurately know the loads because the stresses on the mast for example are so complex and vary widely along the length. Once you start going through the infinite variability of sail plans and weather and sea conditions, it starts to get unmanageably complex. We can sort of guess at a range of loads, but it's really just a guess. We might be able to do a monte carlo type run with all of the various assumptions to start to get a picture of he probabilistic range of loads, but even that could require overwhelming computational requirements that few if any of us would have access to. The computers would probably look like outhouses, not laptops.

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That's back to front I'm afraid. The stresses in any structure are calculated from applying loads to the structure, not the other way around.

Its just simple vector forces. It becomes bit more complex the more sails the vessel has, but the only variable, is the precise load on the sail owing to the camber, exact height above the main deck/sea and true shape of the sail and its variance with the theory of aerofoils. No more so than an aircraft and its wings.

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### JetboySenior Member

I would suggest it is possibly significantly more complex than an aircraft wing.

The typical sail plan will have various options of sails and sizing. All of which apply different loads to the mast. Then you consider every type of wind condition, every angle of wind, every position of the traveler, every tension of the main sheet, every tension of the halyard and downhaul, the outhaul, the tension on the battens, sail twist, etc. and apply them in the range of wind conditions. And then you do that in combination with all of the jib sail possibilities and then the mix and match with a code 0, asym, etc and pretty soon the world of loads on a mast is very big.

Then of course the boat its self is not stationary. You'll need to consider how it moves in a dynamic environment with the forces being applied from the sails. My boat is a trimaran, so you'd need to consider the motion of 3 hulls with a semi-rigid structure. And on top of every scenario above with wind and sail plan choices you need to evaluate each possibility with the world of sea states in which you might sail. How does a confused sea near shore with seemingly unlimited wave forms effect loads?

If you could model all of those variables, we could then know the limits on loads that could be experienced. I would propose that the actual world of plausible loads far exceeds the failure strength of almost every boat's rigging. So now we're moving into a world of having a boat that's safe 99.999% of the time as opposed to one that's only safe 99.99% of the time and judgment calls on what the actual probability of such low probability events occurring is.

We know without any uncertainty that Euler's beam formula doesn't apply to the mast in any accurate way due to the fact that the mast isn't loaded from the top like the formula is designed for. And we know that empirical testing shows beams actually tend to experience failures at lower loads than Euler's formula predicts. For those reasons alone what is probably the most commonly used calculation is nothing more than a rough approximation. And that's without even knowing what the actual loads are.

I think it makes a whole lot more sense to look to empirical evidence for certain answers. This is likely one of those situations. Is a rudimentary model of sail forces better than results derived from testing? When we consider using factors of safety that are often 3-5 times what the calculations would predict, is the calculation really meaningful beyond a rough approximation? Is there value in attempting precision with such large error margins?

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Yup..and that is what the naval architect/designer must do. Some guess at it...those not trained, and then there are those that are trained calculate it. Experience tells one which angles will provide higher loads than others coupled with polar plots thus reducing the max number of calculations required. It's why good designers/naval architects cost money to employ them for a design. It is a specialised field. Just because someone can sail and see the sails flapping about, doesn't mean they understand what is required to design such.

They are both equally instructive.

FoS take into account variables in construction variables in wind loads, such as gusts that are above the current methods of calculations (this is where experience comes into play v the button presser), materials quality etc etc...and on top of that fatigue.

Thus one can spends vast amounts of time and effort to establish a more realistic value (by which time the boat should have been built and the client complaining where is my boat) or simply elect to use one that is considered appropriate, based upon previous designs without any in-service issues.

Just like the human body, because we know all the moving parts, doesn't mean a diagnoses is simply black and white..there is still a degree of uncertainly and trail and error. Only a fool or inexperienced would suggest otherwise.

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### JetboySenior Member

So then you agree that we (both naval architects and amateur boat builders) actually use approximations?

(If the examples you've provide are what you call "calculating" then I think we're arguing semantics as those are not even remotely close to what I would consider more than first order approximation)

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Not suggested anything otherwise.

The degree of "approximations" reduces with experience and knowledge. But your initiation question was about strength of masts. That's basic engineering and material proprieties. Establishing wind loads is a rather different question.

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### DCockeySenior Member

"Physicist by education" - my experience is physics education teaches precision and exactness while competent practicing engineers learn to use available hardware and information to find a satisfactory answer within applicable constraints including time and money.

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