Compression Post

[Cats Paw]

 Looking forward to start working on my project boat {1980 --16}I will be having to replace the original post due to decay and softness on the  bottom end along with the platform in the forward berth. I am considering using heavy wall 3 inch rigid PVC pipe as a replacement.  I can insert treated wood plugs into each end to allow for fastening to the tabernacle at the top the bolts at the end. I would be thankfull for your input our thoughts.

[NateD]

Why PVC? A wood (or metal, aluminum?) post would take up less space than 3" PVC.

[Cats Paw]

Well than maybe I should downsize to 2 1/2" dia. as the original post. Pipe form is the choice for structural strength.

[Jon898]

I'd echo NateD...why PVC?  Personally, I'd look for a piece of aluminum mast (or boom) extrusion.  Who knows, if you hang around with racing types, you may even find enough "for free" from a broken mast, boom, whisker or spinaker pole, potentially with end fittings you could adapt.  IMO a lot more "yottie" than risk having everyone wonder why you have that big drain from the missing foredeck head in the middle of the cabin  .

[nies]

Wood is great and easy to work with,  the strength of wood is all you need and wood will last forever if kept dry, nice look and cheap too.........nies

[NateD]

A while ago I tried figuring out what the compression load would actually be on a CP16, but couldn't find a good rule of thumb and didn't want to invest the time to figure it out the hard way.

My 1980 CP16 was missing the compression post, and I never added one back to it. I didn't notice the cabin trunk flexing at all in heavy winds, although some people here say there is some downward pressure on their posts while sailing, so there is probably some load being transferred, but I would guess it is a pretty small amount when you start talking actual pounds of pressure.

[skip1930]

NateD is probability correct ... not a lot of compression on the post.

Forget the PVC schedule 80 pipe. [If you insist, fill the pipe with expanding foam like Great Stuff. Can't crush a tube. That's why submarines are perfectly round titanium rings where it counts.]

Wood allows for a flattened area to mate with the lower cross frame with lag screws.
It's important that the bottom of the post NOT sit on the bottom of the boat's keel.

AND; the post is in no way attached to the tabernacle.

skip.

[nies]

Why not sit on keel Skip?........I filled the space between the bottom of post and keel with a small block...........nies

[skip1930]

Something about expansion and ease of fit as not all hull and deck/cabins have the same distance and who wants to waste a lot of time fitting the posts?
Slap her up against the cabin top around the area of the tabernacle and fasten the bottom of the post with two lag screws. Give yourself some wiggle room.

That's what I heard.

skip.

[Cats Paw]

A stick of 2-1/2 x 2-1/2 treated timber will probably work best. Also would be a good mounting surface to hang the Fire  Extinguisher that I have planned to install.   I recall that the post was fastened at the topside with a single brass wood screw that was under the tabernacle. I had removed the tabernacle in order to fill and restore the mounting area as the original and proper screws were pulled out and the tabernacle was than re- installed with bolt and nuts  through the deck by the PO. Not a good thing for a breakaway if needed if a de-masting would occur.

[skip1930]

" Also would be a good mounting surface to hang the Fire  Extinguisher "

Humm  Would have to reach into a cabin fire to acquire the extinguisher ?

Might be better hung from the overhead near the drop boards. And another under the settee covers if the bait has them.

skip.

[Cats Paw]

Well the way I see it ... if a fire were to be that extreme in the cabin on a 16.. than forget the extinguisher and  abandon ship. I would be more concerned with the outboard motor going up in flame.

[kickingbug1]

    i would use oak or ash and keep the fire extinguisher (it is probably required by the coast guard or local conservation authorities.

[Cats Paw]

    i would use oak or ash and keep the fire extinguisher (it is probably required by the coast guard or local conservation authorities.

Thanks......That's a good suggestion. I will get by the local building supply and see whats available. I would soften the squared edges of the square. I plan to coat it with Sikkens along with the teak after it is cleaned up.

I can see where the downward force of the mast when the bow comes off a wave hard and the force would not be transferred below putting all the stress on the cabin top without the post in position.

[NateD]

After you posted this question I tried sitting down and doing the math to figure out what the actual loads on the mast step are, but once again, the math has beat me and I gave up. However, as a rough estimate, you add together the tension that you've put on each stay and shroud to get the resting force, then you add 50% for the force the boat will experience when sailing healed at 30 degrees. It isn't anywhere near an exact calculation, but it is a rough estimate.

I don't have a Loos gauge, but I'm guessing shroud/stay tension is probably around 100 pounds each. So that is 300 pounds of pressure on the mast step when you''re at the dock, and 400 under sail. That's my guess, some day I'll try to do the serious math again.....