Mast compression post calculations.

[GILow]

Originally Posted by 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.

Most of the hard work has been done for me by the people.who did the tabernacle conversion. Coachhouse roof is plenty strong and load is spread nicely via a.big alloy plate. I'd take that out and put steel in its place to avoid dissimilar metals.

Bilge arrangement is very easy, big load bearing surface to work with. Again, I'd go steel as it will be a dry bilge.

On balance, I think I'll just copy the Swanson set-up, size for size, weld for weld. Then send the whole thing off to be galvanised when I am done.

[MicHughV]

Just thought I'd mention this last point, if you haven't already thought about it.

Fabricating the steel compression post to the exact height needed, together with the end plates, may not allow you to install it like that, unless you can slide it in sideways, even so there might be a gap, which you will need to fill.

I'd recommend making a mockup out of some 2x4's and plywood end plates to mimic the final design and see if you can get that in.

If you have to rotate the post for any reason, ie, set the bottom and rotate into place, you might not be able to do that either.

See where your problems lie with the mockup. It's possible you may have to make the post a bit shorter to get it in place, and then shim up the difference.

Finally, the deck is probably cambered, so the upper plate should have the same arc template.

Stuff that keeps me awake at night

[GILow]

Originally Posted by MicHughV

Just thought I'd mention this last point, if you haven't already thought about it.



Fabricating the steel compression post to the exact height needed, together with the end plates, may not allow you to install it like that, unless you can slide it in sideways, even so there might be a gap, which you will need to fill.



I'd recommend making a mockup out of some 2x4's and plywood end plates to mimic the final design and see if you can get that in.



If you have to rotate the post for any reason, ie, set the bottom and rotate into place, you might not be able to do that either.



See where your problems lie with the mockup. It's possible you may have to make the post a bit shorter to get it in place, and then shim up the difference.



Finally, the deck is probably cambered, so the upper plate should have the same arc template.



Stuff that keeps me awake at night

I’m kinda direct in my approach.

If it doesn’t quite fit there’s always a couple of acro props.

[Uncle Bob]

Originally Posted by GILow

I’m kinda direct in my approach.

If it doesn’t quite fit there’s always a couple of acro props.

Ahhh, acro props, do you vant some props, very spec price just for you.

[GILow]

Originally Posted by Uncle Bob

Ahhh, acro props, do you vant some props, very spec price just for you. [emoji3][emoji3][emoji3][emoji3]

OK, I'll drop by and we can replace the post on Lake Macquarie. [emoji16]

[Wotname]

Originally Posted by GILow

I’m kinda direct in my approach.

If it doesn’t quite fit there’s always a couple of acro props.

I suppose if the old one comes out (with or without the acro props) then the new one should go in, especially as the new one will be 'smaller' but not shorter.

[Uncle Bob]

Originally Posted by GILow

OK, I'll drop by and we can replace the post on Lake Macquarie. [emoji16]

Matt, all jokes aside, I would suggest that you couldn't go far wrong mimicking the swanson design. Tried and true over a large number of builds.

[catsketcher]

Originally Posted by 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.

Actually Benny died in 1988. He designed the Twelve Metre Australia in about 1976 and then Australia 2 in about 1982. One Australia was an IACC boat and Benny died before their rule was worked out. Simple error - One Australia IACC and Australia 12 metre (Number 1)

Some good points about Euler buckling here. I would not use a prop shaft for a compression strut - they are far too thin. As has been shown above, the resistance to buckling comes substantially from a property called second moment of area or sometimes called moment of inertia. So make the tube larger in radius to maximise this. But Euler buckling is related to the length of the column squared and with the mast compression post so short you can get away with a much reduced moment.

It is very simple maths and can be done by any person with a calculator. Do the maths using say twice the boat's displacement as the force applied - then spec up from commonly available tubes to get more stiffness. If you weld plates on the end the fixity increases. No need to be cute, do the maths and then add safety factors as you desire.

cheers

Phil

[MicHughV]

I'm a retired structural marine engineer....

All this math talk is hurtin' mah head...

[Toccata]

Quote: Catsketcher
"It is very simple maths and can be done by any person with a calculator. Do the maths using say twice the boat's displacement as the force applied - then spec up from commonly available tubes to get more stiffness. If you weld plates on the end the fixity increases. No need to be cute, do the maths and then add safety factors as you desire. "

Interesting that you reiterated the practical conclusion relayed to me by John King. Possibly based on more science than Johns' yacht rollover (bounce over) example?

[bil56]

an approximation used in first calculation: compression force equal to boat displacement ...

[RaymondR]

I'm typing this in an Island Packet 40 and it has a keel stepped mast and the loading dynamics are substantially different than what they would be if it was deck stepped. However, my guess would be that were the section I can observe a component of a compression post column with rigid end fixities it would most probably fail through yield of the aluminium material well before it buckled, that is,that it would fall within that section of the spectrum of slenderness ratios where the Euler formula does not apply to column design and that the OPs circumstances would be similar.

Consequently, if the OPs desire is to replace a large and obtrusive column with one more slender, and consequently less obtrusive, calculating the probable failure of the existing column in yield and using this as the critical buckling load and transposing the terms of the Euler formula in order to extract the required moment of inertia for the selection of a alternative section in another material would be an appropriate design paradigm.

[chris95040]

Originally Posted by MicHughV

Just thought I'd mention this last point, if you haven't already thought about it.

Fabricating the steel compression post to the exact height needed, together with the end plates, may not allow you to install it like that, unless you can slide it in sideways, even so there might be a gap, which you will need to fill.

I'd recommend making a mockup out of some 2x4's and plywood end plates to mimic the final design and see if you can get that in.

If you have to rotate the post for any reason, ie, set the bottom and rotate into place, you might not be able to do that either.

See where your problems lie with the mockup. It's possible you may have to make the post a bit shorter to get it in place, and then shim up the difference.

Finally, the deck is probably cambered, so the upper plate should have the same arc template.

Stuff that keeps me awake at night

This is a fantastic point. When I had mine replaced I had the welder make it 2 inches short, now it sits on g10 plates. I did it so that every trickle of water in the bilge wouldn't run past it and rust it away like the original, but it certainly made installation easier too!

[El Pinguino]

Mine is 2.5 inch diameter.
Ping, Ecuador.

[MicHughV]

I've done a wood (teak)compression post installation twice now.
First on my steel boat, I welded the base plate to a frame, so it was well clear of any bilge water. The teak post had a groove cut thru' the middle of the top end, this allowed me to place the post on the plate and rotate the post and slide the post thru' a steel member, where it was bolted into place. Then I jammed some teak shims between the top of the post and underneath side of the deck. The bottom I kept in place by bolting some teak strips around the post on the bottom plate.
The second one was similar, but a fiberglass boat, so this time I fabricate a teak plate which was fixed to the to the top of the keel ballast, as the ballast surface area was a bit wonky. As before, I rotated the post into position and used shims to fill the space between top of post and underside of deck. The bottom was held in place as before.
Never had a problem with either installation.