Bulb to Fin Attachment

Okay, there has to be an easier way. Please feel free to post your favorite bulb to fin attachment method(s).

I tried cutting a groove in the lead bulb with a Dremel…first mistake. Next I started drilling a series of holes with a 1/8" bit. The bit siezed, broke and I never did manage to get it out. I finally manged to remove enough material through a combination of a rotary zip bit in the drill and a tiny high quality (my wife’s now broken) carving chisel.

I glued the fin into the ersazt slot and faired it with hot glue. I’m thinking I may be able to remove it later if necessary but I’m probably deluding myself. I did find a reasonable method of fairing hot glue though.

Any tips on casting your own bulb would also be appreciated as I don’t want to be stuck with the hammered and filed sinker I used for this one.




Easiest way I have found is to drill 2 4mm holes in the bulb. cut tenons on the bottom of the fin that fit into the holes. glue in with epoxy.
Its quick ,easy and preserves maximum lead volume ensuring the bulb is as small as possible for a given surface area.

If your using a dremel tool, use low speed and use a round carbide burr. If your speed is too high, it heats the lead and slightly melts it. Pictures show my bulb for a M505 hull using a 8 oz. trolling sinker and an Estes model rocket cone.

Ends of the cone are filled with lead shot/epoxy mix and everything is glued together.

I love Brett’s solution - very direct.

I have drilled 1/8 holes in the lead and opened them out into a slot - seemed like hard work to me.

Another (off the wall) attachment method pictured - doubles as a boat stand indoors.
Actually its a very highly technical wing keel resulting from from seconds of design time and research

Similar to Brett’s approach, I drill a hole in the bulb, hammer in a bolt with the head cut off (I think the threads make a better purchase of glue than a smooth rod) insert that into the fin, and make liberal use of my favorite glue.

Haven’t had one fall off…yet.

Bill H

I like Brett’s idea too. Think I will give it a try next time. My way was a bit more time consuming.

You can machine and drill lead quite nicely using Turpentine as a lubricant if you drill or cut quite slowly. I’ve been using the new lead free solders intended for drinking water pipes, and this material machines quite readily. In additon, it is safe for use in your favourite pond, and will poison neither fish nor flesh.
Even fishing sinkers are now made from it, and to be totally responsible, no lead should be put into water.
Footy 08 CAN

I’ve seen people using gasoline on hotglue to get it to let go. That’s why the powerboaters stopped using it on engine frames.

Also, i think if you are going to mount the ballast using Brett’s metrhod, you should extend the fin enough to keep the total keel depth correct.

yea brets metod+bondo:D

Always use the simple method. This way, if the bulb is over or under weight, you can recast and change it.

I have just been using a cheap drywall saw to cut a slot for a brass tang, epoxy the tang into the slot, and GOOP it all onto a fin. If I like it, I’ll cut a slot in the fin and glue it on permanently.

I just bolt mine on. bulb tab slides into keel slot, two bolts go through both.
it lets the bulbs be interchangable.

My solution to this problem was to insert an ‘eyescrew’ into the bulb and then associate this with a keyhole cutout in the lower edge of the keel (see attachment) then add epoxy to secure. Two such inserts would add strength to the assembly.

Philvin - like it!