Lifting gear

[Bayboat]

Here's one for the engineers in the group. I would like to have a lifting ring at the balance point instead of using slings to haul the boat. How would it work to drill a hole down through the wood keel and deadwood and into the lead ballast; the hole would be appropriate diameter to tap for 3/4" NC threads in the lead; insert a threaded 3/4" bronze or stainless rod and turn it in; at the top, make threads for a standard 3/4" galvanized eye nut backed up by a jam nut.
The drill bit and the tap would be welded to steel rod of the appropriate diameter and lubricated with kerosene.

The questions are (1) is this strong enough to lift the boat, about 1700 lbs; (2) about how much would the saftey factor be; (3) how deep should the threads in the lead ballast be; (4) bronze (preferred if strong enough) or stainless rod?

Alternative: Put a 1/2" eye nut on an existing (about ten years old) 1/2" bronze keel bolt. But my gut tells me this would not be strong enough for about 1700 lbs????

[mmd]

"The questions are (1) is this strong enough to lift the boat, about 1700 lbs; (2) about how much would the saftey factor be; (3) how deep should the threads in the lead ballast be; (4) bronze (preferred if strong enough) or stainless rod?
" 1.) Yes, for the bolt.
2.) About 5 for steel, 4 for silicon bronze
3.) I'd dispense with threads and drill right through, then pocket the bolt head/washer/packing assembly in the bottom of the ballast keel. But that's just my opinion.
4.) Silicon bronse would be fine, but an 3/4" nom. lifting eye may be difficult to source.

[Dan McCosh]

3/4 ins. would be plenty strong. Might look at what happens when you try to lift at a single point, though. I've seen this tried, and it gets pretty wobbly when the boat is in the air. Might be better to have two points--0ne fore and one aft, to stabalize the lifting process.

[bainbridgeisland]

Originally posted by Dan McCosh:
Might look at what happens when you try to lift at a single point, though. I've seen this tried, and it gets pretty wobbly when the boat is in the air. This is an important point. Chances are the center of gravity of the boat is well above the top of the keelson. This means the load would be unstable if the lift point is below the center of gravity, i.e. just above the keelson. If you extend the rod to above the center of gravity, you will induce a bending moment into the rod. MMD may not have included this when he told you 3/4 dia is big enough.

Now that we have the caution out of the way. You can certainly install a lift on top of your keelson but should also include guys fore and aft as well as athwartships to stabilize the load. This is in fact common practice for many race boats that are launched by hoist.

[imported_Dutch]

http://www.fredtberry.org/images/WhaleBoat.jpg

[mmd]

"MMD may not have included this when he told you 3/4 dia is big enough." - bainbridgeisland True, I didn't. With all due respect, he asked for an opinion on materials strength, not an engineered solution. Of course one must consider longitudinal as well as transverse moments about the C of G, bending moments in the lifting bolt, shear stresses in the bolt and ballast threads, torque moments induced by the lift rotating while "on the hook", inertial loads imparted by errattic slewing and lifting/lowering by the crane operator, etc.

Sometimes I answer simple questions with simple answers because I fear that I give lectures where they are unwarranted and unwanted.

Now if one were to consider a two-point bridle with jumper stays to the chainplates ... ;)

[ 12-27-2004, 09:24 AM: Message edited by: mmd ]

[Great_Lakes_Bill]

Originally posted by Dutch:
http://www.fredtberry.org/images/WhaleBoat.jpgDutch
This looks like the whaleboat being lowered from the starboard side davits on the oiler that I was stationed on when I was in the navy.
Bill

[RonW]

To add some vital info. to this, I will say first off, I am not a engineer and can not calculate the stress. But- I have spent a little time running cranes, so do know about that point of it.

First off you are going to find out that the guy running the crane will refuse to pick it up. You can not pick up a load that is 2 or 3 times as long as it is wide from one central point(without straps at each end). You have to have a squarish load to do a pick up from one hook. As soon as the boat is airborne, one end or the other will do a nose dive and be hanging dangerously swinging in the wind. May even do damage to the boat. So I would advise to rethink the one point pickup eye.A eye forward and a eye rearward, with a strap hooked to the 2, and then a central pickup would work, if things are reasonably balanced.But not one central eye.

[Bayboat]

Many thanks for the useful responses. What made me think that a single lifting ring might be feasible is that Solings, Ynglings, J-24's and other similar-sized boats are lifted this way. All have fore-and-aft and thwartships bridles that take care of any imbalance, which I would plan to install.

Jamestown Distributors has a 3/4-10 galvanized eye nut.

The suggestion that I install the rod all the way to the bottom of the ballast is a good one. It would act as a stronger keel bolt (the present ones are 1/2" bronze). Would a recessed washer and nut (or two), peened over, at the bottom be sufficient?

One question that isn't answered in the responses is how much thread in the lead would be adequate, if I did not drill all the way to the bottom of the ballast for the rod?

[mmd]

I didn't answer that question 'cause I don't like the idea. IMHO, for several reasons, I think that threaded lead is not secure enough for a single-point load of that magnitude.

But that's only my opinion.

[Bayboat]

One lingering question about the lifting gear for a ca. 1700 lb. boat. I've about decided to lift with a 3/4" bronze rod through to the bottom of the ballast, with a recessed and peened nut and washer at the bottom and an eye nut and jam nut at the top. There is an existing 1/2" bronze keel bolt near the balance point. I could remove it and re-drill for the 3/4" rod, using the 1/2" hole as a guide. With an appropriate bridle (fore-and-aft vectors to other keel bolts and thwart-ship vectors to hefty pad-eyes} this should work.

Before doing this, the lingering question is, would the existing fairly new 1/2" bronze keel bolt with a 1/2" eye nut and jam nut bear the 1700 lb. load with a sufficient safety factor? This would save a lot of fuss, but I would not do it without expert assurances that this would be strong enough. Engineers, please get out your handbooks and look up the safe load for a straight vertical lift and possibly some minor shock load.

[ 01-08-2005, 01:52 AM: Message edited by: Bayboat ]

[Bob Smalser]

First off you are going to find out that the guy running the crane will refuse to pick it up. You can not pick up a load that is 2 or 3 times as long as it is wide from one central point(without straps at each end). You have to have a squarish load to do a pick up from one hook. As soon as the boat is airborne, one end or the other will do a nose dive and be hanging dangerously swinging in the wind. May even do damage to the boat. So I would advise to rethink the one point pickup eye.A eye forward and a eye rearward, with a strap hooked to the 2, and then a central pickup would work, if things are reasonably balanced.But not one central eye. If you do this, then overbuild it by a wide margin.

If it got away from me using one of my machines, I might wind up bouncing it on the hook once or twice to regain control of the swing....doubling your minimum calculated load or worse.

[George Roberts]

Bayboat ---

I read all of the above posts ...

I agree that the stability problem is too difficult to trust to be handled correctly.

This makes the engineering of a proper bolt too difficult to handle also.

A 3/4" bolt is not large.

[Mike Dawson]

My engineering books are buried really deep in storage so I didn't look anything up.

A 1/2" eyebolt probably has a SWL of around 2000# depending on material and cast or forged.
The question is how well are your keel bolts anchored. Sometimes they are just in deep enough to take their share of the load and asking one to hold everything could be risky if you are not sure of their bury. How many bolts are there? Placed in pairs or all inline?

Not knowing the boat's construction, I would want to have the load shared by two bolts, either by a wire rope bridle or a structural metal bridge between them.

A friend had an Express 27 (2500#) that had a single lift point which was a metal eye strap that bridged 2 bolts, IIRC. The lift cable went out the hole for the 3" PVC vent in the cabin top.

LFH's Rozinante plans show a lifting bridle. The lead keel alone weighs 3360#. He shows two 5/8" bolts all the way through the keel (approx 8' apart) with 1 1/2" washers under the nuts. A 7x7 wire rope goes from each bolt to a bow shackle inline with the CG above the cabin top. You could scale down accordingly.