Bulkhead vs compression post

[tinkrman69]

As mentioned before, it appears that you don't want to hear the right answer. But maybe you don't understand the full scope of the situation.

In a boat all the bulkheads,,,both for compartments, cabinets and seating and births... are part of the frame work the keeps the shape to the hull. hence a bulkhead used as compression post for a deck stepped mast. If you want to redesign the floor plan of the boat then you'll have to figure in redesigning the support points. As mentioned the shrouds will pull the hull in and the deck up. so by moving a bulkhead that was supporting those point,,,mast step, hull at the chainplates and also the hull below the water line.... you'll have to redesign new ways to support those points.

It's really common sense, but common sense, takes patience and without an open mind.......well that I'd call senseless

So if you'll take the time to observe "why" an engineer built something one way you'll be able to modify it to fit your plan. I've gutted and redesigned whole interiors,,,always keeping in mind the original support theme and sailed many rough seas with no problem. I've also seen people move stuff around and fracture hulls. one such boat sunk in a matter of minutes. And that was in a squall that passed in 15 minutes in Tampa bay. that was back in the early '70's it's probably why I'll never own a,,,,,,,no I won't go there ;-)

Take yer time man and think it through with an open mind.

PEACE LUV & HAPPINESS
Lance

[Sailorbob8599]

In the construction of my particular boat, a 'Passage 24', a full keeled coastal cruiser (cut away forefoot) ... I used a 2 x 2 laminated overhead beam under a 1" foam cored cabin top, over a 1" plywood main bulkhead, having solid 1 x 2 teak half-posts, (through bolted) on fore and aft sides to beef up the plywood B'head, directly under the spar. It is considered 'overbuilt' for this vessel for added strength, security and peace of mind.
The spar is 4" x 6" x 30' oval Selden aluminum, with a 12' boom carrying 165 sq/ft of sail. It is Cutter rigged (2 head-sails).

[thedude0]

Reason I asked was since my boat... which is generally considered solidly built suffered from mast compression due to rotten core beneath the mast...even though there are 4 bulkheads beneath it which go nearly 4/5 of the way across but no compression post ...compared to modern boats the downside of having bulkheads is light and room...i see some photos of other boats of the same model..who have got rid of the bulk heads and just put a post which frees up a lot of room... many boats seem to do well with just a compression post... so i was wondering whether a solid post would suffice...thanks for the replies...

[thedude0]

there are many aspects of a boat which as a result of time may show that they could have been built in a different way...some designers may have designed some aspects with very good intention but experience then may prove that it could have been designed better or it was over built...a look at bluewaterboats.org always lists some shortcommings/weaknesses of a boat which i am sure that the designer did not intend them to feature in the design...furthermore in the past many a fine boat where built by builders who where not architects but had lots of experience first hand...which is the case here with users of the forum...also many designers had different points of view on what is best so even just by knowing the intentions does not make it the best solution...so the designer is the starting point but not a means to an end in my opinion...plus that is a taken for granted reply which would not have generated different points of view..

[Sandero]

The compression post is placed directly under the mast which is a point load with considerable axial force because of the tensioned standing rigging. The mast is like an arrow and the hull is like the bow!

The post is on the very strong center spine of the boat where the keel is anchored.

Bulkheads are stiffening plates or gussets to hold the hull shape... sort of preventing the bow from ... bowing.

A bulkhead can only replace a compression post when it can transfer the concentrated point load of the mast (via the step) laterally to the entire bulkhead plate. Typically a thin plate does not do well with a concentrated load... experiencing location compression failure. An integral stiff beam at the top of the bulkhead plate may be able to transfer the axial mast load to the entire bulkhead. Of course penetration/passage fore and aft becomes an issue.

A flexible hull.. requires bulkheads for stiffness to hold the hull form. That is NOT what a compression post does. It transfers the axially mast loads to the stiff bottom part of the hull.

[Sandero]

You can begin with a 2 D analysis but a column / post is need to be stiff in all axes. It WILL buckle if does in the weakest axis. And for a sailboat that is athwartship. So you will often see a wood post composite with an athwartship bulkhead because the athwartship axis is the one usually more light loaded... the stanging rigging. Note that the sail forces are usually to one side because the main sail is carried to one side of the mast.. and this is why the lee shrouds slacken a bit.. the mast is actually falling off a bit to lee.

Bulkheads are usually with an athwartship orientation. All the bulkheads are composite with the mast in maintaining hull form. That's what they are there for... resisting lateral athwartship compression loads. Post is for axial loads!

[denverd0n]

Originally Posted by thedude0

...so the designer is the starting point but not a means to an end in my opinion...

I understand what you are saying, but the real point here is that there are a hundred different design details that are involved in determining what can, and cannot, work in a particular boat. Depending on all the other factors, one or the other--a bulkhead or a compression post--might be the ideal solution, or a complete disaster. Just asking which is stronger doesn't tell you anything at all about what will work in YOUR boat. Even if you think the designer made mistakes, you still have to take into consideration all of the design decisions that he made when contemplating a change like this.

[reed1v]

Originally Posted by Sandero

The compression post is placed directly under the mast which is a point load with considerable axial force because of the tensioned standing rigging. The mast is like an arrow and the hull is like the bow!

The post is on the very strong center spine of the boat where the keel is anchored.

Bulkheads are stiffening plates or gussets to hold the hull shape... sort of preventing the bow from ... bowing.

A bulkhead can only replace a compression post when it can transfer the concentrated point load of the mast (via the step) laterally to the entire bulkhead plate. Typically a thin plate does not do well with a concentrated load... experiencing location compression failure. An integral stiff beam at the top of the bulkhead plate may be able to transfer the axial mast load to the entire bulkhead. Of course penetration/passage fore and aft becomes an issue.

A flexible hull.. requires bulkheads for stiffness to hold the hull form. That is NOT what a compression post does. It transfers the axially mast loads to the stiff bottom part of the hull.

Best analogy is to a bow and arrow. The arrow being the mast, the bow the hull, and the stays the string. As the arrow moves backwards towards the bow(compression), the tension forces distribute throughout the entire bow. Boat is just a big 3D version. Unless those forces are distributed well, you will get uneven flexing, and thus a broken bow, a sunk boat, and worse.

BTW: before a commercially developed, mass production sailboat can be attempted, the designs are reviewed by an underwriter for insurance and liability purposes. Some designs are robust enough to withstand altering key components; others are fragile like an egg and one change will sink the ship. Westsails you can do what you want with; J-boats need be left alone.

[Sandero]

You need to do a complete structural analysis if you intend to alter the structure. You simply cannot replace a post with a bulkhead. PERIOD end of STORY. Now the caveat would be the stiffness of the hull. Assuming perfect stiffness.. there is no need for bulkheads and if the mast is deck stepped you need to prevent the deck from sagging under the point concentrated load of the mast. Solution is a post directly under the mast.

I wouldn't attempt structural alterations without consulting a good marine architect/engineer.

[cal40john]

Originally Posted by thedude0

Reason I asked was since my boat... ~ suffered from mast compression due to rotten core beneath the mast... ~

What you described as the failure can be completely independent from boat design problems. Rotten core is due to water intrusion. Did anyone drill holes in the deck and not properly seal them, including holes for wiring inside the mast?

If lots of other boats of the same design have deck failure then it becomes more likely that there was insufficient support allowing the deck to flex enough to crack and allow water intrusion.

[reed1v]

Originally Posted by Sandero

You need to do a complete structural analysis if you intend to alter the structure. You simply cannot replace a post with a bulkhead. PERIOD end of STORY. Now the caveat would be the stiffness of the hull. Assuming perfect stiffness.. there is no need for bulkheads and if the mast is deck stepped you need to prevent the deck from sagging under the point concentrated load of the mast. Solution is a post directly under the mast.

I wouldn't attempt structural alterations without consulting a good marine architect/engineer.

Not true, a lot of boats are built with a bridge that distributes the load to the opposing bulkheads. Works well on Rangers, Cals, etc. Over time, the bridge may need to be replaced due to continual flexing if of wood, or rust if of metal where water intrusion occurs, etc. A bridge is actually better than a post for sailboats under 30 feet since the loads are distributed to the bulkheads and then to the hull.

One does not need to consult with an architect nor engineer. Simply do some research about replacing the bridge on one of the thousands of boat sites; or just go down to your local boatyard and ask what they would advise. Gee, this is not rocket science.

[valhalla360]

Originally Posted by 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

You are asking a question and then telling us we can't give you the correct answer.

If you just want to slap a post in and hope for the best at least leave friends and family ashore while you test it in rough weather. It might be fine, it might punch a hole in he bottom first stiff breeze. No way to know without a good analysis or destructive testing.

[reed1v]

Originally Posted by valhalla360

You are asking a question and then telling us we can't give you the correct answer.

If you just want to slap a post in and hope for the best at least leave friends and family ashore while you test it in rough weather. It might be fine, it might punch a hole in he bottom first stiff breeze. No way to know without a good analysis or destructive testing.

Correct. Depends on what the boat was designed to carry. Some small sailboats are not design to carry mast loads down to the bilge area(centerboards, thin fin keels, thin hulls); others are not designed for deck stepped masts(thin bulkheads, no bulkheads in mast area but rather knees, etc).