Masts: Thicker , or wider

[Mike Banks]

I would use neither. I like a canted bipod mast with a furler to take care of the mainsail, and either a wishbone boom or a standard boom swivelling off the other side of a looped cable, the other end of which fastens to the top of the mains'l furler.

Simple and trouble free, with no mast turbulence at the sail itself to worry about. One can use a double-bipod self-tacking wishbone schooner rig, and single-handed sailing on even a large vessel becomes a hell of a lot easier to manage . Best used on a cat or a tri, but also good on any mono. FAR less strain on cables and decks, and using a simple jury pole, bipod masts are quickly and easily lowered to pass under bridges which bar so many inland waterways to sail craft.

I would not be messing about with anything else if I was building from new or re-rigging an older vessel..


,

[Cliffhanger]

Wow, a complicated question with complicated answers.

I think if you want a mast that can withstand a rollover you might want to look at the fixed rigging instead of the mast profile. The mast is designed to take mostly compressive loads and the rigging takes the tensile loads that result from lateral (horizontal) forces. A rollover increases lateral forces, increasing tensile loads far more than the compressive loads. This results in rigging failure leading to the mast breaking.

[Dave_S]

Masts are designed to minimise the disruption of air flow onto the sail therefore it is made longer forward/aft than it is wider across the boat. To give you the required strength across the boat with less than ideal width it must be made longer. If you were just worried about carrying the loads it would be close to round.

You could reduce the height of the mast to lower the centre of effort or you could reef to the same effect when the breeze gets up and keep the benefits for less breeze.

Originally Posted by Cyrus Safdari

I have the opportunity to get a new mast and was considering the engineering aspect of strengthening the mast in case of a roll-over/knockdown/pitchpole events etc. This seemed to be a real problem in the GGR.

Anyway, I'd like to know the opinions of actual boatbuilding engineers on the issue: Thicker, wider, shorter (masts, that is)

Of course masts can be strengthened with stays and shrouds, and lets take it for granted that the standing rigging and chainplates etc are all functionally in good shape etc. The pictures I see are of masts buckled in the middle-ish parts, which is contrary to the general rule that things break at joints (but there's compression, so that may explain it) And lets assume the stays and shrounds can't be put out farther either, because they just can't. Masts are also tapered sometimes but also putting that aside...

There is a belief that a keel-stepped mast is inherently stronger than a deck-stepped one. A stick being held up in two places is inherently more stable than one held by compression alone. That may be relevant in case of a broken stay but on the other hand, it seems to me that if we're talking about breaking a mast in a knockdown then the bending momentum builds from the tip of the mast (negligible bending momentum) to whereever it connects to the boat, regardless of the keel or deck (maximum bending momentum) So the force there is the pressure against the mast times the length of the mast to that point. There is no bending momentum on the part of a keel-stepped mast that is below the deck, right?

Thus the most obvious way to strengthen a mast is to shorten it. A shorter mast of course also reduces capsize chances too since it allows the boat to heel more before the center of mass moves to leeward of the center of buoyancy - so not only is the shorter mast itself less likely to break, there's less chance of capsize to start with. Right?

On the other hand, not being a structural engineer I have no idea to what degree other methods can strengthen masts.

So for example, would a thicker mast wall help more, or an overall wider diameter mast? I'm guessing a small increase in diameter would dramatically strengthen the mast. On the other hand I'm guessing a wider mast has more drag underwater. There must be some sort of equation that shows the relationship between the strength of a hollow pole to its diameter and/or wall thickness, right?

(How about the people who weld plates to their mast sides -- a sort of lamination. would that be the same as thicker walls? But then, would the weld itself would distort and weaken the mast?)

Anywho, just some thoughts...

[Eigenvector]

The real answer to the engineering question (not wether or not to do it)



Axial and Shear Stresses - Cross Sectional area is used to calculate AXIAL STRESS and SHEAR STRESS. For a rectangle A=Base x Height. (Equation 1)


Bending Stresses – Calculation of bending stresses utilize the Area Moment of Inertia. For a rectangle I=1/12 x Base x Height^3.

Stress = Moment x Outter Fiber Distance / Area Moment of Inertia

Note that there is a strong axis and weak axis. Normally designated as Ixx or Iyy in Cartesian coordinates. (Equations 2 & 3)


Buckling is also a calculated utilizing a suitable equations for the slenderness ratio. Eulers is probably the most common. Critical load = pi()^2 * E * I / Le^2. Note the area moment of inertia is in this equation also. Le depends on the end conditions (stayed/unstayes/deck stepped, ect) Thus Equations 4 & 5 for strong and weak axis.


The answer to the question is going to be the one that fails first in the use of all 5 equations. But this is an ideal case where everything is mathematically straight (small deflection theory). But it would tell you which profile is stronger overall.


In reality the mast is not straight when loaded due to sail loads at the luff and boom attachment point. This is where the linear elastic buckling equation falls apart. The long slender column is bent. Spreaders are used to increase the area moment of inertia thus increasing the buckling strength.


Additionally you can add equations 6 & 7 for checking if the rigging fails before the mast.


If the OP has drawing of the 2 profiles, send them to me and I'll run the calcs and post.

[MartinR]

My best guess is the reason the masts fail when hitting the water is because the tension load on the cap shrouds make these part. Then the mast is loaded for bending, something most masts are not designed for. So strengthening the mast profile will probably have little effect without strengthening the rest of the rigging.

And then there is weight. More weight aloft means less stability and higher risk of knockdown (very simplified). Also very simplified, if you add 10 kg to the mast, you need to add 100kg to the keel to keep stability.

I would be very careful changing to a heavier profile without having stability calculated by a boat designer.

[TeddyDiver]

The most cost and weight effective means against buckling are internal transversal plates (bamboo structure).

[Mlp48]

There are many factual reports out there on the argument of thin wall/big section v. thick wall/small(er) section and if mine to chose, I'm with the latter. No question.

[Eigenvector]

Originally Posted by TeddyDiver

The most cost and weight effective means against buckling are internal transversal plates (bamboo structure).

This is only true for localized buckling caused by bending stresses. Get 1/2" away from the transverse stiffener and it will buckle there under column loading.


Example: Stand on a beer can. Where does it buckle. Now epoxy two beer cans end to end. Stand on this with its transverse mid span stiffener. Same load, same buckling location.

[Eigenvector]

Originally Posted by Mlp48

There are many factual reports out there on the argument of thin wall/big section v. thick wall/small(er) section and if mine to chose, I'm with the latter. No question.

It all depends on the loading and which equations are satisfied. Here's a good example:


Given 2 sections called Fat and Thin. Both weigh the same per unit length. Because of this, both must have the same cross sectional area and therefore have the same tensile/compressive/shear load capacities.


However, Fat has a much higher area moment of inertia (^3 term) which increases the bending and buckling capacities significantly.


Since both weight the same, aluminum is sold by weight, which one do you choose?

[AiniA]

I would focus more on the standing rigging than the mast section. When we arrived in Durban, SA after about 30k miles on new standing rigging we had an aft lower break. This was on a Bristol 45.5 with a massive single spreader rig. The rigger there suggested we oversize the lowers to the same size as the cap shrouds. He said that aft lowers were by far the most common problem he saw on boats that had crossed the Indian Ocean from Oz or Indonesia. Not a surprise when you think of broad reaching for a couple weeks in winds more that 25 knots. With the larger wire we did not have problems.

[TeddyDiver]

Originally Posted by Eigenvector

This is only true for localized buckling caused by bending stresses. Get 1/2" away from the transverse stiffener and it will buckle there under column loading.


Example: Stand on a beer can. Where does it buckle. Now epoxy two beer cans end to end. Stand on this with its transverse mid span stiffener. Same load, same buckling location.

Luckily masts aren't made of beer cans. Now cut the ends of your two beer cans and try to weight with something lighter, like a sixpack

[MartinR]

Originally Posted by Dave_S

Masts are designed to minimise the disruption of air flow onto the sail therefore it is made longer forward/aft than it is wider across the boat. To give you the required strength across the boat with less

Part of the reason. Stiffness, comes also in here, though. As masts are normally pinned/stayed in several places athwartship, often in fewer places alongship. So the mast profile has to take more bending moment in this direction.

For example, my mainmast has no fore and aft support from the lower spreaders to the top. An elongated profile is needed here to resist pumping.

Of course, you could get away with a round profile, but at heavier weight, and this is something that you absolutely do not want, normally.

[MartinR]

On another note. Maybe a thinner mast profile, tapered at the top would be better in the case of a knockdown. Less area means less impact force. Also, a smaller profile means more spreaders and the load of the impact will be shared among more shrouds.

Unstayed masts are probably the best option, as these are actually designed to dissipate applied force by bending. Like airplane wings.

[Pauls]

An important point on designing a rig to survive capsize is to pay attention to the spreaders. If the spreader fails the cap shrouds become useless and the mast will almost certainly fail.

Spreader attachment to the mast is designed to take the spreader compression load. Hitting the water can create high loads in other directions. Pay attention to this.

If the spreader clamping against the caps is not really strong the spreader can slip up/down the shroud. It will then fail.

Water impact can create bending loads on the spreader far higher than what they see in normal conditions. A rigidly mounted spreader can then fail either the spreader or its mast connection.

As in all engineering, the devil is in the details.

[valhalla360]

With the exception of unstayed masts which are a totally different animal...

Stayed masts have a critical design to resist buckling as they are primarily in compression loading...if the stays fail, even with double the strength, the mast will likely fail in a roll over.

But again, what issue is there with the original design that you are trying to fix. So far it's an emotional appeal for "better" with no issue being addressed. Either repeat with the same design as the mast you are removing or find out what the manufacturer spec'd.