Re: Project Rubberbelly Rebirth - 1966 Larson Lapline
IF it's exterior grade, or made w/ waterproof glue, most plywood would work. Auraco (sp?) was quite popular but apparently a fire at 1 of 2 manufacturing sites, has made it scarce in some areas, underlayment, douglas fir plywood and possibly others......
My biggest 'complaint' of dimensional lumber use, would be it is typically less straight when you 1st buy it, and it is much less stable (prone to warp & twist) after you buy it. But left un-secured, or exposed to heat & moisture, especially cycling thru both, ply will go all potato chip looking fairly quick & fairly easy too.
It may require a trip or 2 to a local lumber place(s) to see what they've got & then check for yourself as to it's makeup. Not that the guy at the lumber store was deliberately mis-leading you @ the poplar. He either didn't know, or had been mis-informed, specifically to use in a boat under the deck. Poplar isn't particularly strong, or stable and also isn't particularly dense or resistant to rot.
If you don't hear back from YachtDr, WOG, oops or someone else about the specific makeup of your hull~stringer system, you'll need to PM 1 of them & just ask them to look in on your thread that your stumped or that you have questions on how to proceed. Don't ask the questions in the PM, they may not be answered & infact cause your PM to be ignored. Many of the "pro's" prefer to keep Q&A's in the public forum. In a PM they are not serving the masses & available later to others that follow.....