moving support for deck stepped mast - SailNet Community
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post #1 of 12 Old 11-30-2009 Thread Starter
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moving support for deck stepped mast

My mast is stepped on the deck, and there is a pole, about 4 by 3 inch, that goes from the cabin floor to the top of the cabin under where the mast is stepped. This pole is directly in the middle of where i want to put a removable bunk and also in the middle of the seating area and is a bit of a nuisance..

How can i find out the correct way to "jackpost" this load ?
I dont want to add any weight if possible.

The layout works well for the two new posts that will carry the load to the floor, to put one post up by the forward bulkhead and the other post back by the transition from gally to main cabin, a span of about 6 ft 6 inches. I would like to use an aluminmum beam or rectangular tube; across the cabin celing to take the mast load to the two jack posts ;as it would save weight.

There is no keel as this is a trimaran and in the bilge the supporting framework looks similar to where the load is now to where the two new places for the load will move.

Thanks for any help .
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post #2 of 12 Old 11-30-2009
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That 'support' piece is called a compression post as it takes the compressive load of the mast an transmits the load to the keel. I am no engineer but I would speculate that what you are thinking about doing will never be quite as stiff and strong as a single compression post located under the foot of your mast as it is now. If it were my boat I would live with the inconvenience of the compression post where it is and try to think of other ways to adapt to it.
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post #3 of 12 Old 11-30-2009
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There's no way I would move a compression post or sail a boat that had it's post 'relocated'.

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post #4 of 12 Old 11-30-2009
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6'-6" is a very long span for a beam supporting a mast. Typically beams supporting a mast would be no longer than a width of a passageway opening (2'-4" at max). On my 38 foot boat my mast comes down on a welded aluminum beam. It is 6" deep by 8 wide made from 5/16 plate. That beam only spans about 14" -16" clear.

You would be way ahead of the game to try to span across the boat to the hull, (rather than longitudinallly) which on a trimarran would typically be a much smaller span. If I had to tackle this I would ideally build a pair of ring frames which would transmit the loads to all sides. If it were me, I would probably build them out of fiberglass in order to get a sufficient bond to be able to transmit the loads into the hull and deck. I would also use a high modulus material like carbon fiber or kevlar for the bottom flange of the beam as a way to minimize deflection.

Normally, masts on Trimarrans come down on one of the cross beams (aka) that tie the hulls together allowing all of the transverse and bending loads to be taken by the beams rather than transmitted through the hulls to the beam.

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post #5 of 12 Old 11-30-2009
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Here's an example of an off-set compression post, in case it might give you some ideas:

Pacific Seacraft Flicka Interior


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post #6 of 12 Old 11-30-2009
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The Flicka's "offset compression post" looks more like a convenient handhold that was stuck on after the real compression loads were dealt with by the cabintop arch that is so evident in the photo. Engineers with licenses that they could lose if they got it wrong probably figured out how big the arch needed to be and what it should be made of. A mast resting on a 6' I-beam sounds like a house of cards waiting to be knocked over.
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post #7 of 12 Old 12-01-2009 Thread Starter
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thanks for the replies
yes going across would be better idea
sounds like not moving it is best
but if i ever have an engineer friend onboard will get some free advice

the guy that designed the boat put it in that awkward position for a reason i guess

thanks again
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post #8 of 12 Old 12-02-2009
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Often an awkward position is chosen for production reasons, not because it is the best or most usable design. Much quicker and cheaper all around to throw a simple post in directly under the mast than to install an arch and it's supporting structures. Look at different boats by the same manufacturer, see what they have inside. Many manufacturers make changes during production, or have optional interior layouts on the same boat. And many have different models that share the same hull. For example, the Ariel and Commander by Pearson use the same hull. The Ariel has a bulkhead and beam, while the commander has a compression post. Either could be modified to use the other type of construction with no loss of strength. Ken.
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post #9 of 12 Old 12-02-2009
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It is definitely possible to move the post. It is unlikely that you will find a solution that does not increase the mass of your boat, eliminating the post will most likely make your boat heavier. If you have access to titanium I-Beam, you might be able to keep the weight down, but it will be extremely expensive. Aluminum I-beam will not actually be much lighter than steel in this application. Steel is stronger than aluminum. Aluminum is great for some things. It is easier to machine, cheaper, and lighter than steel, but it is definitely weaker than steel. An aluminum beam designed for this job will be bulkier and likely the same weight or heavier than a steel beam. This is why I suggested Ti. Ti will result in the lightest, smallest beam that will do the job and have plenty of extra strength.

Not moving the post is my preferred solution. Another possibility is to build a special post. One that begins and ends in the same place as the existing one, but arches like a dutch boom, or splits in two and rejoins itself later in the bilge. It is advisable to make this post either extremely overbuilt, or hire an engineer to make sure it is of sufficient stiffness and strength.

The new solution will need to be sized for stiffness, not strength. That means it will have to be larger than it would be if sized for strength. Use a stiff material, fiberglass or other polymers would probably be the worst choice.

In short, just leave the post where it is.
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post #10 of 12 Old 12-02-2009 Thread Starter
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chrome moly steel might be good, light and rigid

I will be refitting the boat over the next two years
And hope to spend some months each winter living aboard in the caribbean once i retire

maybe the beam could be temporalily relocated if we were at anchor for a week or two , no sailing , then when it was time to move ,re - insert the direct connected compression post.

wonder what the stress is on that compression post between: no sails no wind and full sails with 30 knots wind ?

thanks
peter
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