Centerboard trunk core material

[MEJETSKI2000]

Replacing the centerboard trunk on my old FJ. The original material was a thin approx. .050" thick sheet with glass laminated to it. Trying to decide if using fiberglass sheet or some other form of core material would be better. The part is approx. 1 ft high by 3 feet long. Either way it will have epoxy laminated to it for waterproofing.

Thanks, Pete

[alan white]

Where you are asking a lot of a panel, like a centerboard trunk, I'd suggest you use 1/2" ply as original (you meant .5"). Try not to core with anything lacking strength like foam. Centerboards take a beating. Just make sure to completely seal every surface and edge and hole with a few coats of epoxy.

[MEJETSKI2000]

Thanks for the reply, Alan. The original material is a .050" piece of plastic laminated to a single thickness of woven roving. I was hoping to avoid using wood because of weight since I'm adding at least one layer of mat and cloth to the deck for reinforcement. What material other than plywood or foam would you suggest ? Luan maybe, with laminate on both sides...

[alan white]

OOps! Given what you've said, 1/4" ply would be fine. Seal very well and use a quality plywood if you can. Obviously many core types exist, and others here know more than me about the choices, but 6 sq ft isn't much area. In plywood, maybe 2 lbs total added weight.

[MEJETSKI2000]

Since I'm using some mahogany for the leading and trailing sections (epoxy-coated) I guess plywood would be a sound choice and available locally here. thanks for the advice, Alan !!

[PAR]

Mejetski2000, if you're making these repair using epoxy, then you don't need, nor is it desirable to use mat of any kind. Mat is a "crutch" used in polyester GRP laminates to over come the weaknesses of the resin. Epoxy has no such weakness and you'll just be adding a lot of weight and a hugely resin rich portion to the laminate with mat.

If you used 1/4" Okoume plywood the 6 sq. ft. of plywood will weigh ~3.6 pounds raw. If using Meranti plywood ~4.3 pounds. Luan is Meranti, but get good stuff as it will have the veneer count and void free construction necessary for good strength. 1/4" should have 5 veneers, all being relatively equal thickness. Big box store luan will be 3 layers and full of imperfections.

[alan white]

I ought to have pulled out the calculator Six sq ft is an inch and a half, one eighth of a cu. ft, so at 32 lbs per sq ft, four lbs, and that plywood density might be about that. PAR is correct, Mejet. Sorry to mislead.

[PAR]

A 1/4" sheet of BS1088 Okoume is 20 pounds, making for ~.625 pounds per square foot. Meranti is ~3/4's of a pound per square foot.

[MEJETSKI2000]

Guys, MANY thanks for the advice. My supplier has Meranti BS1088 and also Type One glue lay-up B/BB Okoume in stock. Which is the best for epoxy sealing and longevity ? I'm using Epi-Glass Epoxy incidentally, if it matters, 3 coats enough ?

[alan white]

Probably. It's the closest to the desired thickness based on how most people apply epoxy. Full glossy surface.

[MEJETSKI2000]

Which of the woods I asked about is the better product, Alan ? (or anyone LOL)

Thanks, Pete

[PAR]

Okoume is lighter weight, but weaker and less rot resistant. Meranti is 20% heavier, stronger and more rot resistant.

Rot resistance is fairly moot if properly encapsulated, leaving weight and strength issues. Dagger/centerboard cases can be under a lot of strain, but not knowing what you're actually working with, leaves me clueless about which is more important. In small craft light weight is very important. Seeing as this piece is going to be considerably stiffer then what you had, I'd elect to use Okoume to save the weight.

[MEJETSKI2000]

Both the internal and external "joints" will be sealed (and secured) by epoxy laminated 10oz cloth, which hopefully will prevent both water invasion and be more rugged than the single layer of cloth at the leading/trailing interior sections that ultimately leaked and caused all the damage. Any other suggestions, I'm all ears....

[alan white]

The pivot bolt is a source of problems, at least potentially. Make sure you address any leakage issues there. Beef up the area around the bolt to 1/2" and add some extra cloth tabbing there. The area around the pivot bolt should be dead flat to ensure the sealing washer seats flat. Then use a good gooey pipe thread dope (that white stuff, like marshmallow, I forget the name) to provide a long lasting non-hardening seal.

[MEJETSKI2000]

This boat has 2 stainless plates mounted to the top plate of the trunk, which have a boss that fits into a recess on the board (no bolt thank God). None of the hardware on this boat was sealed with epoxy or anything else (will be addressed). There's some CB hardware mounted on the trunk that was glued down with a plywood tab, looks like I'll need more than a qt. of epoxy LOL...

[BobBill]

I read this thread and have a question.

Why not use a good plastic product instead of marine ply?

I plan to repair and rebuild a dagger board chute between a deck and outer hull and will not use wood or core materials. Synthetic seems the best option.

[apex1]

Why not use a good plastic product instead of marine ply?


It is either good, or it´s plastic, you have the choice!

[PAR]

I agree with BobBill, inert materials like HDPE or other plastics usually make much better centerboards, daggers, rudder blades, etc. Most are neutrally buoyant, some resist marine growth, possibly self lubricating and will not suffer from destruction of internal friction (like plywood does in this use). I dislike the use of plywood in anything but quite small craft. A solid wood, strip plank build is a better choice, if wood is "necessary" as it doesn't suffer from rolling shear like plywood and it's 1/3 or more stronger longitudinally.