Polysulfide vs Polyurethane - Below the Waterline

[Texas Boater]

I started doing some research and now I am confused all over again!

From the data provided on all the marine sealant/adhesive manufacturer’s websites it appears that a POLYSULFIDE would be the best if not only logical choice for bedding the outer bottom planks to the inner plywood bottom of my CC. But when I read advice from ship/boat-wrights they all seem to favor POLYURETHANE (if they favor a modern chemical sealant in this location at all).

So what gives – why use polyurethane that will bond the outer planks tenaciously to the inner plywood and not provide as flexible a sealant as polysulfide that is more flexible and appears to be easier to remove for future repairs?

I could not find any scientific study of sealant properties of polysulfide versus polyurethane for use below the water line when subjected to the pressure, vibration and impact loading encountered by a typical boat bottom.

So what am I overlooking? Why does it seem that professionals prefer polyurethane (namely 3M 5200)?

[Hunter25]

Many people, including myself up until a year ago, were under the impression that 5200 and other polyurethanes where the thing for underwater use. This is true for the most part, but there are exceptions, mainly on wood that will get soaked. Polysulfide is the choice when this is the case. So you are right to think this and the industry is catching up, but there are plenty of people still firmly attached to the great bonding power of 5200, even though it will not stay stuck to wet wood, it does to most everything else.