Wood screws vs bolts

I've always figured that wood screws have limited or no use for loaded fittings but I'm re-considering for a current application and seeking advice.

The application is to mount several small chainplates that will be used for control blocks for a backstay system on a 31 foot racer.. The chainplates mount vertically on the transom. At the top of each is a shackle conecting it to a 30mm double block. I may be able to mount these with bolts/nuts/backing plate etc but it will be a royal pain to get to. The fasteners that hold the chainplates to the glass transom and internal structure will be 100% in shear.

I'm beginning to think that a 1/4 X 2 - 3" SS screw might be just as strong for this application as the bolt because all of the load is shear. If I do use bolts they'll need to be about 4" long. At the end of the day it seems to me that all of the load will be on about 3 mm just below the head of the screw.

The shear load will be approximately 150 - 200 pounds per block. There are several more highly loaded lines in this backstay system but they are all to be mounted in places where I can easily use bolts, backing plates, washers and nuts.

Am I kidding myself or would this use of screws rather than bolts for the lighter loaded chainplates work?

Follow up question...

If screws can work should I consider drilling an oversize hole, filling it with epoxy mud and then drilling a pilot hole when the epoxy cures?

 

My experience is that screws or even nails can hold huge loads perpendicular tho their axis. You can even give the screws a few deegres angle upwards so they workmlike "hooks".

200 punds is almost nothing
I can safely stand on a plank supported by a 3.5mm nail!

 

Try screwing a couple of 2x together. You'll never pull em apart. Try putting a screw into a 2x leaving a 1/2" of the head exposed. Whack it with a hammer side to side a few times, I'll bet the head snaps off, there's your shear.

 

I think two pieces hold tigether with screws are locked because of friction. So the srew or the wood arond the scruew is not subject to a lot of shear stresses. In this case, the screws will keep the pressure high enough to lock the plates with friction, and nothing is pulling the plates out from the hull.
If you add some bedding compund it will work even better.

If you wack the head of a screw with a hammer and the screw is 12mm out of the wood, isn't it also a large bending moment?
In your example, try screwing a couple of 2x together, the fasten one of them to the floor and try to move the other one?

 

Screwed & glued is the strongest.

Perhaps using a adheasive bedding compound like 3M 5200 would help.

With a new removal product it can (slowly) be deglued if ever necessary.

FAST FRED

 

Chadler: You are confusing shear force with repeated bending fatigue. If the fitting is well set, the fasteners won't bend. If they did, they would eventually break wheter they are screws, bolts or nails.

If you use screws, use wood screws and not self-tapping. The first have a tapered shank that jams in the wood and prevents it from moving. Self-tappers are threaded to the head, and the shank is much smaller.

 

I've been trying to post this for three days, d*&m error messages. lag bolts, the strength of bolts and the ease of screws.

Steve

 

I've seen a very rare, few screwed chain plate assemblies, most being bolted and having substantial packing plates to distribute the loads to the hull (which is the whole point). The few I have seen that were bolted, were in fact, lagged to very heavy hanging knees or frames and I still had a problem with the method.

Personally, I'd through bolt, with a backing plate and bond the holes in the transom with epoxy. It's a removable system that doesn't compromises the ability of the re-attachment (stripped screw or lag threads) and bolts/backing plate systems are stronger then screws or lags. The bonded fastener holes will prevent moisture getting past the bedding and it's the preferred method of custom/production builders and repair experts.