Mast compression post calculations.

[Rumrace]

As for a new post call up Pappi Lighting in Mississauga. They manufacture T6 aluminum street lighting poles. The poles have internal structure to hold RF shields or components but primarily for strength. A 4” square or round pappi pole is more than 10x the strength or aluminum tube twice it’s gauge. They have cut pieces all the time. The President is a boater too. He might have parts to make it adjustable. They are pretty crafty with T6 extrusions.

[Wotname]

Let the pictures tell the story...
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I don't think so.

[Toccata]

Again for historical interest, One Australia was designed by Ben Lexen in conjunction with Johan Valentijn. The following article is written in the first person by the boat builder.

The dramatic moments on the 5th of March 1995, when during round four in the round robin stage of the America’s Cup challenger series, and in a match race between One Australia and Team New Zealand, the Australian boat split and sunk within two minutes off San Diego, CA.

This was an unprecedented event, and while it occurred when the internet was new, significant commentary now exists online. Alongside this YouTube video is a post by King Cliff which seeks to offer closure to the speculation on how such a complete failure could happen:

I built this boat. Yes, the primary winch failed and they transferred the load to I think the running back stay winch. I’m not a sailor, just a boat builder. Either way, it’s like trying to break a stick with your hands close together and then moving them further apart. The boat wasn’t designed to take the load applied at such a distance.

Plus, these boats weren’t designed to be in conditions like this (18-20 knots). They are flat water boats. The race should have been called off. If it was flat it probably wouldn’t have broken even with the winch failure.

A design flaw? In a way yes and no. It only had two bulk heads. Mast bulkhead and a keel bulkhead and both were mealy ring frames and not full bulkheads. But that wasn’t the true problem IMO. The boat was as hollow as a drum. It sure was cutting edge but it had no longitudinal strength. No beams running fore and aft to stop it breaking in half.


I brought this up three times but was told I wasn’t being paid to think. All I wanted to do was put two short longitudinal beams running maybe a few meters fore and aft of the keel box. But no. Weight was the key factor and the order of the day. I think if they were in it may not have broken. It certainly wasn’t too thin a carbon layup.

Why did it go down so fast? The hull and deck finished weighed only 1.1 tonnes. Incredibly light for an 80 foot maxi. The mast was 135 feet long and the single longest carbon structure ever produced. The mast had 40 tonnes of load pulling down on the mast bulkhead. But the cause of it sinking so fast was the foil and bulb attached below had over 17 tonnes of metal combined.

The foil was solid stainless steel weighing 5 tonnes alone attached to a 12 tonne lead bulb below it with small stainless steel wings off of it. So a 1 tonne broken cork being pulled down by 17 tonnes of steel and lead. Down she went!

It certainly wasn’t badly built. It was a masterpiece of construction built to incredibly high standards. NASA standards. In fact NASA took interest in what we were doing as it was built to the same layup as the space shuttle and we were treading new ground and finding breaking points with new technology.

Why did the three fellas stay on the front for so long? Because there was confusion as to if someone was still down below. You can see one guy leaning down shouting through the forward hatch. Inside was full of sails and nothing else. Up to two people are down there at any one time feeding sails up through the hatches or dragging them back down below. Throw in a few hundred tonnes of water and a recipe for disaster for anyone below. Once they were somewhat sure no one was below, they jumped off. They had no choice anyway.

But the key point to make is that the Kiwi boat had the boat speed on us from the start. Even if nothing went wrong they were going to beat us hands down.

[GILow]

Originally Posted by Wotname

Let the pictures tell the story...
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I don't think so.

Somewhat symbol-minded explanation I think.

[Bowdrie]

Originally Posted by Toccata

Plus, these boats weren’t designed to be in conditions like this (18-20 knots). They are flat water boats. The race should have been called off. If it was flat it probably wouldn’t have broken even with the winch failure.

It wasn't always so.
I remember reading an old article about the first 12 meter boats that came to the US.
They were six of them built in 1928 by Abeking & Rasmussen and shipped over on a freighter to Canada.
The article told the story of one of the boats that was being delivered from Canada to New York, and was caught in a gale, they were down to the smallest of sails and the boat was driving hard in bad conditions but made the trip with no damage.
Wasn't the wind pretty strong when Dennis Conner regained the Cup in Australia with Stars and Stripes?
Or, how about the French sailor that took on of the Bich aluminum 12s from France to Tahiti singlehanded, he said it was "A fine sea-boat".
But you're right, a naval architect that I knew said that longitudinal strength was most needed in boats.

[Toccata]

we've sort of hijacked the thread, but hopefully some will find this interesting nonetheless.
Another example:
John King designed many of the yachts built by Kanga Birtles of Jarkan Yachts (Jervis Bay NSW)
This included the 60' yacht that Kanga sailed around the world in the single handed BOC Challenge in 1990. That yacht rolled in the southern ocean, the rig (and boat) survived.
I think John King has credentials/knowledge when he specifies a compression post. The incident I refer to in post 26 is a practical application of his commonsense approach to yacht design. The yacht that rolled over its mast stump was saved.

[RaymondR]

Originally Posted by Chotu

What are the mast dimensions in x and y and thickness? You can calculate the moments from there and then check moments of various standard tubing to see what meets or exceeds.

Did this for my bow tube/beam.

Moments and moment of inertia are not the same thing.

The C factor in Euler's critical buckling formula are vastly different between a keel stepped mast and a compression post and whilst using the moments of inertia for the mast to design a compression post would provide a very conservative design it would not be considered a correct use of the property of the sections. The differences in E, of say an aluminium mast and a steel compression post, would tend to also emphasize the conservative tendency.

[El Pinguino]

I don't think I would be bothering with stainless.
Either stainless or mild steel you want to cover it otherwise in a cold climate it will be sucking the heat straight out of your boat -and if it is really cold your hand will stick to it and you will lose flesh.

I put some french whipping on mine. Do not - I repeat - DO NOT google that.
Lookee here instead https://vivasanpatricio.wordpress.co...ench-whipping/

Ping - Tulla

[AndyOz]

Originally Posted by boat driver

not to over complicate the discussion, but backstay compressions do not generally exceed 3000 lbs.
so a column of any material able to support compression of 6K is probably fine if the top and bottom plates can spread the loads.
The bending or bowing issue under compression would need to be addressed just to be sure it is stiff enough.

Careful.

Backstay of 3k means forestay also roughly 3k. Plus side stays could be 3k or more each, and you could have lowers as well. 6k wouldn't be enough.

[Toccata]

Boatdriver, careful, it totally depends on how big the boat/rig is.
I've already quoted a qualified naval architect advising that on my 40'/12m yacht that he designed, (with a 17m mast) the compression load on the mast/compression post, with everything up and tight is 15 ton.

[Rumrace]

Anodized T6 extruded aluminum will out perform any tube. Since the couch roof was not built for the mast, is the couch roof threatened?
I noted on cored couch roof which seem to include everyone. Balsa failure around the boot is a threat. Have manufactures addressed this and added a solid pad?
My Dads 1930 something boat had a coach mount mast. Inside was an extra rib between the normal spacing. The ribs were more robust than the other. Blocks on the edges of the bilge had adjustable mounts for the rigging?
When you read the slow advancement from wood to glass the disasters with big brands is well recorded. Resins we use to use simply don’t have structural strength after years have passed.
4 of my favourite shipwrecks are boats which were modified which ultimately caused its sinking.

[GILow]

Originally Posted by Rumrace

Anodized T6 extruded aluminum will out perform any tube.
…

What on earth is your justification for that statement?

[Cheechako]

Originally Posted by GILow

Hi all,

Our Kelly Peterson 44 was originally designed with a keel stepped mast. At some time in the past a very neat and professional job was done to convert the mast to a deck stepped tabernacle system.

The only bit I do not like is that they used the lower section of the original mast as the compression post. The mast itself has in mast furling so this means we have a pretty huge mast section in the cabin when a much smaller steel tube section would work just as well.

How much smaller is my question.

Can anyone point me to some numbers or formulas for calculating an appropriate tube section/wall thickness etc?

Matt

I have it at home but out of town for a few days. There is a formula for a "tube in compression". It often comes down to "slenderness ratio" (A larger diameter of thin wall tube is inherently better that a solid bar of much smaller diameter.)

Most mast forces are not nearly enough to collapse metal from compression. But if the mast support , or mast, flexes sideways, then it fails, but not from metal compression.

It's the diameter that keeps it straight and not failing. Otherwise a small diameter rod could actually support the relatively small compressive force of the rig.

Just for some gut feel: The tensile strength of a grade 8 bolt is 150,000 pounds per square inch.
A 1" grade 5 bolt can lift over 50000 lbs. .... and Steel is stronger in compression that tensile.

[Rumrace]

Originally Posted by GILow

What on earth is your justification for that statement?

The OP wishes to minimize footprint. A 4” extrusion T6 with internal structure would outperform 8” steel tube.

[Rumrace]

Originally Posted by GILow

What on earth is your justification for that statement?

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