Securing REALLY Large Fins?

[jmmome]

This is for my 6 feet tall Marvin Martian Jr. project. The fins are 22" wide by 28" long and are made of 1/2" Baltic birch plywood The two circular plywood pieces are 24" and 12" in diameter respectively, and are also of the 1/2" Baltic birch.

I have fiberglassed the fin/body tube connections with one layer of fiberglass. I will do the same for the circular pieces/ fin connections. I also plan to use (2) 2 1/2" steel corner braces to secure the 24" circular piece to each fin, and (1) 2 1/2" corner brace for the 12" circular piece to each fin. Lastly, I'll epoxy fillet each of these joints.

What additional methods would you suggest to secure the fins to the body tube and to the circular pieces? Additional layers of fiberglass? Maybe a 1" x1" piece of poplar wood "screwed & glued" to either side of the fin/ circular plywood piece contact points at the edge of the circular pieces?

Once the foam body shell goes on it can't come back off, so I'm open to any and all suggestions. And I do plan to reinforce the skinny bottom of the fins with a triple layer of the 1/2" plywood extending up into the fatter part of the fin.

Thanks in advance!

 

[Speaknoevil]

What you have actually looks good. In the past with large fin, large body designs I have used expanding foam to provide a bit of leverage/support within the fin can. But given the dimensions on this body and the nice large upper centering ring, your use of corner braces makes sense and so foam would not be needed. I am not sure about your exact corner brace placement/location, but make sure you put one on each opposing side of the fin. Also, not really sure you gain much from putting any on the bottom/smaller centering ring given you will have a laminate layer of fiberglass also, and the leverage at the outside diameter of that ring is so much less.

 

[jmmome]

Yup- will have one on each opposing side- left off that important piece of info in my description.

 

[jqavins]

Given where you are and already intend to go, how much more do you think you need? I guess you could epoxy some rebar into holes you drill in the fins and rings, then stud all the surfaces with oversize jacks and fill the space up with resin, but it seems overbuilt already.

 

[jmmome]

It probably is overbuilt already- at least, that was my hope, as opposed to underbuilt. I've never built anything with fins this large, so I wanted other sets of experienced eyes to look at this.

The Kaboom Krewe's original version of this weighs about 400 pounds more than mine, so I wanted something I can lift myself- maybe 50 pounds or less. Unless someone points out a fatal flaw in my original construction proposal (just the addition of the corner braces and the epoxy fillets), I won't add any other fortification.

 

[BABAR]

This may sound crazy, but you can reinforce it with pretty much zero weight penalty with dental floss.

Drill holes, say three, in the bases of each fin between the forward and aft bulkheads.

Lace it around with dental floss, pulling it as tight as you can.

Tie it off and wick thin CA into it.

You can probably foam over the floss if you want to.

The fins will break before the floss does.

 

[David Schwantz]

1/2" ply, i would run some dowels down through the CRs into the ply fins nd then glue in place. Not much for weight, but would add great strength.