Replacing Traditional Wood Masts with Alloy

[mebccb]

Originally Posted by Terra Nova

Plain bull.

It's a column under compression, not a beam in bending. The one exception is a free standing mast.

[Terra Nova]

If there were no bending stresses they wouldn't need all that rigging to prevent them from bending.

[mebccb]

Originally Posted by Terra Nova

If there were no bending stresses they wouldn't need all that rigging to prevent them from bending.

Wow, the total ineptitude.

[mebccb]

Originally Posted by Terra Nova

If there were no bending stresses they wouldn't need all that rigging to prevent them from bending.

"All spars are in compression. Masts (except unsupported ones), spinnaker poles, and spreaders may be considered to be in pure compression, while booms are subject to bending."

From Skene's Elements of Yacht Design

Since the advent of loose footed mainsails, even booms could be considered under compression depending on what type of
Vanging arrangement is used.

[mebccb]

Originally Posted by Terra Nova

If there were no bending stresses they wouldn't need all that rigging to prevent them from bending.

The caption under the diagram says "Yacht mast (under compression)"

[zeehag]

keep that balderdash.
i learrned i will be changing my mizzenmast that cane patricia broke with a mast from an aluminumtree. oops. now i need spartite and i need other malarkey to keep the damned thing in place. gggrrr.
it is more cost effective to me to convert as the aluminum tree is here, and thai cedar aint here. oops and i am not changing to spruce or fir--both rot much faster than thai cedar,. stuff is awesome.
the only reason i am changing my mizzen, which until patricia was perfect, is because the windgenerator atop the masthead made the mast like twistoflex straw and cracks happened. water got inside the bolt holes and compound the issue. ok. waah.

btw-- to protect fittings on masts from becoming one with each other, use rubber gaskets and seal masts with the likes if butyl tape. make no connection between aluminum and the other metal. use antisieze with bolts and washers and nuts. have fun.

[Pelagic]

Interesting!
Is the reality of sailing masts that they are stayed primarily for compression loads as in diagram but also experience bending stresses, especially when on a close reach?

In most boats I have sailed hard to windward, I would feel the leeward stays go slightly slack.

Does that not imply some bending and/or twisting stresses in a stayed rig?

I know when doing the repairs, the box masts had a lot of bending flexibility when being handled.

[mebccb]

Originally Posted by Pelagic

Interesting!
Is the reality of sailing masts that they are stayed primarily for compression loads as in diagram but also experience bending stresses, especially when on a close reach?

In most boats I have sailed hard to windward, I would feel the leeward stays go slightly slack.

Does that not imply some bending and/or twisting stresses in a stayed rig?

I know when doing the repairs, the box masts had a lot of bending flexibility when being handled.

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.

[SV Windrush II]

Ok I will chip in here.

From my experience after about 20,000 miles sailing the pacific ocean and going through some gales going to Hawaii and also through Micronesia I have stressed my rigging probably to the max. Also just a FYI I worked for several years as a structural design engineer.

My Mariner 40 is a ketch rig with 2 wooden masts, Main has 1 spreader, fore and back stays and 6 upper and lower shrouds.

When the boat is being knocked around in the swell and it is blowing any more than 20 knots, the mast is experiencing bending and shear stress. It is experiencing compression but not as much as the other two. Here is why.

When you have a sail applying pressure to the mast and it is also whipping around as the boat rolls, the applied pressure is trying to bend the mast between the stay points. Also at the mast head, spreader join and deck, it is trying to shear the mast in half. Also with a mast you have vibration and frequency stress that is trying to splinter the mast. (On a aluminum mast it creates cracks and the main reason why aluminum fails)
Next time you go out on a stiff wind and are rolling through the waves, (Specialty down wind) take a look up the mast and at times it will have a S curve to it, and whats worse it will whip back and forth like someone using a bull whip.

So yes the stays help keep the mast from bending (Thats their main purpose) but you would need to have many more shrouds and stays every few feet going up the mast to eliminate any bending force.

[mebccb]

Originally Posted by SV Windrush II

Ok I will chip in here.

From my experience after about 20,000 miles sailing the pacific ocean and going through some gales going to Hawaii and also through Micronesia I have stressed my rigging probably to the max. Also just a FYI I worked for several years as a structural design engineer.

My Mariner 40 is a ketch rig with 2 wooden masts, Main has 1 spreader, fore and back stays and 6 upper and lower shrouds.

When the boat is being knocked around in the swell and it is blowing any more than 20 knots, the mast is experiencing bending and shear stress. It is experiencing compression but not as much as the other two. Here is why.

When you have a sail applying pressure to the mast and it is also whipping around as the boat rolls, the applied pressure is trying to bend the mast between the stay points. Also at the mast head, spreader join and deck, it is trying to shear the mast in half. Also with a mast you have vibration and frequency stress that is trying to splinter the mast. (On a aluminum mast it creates cracks and the main reason why aluminum fails)
Next time you go out on a stiff wind and are rolling through the waves, (Specialty down wind) take a look up the mast and at times it will have a S curve to it, and whats worse it will whip back and forth like someone using a bull whip.

So yes the stays help keep the mast from bending (Thats their main purpose) but you would need to have many more shrouds and stays every few feet going up the mast to eliminate any bending force.

Yes masts will bend a little bit, BUT the bending is negligible and is not considered in mast design unless it is a free standing mast. All the yacht design books I own and any others that I've seen are in agreement with this, and the fact the your masts have not fallen down are testament to this.

[Pelagic]

Thanks Windrush... that has been my observations also.

Do we all agree?
Primarily large compression forces, but with dynamic bending forces of a considerably lesser degree, that is compensated by the flexibility of the mast?

[mebccb]

Originally Posted by Pelagic

Thanks Windrush... that has been my observations also.

Do we all agree?
Primarily large compression forces, but with dynamic bending forces of a considerably lesser degree, that is compensated by the flexibility of the mast?

Sorry I disagree with wind rush. The primary load on a mast is compression. That is why column compression formulas are used for designing masts.

I can't wait for my back to get better so that I have something better to do than be on this forum.

MB
Out

[ur2slo]

Well might as well chime in now.

I'm reading the thread now, being as in April I am yanking mine out for IRAN time. (No not the country.)

I'd like to pick your brains.......... ( just the good ones)

[SV Windrush II]

Originally Posted by mebccb

"All spars are in compression. Masts (except unsupported ones), spinnaker poles, and spreaders may be considered to be in pure compression, while booms are subject to bending."

From Skene's Elements of Yacht Design

This book is clueless. Yes I agree, all spars are under compression, but to say "Pure Compression" ?? he must have never sailed. I have worked with Highly educated engineers and some of them seam like they need a reality check, so I'm not surprised this kind of wordage came out of a book.

Ive seen my Spinnaker pole bend like a bannana. yes of course it has compression stress, but the compression turns into bending stress the more compression you apply! so using my example, if the pole snaps in two, its because of the compression causing the pole to bend beyond its limits, therefore snapping the pole in half.

The writer of this book i am sure has a lot of knowledge in certain areas but not in this particular area obviously.

[ur2slo]

Originally Posted by mebccb

Yes masts will bend a little bit, BUT the bending is negligible and is not considered in mast design unless it is a free standing mast. All the yacht design books I own and any others that I've seen are in agreement with this, and the fact the your masts have not fallen down are testament to this.

I have to ad some info on above, back in the day, Bell 47 heliwoppers used wood blades. (real pain to dynamic balance when moisture was high.)

Bell put out a training video, its on youtube I think, that had a mast mounted camera on it.

When you saw the blades rotating and flapping and leading and lagging the rotor head, the blades actually took on the shape of a wet linguine noodle! Yikes! and it flys?

We didn't let first time student watch it, because it would scare the pants off them.

So yes, wood moves a lot under loads.