Masthead Spinnaker Crane

[Dave22q]

I an rigging for an asymmetric spinnaker so need a crane to get the halyard forward of the fore stay and the jib halyard. I am told my masthead is not flat but sloped forward. It seems to me that if I mount an ordinary crane (u-bolt welded to a plate) I could cope with the slope by using longer screws with extra nuts to keep the crane roughly level. My thinking is that all the force is down on the bolt trying to rotate the plate up so the extra screw length is no factor as long as the attachment to the mast cap is good. My rigger is not too excited by this thinking, possibly because making or buying a stock crane would be under $ 100. Opinions welcome? Am I nuts?
If the sail is 800 sq ft would I need a 5/16 u-bolts and 3/8 attachment screws?

[UNCIVILIZED]

Why not simply build the crane in the form of an I-beam, to level things out. And then attach your U-bolt to it's forward end? It would be stronger, & by properly attaching said crane to the masthead, the normal way with bolts, you'd be avoiding using nuts as spacers.

I'm guessing his reluctance to build it your way is because there are strong sheering loads on spinnaker cranes, not just loads pulling on them in the vertical. So mounting things in the fashion which you describe is putting unnecessary loads on the masthead/crane fasteners. And in particular, on the threaded sections of the bolts, which is where, they are, by far, at their weakest.

If his concerns are about strength, it's likely because a kite is the one sail onboard which is FAR more likely to induce a full knockdown of the boat. Which makes for a lot of load on that one fitting.

Also, BTW, why does the crane need to be level to begin with? And if you're paying him $ for his expertise, why ignore it?
If you like, do as is common in any other field. Get a 2nd opinion, running all of these ideas, discretely by another rigger or three.

PS: Consider just bolting the U-bolt in place, through a piece of the crane which has been doubled up/thickened. That way you can change out the U-bolt periodically with just a wrench, from the bosun's chair. As opposed to having to drop the rig when a welded one shears off. And then having to hire both an expert with a torch to cut away the mess. And a machinist, to make you some new holes to start over with.

Also, a crane for a kite halyard is something where you want Far more strenght in things, than in most other apps, relatively speaking. So the less work which is needed with a torch the better, as all that heat, alters the metal at the molecular level. Though that can be compensated for.

[Dave22q]

Thanks Uncivilized for the interesting thoughts. Bolting a u-bolt between 2 plates certainly sounds easy and is a lot cheaper then welding. Shearing of the bolt is improbable. U- bolts are cheap when priced as bow eyes and using 3/8 or even 1/2 would add less than $10 in costs over the 5/16 used commercially on boats up to 37' so the only risk would be the 1/4 anchor bolts breaking before the halyard block or the halyard. Sadly it is looking like I might have to go to Miami to get the work done.

[UNCIVILIZED]

Originally Posted by Dave22q

Thanks Uncivilized for the interesting thoughts. Bolting a u-bolt between 2 plates certainly sounds easy and is a lot cheaper then welding. Shearing of the bolt is improbable. U- bolts are cheap when priced as bow eyes and using 3/8 or even 1/2 would add less than $10 in costs over the 5/16 used commercially on boats up to 37' so the only risk would be the 1/4 anchor bolts breaking before the halyard block or the halyard. Sadly it is looking like I might have to go to Miami to get the work done.

For the thoughts, & ideas, you're more than welcome. Regarding how you mount the U-bolt, what I meant was, to simply bolt it through one plate of the I-beam which has had a doubler plate mounted to it. Although, you could mount it through both horizontal webs of the beam. But if so, at a minimum, put some compression tubes in there. If not flat out have them welded in place.

Also, my suggestion for using just a standard U-bolt vs. a welded bail. Is that, at least in racing circles, it's common to shear such bolts/bails. Due primarily to metal fatigue, & crystalization of the metal.
Thus, being able to swap them out easily, on the cheap, only makes sense. As does your oversizing idea, within reason, as you stated.

PS: If you're handy with tools, you might try the following:
Ask a few riggers in Miami, or just plain old machine shops (non-marine type, as they're Far cheaper), if they could fabricate a crane for you, if you bring in a mock up. And if so, get some plywood, & a hot glue gun, & make a full sized model. Then, once it's finished, bolt it in place.

[Dave22q]

Have not seen an inexpensive source for short stainless I-beams unlike plate, channels or tubing but I don't see a major advantage would be over a plate. Thinking resistance to side pulls I suspect a welded bolt to a plate would be stronger then the bolted plate sandwich but I like the idea of not needing a welder. On reflection it seems clear the the strength of bolting the plate tight to the sloped mast cap would be much stronger by adding friction to the shear strength of the bolts to side pulls at a minimal cost of losing a inch or so in height.