Bulkhead vs compression post

[thedude0]

Hi i was just wondering what the opinion is on whether a direct compression post would be stronger than bulkheads...please refrain from answering with...depends what the designer intended ..cheers Ed

[Stumble]

They are both acceptable ways to achieve the required compression stiffness. Each has advantages and disadvantages. Without more information the question does not have a meaningful answer.

[Sailmonkey]

We've got both.....the bulkheads support a beam that carries compression to the hull, as well as supports hull shape when the shrouds are tightened, as well as a compression post the sends the loads directly down to the keel.

So both.....as our boats designer intended...


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[tkeithlu]

Think of a bulkhead as a compression post in all directions, but not as strong as a post in any of them. So, for keeping the general shape of your hull, a bulkhead is the right choice, but for a greater and known directional force, such as supporting your deck-mounted mast, a post is best.

[Stumble]

Originally Posted by tkeithlu

Think of a bulkhead as a compression post in all directions, but not as strong as a post in any of them. So, for keeping the general shape of your hull, a bulkhead is the right choice, but for a greater and known directional force, such as supporting your deck-mounted mast, a post is best.

This is pretty much my point from earlier. Without knowing how the bulkhead was built, and how the compression post is built, there is no way to tell which is 'stronger.' There are a lot of ways to control the compression loads of a mast, and these are just two. You can also use a ring frame, or place the entire deck in compression. None of these is necessarily 'stronger' than the other. It depends on how strong they were designed (then built) to be.

Like any engineering trade-off there are advantages and disadvantages to each.

Compression posts are likely the easiest way to handle the compression loads from the mast, BUT for every pound of mast compression load there is a lifting load on the chainplates. Compression posts do nothing to keep these loads contained. This lifting force also acts to pull the sides of the boat inward. Again, the compression post does nothing to constrain this.

So what's strongest? It just depends.

[Lojanica]

Beef up a weak point by rebuilding in heavier or with stronger materials will uncover the next weakest link in construction if the boat is stressed either by max loading or duty cycle. That's how it works.

[tkeithlu]

Excellent post, Stumble. Thanks.

[denverd0n]

Neither. Either. Both. Flip a coin. Take your pick.

You already said that you don't want to hear the REAL answer, so pick whatever answer you like.

[Lojanica]

Originally Posted by denverd0n

Neither. Either. Both. Flip a coin. Take your pick.

You already said that you don't want to hear the REAL answer, so pick whatever answer you like.

True. My boat has many different types all on one boat.

Mast compression post to keelson which is attached to 4 bulkheads.

Two cabin compression posts to ring frames.

Even a few simple bulkheads and compression posts.

I am far from a naval architect but I am guessing the designer used the best set-up for the application.

If you give more info these folks on here will very likely give you a great answer.

[tkeithlu]

And if you really want to get complicated, take a look at two of my bulkheads, which have compression posts welded to them, both increasing the vertical compression capability and stabilizing the bulkheads from buckling and then collapsing.........

[Dulcesuenos]

We looked at a Antigua 44( think csy molds) they changed to a deck stepped mast with the beautiful wood bulkhead as the compression post. We passed as it had rot at the bottom of the bulkhead( which went well below the sole)and felt it was only a matter of time before something really bad happened. Also seemed to be a major job to repair.

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[DumnMad]

Depends on what is at the bottom of a compression post. Big thick heavy keel - no problem, but skimpy plastic hull you could be in heavy weather and have the mast rigging thrust it through the bottom.

A lot of dead sailors had wooden boats with rotten keel timbers.

[barnakiel]

These may be equivalent.

A bulkhead may transfer AND distribute the load.

A post may only transfer a load.

If the mast is where you do not want a bulkhead, your only option is a post.

etc.

b.

[gerson]

Well, you need to define The loads before: the wetted area under the water pressure distribution, the weight loads, the buoyancy loads, the ballast and center of efforts(lateral), the waves and more and the how the designers combine sustaining the different loads (horizontal vertical and lateral in and over a vessel
The material used in the boat construction and the production methods count too

[reed1v]

My old ranger 26 had bulkhead support with a laminated bridge between to hold up the mast. Inevitably the bridge would slowly compress and the mast sink.

The Luders 36 had a fat compression post and no problems. The mast sat in a metal shoe that sat on solid wood/fiberglass sandwich that sat on the compression post. Very strong. The post sat on top of the keel.

The Westsail 43 was a combination setup: Compression post(old part of mast) and a long boot that held the mast so it could be rotated down on deck. Worked well.