Stainless bolts strong enough for mast support?

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BabaYaga

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Hi all,

I would be grateful if some of you would care to comment on a project I am pondering, especially concerning the strength of stainless steel in this application.
First some background:

The mast compression post in my 9 m masthead sloop is a solid mahogany beam (square cross section, circa 65 x 65 mm) that is screwed and glued to the forward face of the main bulkhead (plywood circa 15 mm thick).
The post stands on a GRP laminate that covers the internal ballast lead keel, some 150 mm below the floorboards.
The top end of the compression post supports the coach roof under where the mast foot is situated. The coach roof in made of wood, with laminated beams from side to side.
Over the years the compression post has given way to the downward force of the mast, so that the center section of the bulkhead has sunk some 5 to 7 mm.
The cause is most likely that the compression post has become a little soft in the end grain, as it stands on the keel laminate where bilge water has been present from time to time.
I now intend to cut away the bottom section of the damaged post, maybe 50 mm, and insert something that can

1) be precisely adjusted to the available space (that is the 50 mm cut away plus the 5 – 7 mm that the post needs to be raised).

2) be left in place and be able to take the compression load from the mast for another 30 years (at least).

What I am contemplating is in effect a jack that is very strong, durable and with a footprint that is not much bigger than the post it replaces.
My idea would be to use 10 mm stainless steel plate and M 12 stainless steel bolts and nuts and construct something along the lines of the photo attached (which I have stumbled upon on the internet).

First question: Regardless of solution, would it be sufficient to expect a compression load at 2 x boat displacement (in my case circa 8 tons)?

Second question: If so, would four stainless steel M 12 bolts, threaded through 10 mm stainless steel plate (and possibly backed by M 12 nuts) be able to safely take that load?

Thank you for reading this far. Your comment would be most appreciated.

http://www.ybw.com/forums/attachment.php?attachmentid=15675&stc=1&d=1327063288
 
Last edited:
It seems a bit over the top to me. The owner of the boat in the picture seems to have cut a much bigger section out than you propose. Can't you repair it with wood? I'm no woodworker but that doesn't seem a very difficult repair to me, a replacement hardwood section, two thin opposing wedges so it can be fitted tightly and clamped while the glue dries and then plenty of waterproofing. I stand ready to be shot down in flames :D
 
I agree with Creaky Decks - use wood, Iroko or something similar which can tolerate some dampness. Certainly cut off the base - assuming there is good access and you can make a square cut. Then put a wood pad as large as possible under the post, if possible jacking up the deck with an Acrow prop? - if it will jack up then try and knock a piece of wood in of the correct thickness then release the Acrow. Or do as said knock in wedges which after trialling could be cut to the correct length, covered with wood glue and knocked in flush. Perhaps with small pieces of wood glued & pinned around the mast base to hold it all together.
I think the stainless method is OTT and expensive.
 
Whether it is over the top or not, any design needs to avoid point loading of the wood's end grain. In the picture, the load appears to be being transferred into a wedge shape at the bottom of the compression post and in time it will at least distort if not split the post. Although probably adequate to the task, it is not good practice to bear the load on a screw thread of the form used on a bolt, notice how on a jack the screwthread is square in section. The suggestion of opposing wedges (quoins) is a good one and could conceivably be built with screws driving the wedges against each other.

Although you are looking at jacking the base of the compression post, with the benefit of raising the base out of any bilge water, you may need to pay equal attention to the end of the post beneath the deckhead. A similar system of quoins could be fitted there, albeit it would look neater in wood. The post itself is bonded to the bulkhead, giving it much greater stiffness than its cross section would otherwise offer, so you only want to take up the "slack" at either end or you will distort the bulkhead. I have seen a number of designs in which the point loading on the bulkhead has been distributed by an arched metal plate screwed across the top edge of the bulkhead, thus distributing the load.

A rigger may be able to calculate the loading based on the dimensions of your rig, but it is not directly proportional to the displacement of the boat.

Rob.
 
PetiteFleur said:
I agree with Creaky Decks - use wood, Iroko or something similar which can tolerate some dampness. Certainly cut off the base - assuming there is good access and you can make a square cut. Then put a wood pad as large as possible under the post, if possible jacking up the deck with an Acrow prop? - if it will jack up then try and knock a piece of wood in of the correct thickness then release the Acrow. Or do as said knock in wedges which after trialling could be cut to the correct length, covered with wood glue and knocked in flush. Perhaps with small pieces of wood glued & pinned around the mast base to hold it all together.
I think the stainless method is OTT and expensive.
Click to expand...

Or even replace it with an Acrow prop full stop - you could always box it in with some ply if you wanted to make it look neat... :D
 
I think the stainless bolt proposal is unnecessarily hard work. When the deck-level mast step deteriorated on my boat, it was rebuilt with load-bearing resin (perhaps called Chockfast??) which is extremely strong. You could cut off the weak end of your post, make up a little dam of plywood, then pour resin in and let it set.
 
Waa-aay over the top.
But to answer one question, 4x M12 or even M10 are miles stronger than necessary: they'd hardly be doing anything since there are no lateral loads so speak of. (OK, there might be if you were T-boned by a trawler, but then I suspect your concerns might be elsewhere.)
 
The wings of Beech Kingairs are held on by two bolts of about that size - I wouldn't worry too much!
 
I think some of the respondants have missed the point.

If I read it right, the OP wants to jack up the remains of the post some 5mm to make up where it has dropped. Thus the threads have to take the full compresive load , not just locate the gismo.

That is why wooden blocks or resin would not do the job.

No engineer but I would think his proposal would be fine.
 
Are you meaning to have the bolts holding down a plate, or the bolts adjustable so that they can be screwed down through the block to lift it up off a base plate?
I would have thought that you'd be better looking for a permanent fixed solution rather than an adjustable one. If its taken 30 years for the last one to move 5mm then a well engineered solution will take the boat to 60 years old.... and I'd have thought other things would be failing by then.
Mast compression posts are often made of stainless tube now. :)
You're not thinking of a jack so you can do it with the mast still up are you? I wouldn't take away the support with the load still on.
 
Personally I would try cuting a pair of folding wedges out of some oak with a shollow angle. Hammer them in and see if the will generate the 5-7mm lift. If so take out put back an un damaged pair with suitable adhesive (epoxy, extramite, resoucinal), leave to dry and trim. I am sure the bolts are strong enough on the basis they will compress the wood before breaking? I would also like to see the load on the wooden support spread better by forming a cup around the end.

I guess whatever you do will be better than you have now?

Good luck and think how much better you will feel this season bashing in to a six.....
 
Thanks everyone for your replies, quite a few very helpful suggestions.
I would like to comment and clarify a bit further.

I agree that the construction in picture that I posted is somewhat puzzling, as the compression post does not seem to rest on a flat surface, which is desirable to spread the load.
What I have in mind is something that resembles the lower part – i.e. simply two thick s/s steel plates, one upon the other, the top one with threaded holes in each corner to take the bolts. By turning the bolts the distance between the plates increases. Possibly a nut between the plates, to lock the top plate at the desired height.

I take the point that bolt treads are not ideal and that a square tread profile, as on a real jack, would be better. But might be strong enough at 12mm perhaps...?
As for over-engineering and OTT I don't quite agree. Two plates, four holes, four nuts & bolts – pretty simple I think. I'm more worried by the opinion of pvb that my proposal is not a good engineering solution – feel free to expand if you like.

The suggestions about using opposing shallow wedges to raise the post are very good and something I have already thought about. What makes me reluctant is that I would like to avoid using wood down there and also that I would have to glue them in place, giving me just one chance to get the height of the post right.

But I will certainly look more into this alternative and also the load bearing resin route.

And no, I will not do anything with the mast in place.

Here is a picture of the compression post foot as it looks today. Thanks again everyone.

http://www.ybw.com/forums/attachment.php?attachmentid=15689&stc=1&d=1327151804
 
I think if the bolts were in compression there'd be risk of them bending. The jack thread you're referring is an Acme Thread. Is there anyway you can use one large bolt I'm thinking of around 30mm, sandwiched between the 2 plates?
 
colhel said:
I think if the bolts were in compression there'd be risk of them bending.
Click to expand...

I agree bolts are supposed to be used in tension only, they are not designed for compression or shear applications, they work by compressing the two joining pieces together, the friction of the mating faces carries the load the bolt only remains in tension. Bolts fail when they become loose as they become subject to shear or compression.

The solution shown seems seriously over engineered for your case. If the bottom of your post is only soft for the bottom 10mm or so, can't you simply use a couple of stainless plates as shims, held in place with stainless bolts? If you need to do this again in the future you can simply add another shim.

I would use a scissor jack with a post on top instead of an acro prop, you can exert far more pressure more accurately than with an acro.
 
I have jacked up a sagging mast support with an acro whilst making good. They're pretty good and once taking the load can be adjusted to quite a fine precision by tapping the handle round with a mallet. If you're going to use a temporary second post to transfer the load, then I'd prefer a hydraulic bottle jack to a scissor jack.

As an aside, I knew one boat owner who found that the bulkhead and attached post flexed when really bashing into a choppy sea ( first time he'd gone down to use the heads in such conditions). To beef it up, he screwed a large stainless angle the length of the post.

I couldn't make out from the photo how much space you have around the base of the post to construct your new support. I'm guessing its below the cabin sole, so out of sight? If there is room and you have the capability to fabricate it, a box could be constructed containing one fixed wedge and one with a tapped hole in the narrow edge so a screw could draw it under the fixed wedge to raise the overall height. A little complex, maybe, but neat.

Rob.
 
I'm not an engineer either but my boat has a wooden compression post backed up by a 2" diam stainless steel post alongside it. Not sure which is actually carrying the mast load, but I reckon either could quite easily.
 
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