When to tell if a fiberglass bass boat transom is bad(spider cracks in gel coat)?

I would be more concerned about the loose jack plate than the crazing. How did the bolts get loose? If the weight and force of the outboard is causing the transom to compress, this will cause the bolts to be loose. If your jack plate has been installed with only bolts and plate washers, this may be the case. Have a really close look at the transom for signs of indentations caused by compression. You may have to add plates to both sides of the transom to spread the pressure over a larger surface. I have a high hp outboard mounted to a CMC plate on a river racer style hull, and made plates approximately twice the size of the jack plate. I made mine out of 3/16" 6061-T6 aluminium plate, and had to form and weld the inside plate to fit around a transom support. I also ran diagonal supports from the inside plate to the hulls' strongbox, for added support.

The boat has been around almost 12 years I would expect to see some wear and tear somewhere. Spider cracks are not an indication of the boat self destructing there would be more visible damage somewhere such as separating stress points perhaps rust and large cracks around any holes that go through the hull itself Fiberglass is some really durable stuff. It had a gtendency to driy out over time like in an old car. But it should last dang near forever in a boating environment where the gel coat has been well kept on the boat.

if you're going to do it, do it right. don't worry about the value of the boat, with a strong transom it'll last you another 30 years or more. first, understand that rot is a bacteria, it keeps spreading unless killed or removed completely. you have to remove the top cap and dig all the old wood out of there, 100% of it, using the tool of your choice. I've known people to use a chain saw but don't recommend it -- you don't want to damage the external fiberglass hull. pound for pound, plywood is stronger than steel. You want marine grade ply, which is hard to find and very expensive. Exterior grade A/B or B/C will do, but don't use interior grade, it'll start delaminating almost before you get it home. More than double the strength of the transom by laminating together 2 pieces, each 1/2 the width of the transom, cut on a cross bias (vertical grain on one piece, horizontal on the other). Cut them to fit the transom, then saturate your wood in a high-quality marine epoxy fiberglass resin (without catalyst), just keep painting on coats until it won't absorb any more. It'll absorb more resin at higher temperatures, so if you have heat lamps, use 'em. Glue the two pieces together with your epoxy resin (catalyzed), and clamp 'em together using every clamp you own plus all you can borrow or steel. Drill a couple holes and use nuts bolts and washers to hold 'em tight in the middle. Once the glue is cured you'll remove the bolts and fill the holes with fiberglass. You now have a panel that is hundreds of times stronger than a hardwood panel of the same size. Measure carefully and drill the holes for splashwell and bilge drains, livewell intakes and drains, and any other through-hull fittings. Drill out the holes a little larger than needed, fill with epoxy and, when cured, drill the size hole you need. Now the edges of the holes are sealed and protected from water intrusion. Slide your new transom into place and use fiberglass tape and gussets made from fiberglass putty to affix it to the hull. With a little bit of research you can find some illustrated how-to's on the subject, it's too involved for me to type it out here. Don't be tempted to use polyester resin, it's find for interior floors and decks, but not the transom, for that you want a good epoxy resin. It's hot, dirty work, but you can do it.

Gel-coat cracking is common around there .

A good test is to belt it with the side of your clenched fist and see if it is solid .

For a quick beef-up repair just get a big offcut of waterproof ply about 9mm thick and through-bolt that to to the transom ( use stainless steel bolts with nyloc nuts and big penny washers , all in stainless steel , washers inside and out , at least four nuts and bolts and washers sets .

Your transom is only bad if the plywood sandwiched in Fiberglass is rotten