Damp framing, replace like for like or ?

[Spot]

I want to replace some 'damp' and 'delaminated' CSM-tabbed marine plywood floors and keel bolt plate on my 21' (6.4m) trailer sailor. I am looking for additional data points, best practices, reasonable lamination schedules, etc. for this repair to my own project boat. I will not be getting it surveyed, writing it off for insurance, or selling the boat until I have completed, enjoyed, and vetted the repairs. There is a longer thread about the boat here:https://www.cruisersforum.com/forums...ml#post3182241

Water seems to have been from a failed cockpit drain that soaked the wood through 'holidays' in the glassing at each floor and there are no other signs of 'grid failure' in the tabbing fiberglass except a couple cracks that appear to be freeze/thaw related and discoloration of the wood from the water from the cabin flooding and slowly leaking keel bolts. That RTV applied by the previous owner did not solve the issue...

Current lams are 3/4" (19mm) plywood w/.180" (4.5mm) polyester and chopped strand mat tabbing. Hull is 5/8" (17mm) thick between keel and keel bolt board (keelson?). Area of repair is 3' (.9m) transverse by 4 feet (1.3m) long and there are other framing members to help support this area that are sound.

Current plan is to use matching marine ply (spoils from helping a buddy fix a power boat) with 3 layers 1708 stitched biaxial +/- 45 degrees w/ mat with epoxy and epoxy over freshly sanded and prepped existing hull. Also planning to make the keel bolt washers 1 size bigger and adding fillets before tabbing and sealing the ply at the keel bolts. The biaxial will be laid to cover fillets and to taper into the hull to reduce 'hard spot' effects.

I saw an early video from Sailing Uma, perhaps #6, on Youtube where they replaced/added floors in a similar fashion using non-structural foam cores to solve a worse problem on a bigger boat with success.

Thanks in advance!

[formosa aus]

I would avoid plywood and use structural foam with glass and polyester or epoxy and consider vacuum bagging. Why use timber which will rot again when there are better alternatives. Tapering edges to avoid stress concentrations. Large area of overlap and roughen for mechanical retention. Use similar thickness of laminate to original but with vacuum bagging you will get a better laminate to resin ratio and hence stronger structure. Bagging is relatively straight forward and there are good videos. Using peel ply over laminates gives good mechanical retention for next layer and epoxy will form chemical bonds to itself. Should try to get existing laminate dry by airflow and heat. Epoxy will resist moisture ingress better than polyester and make sure no laminate is exposed ie consider flow coat.

[Spot]

Originally Posted by formosa aus

I would avoid plywood and use structural foam with glass and polyester or epoxy and consider vacuum bagging. Why use timber which will rot again when there are better alternatives. Tapering edges to avoid stress concentrations. Large area of overlap and roughen for mechanical retention. Use similar thickness of laminate to original but with vacuum bagging you will get a better laminate to resin ratio and hence stronger structure. Bagging is relatively straight forward and there are good videos. Using peel ply over laminates gives good mechanical retention for next layer and epoxy will form chemical bonds to itself. Should try to get existing laminate dry by airflow and heat. Epoxy will resist moisture ingress better than polyester and make sure no laminate is exposed ie consider flow coat.

Thanks for the reply!
Lately I have been considering giving the floor priority down to the hull and then filling in the gaps in between the floors (old keelson) with FR4 plates at the bolt locations. I would do this by removing one or maybe 2 floors at a time and 75mm (3") keelson either side, leaving the keel bolted in place and lightly brace the keel with a board to the trailer, all in an effort not to deform hull. Dig out damp wood and old lam, sand the polyester resin under the floor(s), cut core materials to fit, butter with resin, place in place, fillets to hull, 3 layers 1808 45/-45 +mat with epoxy and a 25mm (1") stagger each side of floor, widest layer down first. I was also consider vacuum bagging it since I have a pump and bagging materials. We are in our summer here so temps are good. It's a deceptively small area but continuously under load so I want to get it right and get back to other repairs and sailing (!)

Flow coat - is that the same as fill coat, hot coat, gloss coat? The resin applied after the initial lamination is done so as to fill in the weave, pinholes, etc?

[psk125]

This boat is a tired (1970's Luger 21?) trailer sailer, not last year's carbon fiber foiler. If the plywood held up for 40 years until needing replacement where it got saturated (and the rest of the original ply and internal structure apparently seems OK according to the OP), there is no need to go exotic with fancy materials or techniques. They might take time to assemble and learn how to use, besides adding costs. The plan that Spot outlines seems reasonable, cheap, and effective.

[Spot]

Originally Posted by psk125

This boat is a tired (1970's Luger 21?) trailer sailer, not last year's carbon fiber foiler.

Hey! I resemble that remark!
Yes, it's the Luger Southwind 21.

Originally Posted by psk125

If the plywood held up for 40 years until needing replacement where it got saturated (and the rest of the original ply and internal structure apparently seems OK according to the OP), there is no need to go exotic with fancy materials or techniques. They might take time to assemble and learn how to use, besides adding costs. The plan that Spot outlines seems reasonable, cheap, and effective.

Beautiful day, spent some time in the hull. The Rockwell deconstruction blades (for wood and metal, not specifically carbide) were cutting CSM at about 1 inch (2.5cm) per second. The ply still had a bond to the CSM but all the ply was damp and de-laminated at all inner levels. There was one spot under the keelson that had standing water. I can only imagine the freeze/thaw forces at play with wet plywood...

The original 'floor' was just a strip of 3/4" ply. It was not glassed underneath, not connected to the hull save for the tabbing. It was not straight across the sole, it was bowed towards the keelson with a screw. Seems like 5 minutes extra work, either shimming the center or tapering underneath on the ends could have made for a better job, along with sealing the bottom of the floors.

[formosa aus]

Yes flow coat is the last coat usually painted using a brush. When you remove the peel ply it leaves a rough surface improving the bonding of the flow coat to the laminate. The peel ply also reduces the need for sanding the laminate.

[Spot]

Originally Posted by formosa aus

Yes flow coat is the last coat usually painted using a brush. When you remove the peel ply it leaves a rough surface improving the bonding of the flow coat to the laminate. The peel ply also reduces the need for sanding the laminate.

Thanks.

Donned my Tyvek suit and did some sanding. Realized I do not have the right consumables for heavy grinding. Discovered another strata or two of 'wet' under the keelson area. Chipped away until pieces started breaking off dry and will check again tomorrow.

[formosa aus]

I would not skimp on materials that hold my keel bolts. See Farr x2 lost its keel on a recent thread. If a brand new yacht can loose its keel, would you want plywood which is going to rot holding yours on? If you are going to the effort of repairing the keel frame I would not use wood in this day and age when there are alternatives such as Coosa board. This is just my opinion

[Spot]

Originally Posted by formosa aus

I would not skimp on materials that hold my keel bolts. See Farr x2 lost its keel on a recent thread. If a brand new yacht can loose its keel, would you want plywood which is going to rot holding yours on? If you are going to the effort of repairing the keel frame I would not use wood in this day and age when there are alternatives such as Coosa board. This is just my opinion

Here is a plan I am considering, removing plywood at keel bolts and replacing with FR4. Along with that I'd change the floors from strips to full depth floors.

[Spot]

The fiberglass seems to have dried out nicely.
I did a test template for the full depth floor using scraps and a hot glue gun to capture the curves and transfer them to a scrap of thin plywood. I like the look so far.

[formosa aus]

Looks awesome!

[Spot]

Originally Posted by formosa aus

Looks awesome!

Thanks. I've put in a brace between the side berths, which are tabbed to the hull, and pulled out 2 more floors going aft. My oscillating blades are dulled so I want to get a couple fresh ones in the morning to finish up the forward floor and the ends of the keelson.

[Spot]

I have the rest of the floors cut out. The last floor let out 2-3 shots worth of water as I cut through the tabbing. I forgot to brace the hull and it sagged about 3/16" (4.5mm) as compared to the template I made earlier. It is now jacked up and shimmed into position.

I also 'failed' with dust control. I was wearing a 'bunny suit' and mask but should have installed plastic sheets to keep the ground resin and CSM dust from going 'tip to tail'

I am still thinking of using G10/FR4 for the keel bolt locations but (re)considering plywood versus foam for the floors. Since those will no be in compression. would a 5-10-15 PCF foam work as well?

[formosa aus]

I am not familiar with your type of boat but from your photos it looks like the “floors” are transferring torsional forces both tensile and compressive from the keel to the hull. They seem to be spreading the forces on the keel over a much larger lateral surface of the hull. I am not an expert but would think you would need structural foam with similar properties to wood as a core. I am sure there are lots of boat builders on the forum who could advise you better than me. Good luck!

[Spot]

Thanks, I appreciate the thoughts and well-wishes.