Encapsulating Marine Ply-- Cloth, Mat, what?

[John F.]

The decks are up and some repairs are in order. I've made the bulkhead out of 3/4" marine ply, and need to glass it and set it in. In the past, on small stuff, I've used West epoxy and layers of cloth. So far, everything has worked out fine. What should I use on this? I'm going to use West Epoxy, but should I use cloth, 1708, or something else? What to I tab the bulkhead into the hull with? Thanks.

John F.

[CaptPatrick]

John,

The overall covering will be fine with one layer, each side, in 6 oz 0/90º cloth. Your tabbing should be 1208 or 1708, but plain 0/90º roving would be fine also.

Br,

Patrick

[John F.]

Capt. Pat-

Thanks. Once I have the bulkhead ready, my plan is to lay it in a thickened epoxy mix, do fillets, and then go back and tab the whole length to the bottom of the hull. The bulkhead will sit off the side about 1/2", and will tab there also. I remember somewhere reading that you advise against having bulkheads hard up against the hull side to prevent hard spots. Does that sound about right? Any suggestions?

How many layers of tabbing?

Is there any source so that I can read what the different f'glass (cloth, mat, roving) is for, or can provide a quick rundown. Thanks-- I know I'm asking alot.

John

[CaptPatrick]

John,

Right about hard spots against the hull sides. Where ever a hard spot is created along the hull, especially from a vertical bulkhead, normal flexing of the hull will stopped at that point. All forces will be concentrated to the line of the intersection and things are going to eventually break. In extreme cases, the bulkhead can act like a knife and cause a complete cut through the hull. The same thing holds true for stringers, but here, when the stringers DO have a gap between the stringer and hull.

As a general rule of thumb, 0/90º plain weaves have less strength than 45/45º plain biaxle weaves. Mat backed fabrics have better bonding capability than non-backed fabrics and create a filler layer between multiple layers of fabric, (this is more true as the weight of the weave increases). Heavier weaves have more strength than lighter weaves. You can search through manufacture's sites and get really more information than folks like us need...

For those of us involved here, just break it down to two categories: Structural & Non-structural. Fabrication, Tabbing and bonding fall into the structural realm, and Covering, coating, and encapsulation are non-structural items.

For Structural, stick to biaxle weaves, (nothing lighter than 1208 or heavier than 1708). for Non-structural stick to 0/90º weaves. For non-structural surface treatment I range from gossamer like veil to 1 1/2 oz. mat to 6 oz. 0/90º plain weave. I rarely go heavier than 6 oz. cloth, rather, just add more layers where needed. I use the veil primarily as the outer most skin layer between the gelcoat and structural layers of molded parts. I use it sometimes as the final layer over a mat surface to further hide the chopped strands from printing through and to obtain less surface porosity that would have to be filled otherwise before spray coating.

Mat normally comes in only two weights: 3/4oz. and 1 /1/2 oz. 1 /1/2 oz. mat is actually just two layers of 3/4 oz. that are sized together at the factory. Note that all chopped strand mat and some veils are sized with a binder that dissolves when subjected to styrene. Epoxy will not cause the sizing to dissolve, so the wet out of the mat is not as complete when using epoxy, like it is when using polyester or vinylester. Mat still works OK with epoxy, it's just not quite as strong and will have a cloudy appearance from trapped microscopic air.

Main bulkhead tabbing, where tight to the hull bottom, and perpendicular to & between stringers can be done nicely with one layer of 1708 or, the most, 2 layers of 1208. Tabbing to hull sides can be as light a one layer of 1208. Be sure that your tabbing over the gap remains in a nice flowing fillet pattern and doesn't get tucked into the gap. The larger the radius of the fillet, the more forces will be distributed evenly over a broader srea. A tucked tab will always eventually fail...

Thickness of face coverings will depend on the amount of abuse the surface has to endure. Bulkhead facings can be quite thin. The necessity for a fiberglass fabric or mat to be included in the facing over plywood is to prevent checking and cracking of the resin as the wood expands and contracts with temperature changes. Cracks in the resin are open doors to moisture.

Things like cockpit panels are more prone to impact damage from gaffs, rod butts, and the like. A heavier covering here will decrease minor dents and abrasion. Heavy traffic areas like decks need to be protected even more, not so much from foot traffic, but from gravity related damage from falling objects such as downrigger weights or your buddy that forgets to change out of his golf spikes before boarding.

I know I'm asking alot.

No big deal this time of the year, I stay holled up in the morning, at least until the temperature reaches 50ºF or so. Opps, it's now up to 64º, so I guess it's time to get back to the outside grindstone...

Br,

Patrick

[John F.]

I know what you mean about waiting. Last weekend i waited to go down to the boat unti it got out of the teens and into the 20s , and yesterday it got all the way into the 30s.

Thanks again. I'll print this out so I don't forget.

[CaptDana]

I just need a little clarification on something you put in the reply- If I understand you correctly, if you are replacing stringers, the material should lay up directly onto the hull?
My next question would be- if you are repairing rotted stringers by cutting the tops off, and then pulling the wood out---would you recommend putting thickend epoxy between the hull and the bottom of the new wood/material for the stringer?
Also- I am assuming that a thickened epoxy would be used on the sides of the wood between the new wood and existing stringer on the inside?
Finishing this off would require glassing from the hull, over the repaired stringer and then back down to the hull.

Lots of questions---just getting it straight for once-
Thanks
Capt Dana
Thanks in advance

[CaptPatrick]

Capt Dana,

Hope you're not too far into that job... The only reason that you'd want to replace a stringer would be if it broke loose from the hull. The plywood inside of the stringer, which does often get moisture intrusion, was used only as a form to lay the fiberglass over and has no structural value in and of itself. Bertram had not figured out how to pre-make hollow stringer cavities when these boats were in production, so a plywood form for each stringer was the technique of the day.

The cross section of the stringers will show that the fiberglass thickness is quite thick. And glassed directly to the hull, with the number of courses necessary to build that thickness, it becomes a very strong, integral structure that does not rely on the core for additional strength.

Generally the stringer glass thickness, at least on the B31s that I've had opportunity to sample, has been between 1/4" & 3/8". Bertram laid the first layer of glass with about 1" of lap onto the hull. Each successive layer lapped further, ending up with a series of tapering overlaps that extended out onto the hull by 4" or more.

The plywood used was 3/4". You can sample your stringers by measuring the total thickness, subtracting 3/4" and dividing the remainder by 2. That gives you the thickness of the fiberglass that was laid up. If that number is at least 1/4" and the stringer isn't coming loose from the hull, there's really no reason to chase the wet plywood core.

By cutting the top off of the stringer, to get to the wood, you'd actually be weakening the stringer, regardless of the re-capping techniques, because you'd be breaking the continuous nature of the original roving layers on the vertical plane...

Br,

Patrick