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Old 24-04-2019, 23:03   #1
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Masts: Thicker , or wider

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...
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Old 25-04-2019, 00:52   #2
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Re: Masts: Thicker , or wider

Normally, the cap shrouds will snap. Or the fastenings of the shrouds to the mast. Or, less common, the lower shrouds, but that is often not a disaster.

Masts are strong enough.

I think best way to strengthen the system would be to change the wires to Dyneema stays.
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Old 25-04-2019, 01:41   #3
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Re: Masts: Thicker , or wider

I didn't see anywhere in your writeup to suggest there was a problem with mast weakness with the current design.

Don't fix what ain't broke.

If you are worried about the boat falling over and breaking the mast, leave a reef in the sail at all times. That will effect about the same thing without getting into complications.
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Old 25-04-2019, 02:56   #4
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Re: Masts: Thicker , or wider

Shortening a mast will decrease the bending moment induced in it. It will also depower the boat, require new standing rigging and new sails.
A thicker wall will strengthen the mast, in proportion to the change in thickness, i.e. twice as thick, twice as strong. It will also increase weight aloft in the same proportion.
Widening or deepening ( in the horizontal direction) a mast section will increase its strength geometrically i.e. twice the width will result in far more than twice its load carrying capacity, but this depends on whether it's a compression or bending load. It also depends on which axis the load is applied (pitchpoling or rolling).
Increasing the size of the mast section will increase windage and weight aloft.
The original design of the boat will have considered all of these factors.
Extreme situations (GGR) should require re-analysis and probable changes in design. Unless you are planning sailing in extraordinary conditions, the original design of the boat is probably best adhered to - unless you have a full analysis conducted by a marine architect.
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Old 25-04-2019, 08:39   #5
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Re: Masts: Thicker , or wider

I would agree with the previous posts. Understand what you are trying to accomplish. What conditions was the boat designed for? What conditions are you assuming (bare pole, large main due to gust or rouge wave, small storm sail as the center of effort will be different)? Have other boats of similar design had problems? Does it have a bendy or tapered mast? If the designer of the boat is still alive definitely call or visit him. Pay a marine architect that has a lot of experience designing sailboats masts to analyze the mast. Calculate the loads on the fittings, standing rigging and reduction of mast head items, radar and radar reflector. Consider a tapered mast, breakaway mast head, adding another set of shrouds, using stronger aluminum, and reinforcing the interior or exterior of just the weak points. This is just the start as this is not a decision to take lightly.
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Old 25-04-2019, 08:50   #6
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Re: Masts: Thicker , or wider

Besides cruising, I have also been windsurfing since the start of that sport. Whn the switch from fiberglass to carbon fiber masts came, I was amazed at how light and strog the new masts were. Too bad they are so expensive for cruising yachts. A stayless mast like the Nonesuch boats, but made of carbon fiber, would make huge improvement in performance for most any boat. Could the cost be less than $30,000 ? I was quoted over $20,000 for conversion to in-boom furling, did not go for that but the carbon mast would be worth much more.
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Old 25-04-2019, 09:17   #7
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Re: Masts: Thicker , or wider

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A thicker wall will strengthen the mast, in proportion to the change in thickness, i.e. twice as thick, twice as strong.
Nope! Wall thickness has a surprisingly small impact on the strength of a mast compared to other factors.

The thing to keep in mind is basic arithmetic about tensile + compressive strength is not appropriate here - the mast is a long slender column and fails due to buckling long, long, long before anything else. This failure point is known as "Euler's critical load", and you'll find that wall thickness has surprisingly less impact on this than you might expect. Other aspects of the cross sectional geometry (how far apart the sides of the mast are from each other) have a much greater impact.

To the OP, you'd want a naval architect, or at least a highly qualified rigger, to aid your investigation into your rig. I think you'll find that such folks have already studied these compromises for a given boat and arrived at a rational decision.

This is an interesting subject, but I think you will almost certainly find that you do not want to modify your existing rig. Seems a bit like trying to design an airplane that can survive a crash in one piece - at least with today's materials I don't believe such a plane could ever get in the air.
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Old 25-04-2019, 09:40   #8
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Re: Masts: Thicker , or wider

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Nope! Wall thickness has a surprisingly small impact on the strength of a mast compared to other factors.

The thing to keep in mind is basic arithmetic about tensile + compressive strength is not appropriate here - the mast is a long slender column and fails due to buckling long, long, long before anything else. This failure point is known as "Euler's critical load", and you'll find that wall thickness has surprisingly less impact on this than you might expect. Other aspects of the cross sectional geometry (how far apart the sides of the mast are from each other) have a much greater impact.

To the OP, you'd want a naval architect, or at least a highly qualified rigger, to aid your investigation into your rig. I think you'll find that such folks have already studied these compromises for a given boat and arrived at a rational decision.

This is an interesting subject, but I think you will almost certainly find that you do not want to modify your existing rig. Seems a bit like trying to design an airplane that can survive a crash in one piece - at least with today's materials I don't believe such a plane could ever get in the air.
I didn't say that twice the thickness would have twice the load carrying capacity. Load capacity is a function of geometry - i.e compression versus bending, combined bending and compression.
Doubling the cross-sectional area of any material halves stresses - i.e doubles strength. This is only for the simplest loading conditions - pure tension, shear or short block compressive loading.

The OP's original question is about how to get some sort of better performance out of a mast in case of a rollover or pitchpole. In this situation, beam bending would be the primary loading mode, not compression. Euler compression loading would be a small fraction of the bending moment induced in a mast in these situations.
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Old 25-04-2019, 09:56   #9
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Re: Masts: Thicker , or wider

The old mast is not original and not good. and since I'm doing a complete bare hull refit changING height is not an issue (I'm keeping the same height: 40ft anyway). Naval architect studies are not an option.

Really, it is a simple matter: I'm looking at two masts, oNE larger diameter and one thicker walled.

Question is: which increases mast strength more.

I have read that mast strength increases to the third power of the diameter. Not sure of this is right and what the function is for wall thickness.
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Old 25-04-2019, 10:07   #10
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Re: Masts: Thicker , or wider

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Originally Posted by Cyrus Safdari View Post
The old mast is not original and not good. and since I'm doing a complete bare hull refit changING height is not an issue (I'm keeping the same height: 40ft anyway). Naval architect studies are not an option.

Really, it is a simple matter: I'm looking at two masts, oNE larger diameter and one thicker walled.

Question is: which increases mast strength more.

I have read that mast strength increases to the third power of the diameter. Not sure of this is right and what the function is for wall thickness.
A competent rigger can give you a precise answer to that question, given the geometry of each of those masts, in probably 15 minutes. Call one.
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Old 25-04-2019, 10:12   #11
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Re: Masts: Thicker , or wider

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In this situation, beam bending would be the primary loading mode, not compression.
I'm no expert here, but I don't think this is true.

When you are sailing, with the sail force pushing sideways along the length of the the mast, it still winds up being a compressive load for the mast due to the standing rigging.

I believe this would, to some extent, also be the case while whipping through the water upside down.

In any event, your words were "A thicker wall will strengthen the mast, in proportion to the change in thickness, i.e. twice as thick, twice as strong." To a non expert, lacking the expertise to infer the retroactive qualifications you've since provided, it leaves them with a breathtakingly flawed view of how to consider the strength of masts. Glad I was here.
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Old 25-04-2019, 10:23   #12
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Re: Masts: Thicker , or wider

Length ("slenderness ratio") is a big thing. Also, more spreaders; The key to keeping a mast intact is keeping the "column" straight. The more it can bend between anchor points the more at risk it is. ("column failure")

"I'm looking at two masts, oNE larger diameter and one thicker walled."
Unfortunately, numbers need to be run. As an engineer my gut says most often larger diameter wins over wall thickness. (hold a solid piece of steel rod, say 1/4" diameter out and observe the sag. Hold a 1/2" diameter thin wall tube the same way and you will have little sag.) A architect would tell you pretty fast and spending a couple hundred dollars for a consult can be interesting as well as good to know. I've done it with Bob Perry years ago.
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Old 25-04-2019, 10:41   #13
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Re: Masts: Thicker , or wider

If you're getting a new mast, the mast manufacturer probably has an engineer to perform the design analysis. They would not generally trust your estimates anyway. If they already have dimensions for your boat, or can find the data online, it can be quick and easy. If not, you have some measuring to do for them, but they will recommend the appropriate mast section, spreader layout, rigging sizes, etc. Contact a manufacturer and verify. I recently worked with Sparcraft and they were very competent, based on my PhD in engineering.
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Old 25-04-2019, 10:50   #14
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Re: Masts: Thicker , or wider

My option to save wait would be to double the stays with dyneema or other cheaper alternative so in case.of broken stay there will be something to hold the mast.
A nice design I saw on a boat had the mast stayed in 3.places 1 cap shrouds 2 3/4 of the mast and then the lower shroud's in case.of broken mast you still have enough to go on .
In case of a capsize the mast is the least of.your concerns .
Engine mounts , tanks , lockers so the boat won't flood , floor boards and thousands other things to secure before you even think about the mast .
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Old 25-04-2019, 11:00   #15
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Re: Masts: Thicker , or wider

Quote:
Originally Posted by SwellGuy View Post
I'm no expert here, but I don't think this is true.

When you are sailing, with the sail force pushing sideways along the length of the the mast, it still winds up being a compressive load for the mast due to the standing rigging.

I believe this would, to some extent, also be the case while whipping through the water upside down.

In any event, your words were "A thicker wall will strengthen the mast, in proportion to the change in thickness, i.e. twice as thick, twice as strong." To a non expert, lacking the expertise to infer the retroactive qualifications you've since provided, it leaves them with a breathtakingly flawed view of how to consider the strength of masts. Glad I was here.
Pat yourself on your back some more while you grossly simplify and cherry pick my responses to the OP.

You are making the most fundamental error in any statics analysis of a structural system - i.e. that each component is perfectly rigid and elasticity has no effect on laoding.
Loads get shed away from the most compliant member to the stiffest.
The least bit of slack in any piece of standing rigging means it sheds load into the stiffer member (the mast, unless it is designed to be "whippy" for some reason).
For the OP, especially if they have a keel stepped mast, the only option to increase the load carrying capacity of their mast is to increase wall thickness - unless they want to re-build the mast partners.
In a rollover or pitchpole, loading is lateral, not axial.
Increasing the moment of inertia (or moment of gyration) is the most efficient way of increasing load carrying capacity, but increasing cross-sectional area is also a valid strength increasing option.
By your argument, a mast made out of aluminum foil would work, if it had the proper geometry. This is of course, nonsense.
Any structural member actually has to have some finite thickness to resist stresses and carry load.
In the simplest example, a 2x6 wood beam can carry a load of X.
Two 2x6s sistered up will carry 2X.
If you load the 2x6 on the wrong axis (at the simplest, beam load carrying capacity is a function of the square power), you would have 1/9 its load carrying capacity.
By the same example, if instead of using two 2x6s sistered up side by side, you could fit a 2x12 into the space you have, you could carry 4 times the load that two 2x6s could.
Columns are even more sensitive to geometry as they carry load as a cubed power, instead of only a square power.
Take a look at unstayed masts - wall thickness is extremely important in the ability of any structural member.
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