Originally posted by hanleyclifford
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- The torque required to drive a 3/4" lag bolt into 8" embedment in lead is more than I'd want to put on a double nut arrangement required with a hanger bolt. My plan includes a properly sized pilot hole, sealant on the threads that I hope will act as a lubricant during installation and a heavy duty impact driver with 4 foot breaker bar and two brutes hanging on to it for the driving operation. Lead is a very gummy material and I expect the operation to be a bear.
- My holding strength calculations were based on 7" (of a 10" lag bolt, 2" loss through the stump bottom and plywood shoe + 1" fudge factor) of the coarse lag threads in contact with lead. I'm not up on the exact ratio of lag threads to machine threads of a hanger bolt but I'm pretty sure I'd lose a few inches on the top to machine threads.
- I haven't seen any 3/4" x 10" stainless hanger bolts available anywhere, not sure they even exist.
I don't recall if I discussed my system before. I'm placing custom stainless castings (aka 'floors', pictures of the casting patterns are attached) in the fiberglass keel stump that transfer the load off the bottom of the stump where the plywood is sandwiched to the turn of the hull at the stump. That area is the strongest portion of the hull due to thickness and curved shape. My stainless lags will bolt through the bottom of the castings through the keel stump into the lead. These are supplemental to the existing bolts.
The holding force calculation of a single lag embedded in 7" of lead exceed the weight of the entire keel by a factor of about two. I'll have 7 lags, if I ever remove my engine, an eighth lag will be installed. The actual holding strength is expected to be greater because the calculation was based on pure lead and external lead keels are cast with 5% antimony for enhanced hardness.
My calculation determined 8800 lbs. holding force per lag. Dividing that by four for a 4:1 safety factor allows 2200 lbs. per lag working load. Seven lags = 15,400 lbs. working load, keel weighs ~4500 lbs. For comparison I had the calculation performed independently by a professional engineer and it came in at 9000 lbs or within 2.2% of my calcs.
I've also included a drawing of cast bronze floors for wood boat construction from Sparkman & Stephens Naval Architects from where I 'borrowed' the concept. It shows the through bolt and lead plug technique Hanley mentioned earlier.
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