after the grinding

So now that I've ground the transom out, I'm getting ready to laminate a replacement.

Here's my plan:

Cut 1/2 marine ply (bs-1088 meranti) to shape and glue in place (with epoxy/wood fiber blend) to glass outer skin.

Fill gaps around edges of new core with epoxy/wood fiber blend.

Build fillet in corners between new core and gunnel and floor.

Tab transom core with 8" 6 oz tape.

Skin core with 3 layers of 17 oz biax cloth - 1st layer with a 6" overlap and each later with 2 additional inches overlap on gunnels/floor/stringers.


Here are my questions:

Should I coat the meranti in epoxy before I glue it in as the core? I guess if I did this I'd do it directly before so it was still tacky enough for a chemical bond.

Through cutting, grinding and pleading with the old core, I've left the inside of the outer glass laminate of the transom with an uneven face. See below.

The outer laminate is 1/2" thick for the most part. What's the best way to fill the uneven surface while gluing the new core in place?

Is 3 layers of 1700 enough for the inside skin - there was only about 1/8" skin on the inside before.

The original core was only 1/2" - any reason to add thickness at this point?

Thanks -.b
32 Bayliner

 

If I were doing this I'd give the meranti 2 coats of epoxy on all 6 sides as a sealant. As long as you reapply your resin with 24 hours or so you'll get a good chemical bond. I suppose the tighter the laminating/coating schedule the better but 24 hours is fine according to the manufacturer I use (system 3).

So you have a couple of seal coats on the new core. Assuming the inside of the transom is clean & dry, mix up some medium/slow resin add some fumed silica to form a glue and trowel the thickened resin into those grooves on the inside of the transom. Before the thickened resin starts to gel, apply additional thickened resin to the inside of the transom and to the new core and install the new core. Clamp, brace, temporarily screw or do whatever you need to do to draw the core and outer transom together. I'd want to see some resin squeeze out to know that I'd achieved a good bond. This installation needs some forethought so that the process runs smoothly. You're going to be time constrained as you want to get the core in position prior to the resin curing.

Once the bond is made between the core and the outer transom the rest is easy and it sounds like you have a good plan.

As far as the 3 layers of 17 oz cloth are concerned, I'll defer to some of the engineers out there. I believe that 3 layers would be sufficient.

MIA

 

after the grinding (long after)

So I've glued in the stbd and port sections in the transom. Below is the port side showing the clamping. Epoxy Products resin and ez thick has been great to work with. Next is the inner "skin".

 

Use milled fibers instead of wood flour in your mix. If you must use the wood flour, use a 50/50 mix with milled fibers, especially in the hull shell fillets.

Excellent use of wedges, BTW. Sometimes you have to be inventive and this is perfect example.

You don't need to skin the internal portions of the core. These do need to be well coated with straight epoxy - 2 coats initially and a thickened third, when ready to bond to the previous layer. Used a notched trowel to apply the thickened goo. The deeper the irregularities in the bond surfaces, the deeper the notches need to be. Simply put, us a 2:1 rule, if the deepest area is 1/8", then use a 1/4" notch.

Use biax for the tabbing, not cloth. If you have to use cloth, place it on a 45 degree bias, so all the fibers will cross the joint or transition area.

I always tell folks to sand after about 18 hours, even though some formulator say you'll get a chemical bond within 24. Use the finger nail test to determine this. If you can dent the epoxy with modest pressure from a finger nail, you're still good for a chemical bond. If you have to press fairly hard, don't risk it and "tooth" the surface and plan on a mechanical bond.

Your transom doesn't need addition thickness, as it's not very highly loaded, with those inboards.

3 layers of 1700 biax will be about 3/32" thick. 4 layers is just shy of an 1/8" and 5 layers is just over an 1/8". Since you're using biax, 4 layers will be fine.

 

now to the tabbing

Thanks PAR, this is very helpful (as usual). The surface I removed just had a thin layer of chopped strand blown on after the core structure had been placed in hull back in '86. So - I wasn't looking to put much back on. I like the idea of a resin coating with a thickened top coat.

When I cut out the rudder mount through hulls I was very impressed with the thickness and bond between the wood and the glass in the transom. (there was a piece of roving laminated to the hull early in the first repair attempt when I was contemplating using multiple layers of glass to patch part of the transom core, instead of ripping the whole thing out, which is why the rudder mount holes aren't weeping goo in this pic)

I took a piece home and tried some distructive testing - the plies in the maranti give up before the bond.

 

chemical bond, decreasing over time, is any time between tacky and about a week. Epoxies take a week or more for full cure - some chemical bonding during that entire period. That said, I've never seen any numbers for improved performance from 'chemical bonding' - we just assume it helps. No doubt more then enough mechanical bonding with epoxy outside the 'recoat window' - I tell customers if you can recoat within the week of curing, great, but don't lose any sleep if you miss the window.

Paul Oman - MS. MBA
A.K.A. “Professor E. Poxy”
www.epoxyfacts.com
epoxies since 1994
Member: NACE (National Assoc. of Corrosion Engineers) -- SSPC (Soc. of Protective Coatings)

 

after the grinding (long afer)

Thanks Paul. My next bit of DB1700 is on the slow truck, so I'll likely be prepping and filling until the 10th or so. I thought I had a bunch, but I opened the roll and found less than a yard (x60")