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How do I fix this?

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How do I fix this?
My fiberglass cover for my DW-15 is cracked on the underside, I want to repair the cracks before they spread to the top.
What type of Fiberglas is going to give me the most strength. I need to wrap around the wood stringers to reinforce the top.
Thanks



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Re: How do I fix this? In reply to
Good morning, Bob~


These stiffeners were cut square and 'glassed in with just CSM (chopped strand mat). The square shape is hard to 'glass to and the CSM has little structural strength.


Here is how I would approach the fix:


1) Grind all surfaces (60-grit) that will get new 'glass and epoxy. This means the stiffeners and about 3 inches all around them. You do not need to remove a lot of 'glass - just expose a fresh surface so the epoxy will adhere.



2) Change the effective cross-section of the stiffeners to more of a trapezoidal shape. You can do this either by filling (resin plus powders) or by adding a filler piece of closed-cell foam - ripped to a 45-degree section on your table saw. (Then epoxy in place.)


3) I would use epoxy resin. For the 'glass, I would either cut strips out of cloth (10-ounce), use tape, or even a biaxial tape (best choice). I would use 6" biaxial tape with mat (which is stitched to the biaxial and so is compatible with epoxy resin - plain mat is chemically incompatible with epoxy). U. S. Composites product number is FG-170806Y:



http://www.uscomposites.com/specialty.html


If I used the cloth or plain tape, I would apply 2 layers, the second at least an inch wider than the first.


As it turns out, I just repaired a hatch on a South Bay Duckboat using the biaxial tape:






Hope this helps!


SJS

Steven Jay Sanford
Pencil Brook Farm
South Cambridge, NY
http://www.stevenjaysanford.com


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Re: How do I fix this? In reply to
Bob, Steve is right on, You need to get rid of whats there, if there is wood in the support now it needs to go. The wood will rot encapsulated in glass even more so being left to the sun with a OD flat color.
Phil
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Re: How do I fix this? In reply to
 Check to see if you have wood rot in the stiffeners. If you have rot, cut it out and start all over.. As Steve said, square [90 degree] is hard to glass.
Try YOU TUBE for visual repair. Tape is easier to work with than cloth. Make sure it is warm enough to work.
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Re: How do I fix this? In reply to
It sounds like I should remove and replace the stiffeners. If I do would I replace them with wood again or hard foam as Steve suggested?
Thanks gents!
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Re: How do I fix this? In reply to
Unless you have a few gallons of epoxy sitting around, I would opt-out of going with Steve's suggestion of using resin and filler to change the cross-sectional shape of these supports and then glassing them in again-expensive and a very heavy end-product. Changing the cross-sectional shape via foam insertion on all these supports will again use a lot of resin, require a great amount of time and effort to achieve the end result, as well as add significant weight to an already likely heavy cover (I have a TDB-17') that is unwieldy and very difficult to handle solo.

You might want to price estimate application of several sections of this material in appropriate widths and simply remove the wood cross-braces completely and replace them. These are only available in 4' sections in a variety of widths, but will take standard resin without damage to the internal foam cores which are the density equivalent of softwood, saving you money over epoxy, and they will easily glass in place with the possibility of some overall weight savings for the total job, with all the wood removed in the end product. Butt the joints in the cross braces by beveling the surfaces to mate completely on a beveled edge rather than just butting them at a 90 and covering with CSM again, which will yield a stronger joint, per Steve's suggestion of use of biaxial tape overlays.

https://www.jamestowndistributors.com/...Preforms+Hull+Lumber

Use of the 3' wide 1708 biaxial cloth US Composites sells, layered over the existing CSM in the lateral areas out to just shy of the cushion tab border would also be something I would consider, particularly if you saved weight by removing the wood and replacing it.

How badly has the gelcoat spider cracked on the topside? This also may need to be addressed, after you stabilize the cover's longitudinal and lateral flex issues.
You can also alter the structural bracing configuration to use less material (see below). This cover is gelcoat/CSM/core mate/cloth laminate, with Prisma structural lumber bracing.members prior wet-out

Last edited by:

RLLigman: Jan 7, 2019, 4:28 AM
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Re: How do I fix this? In reply to
Bob~


Here is a schematic of my approach. It reflects the simplest method. And, it leaves the wood in place - whether or not it's rotted. This may sound sacrilegious, but - I have done it on hull stringers with long-term (decades) success. The new 'glass provides the structure needed for stiffness because it acts as corrugation. It does not matter if the encapsulated wood disintegrates over time.







The foam fillets (from insulation board) are easy enough to mill on table saw - and they do save both weight and cost. In any event, I would bed the fillets in thickened epoxy (add Fairing Compound).


Guessing at 15 l. f. of stiffener, I would estimate maybe two quarts of resin for this job. U. S. Composites gallon-and-a-half kit 635 resin with medium hardener (my standard) is $71.



Other thoughts:



1. Another thing I would do if this were my hatch would be to first run my router around the stiffeners - with a half-inch roundover bit. This would help the new 'glass conform even more readily.


2. If I were to remove the existing stringers, I would replace them with either foam board milled to a trapezoidal section OR I would split 1-1/4-inch PVC pipe. Either way, I would laminate 2 layers of biaxial with mat to achieve sufficient strength.



3. It looks like this hatch is dead flat. I would "encourage" a bit of crown by a) setting the hatch (upside down) on 4 horses so that all edges were level but then place full gallon paint cans (or other dead weights) within the H of the frame. I'm guessing even a half-inch of resulting crown would help shed dew and rain while retaining the fit over the cockpit.


4. BTW: Having molded a new hatch (for a South Bay Duckboat) earlier this Fall, I do use Coremat along with biaxial cloth and mat to get superior stiffness (compared with conventional mat-roving-mat layup). Because it's new construction with no wood, I use polyester resin.


All the best,


SJS























Steven Jay Sanford
Pencil Brook Farm
South Cambridge, NY
http://www.stevenjaysanford.com


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Re: How do I fix this? In reply to
There is not any noticeable cracking on the top of the cover.
I would like to get some ?crown? built into the cover as water pools in the center big time, I think the pooling contributed to the supports cracking.

I am leaning toward removing the current supports and replacing them as I think this may last longer.

I will keep you all posted on my progress.

Thanks again. This forum is such a great resource!! I really appreciate the great advice.
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Re: How do I fix this? In reply to
I suspect your water pooling may have coincided with the break; as the longitudinal stiffness dissipated. The water pooling is driven by the support bracing layout. Keep in mind water does penetrate polyester resin layups slowly, so you may have rot issues unseen and unnoticed. As Steve pointed-out the CSM used over the braces provides very little by way of structural support and stiffness. Also, there are a variety of polyester resins available to boat builders, all with associated quality and pricing that are correlated. Your long axis braces lie well away from the cover's mid-line, far closer to the cover lip, which offers some structural rigidity in the lateral surfaces of your cover-not a well designed set-up. This, too, may have contributed to the break and the sag. Why I posted the alternate bracing layout.

On closer inspection, I also note you have a series of handles attached to the cover in a variety of sites, you might want to consider applying backing plates to these as well during your repair/rebuild interval.

Epoxy resin has several advantages over polyester resin, price and UV light exposure stability are not among these. If you opt to go with epoxy, you will need to paint over it or keep the cover underside out of direct sunlight exposure for long intervals. I often lay mine upside down on the ground when the boat is in use, particularly if it is windy. You may lay yours down with the handle side upright. I like US Composites epoxy resins; what I don't like is their shipping costs, which are pretty pricey. Price other suppliers if you opt to go with epoxy resin in your layup, including shipping.

IF you opt to remove the wood members, inspect the layup cross-section under them. IF there is no cloth layer visible, only CSM, you will want to consider adding a biaxial cloth layer over the entire underside to strengthen the cover, obligating you to either use polyester resin, or exceed Steve's epoxy volume/cost estimate, which didn't include shipping cost.

I forgot two points: 1.) If you opt to use standard blueboard foam sheets ripped to fit, you are obligated use epoxy, since the toluene component of polyester resin is a solvent. 2.) You can get epoxy to adhere well to polyester layups, polyester resin doesn't adhere well to epoxy. IF you opt to lay another layer of biaxial cloth over the inside of the cover you will have to sand-off the paint on the CSM and you can apply this with polyester resin only if you add the structural supports as your last layer if you use epoxy in their layup. The opposite sequence will not yield good results.

Last edited by:

RLLigman: Jan 7, 2019, 5:21 AM
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Re: How do I fix this? In reply to
Good morning, Rick~


Your thoughts re shipping are certainly valid. Living in dairy country - and not boat country - I mail order most of my boat and decoy supplies. I look for "free shipping" whenever I can, but I have been impressed with the cost of delivery - both to and from Pencil Brook Farm. One thing I have learned is that distance matters - whether with USPS or UPS. I always try to keep costs down for my customers, but sometimes my charge for re-habbing a gunning rig is outstripped by the mailing charge.


I occasionally contemplate selling off my shop and machinery and tools and swapping them for a big brown delivery truck....


All the best,


SJS

Steven Jay Sanford
Pencil Brook Farm
South Cambridge, NY
http://www.stevenjaysanford.com