Fixing bulb to fin

[Jamestj]

It might be useful to alter the angle of the lead bulb to carbon fin. I thought about using Woods metal since it melts at 70C. I could heat the bulb and alter the angle and it should stay put when cold. I have not heard of this being done. Are there any problems using this method?

Thanks,

James

[Ralph Knowles]

Hi James,

I've only heard of Woods Metal being used in the manufacture of copper plumbing fittings. In its heated form it is used as a hydraulic medium in the forming of 'Ts' and elbows. I think there would be a problem of how to adhere the fin to the metal which I presume you would be using as a 'grout' in the moulded slot in the bulb. I fixed my lead bulb to the fin by running in thickened epoxy after 'raggling the lead faces in the slot to give a good key. I have been told that if the bulb needs to be adjusted by a degree or two, then the answer is to dip the bulb in very hot water, NOT the fin, and the epoxy 'grout 'will soften enough to allow a slight alteration in alignment. The usual fore and aft cant of the bulb is between 1 and 3 degrees, so very little adjustment is needed if the initial setting is in the middle of this range.

Cheers

Ralph

[Jamestj]

Hi Ralph

I was thinking of sticking copper foil round the end of the fin using epoxy. The copper should be wetted by the Woods metal and presumably it would also wet tha slot in the bulb since Woods metal is mainly lead. I could heat the bulb and soften the Woods metal without fear of melting the bulb.

Could I not fix the fin in the hull using a similar process except I could use a bit of a glue stick (the plastic used in a heat gun) instead of Woods metal? To remove the fin I could use a hair dryer. Would the rules allow this?

One problem would that I would need to remove the glue from the fin completely so that the fin and bulb could be weighed for scrutineering- but the attachment would allow loads of scope for adjustment.

[Bruce Andersen]

Why don't you just plan on attaching the bulb to the fin using a recessed cap screw up through the bottom of the lead into a threaded metal cross pin set into the bottom of the fin?

That's how most of the Marbleheads used to do it, and it allows easy changes to bulb cant by inserting shims in the bulb slot.

[Graham_Snook]

This is how mine is attached...I would have tapped a screw up from the bottom if my fin was thicker and I knew that the filler could take the weight and strain of hitting a submerged object but I wasn't too sure

In the end I've tapped 2 grub screw holes in the base of the bulb that go to the slot, fore and aft, with a small plate to protect the base of the CF fin. Then I tapped an M4 CS bolt through the side of the bulb and the fin - slightly forward to provide a pivot point - I put small brass bush in the fin hole (to protect the fin again). Until I work out an optimum bulb angle I fill the gaps with plastascene

I just adjust the cant by un-screwing and screwing the grub screws (an idea I aquiried from Lester Gilbert's site - Thank you) but I can adjust the bulb without removing it from below

Just another thought

[Bruce Andersen]

To prevent possible pull out by simply tapping the bottom end of the fin, I drill a 1/4 or 3/8 inch hole athwartships through the bottom of the fin. I then take a piece of similar sized brass rod, drill and tap it to accept a 4-40 bolt, then glue it into the fin. Drill an appropriate sized hole up through the bottom of the fin to meet the tapped and threaded hole in the brass insert and use a 4-40 SS hex head cap screw up through the lead into this brass insert. Shim the groove in the lead that the fin inserts into and tighten it up! Simpler than tapping and inserting adjusting screws into the lead and I've used this to hang 10# Marblehead bulbs off CF fins with no pullout problems.