Mast compression post calculations.

[Toccata]

Re top and bottom plates
I made a template of the post (in timber) with end plates glued onto it in situ.
Transferred it to the workshop. Bolted steel plates to the template at each end, then welded these to a strong back. Between these plates I fabricated the stainless steel pipe and end plates.
When it came time to fit the post, the cabin roof had to be lifted by 1mm so the post was tight. This would have been really difficult if the roof and mast step were parallel, fortunately my cabin roof had a slight slope down to the bow.
Alternatively, make the post 10mm short and add a 10 (or 11mm) shim out of the same material.

[GILow]

Originally Posted by Rumrace

The OP wants to reduce his footprint on the post. This is the smallest post 4” square or round. Anodized it’s corrosion proof.

8” steel rolled welded doesn’t come close.



Smallest footprint it’s square 4” anodized T6 aluminum post. Before it’s anidized you can have the internal 4 tubes threaded. This gives you the ability to make upper or lower plate adjustable. Most have a piece of wood wedged in. Kinda primitive

Fair enough. Well argued.

[RaymondR]

T6 aluminium Modulus of Elasticity 68.9 GPa or about 10,000 ksi

from memory steel is about 30,000 ksi

going to make a big difference when applied as the E in the Euler critical buckling formula.

[GILow]

Originally Posted by RaymondR

T6 aluminium Modulus of Elasticity68.9 GPa or about 10,000 ksi



from memory steel is about 30,000 ksi



going to make a big difference when applied as the E in the Euler critical buckling formula.

So… just for a minute pretend I don’t know if a big number is better or worse…

[RaymondR]

Originally Posted by GILow

So… just for a minute pretend I don’t know if a big number is better or worse…

Buckling force = C x 2 x pi x E x I /l^2

top of the line is directly proportional and therefor, if you treble the E you treble the load before the column buckles. Consequently a steel column with the same section properties as an aluminium column will accept three time the load before it buckles.

It's tough to beat carbon steel from a structural practicability viewpoint, pity it rusts.

[GILow]

Originally Posted by RaymondR

Buckling force = C x 2 x pi x E x I /l^2



top of the line is directly proportional and therefor, if you treble the E you treble the load before the column buckles. Consequently a steel column with the same section properties as an aluminium column will accept three time the load before it buckles.



It's tough to beat carbon steel from a structural practicability viewpoint, pity it rusts.

I knew that. [emoji16]

[RaymondR]

To be absolutely certain you could calculate the compressive loading on the alloy mast to make it yield and use it as the critical buckling load on a column of the length it would be in your boat with fixed for C and back out the moment of inertia required.

Doing it that way your mast would end up as a big wide, flat smear on the deck before the compression post failed.

[Rumrace]

Crushing math goes out the window with internal construction in the extrusion.
The poles I referred to are the only thing standing in a Florida part after a hurricane. The light fixtures ripped of and 12” tapered steel poles flattened beside them.
It’s why they make masts of the material.
So a tapered round steel pole with 3X the base diameter is not as strong as 4 Inch extruded pole. The key with T6 is to use mechanical connections and avoid welding. Welding will reduce strength by 40%.
Inside the extrusion are an extruded hole in each corner. It’s designed to thread to mechanically attach to base or cap as well as increasing the over strength. There is also four open slots or channels suitable to attach internal equipment. All this internal structure while small is what makes the 4” so strong. It’s their smallest pole size. 8’ piece would be scrap to them. They return it to the foundry.
Hollow steel poles kinda suck. They twist and tear apart at the weld. The down side is they are too light and on tall poles require a weight pendulum to stop them whipping and they cost more than double of some steel rolled and welded.

[Rumrace]

I’m more concerned with the glass between the mast and post. My boat is 7 years old which is plenty time for a coach roof nick to let water in to the balsa. I know power boat issues well but on the learning curve my second year sailing. I’m not sure if Jeanneau my brand of boat every addressed this area or had a failure. I read about a sister Beneteau failing. Apparently masting the boat some how the roof beside the boot got nicked in previous years and ignored. Seems like an area which few manufactures if any are addressing.
T6 aluminum, cold rolled steel and endgrain balsa in some brittle plastic with a mil of vinyl, make up my current structure I think. The balsa will eventually loose its oil and collapse. I’m sure going to find out before I flip boats.

[MicHughV]

Nowhere in this thread do I see any references to the rigging.
But the prime purpose of the rigging is to keep the mast "in column".
Many mast failures occur when some part of the rigging fails, and the end result is a section of the unsupported mast that buckles.
Additionally, many sailboats, when heeled over, have notable slack leeward shrouds, which also translates to the compression load being exerted on the mast being off center or out of column.
Masts also end up with numerous holes drilled thru' it, which can also affect it's strength.

It's a complicated subject. But determining compression loads on the mast is only one piece of the puzzle. The compression loads on a compression post should face the same scrutiny.

I have added some links to may provide entertaining reading.

HINTS AND ADVICE - Seldén Mast AB

Rigging instructions & sailmakers guide : Seldén Mast AB

[Rumrace]

Steel poles are not made with internal structure. Crushing math of material is a minor issue steel start corroding before it’s installed and adds silly weight and defeats OP s dimensional wishes.

[KevinE]

I am a structural engineer: pretty safe to assume a design compression force equal to the tensile capacity of your windward side stay. As as matter of reference a HSS152x6.4mm wall (6"x1/4") tube (45 ksi yield) with unbraced length of 2.4 meters (8ft) has an ultimate (failure) load of 144 kips (kip=1000 lbs) under pure axial load. You need healthy size bearing plates top and bottom is this will probably be the weak link - you really need to know the compressive strength of the material at top and bottom of the post - plates need to be thick enough to ensure load distribution over the entire area of the plate. Hopefully there is no plywood in the load path.

[MicHughV]

That 6"x6"x1/4" post is going to come in at around 150 lbs. While it is certainly strong enough, it is heavy. My guess, based on calculations I did for my own boat, the compression load on the mast is likely to be in the 10,000 lb range, which makes it a bit of overkill. Suggest a smaller size.

[GILow]

Originally Posted by MicHughV

That 6"x6"x1/4" post is going to come in at around 150 lbs. While it is certainly strong enough, it is heavy. My guess, based on calculations I did for my own boat, the compression load on the mast is likely to be in the 10,000 lb range, which makes it a bit of overkill. Suggest a smaller size.

Yes, this is how my Swanson 42 was constructed and it does feel a little like overkill. But still way better than having the mast section in the cabin.

[MicHughV]

Well, you should allow for a safety factor, for sure. A 6x6x1/4 tube is a pretty big heavy beast.
What is the displacement of your boat ??

The end plates need some thought. I'm not sure how much space you have to fit an end plate? Nor do I know how you plan on fastening this post to the deck or keel. If space is a constraint, a smaller section will likely also do the job, but at this point I'm just guessing at the likely compression load on the mast, so am reluctant to suggest a smaller size. The end plates can be stiffened by welding some triangular gusset plates there.

As the previous poster said, the deck where the post will locate also needs to be checked. This should be solid. Presumable the bottom will bear on the keel somewhere or somehow.