Mast compression post calculations.

[zstine]

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.

for buckling, he needs to keep the slenderness ratio below about 100. assuming a round tube, as long as the physical cross sectional area of the compression post is as strong (keeping in mind different strength of AL vs SS) as the the cross section area of the mast and the slenderness ratio is below 100, he should be ok.

[Carriage Boy]

To the OP, does the compression post on your Swanson have any openings or features where you could measure it's wall thickness? If the sailplan of your two boats are similar, there is the key your answer courtesy of a naval architect.

You could select a smaller alternate tube diameter for your Kelly Peterson, replicating the moment of inertia used on your Swanson by increasing the wall thickness appropriately as you reduce the tube diameter used for your KP.

[rhubstuff]

Unless weight is an issue, you could try to find a used propeller shaft and cut it to the right length, instead of using a tube section. The compression strength would be enormous and the visibility factor very small. You might need a tension rod as well, depending on the design of your vessel.

[GILow]

Originally Posted by Carriage Boy

To the OP, does the compression post on your Swanson have any openings or features where you could measure it's wall thickness? If the sailplan of your two boats are similar, there is the key your answer courtesy of a naval architect.

You could select a smaller alternate tube diameter for your Kelly Peterson, replicating the moment of inertia used on your Swanson by increasing the wall thickness appropriately as you reduce the tube diameter used for your KP.

After a lengthy discussion with an engineer last night, involving Euler and how the heck to pronoUnce his name correctly, we both came to your conclusion and agreed that the Swanson was the perfect model to copy. Everything on the Swanson is over-engineered and I’ve never heard of a Swanson having problems with the compression post. And the 120 mm pipe on the Swanny is very unobtrusive while also being somewhat practical as a hand-hold.

However, I am going to use the info I’ve gathered here and elsewhere for comparison purposes, just to satisfy my curiosity.

[GILow]

Originally Posted by rhubstuff

Unless weight is an issue, you could try to find a used propeller shaft and cut it to the right length, instead of using a tube section. The compression strength would be enormous and the visibility factor very small. You might need a tension rod as well, depending on the design of your vessel.

I feel like this would buckle under load.

But it would look nice.

[Cwens]

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

With a keel-stepped to deck-stepped conversion, my concern would be lateral or fore-and-aft forces on the tabernacle for which the coachroof was surely not designed.

[rhubstuff]

"Unless weight is an issue, you could try to find a used propeller shaft and cut it to the right length, instead of using a tube section. The compression strength would be enormous and the visibility factor very small. You might need a tension rod as well, depending on the design of your vessel."

I thought of that, too, but I was thinking of a pretty large diameter shaft, larger than one would usually think adequate. Would a solid column tend to buckle more easily than a tubular section of the same diameter? It would be stainless or bronze, or maybe monel. All have terrific compression strength, and an old used one might be had for a very low price and be perfectly useful as a compression post even if it was a bit pitted or scored.

[Chotu]

Originally Posted by GILow

After a lengthy discussion with an engineer last night, involving Euler and how the heck to pronoUnce his name correctly, we both came to your conclusion and agreed that the Swanson was the perfect model to copy. Everything on the Swanson is over-engineered and I’ve never heard of a Swanson having problems with the compression post. And the 120 mm pipe on the Swanny is very unobtrusive while also being somewhat practical as a hand-hold.

However, I am going to use the info I’ve gathered here and elsewhere for comparison purposes, just to satisfy my curiosity.

That’s a good idea. Look through the math in a basic way. Essentially, you are trying to copy the thing you have now, but doing it with some thing of a different diameter. That’s all you are doing. How are you doing.

“Oiler”’s equations were in my link as well.

You’re on the right track. It’s probably a lot easier than we are making it, but, it’s worthwhile to go through it so that you can really feel good about it. And so you don’t get something stupidly heavy.

[GILow]

Originally Posted by ChrisOwens

With a keel-stepped to deck-stepped conversion, my concern would be lateral or fore-and-aft forces on the tabernacle for which the coachroof was surely not designed.

That question crops up now and then, but I think it is a bit misleading. It turns out the lower stays take the vast majority of those forces, what takes place at the mast foot is trivial in comparison.

[patprice]

As a rough guide to the load on the post, it will be the tension on the winward shrouds plus any input from other stays?
Offhand I do not know the factor of safety in wire rigging. Maybe 25% say of breaking strain.
The above might be a simple calculation. Be conservative.

[Toccata]

FWIW
Re post 21. The deck does actually take fore and aft loads in a keel stepped situation, via wedges or a product like spar tight.
I too went through the same process of changing a keel stepped mast to a deck stepped mast, (twice, two different yachts). I was looking for a water tight hull and a thinner post in the cabin space.
The naval architect approached the calculation question in an interesting way.
12m racing yacht everything fully tight and sailing "hard" the calculated vertical load on the mast was 15ton.
2nd aspect of the calculation: true story; A yacht washed onto a reef, the mast snapped at the lower spreaders. At some stage the yacht was rolled over, "bouncing" on the mast stump as it went. The boat survived, the compression post survived. He took the view that at the moment of the bounce, the load on the compression past was approx double the weight of the hull, (in that case 10 ton). So the load for calculation purposes was 20ton.
This boat weight was very similar to mine
The selection was a stainless steel schedule 40 pipe, wall thickness at least the same as the aluminium mast extrusion and pipe diameter more than half the current mast section.

[Panope]

Originally Posted by GILow

Hi all,

......a very neat and professional job was done to convert the mast to a deck stepped tabernacle system.......

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

[Adelie]

Given that the deck to keel dimension is probably in the vicinity of 7’ a wood 4x4 would probably be just fine.

Given the short height bowing failure is unlikely.

[RaymondR]

My Roberts, a 36' modified to 40', had a 4" x 4" x 1/4" mild steel compression post about 11' long. However I would advise stainless steel if you could get your hands on a piece of it. My saloon table was mounted on the compression post by a bracket which allowed it to be raised all the way up to the deskhead, very handy for maintenance purposes in a steel boat.

[El Pinguino]

No idea about the calculations - however

I have a compression post - it is round rather than a box section and at about 3/3.5 inches diameter ( 39 foot boat ) makes a perfect handhold in the middle of the saloon when things are 'bouncy'.
One issue I had to deal with in 2015 during her ' half life refit ' was the compression post 'footings'.