Hobie 18 Boom Cap replacement question.

I'm trying to replace a broken boom cap on a Hobie 18. Got several (used) replacement parts shipped in, and drilled out the rivets to remove the old cap.

However, none of the rivet holes in any of the 3 used replacements I've tried line up with the holes in the boom itself. And on closer inspection, none of them match up with the others either. It's like each of the parts was drilled by hand, separately. Worse, the holes are all just far enough out of alignment that I can't force them, but just close enough together that trying to drill new ones would leave me with giant oblong holes that a rivet wouldn't grab onto properly.

Looking in the manual, and on old seller sites, it seems like the boom caps always came pre-drilled. So I'm trying to get a handle on what's going on.

Figured I'd ask if this was a standard/common thing, and if so what some suggestions for how to retool things so that I can attach the cap without compromising structural strength of the join.

Castings are never pre-drilled if internal in an extrusion. They typically are drilled (in production) once inserted in the extrusion using a drill fixture. It is always possible there were changes or different fixtures used. Using a new end cap for a repair, it would not have been drilled and you would have drilled using the holes in your extrusion... drilling fresh holes in the casting.

For these used parts, you can add new holes in different locations and either fill the old with extra rivets or something like JB weld.

_________________
Matt Miller
Former - Director of Parts and Accessory Sales
Warranty and Technical Support
Hobie Cat USA
(Retired 11/7/2022)

I would take all of the caps you have to an Aluminum welder. Have them fill in the holes, and either they or you file/grind back to shape and size. Then use your mast as a templat to mark for drilling. I would drill the new holes with the cap out of the mast so as not slot the holes in the mast. Besure to center punch the middle of the mark, and start with a small drill bit.

Thank you both for the info and the advice. I'll give it a try.

My tip:
- don’t touch the end castings.
- cut off the end of the boom extrusion by the minimum amount so the original holes are cut off, but no more. Both ends.
- then measure and mark the holes to match the end cap castings
- use alloy rivets. Three of them on the front casting. One on each side and one on the bottom.
- drill a 3/8” hole in the bottom of the extrusion about 2” back from the front end. This is a drain hole for any water that may get in.
- use alloy rivets for the rear casting but only one on each side as it is always under inward tension from the outhaul system.
- so long as the gooseneck pin in the front casting is free rotating and doesn’t slop around in the casting hole, there will be no undue load on the front casting, thus alloy rivets will be sufficient. If the gooseneck pin is loose, use 3/8” nylon washers as spacers both inside and outside to remove the slop.
- use a Tren-Tec gooseneck block on the gooseneck pin. It is a perfect fitting for attaching the boom to the mast.
- have a Hobie day……………..

_________________
John Forbes
Hobie 18 Reimagined
Sail # 490
Boat name: 18@heart
http://www.hobie18.fun
https://www.facebook.com/Hobie18catamaran/

I think John’s recommendation is a good one. The boom is a a few inches longer than it needs to be anyway, so cutting off 1/2” or so to eliminate the old rivet holes shouldn’t cause any issues.

I would use a power miter/chop saw to make the cut so it is perfectly square and clean.

sm

John's idea is probably cheaper too. I then, would slide the cap into the cut boom "Partially", to mark accurate orientation of your holes. Then mark the depth. Measure accurately, or, you end up where you are now.