Hey well I just went to the hardware store to try and find something to fix the small crack in my plastic pelican boat and he gave me 3M Marine Adhesive Sealant, but it says fiberglass and wood. I always thought my boat was fiberglass anyways but some on here have told me it was plastic. So will this stuff work? If so how do you use it?

It will work to fix the leak and will flex with the boat but you need to stop the crack from getting any bigger first.  Your best bet is a plastic welder but not everyone has access to someone who can do that.  It will not be a lifelong fix either.  the marine adhesive/sealant (5200) is designed as a waterproof glue.  It will work for a while but this is not it's intended purpose.

3M adhesive sealant isn't intended to seal up structural cracks or leaks.  You need a product called Marine Tex or, at the very least, 5200.  

how would using it make the crack bigger? if it wears off?

It won't make the crack bigger, but it will not stop the crack from growing.  

3m 5200 marine adhesive/sealant is the product.  It is a temporary fix once the crack has been stitched as shown in the other post.

to stop the crack from growing.  you need to drill holes a the very ends of the crack.  Then do what ever repair method that you choose and hope for the best.  

before drilling

 -----------

after drilling

O--------O

Make sure that inner part of the circle is within the crack!!  Very important.  If it is not the crack can move around the hole that you drilled

CGH- I don't understand how that will stop the crack, I've "sewn" one before (plastic canoe) and we did it like stitches.  Crisscrossed all the way up the crack.  Like shoe lacing.

CGH- I don't understand how that will stop the crack, I've "sewn" one before (plastic canoe) and we did it like stitches.  Crisscrossed all the way up the crack.  Like shoe lacing.

It is called stop-drilling like I pointed out in the other post about this boat.  The holes on the end of the crack captures the ends of the crack and stops the crack from growing any bigger.  I do it quite often on aircraft structures. If you drill the hole and don't get the very end of the crack, you will soon have a bigger crack with a hole in the middle of it!

That is interesting, it would make for a great tutorial.  If you can, take some pics next time you have to do it.  How big of a bit would he use?

CGH- I don't understand how that will stop the crack, I've "sewn" one before (plastic canoe) and we did it like stitches. Crisscrossed all the way up the crack. Like shoe lacing.

PS- I didn't mention this before because it didn't work

drilling a crack is an old trick grasshopper but work my stepfather showed me that a while back.  And he showed me how to get out of the house when the women folk are cleaning up after dinner ;D