Mast compression post calculations.

[Toccata]

Further to my post number 26.
Some may be interested in a bit of history:
The naval architect I referred to is John King. He was the "numbers man" behind Ben Lexen. Ben designed Australia 2, (Winner of the 1983 Americas Cup) but John King did many of the numerical calculations.
John is an elderly gent now, but was able to bring his experience to bear on the question of compression posts on both my Jarkan 40' that he designed in the 80's and my recent project, a Savage Oceanic 46'. Both these yachts I converted to deck stepped masts.
The summary of the specifications was:
In stainless steel, use a wall thickness at least the same as the original mast.
The cross sectional area of the pipe to be at least half that of the original mast.
{Please Note: I am not a naval architect, this spec is only suggested as a starting point for your project. No responsibility implied for relying on this information}

[GILow]

Originally Posted by Toccata

Further to my post number 26.
Some may be interested in a bit of history:
The naval architect I referred to is John King. He was the "numbers man" behind Ben Lexen. Ben designed Australia 2, (Winner of the 1983 Americas Cup) but John King did many of the numerical calculations.
John is an elderly gent now, but was able to bring his experience to bear on the question of compression posts on both my Jarkan 40' that he designed in the 80's and my recent project, a Savage Oceanic 46'. Both these yachts I converted to deck stepped masts.
The summary of the specifications was:
In stainless steel, use a wall thickness at least the same as the original mast.
The cross sectional area of the pipe to be at least half that of the original mast.
{Please Note: I am not a naval architect, this spec is only suggested as a starting point for your project. No responsibility implied for relying on this information}

An interesting rule. I’ll see how it compares with the Swanson.

But just to be clear, he didn’t have anything to do with designing One Australia, did he?

https://youtube.com/watch?v=8Yau9A7XDHs&feature=share

[Toccata]

but maybe the compression post was ok.......

[ozsailer]

Just something for thought that I do not see has been raised here. Have you thought of the insurance implications should the system fail. Just a scenario to consider. Mast fails, you make and insurance claim, assessor comes to inspect, views changes to system, asks who engineered new system and where is supporting documentation, "I have none" knowing insurance companies I would think they then have an out to pay out". Also I have noted in ALL our boating policies hidden away in the disclosure booklet one normally finds a clause which states "And advise the insurance company of anything they may be made aware of" I would most definatley consider your changes worthy of the insurance company being notified.

Ozsailer

[Toccata]

The insurance/boat modification argument has been raised many times with regard to other ideas for boat modifications.
A poorly fitted through hull or crappy hose clamp is possibly as much a liability as a compression post.
Are we going to live in fear of insurance companies, or make sensible modifications, defend them by proper research and execution, and stand up to the assessors opinion?? No easy answer to this and can be/will be argued both ways.
Re the compression post:
The bigger modification in the keel stepped/deck stepped conversion is the redesign of the mast, leading to a different position for the spreaders to allow for the different column length created by changing the length of the mast below the original first spreader position.

[Wotname]

To the learned engineers here.

If the compression post was slightly undersized and subsequently failed, would it be more likely to see some early evidence of possible problems (bending, reduction in stay tension, vibrating or some such) or would the failure be sudden and catastrophic?

[GILow]

Originally Posted by ozsailer

Just something for thought that I do not see has been raised here. Have you thought of the insurance implications should the system fail. Just a scenario to consider. Mast fails, you make and insurance claim, assessor comes to inspect, views changes to system, asks who engineered new system and where is supporting documentation, "I have none" knowing insurance companies I would think they then have an out to pay out". Also I have noted in ALL our boating policies hidden away in the disclosure booklet one normally finds a clause which states "And advise the insurance company of anything they may be made aware of" I would most definatley consider your changes worthy of the insurance company being notified.

Ozsailer

Very good point. Not an issue for me specifically but might well be relevant to others stumbling over this thread.

[GILow]

Originally Posted by Toccata

but maybe the compression post was ok.......

If so it was about the only bit of the boat that was.

Not a moment of national pride for an Aussie, that's for sure.

https://youtu.be/3m5qxZm_JqM

[NSboatman]

Wottie- likely, yes; although buckling failure can be instantaneous and catastrophic- it is rarely without some warning signs, as you note.

I’ve done exactly this conversion; and my rule of thumb was ‘design the post for the max displacement of the boat’. This is more - much more- than the mast compression is likely to ever impose, and is reasonably conservative. For a cruising boat.

The reality is that for most cruising boats in the 38-50’ range, with a rigidly constrained column less than 2.3m long; this is no more than a simple piece of sched 40 steel pipe roughly 3-4” dia.

…but you have to do the math and make sure you can get loads aligned and evenly distributed into the column.

[GILow]

Originally Posted by Panope

Matt,



I don't know how yours is configured, but many tabernacles (including mine) are not like a traditional "deck stepped" mast in that bending loads are transferred to the below deck section.



Your installation might require the below deck element to resist some bending loads in the same way a keel stepped mast does.



Can you post a photo of your tabernacle?



Steve

Here it is, at last.

[Toccata]

I've had real trouble posting pictures on this site, so given up.
By description: My mast sits on an aluminium base about 50mm larger than the mast section in all directions.
The mast base (probably properly called a step) is bolted through the deck to the plate on top of the compression post. There is an (approx) 20mm plug going up inside the mast section.
There are effectively only shear loads being transferred to the deck by the step bolts, ie no bending forces at all. Similar to the loads imparted by mast wedges or a product like spartite.
In my case a new mast was then designed and made to suit the new column length (now being about 1.9m shorter than the original mast) Total mast height and configuration is the same as the keel stepped mast. I used the same sails and I could have used the same standing rigging, but chose to update it.
In my opinion, your tabernacle may impart slight lateral/bending/twisting force to the deck, in reality however the tabernacle would provide very little lateral support to the mast as the attachment is by the single pivot bolt (rather than 2 bolts, upper and lower). If the mast was a very tight fit in the cheeks, there may be lateral bending moment applied to the deck, but again minimal. Really the mast is not bending laterally at that point, unless your stays are loose? It may be deflecting forward by pressure from the boom via the main sheet and vang at the level of the gooseneck, but again yours is free to bend in this direction by virtue of the single pivot bolt, so imparting minimal twisting loads to the deck.
A taller tabernacle with multiple through bolts and a loose rig may be a different story.
A complicated reply, sorry.

[El Pinguino]

Originally Posted by Wotname

To the learned engineers here.

If the compression post was slightly undersized and subsequently failed, would it be more likely to see some early evidence of possible problems (bending, reduction in stay tension, vibrating or some such) or would the failure be sudden and catastrophic?

I'm not sure which would be worse.
'Hey Skip! the mast just fell down' while you are having your porridge - bin there.

Or - on a long passage - having lowers slacken - tension lowers - they slacken again - this goes on for a week or more - you know it is going to end in tears - you just don't know when-

Back to what I have - I'm pretty sure it is 3 inch diam. Mild steel - very heavy wall - in fact 'heavy as all get out' as I found out when it was out of the boat some years back.
I'll pop over to Ecuador tomorrow and report back.

[RaymondR]

Originally Posted by Wotname

To the learned engineers here.

If the compression post was slightly undersized and subsequently failed, would it be more likely to see some early evidence of possible problems (bending, reduction in stay tension, vibrating or some such) or would the failure be sudden and catastrophic?

It's considered a stability problem and once the buckling begins second order moments come into play.

Euler,

Pcr= C2pi E I /l^2

where C = end fixity factor.

E = elasticity.

I = moment of inertia.

l = length of column

since in the instance under discussion all the factors except I are fixed then I is all we have to play with. Consequently the tube diameter and wall thickness are what we should attempt to optimize. However to thin a wall will fail in yield.

In the case of a sail boat the maximum load on the compression post would be experienced by the combination of the stay tension imposed loads and those generated by the righting moment generated by the ballast when the mast had been pulled down parallel to the water.

[Chotu]

Originally Posted by RaymondR

It's considered a stability problem and once the buckling begins second order moments come into play.

Euler,

Pcr= C2pi E I /l^2

where C = end fixity factor.

E = elasticity.

I = moment of inertia.

l = length of column

since in the instance under discussion all the factors except I are fixed then I is all we have to play with. Consequently the tube diameter and wall thickness are what we should attempt to optimize. However to thin a wall will fail in yield.

In the case of a sail boat the maximum load on the compression post would be experienced by the combination of the stay tension imposed loads and those generated by the righting moment generated by the ballast when the mast had been pulled down parallel to the water.

Which is exactly the answer I gave in post #3. Somehow, people thought it wasn’t right though. Lol

All you have to do is check the moments and make sure you have the same or greater. Solved in post #3.

No idea how the learned engineers didn’t understand that. I’m baffled at what followed.

So as I said at the outset of the thread, just plug in your dimensions for various off-the-shelf, replacement columns to see what the moments are. Make sure the replacement moments are greater than or equal to the moments for the column that is in place right now. Done.

I think possibly, a lot of people don’t understand what the moments are. In general. Maybe that’s where they went astray reading my post. They are not regarding bending or righting or anything like that, even though they use the same word. They are just regarding the distribution of material from the origin at the center of the profile slice. They are a way to quantify that geometry, which can be complicated, into simple, convenient values you can use to compare to other configurations, just like we are doing here in this case.

Take a look at the following attachments. Each of them has a dot at the origin. Each of them represents a different type of column or hollow extrusion. The moments calculated on them are a measure of the wall thickness and distribution of that wall material from the dot in the middle - conveniently used as a pair of numbers to compare between the different types of profiles and wall thicknesses available.

That’s how Ixx and Iyy moments are used for rigging.

[Rumrace]

I was just reading about core degrading between the compression mast and boot. Apparently in the top is a piece of wood slowly crushing. I’m learning lots on this thread. Thanks