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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 08-19-2006
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Compression Post?

I own an Ajax 28 and would like to install a compression post beneath the deck-stepped mast. Unfortunately, my boat does not have a strongback supporting the roof loads, nor longitudinal and transverse stringers providing rigidity above the bolt-on lead keel. My main bulkhead is nicely tabbed to the hull but not tabbed to the roof (or a strongback). This bulkhead simply slides into a groove moulded into the roof liner. Given these design details, would a compression post on its own transfer mast loads effectively from the cabin roof to the keel? In deck-stepped designs does the downward force of the mast need to be spread eveningly over the cabinroof/strongback/bulkhead/hull structure (that I am missing) or can it be simply transferred to the keelbolts via a compression post? Would point-loading the hull at the keel cause the shroud tension to banana the hull?
Thanks in advance!
Dave
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Old 08-20-2006
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Old Pearson 26 (Alberg) => 2" scd-40 alum pipe

Old Pearson 26 (Alberg) => 2" scd-40 alum pipe
Not much else...
Did have a 3/4" block under the mast step out side and a 1-1/2" block on top of the pipe inside. The bottom of the pipe sat in a smallish block on top of the ballast glassed into the keel.
Spreading the load out and tying things togeather is almost always preferable but I describe the Pearson to show what is minimaly acceptable.

Best,
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Old 08-20-2006
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Compression Post?

Thanks, SailandOar.
Was the 2inch aluminum pipe attached to a bulkhead so it could not bend? Or was it free of the bulkhead so its motion could not crack the bulkhead? Would you recommend aluminum, st.steel, carbonfibre, etc?
Dave
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Old 08-20-2006
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If I were you, I would install the compression post, but add a plate of some sort on both the top and bottom of the post to spread the load out. Concentrating that kind of load is generally a really bad idea, and may cause worse problems than you already have.

Stainless steel is probably the easiest to work with, and far better than Aluminum, which has much more trouble with metal fatigue. Carbon fibre is probably the most difficult to work with and expensive of the materials you've listed. Wood is also a possibility, but my preference would be SS, Al, Carbon Fibre, and then wood in order of preference.
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Last edited by sailingdog; 08-20-2006 at 12:52 AM.
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Old 08-20-2006
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Free standing / unsupported

The alum pipe was unsupported except were it went through the floor and then about another 7" or 8" down to the top of the bilge ballast. Total lenght of the pipe was aprox. 60"

Look for a forum of Pearson 26' commander and ariels and throw out a question to get the exact details from them. I had a 1960's vintage 26' pearson commander (Commander is the big cockpit version) and it has been 5 or 6 years since I thought about it. If I can find a picture I will post it this week.

If you can position the mast step near or on a bulkhead and provide a horizontal frame/strong-back to carry the load to the bulkhead and some vertical frames/stiffeners attached to the bulkhead you will be in good shape.

If you posted some pictures or a more detailed writeup/description of the project you would get MUCH better feedback.
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Old 08-20-2006
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"Would point-loading the hull at the keel cause the shroud tension to banana the hull? " i don't think adding a compression post would change the loads THAT radically. Banana problems come from too much tension, regardless of the mast arrangement.

Remember that a boat is a moving living thing, ideally you need to slack off all rigging (fore/aft/and shrouds) and let the deck RISE as much as it can. Then install the compression post, figure out a way to keep it plumb in column (by attaching it to bulkheads, or using collars/seats at both ends, etc.) and then...ideally you need to make the mast long enough so it is also firmly compressed and held under tension when the rigging is properly all tensioned. If the post is too long, it will be creating uneven stresses. Too short, and it may bounce out if the deck rises (dropping from a wave) or just not provide as much support a it could.

Unless your boat is known to have deck problems, you might just leave out the post and trust the builder to have built the deck well enough.

On post materials...almost anything can do for that much boat. A plain 2x4 in compression can hold up something like 1-2 tons, so a post or beam selected from good clear wood, even pine or redwood, would work. As would an aluminum pipe. I've never seen a stainless compression post, seems like a waste of money to me. But whatever you use, you need to make sure the base of it will stay dry, or be waterproofed, and that it is held in place, plumb and unable to bounce out. Probably means some type of collar at both ends, and those would also spread you loads, at least for the upper one.

Or are you prepping the boat for some typhoon sailing?
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Old 08-20-2006
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If you decide to do it, I would second stainless as preferable to aluminium. My boat came fitted with stainless (by the way).

The reason against Al is that even if tempered to T6, it still disturbs far more easily then stainless. Disturbing is a blacksmithing term for compression loading that causes the pipe to mushroom and get shorter, but develop thicker walls in some spots or in other ways redistribute the metal. For a compression post, it does not need to get much shorter before it is doing virtually nothing at all.

Sasha
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The other thing is that aluminum starts to fatigue under repeated stress. Stainless steel does not fatigue as easily, as it has to be stressed beyond a certain limit before fatigue starts to set in.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 08-20-2006
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Pearson 26' Mast compression post (vintage-1968)

What is GOOD ENOUGH?
Pic is rotated, boat was upright. This was a beat up old boat when I got it and I rode it hard. Had a trysail tack on the mast and used it a few times, fell off a wave in the Chesapeake near the mouth of the Potomac when motor sailing hard in a strong breeze and STEEP CHOP and that old skinny post managed to hang in there.
If you plan on rolling the boat off Terra del Fuego then by all means go with a heavier stainless pipe maybe a bit bigger and larger pad on the top. Does not even look to be 2" OD and as I think back on gripping the pipe it seems that it may have been only 1.5" or 1.75" OD. I said in the first posting that it was Schd. 40 but I really don't know. I seem to remember the sound of tapping it and I was not solid and didn't seem extra thick wall.

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Old 08-24-2006
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Compression Post?

Thanks everyone for your advice. HeloSailor, typhoon sailing sounds like fun but I'm having difficulty keeping the correct shroud tension at the dock! It seems the deckstepped mast is gravitating toward the keel. If one decides to cut into the cabin roof liner is it possible to fibreglass over one's head and later match the liner gel coat aesthetics?
Dave
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