Compression Post

DianaOfBurlington · Feb 1, 2016

Roland, you have hit on the very thing I have written about here about 800 times!

Here is the link to the old post-- http://forums.sailboatowners.com/index.php?threads/post-compression-help.148755/page-2#post-996161

Here, for what it's worth, is my blog post-- http://dianaofburlington.blogspot.com/2013/01/the-crucial-duty-of-compression-post.html

Don't know why the print looks so small. But it's from the 'old' site format.

The 'fix' I have been advocating about this is to make a 'bulkhead' of fiberglass athwartships under the sole there, to strengthen the sole itself to hold the mast. This could be made of fiberglass board (like from McMaster-Carr), could span the whole space, could be fitted with an longitudinal blade for stiffness, could be fitted with limber holes, and, most importantly, be tabbed in (structurally) to the sides of the hull to either side. This would spread the load, sufficiently and to the right places. The point I was making was that standing the mast on the keel itself (or transferring the load directly so that in effect it's on the keel itself) is patently bad engineering. Remember the mast is the arrow, the rigging is the bowstring, and the hull is the bow. You're trying to shoot the arrow through the bottom of the keel. If the bow works at all (and it does, in this case), then it's going to apply a load there-- to the bottom of your hull. And guess what the bottom of the hull is holding on?

In short: never compound loads from separate duties in the same direction... especially in a stressed-skin structure.

Even amongst industry leaders, we have all become soft about idea of fiberglass being a wonder material for boats. We think it can do anything, that it doesn't rot, that it's strong, that it's maintenance-free. All of these are myths, not even close to being true in the real world. Stressed-skin boats receive a variety of loads at a variety of points, often without being understood. Your chainplates pull up on either a (rot-prone) plywood bulkhead or a cored fiberglass deck. Your mast step presses down on a cored fiberglass deck. Your keel pulls on a fiberglass eggshell. And fiberglass isn't as good in tensile, especially when point-loaded (equivalent of clevis pin in hole drilled through 'glass) as most people believe. Where's the strength and the support? 'It's fiberglass; it doesn't need it!' --bah.

Even considering maintaining and rebuilding the existing aluminum block under the H30 sole is, technically, a waste of time. Sure; you can make the existing system work again, as apparently it did for three decades or more. But it's a bad system to start with and, when the occasion permits, should be replaced with a proper support structure.

Some rules of thumb:
1. Avoid the installation of structural metal in the bilge.
2. Avoid the installation of structural plywood in the bilge.
3. Avoid the installation of structural mahogany, teak, oak, ash, and other hardwoods in the bilge.
4. Whenever you face a load of any kind, in a stressed-skin boat, spread it as far and as evenly as possible throughout the skin. This means tabbing large-ish supports into wide areas with adequate 'glass work. Point-loading stressed skins is a recipe for failure (distortion, delamination, and, in worst cases, puncture).
5. Stacking things for strength is child's play. Arranging blades, aligned with the axis of the load, is far lighter, stiffer, and thus better. (Anyone else notice that, if using an aluminum I-beam, that Hunter placed it in the wrong attitude? It should have been standing up, not lying on its side!

)

I conclude that the fabricated fiberglass bulkhead, adequately fitted into the space, in line with the load of the proper thickness (wouldn't go less than 1/2" here; 3/4" better) and massively tabbed in-- and then furnished with limber holes to permit water to flow by-- is the only *proper* way to solve this once and for all. This can be easily made up, worked into place, and shimmed or even caulked in 5200 prior to laying up 'glass on it. Do this, and the problem shall not recur in your and my lifetime put together.

Now if only Hunter had done this from the start!

DianaOfBurlington · Feb 2, 2016

Yes, it does make sense, Roland; but as you say, it's basically a better version of what was done before; so we'll just see how well it lasts over the long run. I just get afraid of the very bottom of the boat any time I'm not sure how thick it is.

When I reset my keel I didn't really remove it, just jacked up the boat a little and so the keel bolts did not fall free of the holes. So I really don't know how strong it is. At great effort, since G-10 is very hard to work with (kills every sharpened edge you put to it), I made 'backing blocks' to catch the two biggest keel bolts to either side and set these into 5200 and drew the boat and keel back together. I also made a triangular one to catch the forwardmost bolt (the one under the greatest tension and shock when you hit bottom) and did the same there. Of course I taped over the threads (the tape is still there) to keep the 5200 from bonding to them, which would make impossible any future tweaking of these bolts. I had noticed some (very minor) delamination and cracking along one side, outside, and it scared me. But it's got 2-1/2 tubes of new 5200 in there now; so, less to concern me.

I still would be wary of standing anything on top of the bottom of the boat. As I tried to explain, on a stressed-skin structure, you want to spread the load. If you ever did any more than you've done now, it would be to fabricate a web (thinner now, since it's not holding the compression) and fasten it to one face of your block and 'glass it to the sides of the hull (don't even worry about the bottom edge). Think of it as a bulkhead, a blade, that won't distort vertically. Properly 'glassed to the hull, this will be a rigid structure.

People (like my brother the aerospace engineer) dismiss rigidity like it's a recipe for failure. On a boat I totally disagree. You always have the boat's own movement in the water to take up any excess loads. Therefore, say I (and said my dad), make it as stiff as you can. Anything that's 'too' stiff, that would break under impact, or vibration (neither of which you have to any appreciative degree on a sailboat), will be mitigated by the boat bouncing in the water or heeling over. And you will sail better with snug shrouds and rigidly-fastened bulkheads. From a design, engineering, component, and utility perspective, I don't see any detriment to that.

RobertLangdirect · Feb 2, 2016

If you can, take an after photo at the same distance and angle as the before shot. Thanks!

kito · Feb 3, 2016

Oh no!....I made a new compression post base using 3/4" solid aluminum plate sandwiched between 2 pieces of 6" structural aluminum channel and bolted together. I also bolted a 3/4" aluminum plate to the top of it. I was just going to use Bondo Glass to secure it in the same place as the old rusted out steel I-beam was. I understand getting it off the keel and do like the idea of the joist system tied into the hull rather than on the keel, but I can't keep taking one step forward and 2 steps back. I figured the original design lasted 37 years.....I'll be gone by then

kito · Feb 3, 2016

Roland5048 said:
OK, so this is really a continuation of several other threads. Pulled the glass off the original I-beam and this is what was left. Removed all traces of it and cleaned all glass off. Had a block made of kiln dried Marine Tex (sp?) plywood epoxied and glassed over. Set in with epoxy and gel coated. The yard actually gel coated the keel bolts as seen in the pic. Have since cleaned the bolts of all gel coat. My question to the experts out there is, does this look like it should work and should I have left the stainless bolts and nuts covered in gel coat or did I do the right thing by removing it? What I know about stainless is that it needs exposure to oxygen to retain the stainless quality. Another question to JC. You mentioned in a previous post about not putting downward pressure on the keel. This block sits on an area that seems to be built up from the keel. Could you elaborate a bit further? Thanks.
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Any reason why they set your post in epoxy rather than a softer polyester resin?. It's only to hold it in place. The next guy to change it out will need a jack hammer

. I was glad to see my original steel beam could be chipped out easily with a chisel.

DianaOfBurlington · Feb 3, 2016

I don't have a problem with epoxy; it's the only thing to use if there's wood involved. My solution would be using 'glass board, like G-10 or something easier (redboard, even koosa) and with adequate grinding and fitting the polyester or vinylester will work fine.

My idea for fitting it in with 5200 is only for the purposes of positioning it. You would 'glass over the whole seam all the way around. If you put in enough 5200 you can form a smooth fillet with your thumb, about 3/8" or even 1/2" in radius, that makes a perfect curve over which you can lay 'glass without air bubbles (which will always scope up water, so best not to have them even with 'glass-on-'glass).