Replacing Traditional Wood Masts with Alloy

[StuM]

Originally Posted by mebccb

The amount of bending that a mast experiences even when close hauled in a stiff wind is considered negligible. That is why they are designed for compression only. The unsupported lengths are kept to a minimum by additional spreaders and intermediates so that the mast is kept as straight as possible, and any bending stresses are minimal. It is very important to use running backs properly for example. If the forestay is allowed to bend the mast forward to much the mast would fail. The running back counters the force of the forestay and translates it into a compression force on the mast.

Don't tell racing sailors that masts don't bend - it's a primary means of changing sail draft.

[SV Windrush II]

Here is something about Skenes Methodology from the book "Sailboat Mast Design" by ALEXANDRE BERGERON


"The Second point of contention with Skene’s Method is its oversimplification of the problem of mast design. It does little to account for different configurations in rigging and shrouds or mast tapers. There is no care taken to account for the sideways forces imparted on the mast by sails or dynamic effects of waves and so on. Everything is neatly summed up as a maximal compressive force which simply leads to a buckling analysis. In This sense, Skene’s Method is very limiting because the designer cannot optimize his or her mast design"

[SV Windrush II]

Also to add to this thought.
Just because a mast or anything else for that matter is not bending or Visually deformed, does not mean the load on it is not trying to bend or deform it. In the case of the Mast, yes I agree that the stays and shrouds cause downforce which leads to compression forces, but that is just ONE of the stresses on a mast. If not, then I could just use a 3 inch solid round rod as a mast because it is about the same compression properties as a 10 inch hollow tube that is .1875" wall thickness. but of course that would fail instantly because all the other forces including compression force would bend the round rod like a noodle and force it to collapse.

[mebccb]

Originally Posted by StuM

Don't tell racing sailors that masts don't bend - it's a primary means of changing sail draft.

This is the "CRUISERS FORUM".

[mebccb]

Originally Posted by SV Windrush II

Also to add to this thought.
Just because a mast or anything else for that matter is not bending or Visually deformed, does not mean the load on it is not trying to bend or deform it. In the case of the Mast, yes I agree that the stays and shrouds cause downforce which leads to compression forces, but that is just ONE of the stresses on a mast. If not, then I could just use a 3 inch solid round rod as a mast because it is about the same compression properties as a 10 inch hollow tube that is .1875" wall thickness. but of course that would fail instantly because all the other forces including compression force would bend the round rod like a noodle and force it to collapse.

Compression strength is psi at yield limit of given material, don't confuse with column strength. Compression force is simply that, compression force applied.

[mebccb]

Originally Posted by SV Windrush II

Here is something about Skenes Methodology from the book "Sailboat Mast Design" by ALEXANDRE BERGERON


"The Second point of contention with Skene’s Method is its oversimplification of the problem of mast design. It does little to account for different configurations in rigging and shrouds or mast tapers. There is no care taken to account for the sideways forces imparted on the mast by sails or dynamic effects of waves and so on. Everything is neatly summed up as a maximal compressive force which simply leads to a buckling analysis. In This sense, Skene’s Method is very limiting because the designer cannot optimize his or her mast design"

Modern design books also handle mast design in terms of column strength, because it is what works.

[mebccb]

Originally Posted by SV Windrush II

Here is something about Skenes Methodology from the book "Sailboat Mast Design" by ALEXANDRE BERGERON


"The Second point of contention with Skene’s Method is its oversimplification of the problem of mast design. It does little to account for different configurations in rigging and shrouds or mast tapers. There is no care taken to account for the sideways forces imparted on the mast by sails or dynamic effects of waves and so on. Everything is neatly summed up as a maximal compressive force which simply leads to a buckling analysis. In This sense, Skene’s Method is very limiting because the designer cannot optimize his or her mast design"

All of the masts on all of your boats were designed as supported columns. Unless of course you have a free standing mast.

[mebccb]

So if your boat pitch poled, the acceleration/deceleration forces on a mast would be tremendous, even difficult to calculate. The bending stress (Yes I said bending stress!!) induced by the inertial mass of the mast alone, could it cause the mast to fail? Or the impact with green water? I believe it could, and certainly has, especially a lightly built racing mast or cruiser/racer mast. Also how would an unsupported mast stand up to this sort of abuse? And cruising, boats, should they be designed to have at least a fair chance of surviving such an incidence? How stiff would such a mast be, and how much bending would it exhibit under reasonably heavy weather sailing?

[mebccb]

Have fun all

[SV Windrush II]

Originally Posted by mebccb

Modern design books also handle mast design in terms of column strength, because it is what works.

There is a reason why it works. Fact is, the larger in diameter the "column" and where they design in the wall thickness the less it will bend, buckle or flex under load.

In any case I am looking at all factors involved. Many boats have been de-masted without hitting the water. and some have capsized without any damage. all depends on the strength verses load of everything involved.

There is so many factors involved with load on masts while sailing that there is still a lot of just estimating based on experience and i believe still a lot to learn when designing a mast, but I think technology in this area really has not been advanced as much as its potential, at least from my prospective coming from the aerospace industry where we have to study every considerable load, stress and strain of every material that's attached to an airplane, and because boats have very similar characteristics.

[Jim Cate]

Maybe this discussion is derailed by semantic differences. Seems to me that masts and their rigging are designed to resolve all the transverse loading into compression loads on the tube. To reduce the chance of buckling, the mast is divided into relatively short panels each of which works towards that resolution. When a mast fails, it is because one or more of those panels has failed to keep the tube in compression, with the attendant buckling and bending, not because the tube itself was not stiff enough. Anyone who has handled a bare mast section in a horizontal attitude knows how very floppy they are athwartships and unaided by rigging... very noodley!

In the fore/aft dimension the tubes are stiffer by far, but still unable to resist bending without some support. Stays provide this support at the head and possibly mid span with an inner forestay and running backs, aided in some rigs by fore and aft lowers or check stays. Manipulation of those stays can avoid or enhance fore/aft bend, and this is done normally on race boats and some cruisers. It is here that the additional fore/aft stiffness of the tube comes into play... controlled bending but in a plane where the tube is designed to absorb it.

This view may be overly simplistic, but seems adequate to understand the basics of rig design.

Jim

[Terra Nova]

Originally Posted by mebccb

...a mast would not normally be exposed to bending stresses...

Originally Posted by mebccb

...masts will bend...

You sound very confused.

But at least you now admit that masts bend and that even your yacht design books say so. You should be proud of the progress you've made.

So let's all agree that masts bend.And, since they all bend, they are all subjected to bending stress. Elementary.

[SV Windrush II]

Originally Posted by Jim Cate

Maybe this discussion is derailed by semantic differences. Seems to me that masts and their rigging are designed to resolve all the transverse loading into compression loads on the tube.

Jim

I fully understand the design principle as to the rigging reduces the bending moment of the mast by using stays and spreaders, but does not eliminate it.
The rigging design and analysis brings it down to an acceptable level of strength and stability so that it will not collapse under intended use.

I guess the point I am trying to make of all this if you repair a mast, it needs to be at least as strong as originally designed.

Just on a side note, I pulled my hollow wooden mast and repaired about a 8 foot section at the Spreader block. the 55' mast was strong enough after the repair to lift it up from the center and only deflected from foot to head by about 4 feet. If designers were not considering bending strength, then when my mast was lifted from the center it would have just snapped in half like a twig.

The OP was asking if anything matters besides compression strength and my answer is YES, you cannot base strength of the mast solely on downward pressure. I think we can all agree on that, otherwise there would be many mast repair jobs that will fail.

[mebccb]

Originally Posted by Terra Nova

You sound very confused.



But at least you now admit that masts bend and that even your yacht design books say so. You should be proud of the progress you've made.



So let's all agree that masts bend.And, since they all bend, they are all subjected to bending stress. Elementary.

Your very good at misquoting and taking things out of context. Not a skill to be proud of.

Elementary

[mebccb]

Originally Posted by Jim Cate

Maybe this discussion is derailed by semantic differences. Seems to me that masts and their rigging are designed to resolve all the transverse loading into compression loads on the tube. To reduce the chance of buckling, the mast is divided into relatively short panels each of which works towards that resolution. When a mast fails, it is because one or more of those panels has failed to keep the tube in compression, with the attendant buckling and bending, not because the tube itself was not stiff enough. Anyone who has handled a bare mast section in a horizontal attitude knows how very floppy they are athwartships and unaided by rigging... very noodley!

In the fore/aft dimension the tubes are stiffer by far, but still unable to resist bending without some support. Stays provide this support at the head and possibly mid span with an inner forestay and running backs, aided in some rigs by fore and aft lowers or check stays. Manipulation of those stays can avoid or enhance fore/aft bend, and this is done normally on race boats and some cruisers. It is here that the additional fore/aft stiffness of the tube comes into play... controlled bending but in a plane where the tube is designed to absorb it.

This view may be overly simplistic, but seems adequate to understand the basics of rig design.

Jim

Thanks Jim