Mast compression post calculations.

[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

[JPA Cate]

What'll you do with the extra 2" space around the mast?

Is it worth the work?

Obviously the present set up is strong enough.


Ann

[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.

[GILow]

Originally Posted by JPA Cate

What'll you do with the extra 2" space around the mast?

Is it worth the work?

Obviously the present set up is strong enough.


Ann

The current setup is visually very disruptive. The mast has a very deep chord thanks to the provision for in-mast furling.

I’m taking the mast off soon so I figured it would be the perfect time to replace the post.

[GILow]

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.

The mast is 260 mm x 160 mm, sort of regular oval. Height of the post would be 2300 mm.

I think I see where you are going with this approach, however I’m not sure bending moments are the issue. In theory at least, this post should only experience direct compression force. (Unless I end up with a seagoing pole dancer on board.). It is firmly attached to the boat, top and bottom.

For reference, or comparison, my Swanson 42 has a 120 mm diameter galvanised steel pole for a compression post, as is standard for the boat. I’ve never heard of any problems with them.

The two boats have very similar masts and sail plans, so I’d happily duplicate it, but I actually think it may be possible to go smaller still.

[MicHughV]

When I built my boat I had a several books on Yacht Design. Several of these included how to determine compression loads on a mast. Skenes Elements of Yacht Design comes to mind, but there were others. A google search will lead you to a way to to do this. You should be able to find a downloadable pdf file.

I remember doing mine. The main mast was 40' tall, single spreader and decked stepped, with a compression post underneath. The calculations were quite involved and required several pages.

It's a step by step process. I don't remember all the particulars anymore, but was surprised to learn that the compression loads can be significant, several 1,000's of pounds. I could probably dig them up from one of many many boxes of sailing memories. I seem to recall a number that was in the 7-8,000 lb range for my boat, a 38' Roberts. Larger boats, obviously will be much more.

Once you have the compression load, you can move to the next step, which is to determine an appropriate mast section. Many variables involved here. Height, number of spreaders, boom location, size of sails, etc.

For my compression post I made up a solid teak post, which if memory holds, was around 3"x5" solid teak. It was braced in the middle by being part of the galley furniture. Never had a problem with it.

[Chotu]

Originally Posted by GILow

The mast is 260 mm x 160 mm, sort of regular oval. Height of the post would be 2300 mm.

I think I see where you are going with this approach, however I’m not sure bending moments are the issue. In theory at least, this post should only experience direct compression force. (Unless I end up with a seagoing pole dancer on board.). It is firmly attached to the boat, top and bottom.

For reference, or comparison, my Swanson 42 has a 120 mm diameter galvanised steel pole for a compression post, as is standard for the boat. I’ve never heard of any problems with them.

The two boats have very similar masts and sail plans, so I’d happily duplicate it, but I actually think it may be possible to go smaller still.

Ok,

For a more thorough look, here is all the info you need, plus a calculator to help with the actual numbers:


https://amesweb.info/Beam/Column-Buc...alculator.aspx

I still think simple comparing the moments of the current mast section (thickness needed) with various column dimensions will work, since buckling of the column is a bending of the column. the same strength in different configurations can be compared using just the moments and the material type. You can see that appear as factors in this much more complicated calculator in my link.

But of course there is also the other failure mode where the metal stays in column and just squashes.

Either way, the link had everything you need.

[GILow]

Originally Posted by Chotu

Ok,



For a more thorough look, here is all the info you need, plus a calculator to help with the actual numbers:





https://amesweb.info/Beam/Column-Buc...alculator.aspx



I still think simple comparing the moments of the current mast section (thickness needed) with various column dimensions will work, since buckling of the column is a bending of the column. the same strength in different configurations can be compared using just the moments and the material type. You can see that appear as factors in this much more complicated calculator in my link.



But of course there is also the other failure mode where the metal stays in column and just squashes.



Either way, the link had everything you need.

Thank you for the link.

That’s a very complex calculator, I’ll take a while to understand it.

[GordMay]

Your compression post is a “Short compression member” with “Fixed-Fixed ends”.
Short columns fail, by crushing, at very high stress levels, that are above the elastic limit (yield point) of the column material.
Column Buckling Calculations
➥ https://mechanicalc.com/reference/column-buckling
➥ https://www.degreetutors.com/column-buckling-equations/

[Spot]

Hi Matt, you will need material type and thickness of the replacement tubing and the same for the spar, including any internal bracing, to finish out the calculations. I saw a patent drawing where the 'sail cover' part of the mast were separate extrusions. Unless they are welded to the main mast I think they would fall off of the compression calculation.

Keep in mind I was often prone to falling asleep in any college class involving math. For one session, that I was awake for, they had us do all the bits for a static beam load and then right at the end of class told us to never do this again, instead go look in the back of the book for strengths of timbers...

[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.

[Rumrace]

I’m just reading to learn. I did ask the broker why the mast was skinny inside. Power Boater assumption all masts came inside.
I gather they are adjustable. My mast has to come off every winter and I’m 2 years off to change rigging. Someone told me the inner support must match the boot on the coach roof.
Wouldn’t a rigger be your best bet to talk to. There is a surveyor in the Forum also. He pointed out some of the faults my French production boat has. I’m not sure if they cut corners on the compression post. I’m pretty sure he’s well informed on them. I think he’s inspected over 2,000 boats. His tag is Boatpoker. I had a rigger come help me with the boat. He inspected everything before the mast got near the hull. He did take up the floor panels. He changed out most the sheets.

[captlloyd]

Go to a steel tubing supply company and order a 8X8 section of square or round tubing 8 feet long with at east a 3/16 wall. Plate the ends with 1/4 inch material and powder coat the whole thing. Done.

[foufou]

I was faced with the same problem as the mast that came with my boat was not the same as the original but decked stepped. I may have over simplified the problem but my reasoning was that if the same amount of material was in the compression post as was in the mast, then it would be fine. So I bought a solid piece of 6061 T6 aluminum from a scrap metal place. It was about the dimensions of a 2X4. I then ripped an angle on it so it would conform to the wall that separated the head from the V berth cabin and bolted it to that wall. I had it checked out by a Naval Architect who approved of the design and suggested adding an athwart ship 1/8" thick G-10 mini bulkhead between the bottom of the hull and the cabin floor. So far so good, no issues. I had done this for the same reason as Gilow mentions; to reduce the bulky section intruding into a small space.

[vpbarkley]

Bending moment is of no consideration here, this part is in compression only.

Measure the section area of the existing mast. This is the measurement of the material thickness by the length of the section. Wrap a tape measure around the mast for length.

Yield allowables for generic steel and aluminum are 50k psi and 40k psi respectively so steel can safely be substituted for aluminum of the same section area.

Once you have selected an alternate structural element (tube, etc.) you need to perform column calculations to ensure the structural element will not buckle under compression.

Here is a column buckling calculator: https://calcresource.com/statics-buckling-load.html

You can find Young's modulus for your column material by searching online.

You can calculate moment of inertia for your column by searching online.

Send me a private message if you need additional help.