aluminum mast strut vs oak post

Hi all
I am restoring a S&S designed Dolphin 24 built in 1960. It has a deck stepped mast with a support strut in the cabin. I have the original design specs which called for an unsupported 5' long aluminum "standard" 1 1/4 " pipe. This pipe had some corrosion near the keel fitting but obviously had met the load needs for over 40 years. The load is non eccentric.

For a number of reasons I want to replace this pipe with an oak post which will be supported by a structural bulkhead on one side. The other side is a door frame for a door to the forward cabin.

I propose to use a 2" x 3" oak post with the 3" dimension position fore and aft - both the ends will be fixed. I would like to know the compression load capacity of the 5' long unsupported aluminum pipe, and the compression load capacity of an unsupported 2" x 3", 5' long white/red oak post, and the effect of the support of the structural bulkhead. Could this post be 2" x 2"? Any input much appreciated. Thanks

Ron

 

You can compute the buckling capacity of the pipe yourself. Pcr = pi^2 E I / (L^2)
E = young's modulus = about 10,000,000 psi for AL
I = moment of inertia of pipe section = (pi/4) * (ro^4 - ri^4)
ro = pipe outer radius ri = pipe inner radius
L = length of pipe.

The yield strength of the pipe depends on the alloy. Some marine grades are in the vicinity of 20000 psi, some high performance grades are much higher. Compression strength of oak depending on grades, quality are around 7000 psi conservatively. You can figure the peak load your mast could impart onto the post will be the sum of all the yield strengths of your shrouds and stays. This might be conservative, but you know then that a shroud would fail before the post does.

 

oak vs aluminum post

Water Addict
Thanks for your reply.
Pcr = Crushing strength of pipe = pi^2 E I/(L^2)
What does the symbol ^ mean?
What does pi mean?
Ron

 

The compressive strength of wood is far less that that of steel or alu; matter of factly the strenghth of wood is in the direction of the fibers, grain. A deckstepped mast is usually supported by a steel or st. steel pipe, that carries the compression of the rigload to the keel. You do not gai anything by changing metal for wood. If you don't loke the metal look in your cabin, cover the pipe with a thin woodveneer. Another item is that when wood gets wet it may loose a proportional part of it's strength. Any calculation in this respect might therefore fail if you don't make sure that this sopprt can't suck up moisture.
Altogether a bad idea.

 

Ron,
^ means exponent, pi is 3.141592654 as in pi r squared for area of circle. If your aren't sure what these are off the bat, you might want to consider just replacing the pipe with another one of the same size. That is probably the safest, simplest, and affordable solution.

 

Water Addict
Sorry I took so long to get back - project was 'displaced. Anyway, I thought pi was pi but wanted to be sure. Using the numbers given I get 39600 lbs capacity for the al post and 42000 lbs for the oak post.

For moral support I talked to 2 marine architects, who freely admitted they were winging it on this. They told me, that the original al post with pipe flanges at the ends was a cheap and easy but over designed mast support system for the load range on this size boat, and that the oak post, if properly aligned, end supported and fastened would be adequate. I did talk to S&S to try and get the designed compression load of the mast but this was not available from their records. I do have a copy of the design drawing calling for a 1 1/4 in al pipe.

Now, what worries me is that I found the old al post I took out of the boat. Wouldn't you know that when I measured that post I found that it had a OD of 1.625 and an id of 1.375! Such a post would have a compression load capacity of 73397 lbs!

D'artois' comments (Thank you) are also disheartening.

Ron