I've just spent the past two days glued to my screen
trying to google
an answer to this, but cannot come up with the goods.. I would greatly appreciate it if the CF community can shed some light on this for me.
Quite simply, I have a bolt-on high aspect fin-keel, lead, with 3/4" stainless keelbolts. Boat is 40', I don't know how much the keel
weighs, but it's certainly a LOT. Boat is originally a racing
design (although I'm converting to a cruiser)
Anyway, I have 13 keelbolts. The story begins two years ago when I had the boat hauled out and had the local shipyard (River Quays, Sydney) replace the washers under the nuts with larger ones...
...several years later, I noticed that one which was in a wet place in the bilge
had rusted solid... after some investigation... much to my amazement, these lovely people had replaced the old stainless washers with galvanized ones!
(No, going back is not an option, asking them for advice is not an option, getting a refund is not an option - this is Australia
, all too hard... "We're licensed shipwrights and have been doing this for 20,000 years... you don't question us, mate.. who do you think you are anyway!?")
After some cursing, I went out to a metal fabricator (Edcon Steel
, great bunch!) and had some very nice washers (plates actually...) fabricated.. IN STAINLESS.. in about 10 minutes (!)... ahem... so here I have a bunch of nice plates, 5mm thick, way better than what I had before for about $60 (which incidentally is 25% less than what those lovely shipyard people charged per hour
over some 10 hours or so to stuff up my boat... ahem..)
OK, now comes the hard part - what torque to apply?!?!?
After googling at length and reading many many many posts on a variety of forums
- these are the only concrete
figures I could find:
Keel Bolt Torques
The short of it is that according to the above, 3/4" keelbolts should be torqued to 250 lbs/ft.
My boat is not a CNC or a Tartan, however these appear to be somewhat authoritative, and I figured the torque figures given above would be standard.. right? (I'm assuming that they use stainless bolts too..)
Anyway, since I don't have much swinging room inside the boat, I invested in a very serious torque multiplier and 250 lbs/ft is a piece of cake - if required, I can even do 900 lbs/ft.
BUT!!!! Just to make sure... I googled some engineering tables etc (as I read during my original googling that bolts should be torqued to 80% of their tensile strength...?) and I found this:
Bolt Torques - Stainless Steel, Brass Aluminum Bolts - Engineer's Handbook
In short - it states that a 3/4" 316 stainless bolt should be torqued to 1582 inch/lbs - which is about 131 lbs/ft - about HALF of what CNC and Tartan recommend.
As a cross-check, I looked up the tensile strength of 3/4" 316 stainless (75,000 lbs), then took 80% of that and threw it into this tool to work out torque:
Which came up with 9000 in/lbs, which is 750 lbs/ft... which intuitively... seems WAY too high.
The other issue here is that reading a little bit further, it appears that if the bolts are lubricated, even less torque is required to achieve the same clamping force.
To add some more complexity just for fun... my bolts/nuts have some burrs etc on the threads, so I believe there's plenty of friction there such that not all of the torque applied is being converted to clamping force.... and since cleaning/rethreading is not possible, I was thinking of lubricating the bolts too...
At the moment, prior to reading the second set of links above, I had already torqued some of the bolts to 250lbs/ft and the bolts certainly didn't snap, but I'm still not at peace - as I'm still not sure if 250 is too high or too low...
So all in all:
a: Is 250 lbs/ft dry correct for my 3/4" bolts?
b: Should I lubricate and torque to 250lbs/ft to overcome thread and nut-to-washer interface friction?
c: Anybody in favour of 750 lbs/ft (dry/lubed?)
d: Why the huge discrepancy between the engineering data and cnc/tartan recommendations?
Thank you all copiously in advance and looking forward to your advice!