TOPIC: 1960 redfish seashark

Hi guys,
I have a 1960 redfish seashark that i started the restoration process. I have stripped the boat down to the fiberglass floor and transum. all of the original balsa wood was rotted, I stipped it all out and ground the floor flat so i can lay a new mat down and replace the wood. The only problem is i'm not to sure if i can just use plywood and then lay another mat over that or not

You are planning on replacing the Balsa aren't you.

FiberGlassic's new head-honcho is a fiberglass repair professional and wrote up the following article on the suitability of plywood for core material:

glassclownboats.com/PlywoodCores.aspx

In past projects posted here on the forum, folks have used anything from balsa to urethane foam for boats such as Redfishes with a cored floor. Try searching the forum with these keywords "balsa" or "urethane" and you will find lots of good discussion including this one:

www.fiberglassics.com/fiberglassics-forums/main-forum/balsa-core-hull-replacement-guide#20239

Balsa is used on the floor. It's not all that hard to replace. Express Composites has it. It makes a fiberglass sandwich that is very strong and light weight. You can't replace it with plywood, and I would never use marine plywood. You could put stringers in which might be the best bet if you don't want to use Balsa, but then you need a plywood floor.

Boat builders used end grain balsa as a core material because, when dry, it creates a very stiff sandwich construction that's also very lightweight.

All good advice in this thread so far.

Balsa commits suicide shortly after any moisture gets at it. For hulls, most builders moved away from balsa core by the 1970s because it was so problematic.

If it were my boat with a cored hull/floor. I would spend a little extra money and use either rigid urethane foam or a honeycombed core like Airex (there are other brands as well).

In a Redfish, the balsa core can be attacked by moisture from two directions; either from improperly bedded hardware secured into the floor, or by holing the outer skin. Either one will eventually turn the balsa to mush.

50s flash is right about the plywood in this case. The reason for the balsa core is to keep the boat lightweight, adding plywood would work against that end.

Regardless of your choice of core materials, the most important aspect of the job will be the surface prep. Make sure to remove 100% of the balsa residue, abrade the clean fiberglass surface, vacuum, and solvent wipe before adding a new core. Without a good bond to the substrate, the core can't do it's job.

I have never used airex or anything else, Depending on cost, or what you want to use the boat for the extra money may be well worth it. I have never seen a good balsa core yet, but, if it lasted for 50 years uncared for, and you don't intend to leave it in the water, or setting outside where water can get at it. it may last longer than I do. I don't like reinventing wheels. I love Larson's system. They used lapstrake design, which in itself had strength, then just laid down some Oak strips and glassed them in. Simple light, and strong. If everything is glassed in, and Gel-coated water shouldn't get at it.

I see you're a big fan of Marine Plywood. CDX and Marine use the same glue, and both woods rot if they get wet. I use CDX and give it a couple coats of resin and Acetone 50/50. This does 2 things 1- it makes the outer layer water resistant, and 2- it adds tremendous strength to the plywood which becomes almost like a fiberglass. Any voids (which really don't affect strength enough to matter) I bondo over before covering with fiberglass. I know if you ever try it you will never buy Marine plywood again.

thanks for the link

Thanks for the help guys ive had it for a 2 years, i have a 1963 merc 850 that i'm also going to rebuild and repaint. Once the warmer weather hits up here in mass i'm going to have at it and will post some pics, and will probably look for some more advice thanks again